The Life and Times of Dissodinium

I've referred before to the position of the minute crustaceans known as copepods as one of the major groups of animals making up the marine zooplankton. Copepods form a significant part of the diet for a wide range of other marine animals: fish, molluscs, jellyfish, you name it. They are also targeted by other organisms coming in at a different scale.

Dissodinium pseudolunula: dinospores waiting to be released from the shell of a secondary cyst, copyright Gabriela Hannach.

Dissodinium is a genus of dinoflagellates, another group of organisms that has appeared on this site in the past. Most dinoflagellates are primarily photosynthetic but not Dissodinium: it's a parasite. Specifically, it's a parasite of copepod eggs. Copepods produce relatively large eggs compared to their body size that are full of tasty lipids and other nutrients so it's hardly surprising that they would attract attention. The free-swimming dinospore of Dissodinium initially looks much like a typical dinoflagellate but once they attach to a copepod egg they produce a sucker-like organelle through which they slurp up the egg's contents, swelling to a globular blob. When feeding is finished, this blob detaches from the remains of the egg to begin the process of reproduction.

There are two species of Dissodinium whose asexual life cycles were described by Elbrächter & Drebes (1978). I haven't found any reference to a known sexual reproduction cycle for Dissodinium. In both species, the replete individual forms a spherical primary cyst that floats free within the plankton. The contents of the primary cyst divide within the cyst wall to form the next stage, the secondary cysts. In the most commonly seen species, Dissodinium pseudolunula*, these secondary cysts are distinctively crescent-shaped. Following their release from the original primary cyst wall, the cytoplasm within the secondary cysts further subdivides to form the actively swimming dinospores. These dinospores presumably function as the infective stage for another round of the cycle but it should be noted that Gómez et al. (2009) were unable to induce infection when they incubated newly released dinospores together with copepod eggs. Instead, the dinospores encysted themselves in a hyaline membrane and Gómez et al. suggested that some sort of maturation period may be necessary before infection can take place. The second species of Dissodinium, D. pseudocalani, differs in that the secondary cysts are not crescent-shaped, and divide to release the dinospores while still themselves contained within the original primary cyst wall so the breakdown of the latter releases dinospores directly into the environment. This compression of the life cycle has also sometimes been observed with D. pseudolunula.

*This species has often masqueraded in the past under the name of Dissodinium lunula. The name 'Gymnodinium lunula' was originally used for crescent-shaped cysts by Schütt in 1895. Unfortunately, Schütt's figured examples of this 'species' included representatives of two quite different dinoflagellates, now classified as Dissodinium and another genus Pyrocystis that is not parasitic. The name lunula has become properly attached to the latter species, requiring a different name for the Dissodinium.

Stages in the life cycle of Dissodinium pseudolunula, from Elbrächter & Drebes (1978), running from a freshly released primary cyst at top left to a newly attached parasitic dinospore at bottom right.

Elbrächter & Drebes (1978) included Dissodinium in the Blastodiniales, a morphologically diverse group of parasitic dinoflagellates. The advent of molecular analyses would later demonstrate this grouping to be polyphyletic with parasitic dinoflagellates evolving on numerous occasions from free-living ancestors. Instead, Dissodinium and another parasite of copepod eggs, Chytriodinium, form a clade that is closely related to the major free-living genus Gymnodinium (Gómez et al. 2009). Gómez et al. also found that D. pseudolunula retains some elements of its free-living ancestry: it still retains chlorophyll (chlorophyll is absent in D. pseudocalani and Chytriodinium). Just how functional this chlorophyll remains is an open question: it appears less concentrated within the cell than in a typical photosynthetic dinoflagellate, and Gómez et al. were unable to maintain a culture of D. pseudolunula under conditions that would support a free-living species. Nevertheless, they suggested that a low level of photosynthesis might supplement the dinoflagellate's nutrient requirements while it waited out the aforementioned incubation period before finding itself a host.

REFERENCES

Elbrächter, M., & G. Drebes. 1978. Life cycles, phylogeny and taxonomy of Dissodinium and Pyrocystis (Dinophyta). Helgoländer wiss. Meeresunters. 31: 347–366.

Gómez, F., D. Moreira & P. López-García. 2009. Life cycle and molecular phylogeny of the dinoflagellates Chytriodinium and Dissodinium, ectoparasites of copepod eggs. European Journal of Protistology 45: 260–270.