Glenodinium and the Horseshoe of Light

Yes, it's another dinoflagellate. The subject of the above photo (from here) is Glenodinium pulvisculus. Glenodinium is a genus of photosynthetic, mostly freshwater dinoflagellates in the family Glenodiniaceae, diagnosed by Fensome et al. (1983) by the possession of four apical plates and six postcingular plates (see near the top of this post for a brief explanation of these terms). Fensome et al. included two genera in this family, Glenodinium and Glenodiniopsis. Glenodinium has a horseshoe-shaped eyespot, but Glenodiniopsis does not. The eye-spot presumably functions in phototaxis, though it is worth noting that Glenodiniopsis is positively phototactic even without one (Highfill & Pfiester 1992).

Glenodiniopsis uliginosa, from here.

The identity of Glenodinium has been somewhat confused over the years, due in part to confusion over the identity of its type species, G. cinctum (Loeblich 1980). As a result, many of the references to Glenodinium in the literature refer to unrelated species, while true Glenodinium appears relatively little-studied. One species of Glenodiniopsis, G. steinii, has fared a little better, and its ultrastructure was described in detail by Highfill & Pfiester (1992). Among the more interesting details they noted was that instead of the multiple chloroplasts this species had originally been described as having, it really possesses a single chloroplast but one with multiple lobes, so that if it is viewed in cross-section the lobes might appear as individual plastids.

REFERENCES

Fensome, R. A., F. J. R. Taylor, G. Norris, W. A. S. Sarjeant, D. I. Wharton & G. L. Williams. 1983. A classification of living and fossil dinoflagellates. Micropaleontology Special Publication 7.

Highfill, J. F., & L. A. Pfiester. 1992. The ultrastructure of Glenodiniopsis steinii (Dinophyceae). American Journal of Botany 79 (10): 1162-1170.

Loeblich, A. R., III. 1980. Dinoflagellate nomenclature. Taxon 29 (2-3): 321-324.