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(To complete all classifications ETI has added the Kingdom and the Phyla of all the different taxa treated on this DVD-ROM without higher classification descriptions. Texts from Lynn Margulis and Karlene V. Schwartz, Five Kingdoms. CD-ROM Copyright 2002 ETI / Freeman & Co Publishers)

The marine protists that Ernst Haeckel traditionally called “radiolarians” and other superficially similar plankton, large protists with some radial symmetry, are grouped as classes in the phylum Actinopoda for convenience and pedagogy. That actinopods represent convergently evolved lineages more related to certain zoomastigotes (Phylum Zoomastigota (including zooflagellates)) than they are to each other is likely but, in the absence of comprehensive information, we retain the traditional actinopod grouping, with its four classes. The first class is Heliozoa, freshwater sun animalcules. The second is the mostly deep dwelling Phaeodaria, and the third is the more open ocean Polycystina, which two classes together constitute the traditional Radiolaria. The fourth taxon is Class Acantharia (sometimes also grouped in Class Radiolaria), with their strontium sulfate skeletons.
Actinopods, heterotrophic protoctists, are distinguished by their long slender, cytoplasmic axopods, also called axopodia. These fine projections are stiffened by a bundle of microtubules running down the axis of the structure called an axoneme. Each axoneme has an often quite elaborate arrangement of microtubules characteristic of that actinopod group, and the microtubules are often cross linked. Electron-microscopic studies have shown that the taxa Class Acantharia and the Radiolaria (including Class Polycystina and Class Phaeodaria), all considered marine “radiolarians,” are products of evolutionary convergence and are only remotely related to one another.
Heliozoans are primarily freshwater plankton, although estuarine, marine, and benthic (seafloor dwelling) species are known. Thirty-four genera and nearly 100 species are known. Many use their axopods to catch prey. The axopods radiate out into the water, surrounded along their length by plasma membrane. In some heliozoans, the axonemes grow out directly from the endoplasm; in others, each axoneme grows out from its own structure, the axoplast, located next to the nucleus. In a group called the centrohelidians, all the axonemes arise from a single axoplast, called a centroplast, whose center often contains a clearly defined organelle.
The rowing actinopod Sticholonche zanclea Hertwig has been an enigma for taxonomists. Its peculiar skeleton, the placement of its axopods on the nuclear membrane, and the hexagonal pattern of the axopods in cross section have justified its placement as the only species in the isolated order Sticholonchidea. That order was originally thought to be radiolarian (as suggested by A. Hollande, M. Cachon, and J. Valentin in 1967), but it is more likely a marine heliozoan (as suggested by Cachon in 1971). Sticholonche is found rowing in the Mediterranean with the splendor of a Roman galley: it has microtubular oars and sets of moveable microfibrillar “oarlocks”. Unfortunately, it does not grow in the laboratory.
Many heliozoans have siliceous or organic surface scales or spines. In a few species, a spherical organic or siliceous cage encloses the entire cell. The cage has bars arranged in a repeating hexagonal pattern through which the axopods penetrate.
Except in the order Desmothoraca (for example, Clathrulina elegans), reproduction in heliozoans by zoospores or swarmer cells is unknown. Sexual reproduction has been rarely seen; cells reproduce by binary or multiple fission or budding. In some multinucleate species, the nuclear and cytoplasmic divisions are not synchronized. In uninucleate forms, the axopods retract so that the organism does not move or feed during cell division. Retraction is caused by disassembly of the microtubules in the axonemes.
A kind of autogamy (self-fertilization) has been reported in some heliozoans. A mature cell forms one or more cysts inside the cell. Meiosis apparently takes place in the cysts, and certain nuclei degenerate. Two of the final meiotic products in each cyst then fuse—their haploid nuclei form a new single diploid nucleus. The only surviving product of the two meiotic divisions and fusion emerges from the cyst as a mature heliozoan. Whether this inbred sort of reproduction is common is not known, because of the paucity of study. Heterogamy (fusion of nuclei from different individuals) may occur. In Actinophrys, two cells (but not their nuclei) may fuse just before they undergo autogamy. Gametes originating from one of the two cells have been seen to fuse with gametes originating from the other. Cell fusion is common in heliozoans, but whether it constitutes meiosis and fertilization is not known.
