Pseudo-worms and such

Yesterday I started with dinoflagellates, and showed a fairly typical example. While the typical dinoflagellates are fairly neat in their own right, this post will deal with some far less typical dinoflagellates - the parasitic members of the orders Blastodiniales and Syndiniales.

In the Fensome, Taylor et al. (1993) classification of dinoflagellates, Blastodiniales were a group of extracellular parasites with a dinokaryon (the distinctive dinoflagellate nucleus - see the previous post) for only part of their complicated life cycles. Blastodiniales were a diverse group, and Fensome et al. made no secret of its probable polyphyly. Most families of Blastodiniales start out as a trophont (feeding stage) attached to the host by rhizoids, a peduncle or a stylet (the exception is Blastodinium, in which the trophont is not actually attached but resides within the gut of copepods). The trophont may produce spores while attached and feeding (palisporogenesis), or may detach first before producing spores (palintomy). The spores are eventually released as motile dinospores, that have been recorded fusing to form gametes in at least some taxa, or give rise directly to the new generation of trophonts. Plastids are present in Blastodininium and Protoodinium, while other Blastodiniales are non-photosynthetic.

In Cachonella (Cachonellaceae), the trophont is attached to its siphonophore host by rhizoids. When it is finished feeding, it detaches and develops long tubular processes within the host's gut (the illustration in Fensome et al. has definite B-grade sci-fi appeal). After being passed from the host, the ex-trophont produces coccoid aplanospores (non-motile spores) that in turn remain attached to each other while shedding a series of cyst membranes, to form a branching structure with a spore at the end of each branch. Eventually the non-motile aplanospores give rise to motile dinospores.

Haplozoon is a very distinctive form that Fensome et al. assigned to Blastodiniales (though Leander et al. (2002) disputed this position, as Haplozoon appears to possess a dinokaryon throughout its life-cycle). Haplozoon initially attaches to its host (a larvacean or annelid) as a unicellular trophont by a stylet. It then undergoes multiple cell divisions to give rise to a flat worm- or ribbon-like (apparently) multicellular form with a single trophocyte (feeding cell), multiple rows of gonocytes (dividing cells) and a distal row of sporocytes (spore-producing cells). The single nuclei of the sporocytes become four, and individual sporocytes are released as cysts (probably eventually releasing four dinospores, but this doesn't appear to have actually been observed). While the mature stage of Haplozoon has generally been interpreted as multicellular (or colonial), Leander et al. found that SEM images of H. axiothellae appeared to show a single continuous membrane covering all "cells", and so interpreted Haplozoon as forming a unique compartmentalised syncytium (multinucleate single cell).

The non-photosynthetic Syndiniales possess dinoflagellate-like flagella, but do not possess a dinokaryon. In the Syndiniaceae, the trophont is a multinucleate plasmodium. In the Sphaeriparaceae, the trophont produces a long chain of aplanospores that are eventually released as dinospores.

Amoebophrya is a member of Syndiniales whose trophont develops a large conical cavity, the mastigocoel, in which multiple flagella are formed before the entire structure flips inside-out to give rise to a long worm-like, multinucleate, multiflagellate, mobile stage, the vermiform. The vermiform then undergoes multiple cleavages to form hundreds of individual dinospores.

REFERENCES

Fensome, R. A., F. J. R. Taylor, G. Norris, W. A. S. Sarjeant, D. I. Wharton & G. L. Williams. 1993. A classification of fossil and living dinoflagellates. Micropaleontology, Special Publication 7: i-viii, 1-351.

Leander, B. S., J. R. Saldarriaga & P. J. Keeling. 2002. Surface morphology of the marine parasite Haplozoon axiothellae Siebert (Dinoflagellata). European Journal of Protistology 38: 287-297.