CLASSIFICATION AND DISTRIBUTION OF SOUTH ATLANTIC RECENT POLYCYSTINE RADIOLARIA1

Demetrio Boltovskoy

ABSTRACT

This paper presents a review of the current knowledge on the identification and distribution of Recent polycystine Radiolaria so far recorded, or presumed to occur, in the South Atlantic Ocean (0° to 60°S, from the South American coasts to the coasts of Africa). However, because the area concerned covers from equatorial to Antarctic waters, and since polycystine radiolarians are geographically (but not environmentally) cosmopolitan, the review covers most common species worldwide. Illustrations, short diagnoses, bibliographic references and distributional data (both geographic and vertical) are included for 164 polycystine morphotypes (species-groups, species, and subspecific categories). Introductory remarks offer general data on radiolarian anatomy, biology, ecology, and reproduction. Methodological aspects are dealt with in some detail, with special emphasis being placed on comparative aspects of the environmental and paleoenvironmental information conveyed by planktonic materials, sediment trap samples, and sedimentary deposits. Known or assumed geographic and vertical species-specific distribution ranges are summarized, as well as available information on absolute abundances in the water-column (plankton and sediment trap samples) and in the surface sediments. The illustrated glossary aimed at the less experienced student defines the terms and morphological details useful for diagnostic purposes.

Demetrio Boltovskoy, Departamento de Ciencias Biológicas, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, 1428, Buenos Aires, Museo Argentino de Ciencias Naturales "Bernardino Rivadavia", and CONICET, Argentina

KEYWORDS: Radiolaria, Polycystina, identification, distribution, Recent, plankton, review

1This article constitutes a modified version of the chapter "Radiolaria Polycystina", originally prepared for the book "South Atlantic Zooplankton" (D. Boltovskoy, ed.), a ca. 2,000 pages guide on the identification and distribution of 30 zooplanktonic groups to be published in 1999 by Backhuys Publishers, The Netherlands.

1 August 1998
Submission: 31 August 1997, Acceptance: 15 April 1998
http://www-odp.tamu.edu/paleo/1998_2/boltovskoy/issue2.htm

 

INTRODUCTION

Polycystine radiolarians are exclusively marine, pelagic, solitary or colonial protists provided with actinopods. Polycystines comprise the Collodaria, a small group lacking a skeleton, or provided only with scattered mono- or polyaxonic spicules; and the Spumellaria and Nassellaria, most of which have a well developed siliceous latticed or spongy skeleton (see "Sedimentary vs. water-column materials."). Solitary species (the greatest majority) range between 20-30 µm to about 300 µm, but colonies (some Collodaria and the spumellarian family Collosphaeridae) may in exceptional cases be as long as 3 m (Swanberg 1979). The siliceous skeletons of the polycystines are a major contributor to the sedimentary flux, their earliest records dating back to the Cambrian. Paleozoic, Mesozoic and Cenozoic sequences furnish detailed records for evolutionary, stratigraphic and paleoecologic analyses.

A distinguishing feature of all radiolarians (polycystines and phaeodarians) is the central capsule, a proteinaceous perforated membrane that divides the cytoplasm into two areas: the endoplasm or intracapsular cytoplasm, and the calymma or extracapsular cytoplasm (Figure 1A). This central capsule is either spherical (in many Spumellaria), or elongated and pyriform (in most Nassellaria, Figure 1B). The intracapsular cytoplasm contains reserve substances and major cytoplasmic organelles (nucleus or nuclei, mitochondria, and other organelles, except for the digestive vacuoles), and is generally believed to be responsible for the functions of reproduction, biochemical synthesis and energy production. The calymma is the frothy or web-like extracapsular cytoplasm where the digestive vacuoles are located. Algal symbionts, when present, are enclosed within vacuoles usually located in the calymma. Colonial forms have a gelatinous sheath containing numerous central capsules interconnected by a rhizopodial network.

Polycystine skeletons are typically constructed of a network of structures which can be either connected at both ends with other elements - the bars (Figure 2F), or formations attached to the rest of the shell by one end only - the spines (Figure 2A-B, 2G). All skeletal elements are composed of amorphous silica (SiO2 nH2O). There is a perplexing variety of shapes in which these bars and spines can be arranged in order to form the skeleton, from simple latticed spheres or a few anastomosed spines (Figure 15.119), to elaborate constructions with several concentric spheres (Figure 2B) or multilocular conical structures with protruding latticed or solid appendages known as wings, feet, teeth, etc. (Figure 3O-P, and 3Q-R).

Very little is known about the reproduction of the Radiolaria. In addition to vegetative reproduction (Hollande and Enjumet 1953), the production of biflagellated swarmers was observed, but it is not known if the swarmers are asexual dissemules or motile gametes (Anderson 1983a). Although no direct estimates have been made so far, it is generally assumed that individual radiolarian life spans are around two to four weeks (Anderson 1983a; Caron and Swanberg 1990).

Polycystines consume a wide variety of prey including bacteria, algae, protists, copepods, appendicularians, and other small zooplankton (Anderson 1983a, 1993; Caron and Swanberg 1990). Algal symbionts, when present, secrete photosynthetic products that are assimilated by the host as a nutritional source (Anderson 1983b).

The first published descriptions of Radiolaria date back to the early nineteenth century. Between approximately 1850 and 1900, C. G. Ehrenberg, J. Müller, R. Hertwig, A. Popofsky, and especially E. Haeckel described thousands of new species and provided the first comprehensive classification systems (Riedel 1967a). After a period of little activity, interest in the Radiolaria was renewed around 1950, and somewhat later further fostered by the rich sedimentary materials recovered by the Deep Sea Drilling Project.

Because of their application to stratigraphy, polycystine studies have traditionally been within the realm of geologists/paleontologists, with biologically-oriented publications representing less than 10% of the overall total produced to date (A. Sanfilippo, personal commun., 1997). The directory included in the 1994 issue of Radiolaria (Newsletter for the International Association of Radiolarian Paleontologists) lists 400-plus names; however, only 100-150 of these are primarily concerned with radiolarian studies. Almost all these workers are geologists focusing their interest on stratigraphic and paleoceanographic problems, especially dealing with Paleozoic and Mesozoic deposits; interest in Cenozoic faunas has been dwindling over the last few years. Biologically-oriented research based on samples from the water-column has even fewer specialists, and at present they are probably less than 10-20 world-wide. Since 1834 approximately 3500 works on polycystine radiolarians have been published (over half of these on Cenozoic faunas, about 35% on Mesozoic, and 15% on Paleozoic; A. Sanfilippo, personal commun., 1997).

 

METHODS

Provenance and collection of materials

Most of the surveys on extant polycystine radiolarians published to date are based on samples of their skeletons preserved in the surface sediments, rather than on plankton samples. Sediment samples have some advantages over water-column materials, but also several important shortcomings (see Sedimentary vs. water-column materials).