Both polycystines and phaeodarians often have strikingly beautiful opaline skeletons made of hydrated amorphous silica; they are extremely common in tropical waters. Of the more than 4000 actinopods described in the literature, some 500 are estimated to be polycystines. Along with diatoms, silicoflagellates, and sponges, they are responsible for the depletion of dissolved silica in surface waters.
Polycystines and phaeodarians differ in many ways. The polycystine skeleton is made of opal (hydrated amorphous silica); the phaeodarian skeleton is made of silica plus often a large quantity of organic substances of unknown nature. The polycystine skeletal elements look solid under the light microscope; however, electron microscopy reveals tiny canals and pores in their skeletons. The skeletal elements of phaeodarians look hollow even under the light microscope: their spines are tubular and the continuous shells of many species have a bubbly “styrofoam” ultrastructure barely visible by light microscopy and conspicuous by electron microscopy. Crystals, but not skeletal components, of strontium sulfate (SrSO4) are secreted by some adult polycystines in their endoplasm and perhaps by all of them in their undulipodiated swarmers, whereas SrSO4 is unknown in phaeodarians.
The capsule enclosing the central mass of cytoplasm in both polycystines and phaeodarians is not a flimsy microfibrillar open mesh net (as in acantharians) but is made of massive organic material. The polycystine capsule, probably composed of mucoproteins or mucopolysaccharides, is made of numerous juxtaposed plates, like the pieces of a jigsaw puzzle separated by narrow slits, whereas the phaeodarian capsule is a single continuous structure. The polycystine capsule grows in diameter during the life of the organism; the phaeodarian capsule cannot increase in diameter after it has formed—it can only thicken its wall.
The axonemes of the polycystine axopods studied so far are made of parallel microtubules aligned in geometrical arrays, with bridges between microtubules. Most species have many such axopods per cell. Polycystines usually have one axoplast from which all axonemes originate, but some groups have other arrangements—for example, individual axoplasts, one per axoneme, are located near the nucleus. In phaeodarians, only two axonemes penetrate the capsule. They originate from separate axoplasts just inside the capsule. The microtubules in the basal part of these axonemes are not linked by bridges. Light microscopy reveals a cortex of many thin peripheral pseudopods, which are perhaps branches of the two axopods. No polycystine axoneme is known to branch.
Polycystine orifices called fusules are complex mufflike structures each filled with a dense plug that permits the passage of the axonemal microtubules, if they originate inside the endoplasm, but that hampers the circulation of cytoplasm between the endoplasm and the extracapsular pseudopodial network. The phaeodarian capsule normally has only three orifices of two kinds: a wide, complex astropyle, which is an opening that ensures exchange between the endoplasm and whatever cell parts lie outside the capsule; and two, rarely more, parapyles. These openings, simpler than polycystine fusules, allow the passage of the two thick cell axonemes. At each parapyle there is a cup-shaped axoplast from which an axoneme originates. Outside the capsule in front of the astropyle of many phaeodarians is a mass of predigested food called the phaeodium. The polycystines lack the phaeodium.
In the phaeodarian endoplasm are numerous strange tubes, called rodlets, about 200 nm wide, having a complex repeating ultrastructure. Their role is unknown (perhaps they take part in the secretion of the capsule). No such rodlets are known in the polycystines.
Polycystines supplement heterotrophy by photoautotrophy in symbiotic yellow or green algae (zooxanthellae or zoochlorellae; Phyla Pr-14 and Pr-28); phaeodarians lack algal symbionts.
Most polycystines and all phaeodarians have only one nucleus, large and polyploid. Only the phaeodarian nucleus undergoes an extraordinary equational division, superficially resembling classical mitosis, in which two monstrous “equatorial plates” are formed, each with more than 1000 chromosomes.
