Silicon Rockets

In a previous post, I spoke of the radiolarians, marine protists renowned for their intricate skeletons, and the major radiolarian group known as the Spumellaria. Standing in contrast to the spumellarians is another major group, the Nassellaria. Like spumellarians, nassellarians have a skeleton of silica but whereas the basic shape of spumellarian skeleton is a sphere, that of nassellarians is a cone, bell or some similar shape, arranged along a longitudinal axis. The origination point of the skeleton is at or near the top of the cone and is known as the cephalis (from the Greek for 'head'). There may be an apical spine rising above the cephalis. Below it, the skeleton is commonly divided into recognisable sections referred to as the thorax, abdomen and post-abdominal segments (if present). The nucleus of the cell is more or less associated with the cephalis, contained within it at least during the juvenile stage of development though it may shift below the cephalis as the cell matures (Suzuki et al. 2009).

Skeleton of a Eucyrtidium sp., copyright Picturepest.

As is commonly the case with unicellular organisms, radiolarian taxonomy has been influenced by disagreements about which features should be regarded as more significant. Some would arrange taxa based on the overal formation of the skeleton. Others would focus on the development of the initial embryonic spicule around which the cephalis develops. A recent phylogenetic analysis of living nassellarians by Sandin et al. (2019), based on both morphological and molecular data, found that overall skeleton morphology was a much better indication of relationships than the internal structure. One well supported subgroup of the Nassellaria is the superfamily Eucyrtidioidea.

Eucyrtidioids have a fossil record going back to the Triassic (Afanasieva et al. 2005). The cephalis is spherical and clearly distinguished from the following segments by a constricted basal aperture. The test is usually multi-segmented; members of the subfamily Theocotylinae may have just two segments but other members of Eucyrtidiidae have up to ten segments. Fossil families assigned to Eucyrtidioidea by Afanasieva et al. (2005) may have up to twenty (but as Afanasieva et al.'s concept of Eucyrtidioidea was not found to be monophyletic by Sandin et al., the affinities of these fossil families perhaps warrant re-investigation). Segments are commonly divided by distinct inner rings. The skeleton lacks feet, the term used for protruding spines around the basal aperture of the skeleton found in many other nassellarians.

The phylogeny of nassellarians indicated by Sandin et al. (2019) places the Eucyrtidiidae as the sister taxon to other living nassellarians. Other living families included in the Eucyrtidioidea by Afanasieva et al. (2005) were placed in more nested positions. The implication is that the multi-segmented condition may be ancestral for crown Nassellaria. Segments are added progressively during the life of the radiolarian, leading the organism to look quite different at different ages. Indeed, this metamorphosis is pronounced enough that one of the earliest influential researchers on radiolarians, Ernst Haeckel (he of Kunstformen der Natur fame), made the mistake of classifying different ages as different species, genera and even families. Our understanding may be better than in Haeckel's time but there may still be a lot to learn about these intricate organisms.

REFERENCES

Afanasieva, M. S., E. O. Amon, Y. V. Agarkov & D. S. Boltovskoy. 2005. Radiolarians in the geological record. Paleontological Journal 39 (Suppl. 3): S135–S392.

Sandin, M. M., L. Pillet, T. Biard, C. Poirier, E. Bigeard, S. Romac, N. Suzuki & F. Not. 2019. Time calibrated morpho-molecular classification of Nassellaria (Radiolaria). Protist 170: 187–208.

Suzuki, N., K. Ogane, Y. Aita, M. Kato, S. Sakai, T. Kurihara, A. Matsuoka, S. Ohtsuka, A. Go, K. Nakaguchi, S. Yamaguchi, T. Takahashi & A. Tuji. 2009. Distribution patterns of the radiolarian nuclei and symbionts using DAPI-fluorescence. Bulletin of the National Museum of Nature and Science, Series B 35 (4): 169–182.

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.

Name the Bug: Sticholonche zanclea

Sticholonche zanclea (photo from here).

Sticholonche zanclea is a very unusual marine protist. In the past, it has been classified among the heliozoans, a group of organisms united by their generally radial arrangement of long cytoplasmic extensions called axopodia. However, both molecular and ultrastructural studies have established that the 'heliozoans' are a polyphyletic assemblage of a number of unrelated groups that have converged on a similar morphology. 'Heliozoans', in general, are passive trappers of other micro-organisms and food particles by means of their axopodia; the helizoan morphology provides for significant area coverage without massively increasing the cell cytoplasm. Compare this to the 'radiosa' form adopted by amoebae that become detached from their substrate and which serves a similar purpose (though in that case the aim is increasing the chance of recontacting the substrate). In the case of Sticholonche, molecular analysis has placed it among the also-axopod-bearing radiolarians with which it shares the production of siliceous spicules. Sticholonche differs from other radiolarians in lacking a central capsule dividing the cell into internal and external sectors; however, said molecular analyses place it nested among rather than sister to radiolarians. Indeed, it may be the sister to the spumellarid family Litheliidae (Kunimoto et al., 2006).

Sticholonche differs from other 'heliozoans' in being flattened with the axopodia concentrated laterally. The axopodia are rigid, reinforced by a central core of microtubules, and anchored on small cup-shaped depressions on the nuclear envelope (Cachon et al., 1977). By flexing the nuclear envelope, Sticholonche can move by rowing itself with the axopodia. If your day has so far provided insufficient awesomeness, follow this link, scroll down the page a bit, and you will find a video of a Sticholonche doing just that.

REFERENCES

Cachon, J., M. Cachon, L. G. Tilney & M. S. Tilney. 1977. Movements generated by interactions between the dense material at the ends of microtubules and non-actin-containing microfilaments in Sticholonche zanclea. Journal of Cell Biology 72: 314-338.

Kunimoto, Y., I. Sarashina, M. Iijima, K. Endo & K. Sashida. 2006. Molecular phylogeny of acantharian and polycystine radiolarians based on ribosomal DNA sequences, and some comparisons with data from the fossil record. European Journal of Protistology 42 (2): 143-153.