A variety of sediment coring and grabbing devices have been used throughout the years for analyses of the polycystines from the upper centimeters of the sediments (Kennett 1982). Gravity and Kasten corers are among the simplest, permitting retrieval of up to a few meters of sediment at a time from practically any depth. Piston corers have been used widely due to their ability to recover long sedimentary sequences, up to 20-30 m in length. However, all these devices tend to disturb the sediments, especially the uppermost layer which is of particular importance for the analysis of Recent assemblages. Box corers (rectangular, shallow, ca. 1 m coring boxes which ensure complete closure of water-flow passages after sampling and before leaving the seabed, thus minimizing sample washout during ascent) are preferable for retrieval of the top layer of the sediments. However, the fact that box core samples usually lack the thin uppermost phytodetrital film characteristic of most sediments (Billett et al. 1983) suggested that the bow-wave of the device is strong enough as to wash away any mobile particles before hitting the bottom. Multicorers, an arrangement of several short coring tubes mounted on a rigid frame seem to overcome this problem successfully as they have been shown to collect phytodetritus, as well as significantly higher numbers of macrobenthic specimens than box corers (Bett et al. 1994).

Plankton samples for radiolarian studies are usually collected with nets. However, this group, as well as a few other microzooplanktonic taxa, pose serious methodological difficulties. Indeed, they are too small (around 20-30 to 300 µm) to collect effectively with standard zooplanktonic nets (100 to 300 µm in pore size), yet too scarce in most areas to yield adequate catches with water-bottles or low-powered pumps. Thus, fine-meshed nets have to be employed, which significantly complicates not only the concentration of the radiolarians (due to the concomitant retrieval of other organisms, some of which, like the diatoms, cannot be fractioned out later; see Swanberg and Eide 1992), but also because net clogging jeopardizes subsequent estimations of the volume of water filtered (Tranter and Smith 1968; Boltovskoy 1981b). In order to avoid clogging by smaller particles, thus ensuring better estimates of the volume of water filtered and larger sample-sizes, meshes ranging between 60 and up to 100 µm are traditionally used for polycystine studies in the water-column. It should be stressed, however, that both absolute quantitative estimates of radiolarian abundance, and the proportions of at least some species and developmental stages may be seriously biased in these collections: Boltovskoy et al. (1993a) reported that in sediment trap materials from the tropical Atlantic shells below 40-60 µm represent roughly 50% of the overall polycystine fauna.

Estimates of radiolarian abundances in the water-column must be performed with flow-metered nets; clogging of the meshes, in particular of those with small pores, makes assessment of the volume of water filtered based on distance towed and mouth diameter extremely unreliable (Tranter and Smith 1968; Boltovskoy 1981a, b). Thus, whenever unflowmetered nets are employed, such as those derived from Tucker's (1951) opening-closing mechanism (e.g., the Multinet, based on Bé's 1962, design; the MOCNESS, Wiebe et al. 1976; the RMT 1+8, Baker et al. 1973), it is strongly recommended that evaluation of radiolarian concentrations be avoided (species proportions, on the other hand, are in principle unaffected in these samples).

For assessment of the delicate colonial forms, as well as for studies of feeding, growth, metabolism, etc. of live individuals, specimens are collected by divers (e.g., Swanberg 1979), or by means of very short and slow plankton tows, thus ensuring a better preservation of the protists (Matsuoka 1992).

Sediment trap techniques have undergone major improvements in the last years, thus constituting a very useful tool for the collection of polycystine materials (US GOFS 1989; Lange and Boltovskoy 1995). Simple sediment traps consist of a concentrating cone or funnel which tapers into a collecting jar; the array, which can have either one or several traps, is moored to the bottom or drifts with the current suspended from a buoy at the surface. Time-series models are deployed at different oceanic locations for periods up to a year or more, and are provided with a mechanism which replaces the collecting cup at predetermined intervals thus yielding a detailed record of the changes in the amount and type of flux throughout several seasons (Honjo and Doherty 1988; Lange and Boltovskoy 1995).

Sediment-trap materials have some important advantages over planktonic collections. Sample-size is usually much larger in sediment traps than in plankton nets, with fluxes as high as 200,000 shells/m2/day having been recorded in the equatorial Atlantic (Boltovskoy et al. 1996; see also table 3 in Boltovskoy et al. 1993a). Seasonal plankton collections are composed of a sequence of snapshots which represent but an insignificant proportion of the total time elapsed between tows, and may therefore not only under- or overestimate mean protist abundances (e.g., Bé et al. 1985), but also yield "atypical" specific assemblages. Time-series sediment trap samples, on the other hand, integrate over preselected depth and time ranges, thus averaging the overlying plankton over restricted periods which yield adequate chronological resolution to allow pinpointing the relative importance of limited offsets of the yearly cycle. Furthermore, since seasonal variations in total mass flux are usually closely coupled with primary production in the upper mixed layer (Honjo et al. 1982, 1988; Deuser et al. 1983, 1990; Wefer 1989), comparison of total flux vs. radiolarian numbers and specific makeup can furnish first hand information on indicators (and paleoindicators) of the biological productivity of the associated water masses.

Sediment trap materials, however, also have some shortcomings. Because of limitations associated with the hydrodynamic properties of particle accumulation in the traps, these devices are most effective when deployed at depths in excess of 500-700 m (US GOFS 1989; Lange and Boltovskoy 1995). As a result, they integrate the flux from several biologically dissimilar layers (e.g., Kling 1979; Kling and Boltovskoy 1995). Furthermore, sinking skeletons intercepted at these depths may not adequately reflect their standing stocks at the surface, nor their specific composition. Boltovskoy and Alder (1992) concluded that, in the Weddell Sea, over 90% of the polycystines that inhabit the upper 400 m are destroyed (probably due to fragmentation by grazing) before reaching 400-900 m of depth. Subsurface advection of shells produced at higher latitudes and integration of low protist abundances over large depth intervals may be responsible for the fact that, in the eastern equatorial Atlantic, polycystine assemblage compositions recorded in plankton samples at 0-300 m are totally different from those recovered in traps at 800-2000 m (Boltovskoy et al. 1995; Figure 4A, 4B).

It should be borne in mind that the yields of sediment trap samples are not amenable to direct comparisons with those of plankton samples: while the former are an expression of the downward flux, which in turn is associated with productivity and preservation, quantitative plankton samples give information on standing stock only. Hence, compositional differences may not only reflect advection, destruction by grazing, etc., but also biological traits of the species considered. Thus, a scarce species with high reproduction, mortality and output rates may be rare in the plankton but abundant in the underlying sediment trap (Kling and Boltovskoy 1995; Boltovskoy et al. 1995).

As with other zooplanktonic groups, analyses of radiolarian vertical distribution patterns are usually performed with the aid of vertically stratified plankton tows (e.g., Renz 1976; Kling 1979; Dworetzky and Morley 1987; Kling and Boltovskoy 1995; Abelmann and Gowing 1997). However, because their identification is based on the siliceous skeleton which preserves after the death of the cell, in order to discriminate live vs. dead protists in the subsurface layers the cytoplasm is often stained with Rose Bengal, Sudan black B, or eosin (Petrushevskaya 1971b; Swanberg and Bjųrklund 1986; Abelmann and Gowing 1997). Although this technique can furnish some clues on the living depth ranges of the species, it does not provide unequivocal information because of uncertainties associated with the speed of decomposition of the protists' cytoplasm. Boltovskoy and Lena (1970), for example, concluded that specimens of several planktonic foraminifera still contained protoplasm in their shell 98 days after death. Bernhard (1988) compared estimates of the proportions of presumably live benthic Foraminifera as indicated by Rose Bengal and Sudan black B staining and by ATP assay, concluding that stained protoplasm was present in individuals up to four weeks after actual death of the cell. These lapses are significantly longer than the time it takes a radiolarian shell to reach the sea-floor (Takahashi and Honjo 1983).