Class Polycystina is divided into the orders Spumellaria and Nassellaria. The spumellarian has fusules scattered all over its central capsule membrane; thus, its axopods radiate in all directions. The protist is usually spherical, ellipsoidal, or flattened, and so, naturally, is its skeleton. Some spumellarians form large colonies in which hundreds of individual organisms are embedded in a common mass of jelly. The fusules of nassellarians, which never form colonies, are clustered at one pole of the capsule membrane; their axopods are grouped in a conical bunch that leaves the cell at that pole.
Acantharians, generally spherical organisms, have a unique radially symmetrical skeleton composed of rods of crystalline strontium sulfate (SrSO4). The skeleton usually has 10 diametrical (20 radial) spines, called spicules, inserted according to a precise rule, known as Moller’s law, which was discovered by Johannes Moller in the nineteenth century. The acantharian cell is a globe from whose center the spicules radiate and pierce the surface at fixed “latitudes” and “longitudes.” If there are 20 spicules, then there are five quartets—one “equatorial,” two “polar,” and two “tropical”—that pierce the globe at the latitudes 0°, 30° N, 30° S, 60° N, and 60° S. For the equatorial and both polar quartets, the longitudes of the piercing points are 0°, 90° W, 90° E, and 180°; for the tropical quartets, 45° W, 45° E, 135° W, and 135° E. Even in acantharians that do not have the general shape of a globe, these orientations are strictly observed, although some spicules are thicker and longer than the others. Some species have many more than 20 spicules, as many as several hundred, but they are always grouped by some elaboration of Moller’s law.
Acantharian cells are made of distinct layers. The innermost layer, coarsely granulated with many small nuclei, is the cell’s central mass. Immediately surrounding the central mass is a perforated, flimsy network of microfilaments called the central capsule membrane. Through the central capsule membrane, the central mass extends several kinds of cytoplasmic outgrowths: cytoplasmic sheaths surrounding the skeletal spines; reticulopods, which are cross-connected netlike pseudopods lacking axonemes; filopods, which are thin pseudopods stiffened by one or very few microtubules, and a number of axopods (usually 54, but in some acantharians there may be several hundred) arising from axoplasts between the spines.
At the periphery is the cortex, a thin, flexible layer of microfilaments, which may be arranged in intricate designs. The cortex is underlaid by a network of reticulopods. Where the strontium sulfate skeletal spines pass through, the cortex is pushed out, like a tent stretched out over tent poles. At these points are filaments, the myonemes, that apparently control the tension of the cortex and bind it to the skeletal rods.
The delicate axopods increase the amount of cell surface exposed to the sea. They retard sinking and perhaps allow efficient scavenging of nutrients from the water. Prey, generally protoctists and small animals, adhere to the axopods. Cytoplasm from the axopods then engulfs the prey and cytoplasmic flow transports it down the axopods toward inner parts of the cell, where it is digested.
Acantharians produce many small swarmer cells, each containing a drop of oil reserve and a crystal and bearing two [9(2)+2] undulipodia. The undulipodia originate from kinetosomes in the anterior part of the swarmer cell. Some acantharians round up to form cysts in which they undergo mitotic divisions. Swarmers develop and are later released from these cysts. Little about the development process is known because swarmers have been devilishly difficult to culture in the laboratory. Meiosis has not been seen.
Most acantharians are effectively photoplankton because they harbor many haptomonads (Phylum Pr-10) that live and grow in them. The haptomonads are grass green in color, and photosynthetic. The symbiotrophy permits the acantharians to obtain their energy and food by photosynthesis in the nutrient-poor open ocean. The acantharian wastes provide nitrogen and phosphorus for their haptomonad symbionts.

Literature
Hollande, A., J. Cachon, and M. Cachon-Enjumet, “L’infrastructure des axopods chez les Radiolaires Sphaerellaires periaxoplastidies.” Comptes Rendu Hebdomedaire Seances Academie des Science (Séries D) 261:1388–1391; 1965.


Copyright 2002 ETI / Freeman & Co Publishers

Phylum Actinopoda