INVERTEBRATE ZOOLOGY - H. I. Shcherbak - 2008

PROTISTS - PROTISTA

PROTOZOANS (PROTOZOA)

SARCODINES

PHYLUM AMOEBOZOA

RADIOLARIANS

Radiolarians are marine, thermophilic planktonic animals (ranging in length from 40-50 µm to 1 mm, occasionally larger).

Radiolarians are typically characterized by radial Symmetry, which is an adaptation for suspension in the Water Column. Pseudopodia (axopodia and filopodia) radiate outward from the body (Fig. 25). Most species possess a mineral Skeleton that serves not only a protective function but also a skeletal-support and locomotor role. There are a total of about 8,000 known species.

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Fig. 25. Radiolarians (a - from Dogiel; b, d - from Petrushevskaya; c - from Hausmann):

a - Acanthometra elastica (class Acantharea); b - Calocyctus monumentum (class Polycistinea);

c - Challengeron sp., d - Castanidium variabile (class Phaeodarea)

Unlike other Protozoans, radiolarians possess an intracellular skeleton consisting of an organic central capsule and mineral structures. The central capsule encloses the inner, organelle-rich region of the Cytoplasm containing one or more nuclei, separating it from the extracapsular region—a highly vacuolated outer layer of cytoplasm (Fig. 26). The capsule features one large and several small pores through which these cytoplasmic layers communicate.

Fig. 26. Diagram of the body Structure of a radiolarian (from Dogiel):

1 — central capsule; 2 - Nucleus; 3 - layer of dense cytoplasm surrounding the central capsule; 4 - base of pseudopodia;

5 - spongy layer of cytoplasm; 6 - condensed layer of cytoplasm; 7 - axopodium; 8 — filopodia

The mineral skeleton varies in appearance: radially arranged needles joined at the center; perforated, nested spheres; umbrella-like structures; crowns with long outgrowths, etc. In species of the class Acantharea, it is formed of strontium sulfate (SrSО₄), whereas in others it is composed of silicon oxide (SіO₂).

Straight axopodia containing internal supporting microtubules radiate outward from the intracapsular cytoplasm, increasing the body surface area. Thin, thread-like filopodia extend from the surface layer of the cytoplasm, forming a capturing net around The Cell (Fig. 26). Radiolarians feed on various unicellular organisms that adhere to the filopodia.

In mononucleated species, the nuclei contain a highly polyploid (approximately 1000 n) set of Chromosomes.

The reproduction of radiolarians has not been studied in detail; both asexual reproduction and a sexual process occur. Asexual reproduction in species with a skeleton built of separate needles occurs by binary fission with the distribution of skeletal elements between daughter individuals, whereas in species with a complex solid skeleton, numerous small daughter Cells (zoospores/swarmers) are formed, each of which regenerates the central capsule and skeleton. A sexual process is known in some species, during which flagellated Gametes are formed that copulate either isogamously or anisogamously.