Unless special cytological studies are required (e.g., Petrushevskaya 1986), plankton and sediment trap samples can be preserved in 4-5% formaldehyde; the addition of picric acid to the solution enhances the preservation of the colonies, yet acidification should be avoided if the calcareous plankton is to be saved from dissolution.

Sample preparation and analysis

The following section offers some general comments on the preparation of whole samples for routine counting and identification procedures. It does not review the methods involved in special cytological and ultrastructural studies (see Anderson 1983a, for a review of these topics), as well as those used for detailed taxonomic work, which can involve thin-sectioning, etching and polishing, etc. (Riedel and Sanfilippo 1977; Boltovskoy et al. 1983; Petrushevskaya 1986).

Pelagic surface sediments are usually clean enough as to require little treatment before preparation of the slides. Elimination of the organic matter and disaggregation of the materials is achieved by boiling the sample (5-10 g) for a few minutes in a beaker with water to which hydrogen peroxyde (10%, 300 ml per liter) and tetrasodium pyrophosphate (10 g per liter) have been added. Disaggregation, cleaning and removal of clay coatings and infilling particles can be aided by treating the sample in a gentle ultrasonic bath. For further disaggregation of heavily indurated sediments various products, such as kerosene, paint thinner, or ammonia can be helpful (the sediment is dried, soaked in the solvent, and then immersed in water, upon which disaggregation usually occurs rapidly). If calcareous material is abundant it can be removed with a few drops of hydrochloric acid (after eliminating the hydrogen peroxyde by wet-sieving). The resulting clean material is then sieved with abundant water in order to eliminate the reagents and smaller particles. The mesh size used depends on the aims of the study; most surveys routinely employ 40-60 µm-meshes, yet these, as described above, miss many of the smaller species, as well as most developing forms. If precise abundance estimates are sought, mesh openings around 15 to 20 µm should be employed, although these will retain large numbers of unidentifiable skeletal fragments, as well as non-radiolarian material (especially diatoms), which can make subsequent observation more laborious. The clean residue in the sieve is pipetted onto glass microscope slides, dried, and soaked with a few drops of xylene; before the xylene has evaporated the mounting medium is added and covered with a cover glass. Canada Balsam is most often used for these preparations, although it takes longer to harden than some other synthetic materials, commercially known as Norland, Pleurax, Hyrax or Depex.

Moore (1973) proposed a convenient method which allows quantification of the number of radiolarian shells per unit weight of sediment. Before processing as described above, the sample is dried and weighed. This weighed sediment is then cleaned and sieved, and all the resulting residue is poured into a large (e.g., 5 l) beaker full of distilled water, on the bottom of which one or two cover gasses have been positioned. The water with the sediment in the beaker is then thoroughly stirred (avoiding rotational motion, which will result in centrifugal fractionation) for achieving a random distribution of the particles, and the sediment is allowed to settle. With the aid of a siphon all but 30-50 mm of water are removed, and the remainder is evaporated with an overhead infrared lamp. When the surface of the cover glasses is dry they are removed from the beaker and mounted as described above. The slide thus prepared will contain a fraction of the radiolarian shells present in the original sample, this fraction being equivalent to the proportion that the surface of the cover glass makes of that of the surface of the bottom of the beaker.

Preparation of plankton and sediment trap samples is somewhat more labororious due to the large amounts of organic material they contain. When both absolute radiolarian concentrations and specific inventories are sought, it is recommended that counting be performed separately from the identifications. Polycystines can be counted (although not identified) in whole, unprocessed samples in counting chambers under the inverted microscope (Hasle 1978; Boltovskoy 1981c; Villafańe and Reid 1995). Subsequently, either the entire sample or a subsample can be treated in order to eliminate all organic matter leaving the clean siliceous skeletons that will be mounted as described above for sedimentary materials. It should be born in mind, however, that radiolarian cells are often very difficult to recognize in preserved, unprocessed plankton samples. The siliceous skeleton, usually the most conspicuous distinguishing feature, is obscured by the cytoplasm to such an extent that radiolarians are easily confused with other planktonic protists, fecal pellets, eggs, various organic aggregates, debris, etc. Adding a few drops of hydrogen peroxide and/or hydrochloric acid, which slowly digest the organic matter, and comparing the dubious particles before and after treatment can greatly help to pinpoint radiolarian cells (Alder, personal commun. 1997).

Several different methods have been used for eliminating organic material from water-column samples, including high- and low-temperature ashing, oxidizing with hydrogen peroxide and/or ultraviolet light, etc. (see review in Boltovskoy et al. 1983). One of the most widespread, however, is that proposed by Simonsen (1974) for cleaning diatom frustules. The plankton sample is rinsed with abundant fresh water (wet-sieving), and placed in a beaker to which an equal volume of saturated KMnO4 is added; it is then left for 24 hs. A volume of concentrated HCl equivalent to that already contained in the beaker is subsequently added to the sample; the dark brown liquid is gently heated until it becomes transparent or light yellow. Once the sample has cooled, it is sieved again thoroughly with fresh water and rinsed with distilled water. The residue is pipetted onto microscope glass slides as described above.

Analysis of the specimens is best performed in mounted slides, which by transparency permits observing the internal structures (such as medullary shells, spiral structures, etc.), and the wall-thickness. In addition, slight variations in the depth of field allow one to determine whether a shell of circular outline is a disc (in which case most of the surface is in focus simultaneously), or a sphere (either the central part or the periphery are in focus). Photographs taken in the light microscope have the advantage of being readily comparable to mounted specimens. The scanning electron microscope (SEM), on the other hand, is especially suitable for analyzing the surface morphology, but only in specimens with large openings in the outermost shell, or in those partially broken, can internal structures be observed. SEM photographs produce very appealing results, but their comparison with routine collections mounted in slides is tricky (see Figure 5A, 5B, 5C). Ideally, both techniques should complement each other (Boltovskoy et al. 1983, described a method which allows performing light and SEM observations and photographs of the same radiolarian specimens).

