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).