Radiolaria Phaeodaria
Introduction

Phaeodarians are exclusively marine, planktonic organisms that differ fundamentally from the Polycystina (the other superorder of Radiolaria) by distinctive characteristics of the central capsular membrane, as well as by a mass of pigmented spherules, the phaeodium. The central capsule is a unique part of radiolarian soft anatomy that encloses an inner zone (intracapsulum or endoplasm), separated by the capsular membrane from the outer zone (extracapsulum or ectoplasm). Phaeodaria are generally larger forms whose skeletal elements are often hollow tubes rather than solid bars. The skeletons are composed of opal, but with some organic content or matrix that is still poorly understood. Almost all the species are solitary, except for a few genera (e.g., Nationaletta, Tuscaretta, Tuscaridium) which have been observed to form colonies of up to 16 individuals enclosed in a common meshwork sphere (Phaeodarian cell features 12, Tuscaretta globosa chuni; e.g., Haecker, 1908; Ling and Haddock, 1997).

Phaeodarian cell features, Phaeodarian cell features 2, Phaeodarian cell features 3, Phaeodarian cell features 4, Phaeodarian cell features 5, Phaeodarian cell features 6, Phaeodarian cell features 7, Phaeodarian cell features 8, Phaeodarian cell features 9, Phaeodarian cell features 10, Phaeodarian cell features 11, Phaeodarian cell features 12.

Phaeodarian skeletal geometry is quite varied and complex, with a number of basic plans that are the basis for recognizing families. Some families produce fairly simple latticed spheres, very similar to those in many spumellarian polycystines (Cannosphaera antarctica, Castanidium variabile). In another group with similarly latticed walls the shells are divided into two symmetrical, mirror-image halves or valves, as in pelecypods (Conchellium capsula 2, Conchidium caudatum). Other bivalved forms develop radially into elaborately branched structures that eclipse the inner valves (Phaeodarian cell features 8). Several groups build an open polygonal meshwork resembling the architectural structures known as geodesic domes (Aulastrum monoceros, Aulatractus fusiformis, Sagenoscena irmingeriana, Sagoscena castra). Species of one family construct skeletons by accreting foreign materials, usually the siliceous skeletons of other protists, such as diatoms, polycystines, and silicoflagellates (Aulokleptes flosculus).

The phaeodarian central capsule is double-walled, with a thicker outer wall placed very close to a thinner inner wall. One main opening in the wall is surrounded by a characteristic phaeodarian anatomical feature, the astropyle (Phaeodarian cell, Phaeodarian cell-capsule, Phaeodarian cell features 8). The main part is the operculum, a flatly conical or cap-shaped, radially striated structure, which extends into a reduced part, the tubular, straight to sinuous proboscis (Phaeodarian cell, Phaeodarian cell-capsule, Phaeodarian cell features 8). Electron microscopy reveals that the striations are reflections of complex folding (see Cachon and Cachon, 1973; and Anderson, 1983a for more detail). In addition to the astropyle, there are usually one or more accessory pores termed parapylae. Typically there is a pair of such pores (Phaeodarian cell, Phaeodarian cell-capsule). A combination of astropyle and paired parapylae is characteristic of many phaeodarians, resulting in the name Tripylea which Hertwig (1879) applied to the Phaeodaria. However, Haeckel (1887) warns that parapylae seem to be lacking altogether in several families (Challengeridae, Medusettidae, and Castanellidae), and in a few species in other families. Rarely, there may be only one, or several (in access of two), parapylae.

The phaeodium usually occupies the ectoplasm, but may also extend into the endoplasm, and may rival the rest of the cytoplasm in volume. It consists of spherules of various yellow, brown, or green hues, and occasionally some red granules. Recent studies of phaeodia (Gowing 1986, 1989, 1993b; Gowing and Bentham, 1994) show that it is an assemblage of food and waste vacuoles containing a wide range of materials including: bacteria, prokaryotic and eukaryotic algae, algal cysts, dinoflagellates, microheterotrophs, large virus-like particles, loricae, coccoliths, diatom frustules, trichocysts, cuticlar remnants and nematocysts of metazoans, amorphous material, and siliceous skeletal fragments. It is presumed that these are representative of nutritional sources, although some of the debris may have entered the cells as remnants of secondary food sources such as fecal pellets or organic aggregates.

Phaeodarians are heterotrophic feeders, and most species so far investigated are generalists, with vacuoles containing a wide variety of materials, as listed above. An occasional specimen bearing vacuoles mainly with a single food type has been observed in a species which in other cases contained various food types. And the concentrated resources in organic aggregates are considered a potentially important food source (Gowing and Bentham, 1994). Unlike many polycystines, phaeodarians apparently lack symbionts.

Reproduction in the Phaeodaria is still poorly known. Flagellated swarmers have been observed and are generally presumed to represent a sexual stage, but their exact function remains elusive. Apparent dimorphism in some species may be related to a sexual reproductive stage (Kling, 1971). Fission also occurs, often accompanied by large and numerous polyploid chromosomes that are visible even in the light microscope. At least some species undergo an unusual mode of mitotic division (Anderson, 1983a; Grell, 1973).

The first phaeodarian species were described by Haeckel (1862), who placed two of three new genera in new families but did not otherwise separate them from sphaeroid polycystines. In 1878 Haeckel recognized these species as a separate group, the Pansoleniae, based on the tubular structure of their skeletal elements. Hertwig (1879) placed Haeckel’s genera in the Tripylea in reference to the distinctive arrangement of pores in the central capsule. Haeckel (1879) noted Hertwig’s work of the same year, but observed that the set of three pores was not universal, renaming the group Phaeodaria after the typical anatomical structure.

Phaeodarians have received little concentrated study since the large expedition monographs of the late nineteenth and early twentieth centuries (see RaPh. 2 Geographic and vertical distribution). Some recent studies of one or more families based on sampling patterns approaching oceanic scale are: Kling (1966, 1976), Tibbs (1969, 1976), and Tibbs and Tibbs (1986). Reshetnjak (1966) reviews collections from the northwestern Pacific and provides a detailed account of the biology, morphology, systematics and world-wide distribution of the entire group.

Phaeodarian skeletons are rarely preserved in sediments. Either the peculiar siliceous shell material or the thinness of the skeletal elements, or both, result in such rapid dissolution rates that many species disappear from the water-column even before reaching the sea-floor (Takahashi, 1981). Nevertheless, a few species have been reported from sediments in some areas, and rarely in formations as old as Eocene (e.g., Dumitrica, 1964, 1965, 1973; Ling, 1991; Petrushevskaya and Reshetnjak, 1980; Reshetnjak, 1971; Stadum and Ling, 1969). Because of the limited fossil record, there is as yet no paleontological evidence of evolutionary relationships.