Assessment of radiolarian species-specific absolute and relative abundances are based on identifications and counts. Since any given slide often contains thousands of polycystine shells, the researcher is forced to decide how many specimens should be identified and counted in order to achieve an adequate estimate of overall numbers and species proportions. Several methods have been proposed for the assessment of bias in sample-based particle counts (see reviews in Venrick 1978, 1995; Frontier 1981), and in the appraisal of species proportions (Patterson and Fishbein 1989; Buzas 1990). Patterson and Fishbein (1989) concluded that for species representing >50% of the overall taxocoenosis at least 50 specimens should be counted in order to achieve reliable percentage data, 300 counts for species which comprise approximately 10% of a sample, 500-1000 counts for species that make up 5%, and counts of several thousands for those that comprise 1%. Unfortunately, in the case of the polycystines these efforts are unrealistic because in any given sample containing 100-150 species only one-three are above 10%, and 70-90 occur at levels below 1% (see "Geographic and vertical distribution"). In terms of the amount of information attained, it is more profitable to analyze more samples at a lower resolution, than to examine fewer sites at these statistically more reliable levels. Thus, in practice proportions are estimated in bulk, regardless of the individual species abundances, usually scanning 300-600 specimens per sample. It is common practice to identify the first 300-600 individuals on the slide, and then check the rest of the slide or slides for the given sample in order to account for the rarer taxa. The relative abundances of the latter are estimated approximately, and they are usually excluded from subsequent general numerical analyses (e.g., multivariate techniques, such as cluster and factor analysis) because of the uncertainties associated with their assumed absences. It should be stressed, however, that the counting effort necessary for reliable estimates of the fractional abundance of the rare species is inversely proportional to the equitability of the assemblage. Thus, when the sample is strongly dominated by a single or only a few taxa, such as in polar areas, chances of recording the rare polycystines in random sequential counts are low because the observer repeatedly hits the dominant species. On the contrary, as equitability increases so does the probability of logging a so far unrecorded species with every new specimen scanned.

 

GEOGRAPHIC AND VERTICAL DISTRIBUTION

Sedimentary vs. water-column materials

As opposed to other zooplanktonic groups, studies on the geographic distribution of extant polycystines have been chiefly based on sedimentary - rather than on planktonic - materials. As mentioned above, sediment samples present some advantages, but also several important shortcomings.

Whereas polycystine abundances seldom exceed five cells per liter in the plankton (e.g., Caron and Swanberg 1990), one gram of (dry) surface sediments can contain thousands to hundreds of thousands of radiolarian skeletons. Plankton samples yield a snapshot-type image of the composition of the assemblages, which does not necessarily adequately reflect long-term trends. The daily, seasonal and interannual variability involved is smoothed out in the sedimentary record, which may be a welcome trait when general patterns are sought. Further, sedimentary materials are more readily available from the various repositories around the globe than plankton samples. In any case, plankton samples not collected for microplanktonic purposes may be useless for radiolarian studies due to inadequate net mesh-size.

On the other hand, interpretation of the geographic distribution of extant radiolarian assemblages on the basis of sediment samples presents several important drawbacks (Boltovskoy 1988, 1994, 1995; Kling and Boltovskoy 1995; Figure 6). On their way to the sea-floor and after settling, radiolarian remains are grazed upon by various consumers thus breaking their skeletons into unidentifiable fragments. Because more delicate shells are destroyed more readily than the more robust ones, specific makeups on the bottom and at mid- depths can differ significantly from the living assemblage in the upper water-column (Boltovskoy et al. 1993b, 1995). Selective dissolution of whole siliceous skeletons en route to the sea-floor and after deposition, although often advocated as an important source of plankton vs. sediments dissimilarities (e.g., Petrushevskaya 1971b; Renz 1976), is probably much less critical than fragmentation due to grazing (Boltovskoy and Alder 1992; Morley et al. Ms). Bottom materials can be reworked after deposition (as a result of which non-Recent deposits, sometimes characteristic of quite dissimilar oceanographic settings, are brought up to the surface layer, or winnowed by bottom currents (dislodging settled skeletons and carrying them thousands of kilometers away; Figure 6). Sediments integrate the imprint of near-surface faunas (which are generally associated with surficial temperature, salinity and primary production fields, as well as with currents and water masses), with the meso- and bathypelagic species whose geographic distribution is uncoupled with upper-water oceanography (Figure 6). In general terms the sedimentary distributions of cold-water species tend to show conspicuous equatorward extensions as compared with their planktonic patterns. This distortion is most probably due to the fact that extended survival of the expatriated cold water taxa is facilitated by submersion (Boltovskoy 1988, 1994; Figure 6); as a consequence, sediment-derived species-specific ranges may wrongly suggest an enhanced tolerance to gradients in the ecological factors.

While these limitations stress the need for caution in biogeographic interpretations based on sedimentary materials, the usefulness of bottom samples for biogeographic, paleobiogeographic and paleoecologic purposes has been confirmed in many reports and is certainly beyond doubt. Furthermore, when compared with water-column materials, sedimentary ones can furnish much useful distributional and ecological data (e.g., Boltovskoy et al. 1993b; 1995), usually unavailable from plankton or sediment trap samples alone.

Geographic patterns

Polycystines are typically open-ocean organisms, occurring throughout the World Ocean. However, distinct coastal associations, while uncommon or absent altogether in areas with an extended shelf, such as the Southwestern Atlantic (Boltovskoy 1980), have been described in various studies. For example, Norwegian fjords host dense and diverse radiolarian assemblages, which differ from those of the open Norwegian Sea (Swanberg and Bjųrklund 1986, 1987, 1992). Interestingly, two of these fjord species, Rhizoplegma boreale (probably synonymous with Spongosphaera streptacantha), and Phormacantha hystrix/Plectacantha oikiskos, have been found to strongly dominate (up to 47% of all polycystines) shallow, coastal sediments around Antarctica (Nishimura et al. 1997). General differences between presumably neritic vs. oceanic radiolarian assemblages have been described occasionally in the literature (Kruglikova 1984), and even used for paleoenvironmental reconstructions (e.g., Palmer 1986). However, with the probable exception of specific diversities, which indeed seem lower in neritic assemblages (Nishimura et al. 1997), and the fact that a few selected polycystines are probably less intolerant to near-shore conditions than the bulk, most other traits (such as Spumellaria:Nassellaria proportions, percentages of Spongodiscidae, percentages of "spiny Porodiscidae", percentages of small "Cyrtoidea"; cf. Kruglikova 1984) need further confirmation.

Polycystine densities are typically around 0.3-1 cells per liter, but values exceeding 50 ind. l-1 have been recorded in some productive areas (Caron and Swanberg 1990). The quantitative distribution of polycystines in surface sediments of the South Atlantic is illustrated in Figure 7. This pattern is probably an approximate representation of their concentrations in the water-column as well, and it also roughly reflects the overall distribution of primary production (e.g., Koblentz-Mishke and Vedernikov 1977), and of phytoplanktonic (Semina 1977) and zooplanktonic (Bogorov et al. 1968) biomasses. Highest numbers of polycystines would thus be expected along the upwelling areas off Africa (Abelmann and Gowing 1997), where the highest radiolarian fluxes have been recorded to day (Boltovskoy et al. 1996), and in the equatorial current system. In the southern part of the ocean high densities are probably associated with the subantarctic belt and its northern extensions, the Malvinas (=Falkland) and the Benguela Currents. In a transect between the Antarctic and approximately 30°S, 10°E (off Namibia), Abelmann and Gowing (1997) recorded highest polycystine densities at 100-300 m in Antarctic waters, and at 0-150 m in subantarctic waters (up to 0.3 ind. l-1; these values, however, may be somewhat underestimating, see Boltovskoy and Alder 1992) . In the Southwestern Atlantic (30-60°S, along 55°W), surface (5-15 m) layers were found to host 0.5 polycystines per liter on the average, with maximum concentrations of three shells per liter (Alder et al. 1997). Lowest numbers are those present in Central Gyre and Tropical/Subtropical waters (see Figure 7).

