Radiolarians of the Globe

Radiolarians are one of the primary groups of micro-organisms to be found among the marine plankton. These unicellular greeblies are justly famed for their intricate mineralised skeletons, leading to their comparison to living works of art. Today's post is covering one particular group of radiolarians, the Spumellaria.

Haeckel's (1899–1904) figure of Hexancistra quadricuspis from Kunstformen der Natur.

Spumellaria are one of the major subdivisions of radiolarians, containing species characterised by a generally spherical skeletal form. Many authors have also included the colonial radiolarians, which often lack a coherent skeleton and may form colonies up to several metres long, in the Spumellaria but these have more recently been treated as a distinct group. The skeleton of radiolarians is entirely enclosed by cytoplasm in life, though in those species in which the skeleton bears radiating spines, those spines may extend beyond the main body of the cell and be covered by only a thin cytoplasmic layer distally. In Spumellaria and anothre major radiolarian group, the Nassellaria, the skeleton is composed of opal, making these living jewels in more ways than one (another radiolarian group, the Acantharea, composes its skeleton of a mineral by the somewhat ethereal-sounding name of celestite). The cytoplasm of radiolarians is internally divided by a fibrous capsule into two structurally distinct sections, the internal endoplasm and external ectoplasm. The denser endoplasm contains most of the cell's primary organelles, such as the nucleus and large mitochondria. Linear microtubular structures called axonemes extend outwards from the endoplasm, passing through pores in the internal capsule and through the ectoplasm. The ectoplasm is often frothy in texture, containing an extensive assemblage of cellular vacuoles. In many of these radiolarians, some of these ectoplasmic vacuoles will house symbiotic algae that contribute much of the radiolarian's nutrition. Otherwise, radiolarians may feed on other small organisms that are captured on axopodia supported by the axonemes, which in spumellarians radiate outwards from the cell body in all directions. Extension and contraction of the axopodia may also help maintain the radiolarian's position in the water column (Cachon et al. 1990).

Schematic diagram of organisation of Didymocyrtis tetrathalamus from Sugiyama & Anderson (1998).

In many spumellarians, the basic skeletal architecture is one of nested spheres and/or globules. Sugiyama & Anderson's (1998) description of Didymocyrtis tetrathalamus stands as a fairly typical example. The central part of the skeleton is a double sphere well within the cytoplasmic capsule with the lobate nucleus contained in the spaces between the spheres. Radiating axes connect the inner shell with an outer shell mostly just outside the capsule (the capsular wall crosses the skeleton at some points). In Didymocyrtis, this outer shell is not spherical but a sort of peanut shape. At each end of the 'peanut', a further cap is added beyond the main shell. In many spumellarians, the outer shell appears spongy in texture, being constructed of densely criss-crossing fine opal fibres. There may be further extensions of the outer shell such as polar spines or funnels.

Not surprisingly, spumellarian classification has most commonly been based on skeletal architecture. Some attempts have been made to construct alternative classifications incorporating cytoplasmic features such as the relationship between the axopods and the nucleus (Cachon et al. 1990) but, as these systems require access to live specimens to place taxa, they have been less popular (especially as most people studying radiolarians are primarily working with fossil material). A phylogenetic study of recent spumellarians by Ishitani et al. (2012) found evidence for two main lineages within the class that differ in ecology. One, including the families Pyloniidae and Sponguridae, contained species found in temperate and cold waters. The other, including the families Astrosphaeridae, Hexalonchidae and Coccodiscidae, was found in tropical waters. Species assigned to the family Spongodiscidae were divided between both lineages, suggesting the need for some further tinkering with the morphological classification.

REFERENCES

Cachon, J., M. Cachon & K. W. Estep. 1990. Phylum Actinopoda. Classes Polycystina (=Radiolaria) and Phaeodaria. In: Margulis, L., J. O. Corliss, M. Melkonian & D. J. Chapman (eds) Handbook of Protoctista. The structure, cultivation, habitats and life histories of the eukaryotic microorganisms and their descendants exclusive of animals, plants and fungi. A guide to the algae, ciliates, foraminifera, sporozoa, water molds, slime molds and the other protoctists pp. 334–346. Jones & Bartlett Publishers: Boston.

Ishitani, Y., Y. Ujiié, C. de Vargas, F. Not & K. Takahashi. 2012. Two distinct lineages in the radiolarian order Spumellaria having different ecological preferences. Deep-Sea Research II 61–64: 172–178.

Sugiyama, K., & O. R. Anderson. 1998. Cytoplasmic organization and symbiotic associations of Didymocyrtis tetrathalamus (Haeckel) (Spumellaria, Radiolaria). Micropaleontology 44 (3): 277–289.