Flux rates of radiolarian shells at depths between 50 and ca. 5000 m vary from 0-4 to over 100,000 ind./m2/day (Boltovskoy et al. 1993a), with highest numbers having so far been recorded in the north-eastern tropical Atlantic (201,064 shells/m2/day; cf. Boltovskoy et al. 1996).

The numbers of species that inhabit the different climatic zones of the World Ocean are difficult to estimate because most authors restrict their scopes to some 20-40 more or less well-defined morphotypes, ignoring the rest of the species. The few surveys that (presumably) did attempt to identify all the skeletons recorded indicate that these numbers oscillate around 100-200 for the tropics and subtropics, dropping to some 50-60 at the poles (Figure 8). This decrease, however, is often punctuated by an isolated peak in the transitional areas which usually host both cold water and warm water taxa, especially in the sediments (see Figure 9) (Boltovskoy 1981d, 1982, 1986).

Despite these rather high numbers, very few of the species are abundant in any given sample. In terms of their relative contribution to the overall polycystine assemblage, usually only one-three species exceed 10%, and up to five represent over 5%; radiolarians whose average percentage abundances are below 1% of the fauna usually comprise 70-90% of all the species recorded (Figure 10). Of the 164 polycystines included in this review, around 10 can attain average proportions in excess of 10% in any given area, 12-15 morphotypes can reach 5-7%, and ca. 50-70 are normally around 1-3% (Table 1). The remaining half of the polycystine species are present at levels below 1%. Highest dominances are associated with polar environments, where a single species or species group can account for 25-40% of the assemblage (e.g., Antarctissa spp. in the Antarctic, cf. Boltovskoy 1987; Amphimelissa setosa in the Greenland Sea, cf. Swanberg and Eide 1992; Phormacantha hystrix/Plectacantha oikiskos and Rhizoplegma boreale, probably synoymous with Spongosphaera streptacantha, in coastal Antarctic sediments, cf. Nishimura et al. 1997; see Figure 10).

Species-specific distributional data for the South Atlantic are scarce and fragmentary. Boltovskoy (1981e) produced a detailed listing of all known Southwestern Atlantic records up to that date, which basically represented 7 reports (Haeckel 1887; Hays 1965; Nigrini 1967; Goll and Bjųrklund 1974; Lozano and Hays 1976; Morley 1977; and Boltovskoy and Riedel 1980), chiefly based on sedimentary materials. This objective compilation produced a spotty picture with no discernible patterns. In the 15 years elapsed since that review several contributions based on South Atlantic materials appeared, but they mostly focused on downcore analyses (e.g., Pisias and Moore 1978; Coco 1982; Weaver 1983; Bjųrklund and Jansen 1984; Grinstead 1984; Charles and Morley 1988; Alperķn 1987), or were restricted geographically to rather small areas (Robson 1983; Dworetzky and Morley 1987; Boltovskoy et al. 1993a, 1993b, 1995, 1996; Abelmann and Gowing 1997). Thus, in order to furnish a more comprehensive insight into polycystine biogeography in the South Atlantic, distributional species-specific data are referred to the 7 distinct areas illustrated in Figure 11). These divisions take into account the distribution of general planktonic biogeographic provinces (e.g., E. Boltovskoy 1970; Koblentz-Mishke and Vedernikov 1977; Boltovskoy 1979, 1981d, 1982, 1986; Dadon and Boltovskoy 1982; Longhurst 1995), as well as radiolarian-based biogeographic patterns (Goll and Bjųrklund 1974; Morley 1977; see Figure 11, insets A and B). For some of the especially abundant and better defined taxa relative (percentage) contributions to all polycystines can be predicted with reasonable accuracy. For most others, however, only a very rough indication of their numbers (abundant, present) can be offered for the time being.

The information used to compile Table 1 was not restricted to data from the South Atlantic Ocean, but was extracted from many reports on various oceanic areas, putting special emphasis on water column-based surveys (see "Sedimentary vs. water-column materials" above). Although very subtle differences between oceanic basins probably do exist (Nigrini 1967; Goll and Bjųrklund 1974), polycystine species are chiefly restricted in their distribution by climatic and productivity fields, rather than by ocean basins, as are most other pelagic planktonic organisms. Thus, with very few exceptions, similar assemblages characterize the equatorial circumglobal belt, the subtropical zones of the two hemispheres, and the polar waters (Petrushevskaya 1971a). Geographic endemics are rare, probably accounting for less than 5% of all the species (one outstanding example is Antarctissa spp., Figure 15.104, which is absent in the Arctic, but dominates both the plankton and the sediments of the Antarctic zone).

It should be born in mind that the degree of mixture between most of the areas shown in Figure 11 is extremely large. For example, in the western South Atlantic the Transition Zone stretches up to almost 15 degrees in latitude (ca. 34-35°S to 47-48°S; Subantarctic species are regularly found here in the same tows as the Subtropical representatives (E. Boltovskoy 1970, 1981a, 1981b; E. Boltovskoy et al. 1996). Because the Brazil current is a southwest flowing branch of the South Equatorial Current, tropical assemblages differ little from the subtropical ones. Central Gyre fauna is also very similar to the Tropical and Subtropical one, yet these oligotrophic waters, characterized by very low overall plankton abundances, host enhanced proportions of several colonial radiolarians.

Vertical profiles

Vertical profiles of total radiolarian abundance in tropical and subtropical waters indicate that the bulk of their populations is usually located in the upper 50-100 m (Petrushevskaya 1971b; Renz 1976; Dworetzky and Morley 1987; Kling 1979; Kling and Boltovskoy 1995; Abelmann and Gowing 1997; see Figure 12A-B, 12C, 12F-G). Quite often several discrete maxima are recorded, one at or near the surface, and a second one between 50 and 100 m (Petrushevskaya 1971b; Kling and Boltovskoy 1995). In the Antarctic, however, peak abundances seem to be associated with the Warm Deep Water and occur deeper, at 200-400 m (Petrushevskaya 1967; Boltovskoy and Alder 1992; Abelmann and Gowing 1997; Figure 12D, 12E).

Many radiolarian species occupy discrete depth intervals of the water column. Kling and Boltovskoy (1995), on the basis of a series of plankton tows in the upper 2000 m in the eastern subtropical Pacific defined the following characteristic layers: (1) surface (with maxima at 0 m, 25 m, 0 and 50 m, 50 m, or 0 and 100 m), (2) subsurface (maximum at 100 m), (3) deep (maxima at 200 m, 200 and 300 m, or 300 m), and (4) species peaking below 300 m. Roughly similar zonations were established by other authors as well (e.g., Renz 1976; Dworetzky and Morley 1987; Kling 1979). Worldwide depth zonations, however, cannot be defined in terms of fixed depths because the distribution of radiolarian species is related to water masses which move vertically as well as horizontally. For example, in the eastern subtropical Pacific inshore and oceanic 0-25 m waters can host a typically warm-water assemblage associated with the Central Water which is advected coastward by the Southern California Eddy, while midway between these two sites the same depths are inhabited by a conspicuously different, colder-water assemblage associated with the cooler waters of the California Current (Kling and Boltovskoy 1995). Many cold water radiolarians that inhabit the upper layers at high latitudes submerge with their corresponding water masses and can be found at depth in mid- and low-latitude areas (Kling 1976; Boltovskoy 1988; Steineck and Casey 1990). Siphocampe arachnea (Figure 15.167), for example, is a dominant component of surface Pacific Arctic and Subarctic plankton; in the central north Pacific it peaks at 100-300 m, and at 300-1000 m in the subtropical eastern Pacific (Boltovskoy 1994).

Changes in the proportions of presumably living polycystine cells with depth have been assessed in a few studies. Boltovskoy et al. (1993a), based on extensive sediment trap data, concluded that numbers of live specimens decrease drastically downwards (e.g., aprox. 100% at 0 m, 50-60% at 100 m, 20-40% at 200 m, 10-20% at 500 m, 5% at 1000 m; see Figure 12H). These results generally agreee with other studies (e.g., Petrushevskaya 1971a; Kling and Boltovskoy 1995). On the other hand, Abelmann and Gowing (1997) estimated much higher proportions of living cells at comparable levels in the water column: over 90% at 100-200 m, around 70% at 300-500 m. It should be noticed that staining techniques, which are usually applied for these estimates, do not adequately differentiate between live and dead cells (see above "Provenance and collection of materials"), for which reason it is probable that concentrations of living specimens below 50-100 m are systematically overestimated in such surveys (Boltovskoy et al. 1993a; see Figure 12H).

As with geographic patterns, data on the depths at which the various species peak listed in Table 1 have been compiled from reports on different oceanic areas. It is anticipated that they are generally valid for subtropical and tropical environments worldwide; at higher latitudes, however, some deep species may occur closer to the surface, while in the Antarctic the bulk of the asssemblages seems to occupy deeper layers (see above).

 

TAXONOMY

Morphology and classification systems

Based on his previous monograph of 1862, and especially on the extensive collections of HMS Challenger, Haeckel (1887) produced the first comprehensive system of radiolarian classification, encompassing over 3,000 species, 2,400 of which were new to science. Although Haeckel's work is still a compulsory reference guide for anyone attempting to deal with the identification of these organisms, it has for some time been evident that it does not satisfactorily represent natural relationships. Indeed Haeckel’s groupings are only based on morphological similarities without the support of continuity in the fossil record, rather than on demonstrable evolutionary sequences. In addition, the rigidity of his diagnoses, based chiefly on strict geometric considerations (Figure 13), ignores the ample intraspecific variability of the polycystines. As a result, many of his described "species" are but slightly different morphotypes or even developmental stages of the same organism (see Figure 13 and Figure 14).

In spite of these shortcomings and the time elapsed, advances in the development of a better classification system have been very limited. Efforts to depart from and improve upon the classification schemes inherited from earlier workers have mainly followed two different approaches: cytological techniques and evolutionary studies.

Hollande and Enjumet (1960), Cachon and Cachon (1972), Petrushevskaya et al. (1976), Petrushevskaya (1981) proposed revisions which use not only the skeleton (as most other classifications), but also cytoplasmic features, in particular the "nucleoaxopodial complex" (sensu Petrushevskaya 1981). Although these schemes are probably sounder in biological terms, their applicability to fossil and subfossil materials lacking the protoplasm is problematic, which is one of the reasons for their very limited acceptance among radiolarian workers.

Analyses of evolutionary lineages in geological sequences were somewhat more succesful than cytological techniques in defining characters applicable to classification. Based on evolutionary evidence, Riedel and Sanfilippo (1986) produced an interesting critical review of the most important skeletal traits used by Haeckel. They concluded that some of them (e.g., number of segments, number of supplementary concentric spheres, number of feet, number of rays and of equatorial spines in discoidal Spumellaria, presence and nature of thoracic wings) have little or no suprageneric value. In contrast, several others (especially cephalic structure, but also pore arrangement, shell terminations in Nassellaria, etc.), traditionally considered as of minor value, are conservative through time, reveal evolutionary lineages and, therefore, are relevant for higher-rank divisions.

Riedel (1967b, 1971), Petrushevskaya (1965, 1971a), Goll (1968, 1969), Sanfilippo and Riedel (1970), Zhamoida and Kozlova (1971), Foreman (1973), Dumitrica (1988, 1989) based on skeletal features alone worked out alternative classifications, either for the entire order or for selected polycystine groups. Of these, Riedel's (1967b, 1971) suprageneric system has become the most widely accepted for extant and Cenozoic radiolarians, and is the one adopted herewith (with slight modifications; see also Kling 1978; Boltovskoy 1981e; Anderson et al. 1996). It should be stressed, however, that this system does not overcome many of the above-mentioned problems, and is therefore a compromise provisional classification. Several of the family-level definitions, especially in the Spumellaria, are rather vague and generally used as a lumping black box for the many forms with complex morphologies and poorly understood relationships (e.g., Litheliidae, Pyloniidae, Tholoniidae).

Specific identification of the polycystines is a time-consuming and frustrating task. With the exception of the few abundant and widespread species on whose names there is fairly good agreement, binomial nomenclature alone very often fails to pinpoint unequivocally a given morphotype because different names are applied to the same species and, conversely, identical organisms are reported under different specific and even generic names (see Boltovskoy and Jankilevich 1985). Because a very substantial proportion of the original species descriptions were published in old and often hard to get monographs, some authors find it faster and easier to create a "new species" for the unusual-looking skeleton in the slide, than to comb the dusty books in search of an adequate, already established name. Ecologically, paleoecologically and stratigraphically-oriented studies often underestimate the importance of a stable and consistent naming system; the lack of species illustrations in these reports allows the wrong designations to go undetected. This not only hinders buildup of useful information, but also significantly degrades the overall quality of radiolarian-based data for other applications. Recent literature has abundant examples of this bias, which introduces even more chaos into the already anarchic situation inherited from turn of the century works. Indeed, this may be a major reason for the waning use of radiolarians in stratigraphic and paleoecologic work.

The following illustrated glossary of most commonly used terms for the description of polycystine skeletons is chiefly based on the listing compiled by Petrushevskaya (1981). Capital letters (in parentheses) denote the group for which the term is used (N: Nassellaria; S: Spumellaria).

Outline classification

The following synopsis, based chiefly on the scheme proposed by Riedel (1967b, 1971), is restricted to extant families only. As pointed out above, it is a provisional classification very likely to change as more structural and evolutionary data are obtained. Taxa in bold characters are those treated in detail in this chapter.

Kingdom PROTISTA Haeckel, 1886
Phylum SARCODINA Hertwig and Lessser, 1876
Class ACTINOPODA Calkins, 1909
Subclass HELIOZOA Haeckel, 1886
Subclass ACANTHARIA Müller, 1858
Subclass RADIOLARIA Müller, 1858
Superorder PHAEODARIA Haeckel, 1879
Superorder POLYCYSTINA Ehrenberg, 1838, emend. Riedel, 1967
Order COLLODARIA Haeckel, 1881

Family THALASSICOLLIDA Haeckel, 1862
Family COLLOZOIDA Haeckel, 1862
Family THALASSOSPHAERIDA Haeckel, 1862
Family SPHAEROZOIDA Haeckel, 1862

Order SPUMELLARIA Ehrenberg, 1875

Family COLLOSPHAERIDAE Müller, 1858, emend. Strelkov and Reshetnjak, 1971
Family ACTINOMMIDAE Haeckel, 1862, emend. Sanfilippo and Riedel, 1980
Family COCCODISCIDAE Haeckel, 1862 emend. Sanfilippo and Riedel, 1980
Family PHACODISCIDAE Haeckel, 1881
Family SPONGODISCIDAE Haeckel, 1862, emend. Riedel 1967
Family LITHELIIDAE Haeckel, 1862
Family PYLONIIDAE Haeckel, 1881
Family THOLONIIDAE Haeckel, 1862

Order NASSELLARIA Ehrenberg, 1875

Family SPYRIDAE (=Trissocyclidae, Acanthodesmiidae) Ehrenberg, 1847, emend. Petrushevskaya, 1971
Family PLAGONIIDAE Haeckel, 1881, emend. Riedel, 1967
Family THEOPERIDAE Haeckel, 1881, emend. Riedel, 1967
Family CARPOCANIIDAE Haeckel, 1881, emend. Riedel, 1967
Family PTEROCORYTHIDAE Haeckel, 1881, emend. Riedel, 1967
Family ARTOSTROBIIDAE Riedel, 1967, emend. Foreman, 1973
Family CANNOBOTRYIDAE Haeckel, 1881, emend. Riedel, 1967

Order and family-level diagnoses

Order Collodaria. Solitary or colonial polycystines without a siliceous skeleton, or provided with simple or branched spicules scattered in the calymma. Due to their fragility, members of this group preserve poorly in net plankton samples, and either do not preserve at all or are represented only by their spicules in sedimentary materials. Partly because of these limitations, information on their classification and distribution is extremely scarce, and no further details are given herein. Detailed reviews of the colonial radiolarians, including several Collodaria, were produced by Hollande and Enjumet (1953), Strelkov and Reshetnjak (1971), and Swanberg (1979). Most of these species have tropical distribution ranges in the three major oceans. In the south Atlantic they are probably restricted to waters associated with the equatorial current system, the Tropics/Subtropics, and the oligotrophic Central Gyre (Figure 11). According to Haeckel (1887), this group comprises four families:

Family Thalassicollida: solitary cells, no skeletal elements; genera Actissa (Figure 15.1), Thalassocampe (Figure 15.2), Thalassopila, Thalassicolla, and Thalassophysa;

Genus Collozoida: colonial, no skeletal elements; genus Collozoum (Figure 15.6, 15.7, 15.8, 15.11);

Genus Thalassosphaerida: solitary, with siliceous spicules scattered in the calymma; genera Thalassosphaera, Thalassoxanthium (Figure 15.3), Physematium, Thalassoplancta, Lampoxanthium (Figure 15.5);

Genus Sphaerozoida: colonial, with siliceous spicules scattered in the calymma; genera Belonozoum, Sphaeorozoum (Figure 15.10), Raphidozoum (Figure 15.12).

Order Spumellaria. Solitary or colonial radiolarians with a well-developed shell of radial symmetry or one derived from the above. Variations in the type of symmetry include spiral shells (e.g., Figure 15.87), asymmetric, discoidal or lenticular (biconvex) (Figure 2H, 2I; 15.60, 15.61, 15.62, 15.63, 15.64, 15.65), triaxonic (Figure 2Q, 2R), quadrangular (Figure 15.66, 15.67), etc. In many cases two axes of symmetry can be clearly differentiated (Figure 2J) but, as opposed to the Nassellaria, the larger axis is homoaxonic. The central capsule (organic) of these cells has many small pores.

Family Collosphaeridae. Colonial polycystines, each individual has a single, thin-walled, spherical or subspherical latticed shell. The Collosphaeridae is the only group of colonial polycystines with complete latticed shells. Colonies consist of a gelatinous mass (which obviously disappears in the sedimentary record, as well as in many net-plankton samples where it breaks down) in which hundreds to thousands of shells are immersed (Figure 1C, 1D, 1E). The shape of the colony is not species-specific; it may be spherical, ellipsoidal, cylindrical, ribbon-shaped, etc., measuring up to several centimeters in length and a few millimeters in diameter. The siliceous shells are always represented by a single perforated sphere (internal spheres are never present), with or without centrifugal (external) or centripetal (internal) tubular projections and/or spines. Spines (when present) are conical (circular in cross-section). As with most other polycystines, specific assignments are based almost exclusively on the skeleton; however, studies of entire colonies, which allow investigating the intraspecific morphologic variability of the collosphaerids, indicate that quite dissimilar shell morphotypes can coexist within the same colony, thus stressing the assumption that at least some of the specific divisions based on the siliceous sphere alone are spurious (e.g., Kleijne 1987; Petrushevskaya and Swanberg 1990).

Family Actinommidae. Solitary species with latticed or spongy spherical, subspherical, or ovoid shells (not lenticular); with or without medullary shells. Surface of shell is often covered with spines, but not tubes. All actinommids posses either single or multiple, concentric spherical or ovoid shells. When several shells are present they are connected to each other by radial beams which pierce the cell. An enormous variety of forms was described in this family whose identification has traditionally been based on Haeckel's (1887) system. Haeckel based the classification of the actinommids (=suborder Sphaeroidea, exclusive of the Collosphaerida) on the following characters (in decreasing order of importance; see Figure 13): 1. Number of primary radial spines; 2. Number of concentric spheres; 3. Position of concentric spheres (intra- or extracapsular), type and relative size of spines, presence of by-spines, type of medullary shell, etc.). However, the number of primary spines varies intraspecifically, whereas the number of main concentric spheres, which within some bounds might indeed be species-specific (Riedel and Sanfilippo 1986), can only be used in the case of fully-grown individuals. It is quite obvious that, based on this trait, Haeckel (as well as many other authors) assigned new names to growth stages still missing the outermost sphere(s) (see Figs. 13 and 14). Furthermore, while growth of an actinommid as far as we know proceeds from the center toward the periphery (Figure 14, upper panel), dissolution works in the opposite direction, innermost, more delicate shells usually disappearing before the more robust cortical ones. Thus, materials from the sediments offer yet another suite of "new species", this time missing the medullary (rather than the cortical) shells.

Family Coccodiscidae (Figure 2H, 2J). Latticed discoidal or lenticular shell enclosing a single or double medullary shell, and surrounded by an equatorial zone of spongy or concentrically-chambered structures (Figure 2H), or forms with an ellipsoidal cortical shell equatorially constricted enclosing a single or double medullary shell (Figure 2J). The formerly actinommid subfamily Artiscinae was transferred to the Coccodiscidae by Sanfilippo and Riedel (1980) due to its phylogenetic affinities with extinct coccodiscids.

Family Phacodiscidae. Lenticular, biconvex, latticed cortical shell, not surrounded by spongy or chambered structures, within which a small, spherical single or double medullary shell is enclosed. The margin (but less commonly the surfaces) of the cortical shell may bear radial spines.

Family Spongodiscidae. Discoidal or cylindrical, spongy or finely chambered skeleton, with or without surficial pore-plate, often with radiating arms or marginal spines. The members of this family are characterized by possessing skeletons which are partly or entirely spongy in appearance. However, as opposed to the Actinommidae, which can also have spongy skeletons, the Spongodiscidae are not spherical. Their overall shape can be lenticular (biconvex discs, Figure 2I), cylindrical (Figure 15.74), quadrangular or subquadrangular in outline (Figure 15.67), or Y-shaped (Figure 2Q, 2R). With the exception of the cylinders, all others are depressed or flattened (rather than circular in cross-section, Figure 2I). Lenticular, quadrangular, and Y-shaped forms may be entirely composed of a spongy mass with no discernible structure (in which case the central part of the skeleton is often thicker and/or denser, and therefore appears darker in the light microscope; Figure 2O, 15.64), or may posses a small central chamber surrounded by concentric or spiral, continuous or interrupted bands (Figure 2R). The surface of some forms may be partly or totally covered with a very thin, porous sieve-plate, which in lenticular forms may extend beyond the central spongy mass forming a delicate equatorial girdle around the periphery of the shell (Figure 2P) (these morphotypes were formerly included in the family Porodiscidae).

Family Litheliidae. The lattice of the ellipsoidal, spherical or lenticular shell is totally or partially arranged along a bilaterally symmetrical spiral. Although very abundant, due to their complicated architecture the litheliids are poorly known, for which reason the morphotypes defined may include several different forms.

Family Pyloniidae. The major part of the shell is composed of a series of successively larger elliptical latticed girdles in three mutually perpendicular planes, with the major diameter of each girdle being the minor diameter of the next larger one (Figure 2L, 2M). The center is occupied by a small ellipsoidal structure - the microsphere (see Dumitrica 1989).

Family Tholoniidae. Completely latticed shell, without larger openings, and with constrictions that define several (typically 6) dome-shaped protuberances (Figure 2K).

Order Nassellaria. Solitary polycystines with a siliceous heteropolar shell, which can be represented by several fused spicules only, by a D-shaped ring and associated spines, or by more elaborate, mono- or multilocular latticed skeletons. With the exception of a few forms lacking a well developed skeleton (Figure 15.101, 15.119), the symmetry of this group is characterized by the fact that the two extremes of the major axis define two morphologically different poles of the shell. One of these, conventionally accepted as the top or anterior end, is where the cephalis is located. A widely recognized, albeit seldom utilized, feature of primary importance for the classification of the Nassellaria is the internal skeleton. The internal skeleton consists of a complex set of spines and connecting bars enclosed in the cephalis (Figure 3A, 3B, 3C), which allow comparison of homologous structures in forms differing widely in their external morphology. Unfortunately, analysis of these features requires dedicated efforts at understanding the complex spatial relationships involved. Furthermore, observation of this internal skeleton is only feasible with well preserved individuals oriented in the right position, which is seldom the case in specimens mounted in permanent slides. In addition to the small scattered perforations typical of the Spumellaria, the central capsule of the Nassellaria is usually provided with a single larger pore.

Family Spyridae (=Trissocyclidae). The skeleton is represented by a well-developed D-shaped sagittal ring (median bar and anastomosed vertical and apical spines), either free (Figure 15.101) or embedded into the latticed cephalic wall, in which case the cephalis is usually bilaterally lobed (Figure 15.93, 15.94, 15.100). Sometimes with thorax, abdomen always absent. The typical heteropolar nassellarian symmetry is often inconspicuous in the Spyridae.

Family Plagoniidae. Skeletons restricted to a simple tri- or tetraxonic nassellarian spicule (Figure 15.119), or a well developed system of main spines enclosed within a fully formed cephalis (Figure 3A, 3B). The degree of development of the cephalis may vary from a few anastomosed bars (Figure 15.120, 15.124) to a well developed, latticed or latticed/spongy chamber. Usually without postcephalic segments. In addition to several fairly well-defined species, the Plagoniidae comprise many probably related forms of obscure taxonomic status usually cited under various generic names (see below). The classification of these forms needs detailed ad hoc studies, for which reason many of them are provisionally lumped under the designation Plagoniidae group in the present chapter.

Family Theoperidae. Cephalis spherical or subspherical, relatively small, often poreless or sparsely perforate. It usually bears an apical horn. Internal spicule small and inconspicuous. With one or more, sometimes up to over 10, usually well-developed postcephalic segments. Generally, cap- or helmet-shaped, or conical in overall outline.

Family Carpocaniidae. The small, rudimentary cephalis is usually totally immersed in the large and well-developed thorax (Figure 3E). Abdomen absent or rudimentary.

Family Pterocorythidae. Cephalis large, divided into three lobes by two lateral furrows directed obliquely and downward from the apical spine to the base of the cephalis. The upper unpaired lobe is located above the two smaller paired ones (Figure 3K, 3L); these basal paired lobes are not always conspicuous. Many pterocorythids are two or three-segmented, lacking postabdominal segments.

Family Artostrobiidae. Spherical or subspherical cephalis, usually with an apical tube directed obliquely upwards (Figure 3J). The pores on all postcephalic segments, or at least on the last ones, are arranged in clearly defined transverse rows (Figure 3S). Usually elongated, multisegmented forms.

Family Cannobotryidae. Cephalis large, with several asymmetrical lobes (sometimes appearing as irregular bulges) (Figure 3M). Mostly 2-segmented forms (cephalis and thorax), but sometimes with post-thoracic segments.

TAXONOMY

Identification of species

The overall total of living polycystine species is probably around 300-600, but this review includes only 164 taxa. Unfortunately, our present state of knowledge is insufficient for a significantly greater coverage. All polycystine families need detailed taxonomic work, but some are especially poorly known (e.g., Actinommidae, Pyloniidae, Litheliidae, Tholoniidae, Plagoniidae). However, the inexperienced student should not be discouraged by the fact that the available literature fails to assist in identifying very significant proportions of the shells present in any given sample. As mentioned above, most radiolarian-based ecological and paleoecological surveys restrict their scope to some 20-40 polycystines, the rest being ignored altogether. Although coverage of an ampler inventory is clearly desirable, these restricted databases allow one to draw very valuable environment-related conclusions. Furthermore, in order to circumvent identification-related problems, the use of family-level assignments has been advocated for environmental and biogeographic analyses (Kruglikova 1981, 1987).

The following section presents succinct information for the identification of the taxa recorded in the South Atlantic Ocean, as well as for several others whose presence in the area is very likely, but has not been confirmed yet.

When applicable, species names are followed [in square brackets] by some of the other most common names under which the form has been recently cited in the literature. Diagnostic information is provided as short remarks stressing the characters which help differentiate the taxon from closely related forms; they are not meant to replace detailed descriptions, but in most cases these remarks, in combination with the illustrations, should suffice for producing adequate identifications of the radiolarians treated. Keys are not adequate for this particular group due to their lack of flexibility, and because they are more prone to lead to misidentifications given the high number of rare polycystine species not included in this review. Shell size is generally of minor value for identification purposes, for which reason no detailed morphometric information is given; the dimensions specified are approximate and are furnished with the sole purpose of a general scale reference. The designation "group" denotes categories of unclear taxonomic affinities, where probably several related taxa are lumped. For each of the species treated one or more references ("Ref.") providing detailed descriptions and illustrations are included.