Coccidinium is a genus of parasiticsyndinian dinoflagellates that infect thenucleus andcytoplasm of other marinedinoflagellates.^[2]Coccidinium, along with two other dinoflagellate genera,Amoebophrya andDuboscquella, contain species that are the primary endoparasites of marine dinoflagellates. While numerous studies have been conducted on the genusAmoebophrya, specificallyAmoebophrya ceratii, little is known aboutCoccidinium.^[3] These microscopic organisms have gone relatively unstudied after the initial observations ofÉdouard Chatton andBerthe Biecheler in 1934 and 1936.

The first species ofCoccidinium that were described wereC. legeri andC. duboscquii, found in the cytoplasm of dinoflagellates in brackish waters nearSète, France.^[4] They have been noted as lackingphotosynthetic stages in their life cycles, which is to be expected given their parasitic nature.Sexual reproduction has been observed “time and again” inC. mesnili as stated by Chatton and Biecheler.^[2]

First discovered by Édouard Chatton and Berthe Biecheler in 1934,Coccidinium was initially observed in the cytoplasm of the dinoflagellatesGlenodinium sociale andPeridinium sp., both taken from the waters of Sète.^[4] They identified two species ofCoccidinium,C. legeri andC. duboscquii. In 1936,C. mesnili andC. punctatum were added to the list of existingCoccidinium species by Chatton and Biecheler. They were found in the same brackish waters of Sète, but in two other peridinians:Kryptoperidinium foliaceum andCoolia monotis respectively.^[2]

Observations ofCoccidinium showed that they werecoccidian-like in their vegetative and replication stages, but theirdinospores, abiflagellatezoospore, resembled syndinian dinoflagellates. Chatton and Biecheler therefore gave this new genera ofendoparasitic dinoflagellates the name ofCoccidinium.^[4]

A complete understanding ofCoccidinium morphology is not yet possible due to insufficient research. There are no other studies completed apart from the initial observations conducted by Chatton & Biecheler in 1934 and 1936, therefore the little information known are based on their descriptions.

InC. duboscquii, thetrophozoite, the feeding form of the parasite, is present in two forms that originate from small uninucleate forms found in close contact with the nucleus of the host and that are distinguished by their load in carbohydrate globules. In the first form, heavy, numerous and large starchy matter are present whereas in the second form this is absent or rare. Recent interpretation of thoseuninucleate forms withoutstarch grains suspect that they might be the parasiteParvilucifera.^[5]

Sporocyte nuclei are large and spherical, with around 4-5chromosomes in total in a general V-shape, which is typical forSyndiniales. The nuclei lie around the periphery of the cell. Dinospore movement is via flagellar locomotion. In the forms rich with starch grains, these parasites grow for extended periods of time with a single nucleus, which Chatton and Biecheler term as “synenergid”. They will surround themselves with a thin cystic membrane before undergoing division, but will not exceed 16 or 32 nuclei.^[4]

InC. legeri, two stages are also observed. The first form consists of smallplasmodia that contain a maximum of 8 nuclei, which are assumed to give rise to dinospores. The second form consists ofCoccidinium multiplying rapidly inside the host, however the nucleus does not undergo division until after the death of the host and the encystment of the parasite in its remains.^[4]

Coccidinium arehaplontic, meaning that for the majority of their life cycle they arehaploid. Reproduction can occur eitherasexually or sexually. Sexual reproduction has been clearly observed in 1934 when Chatton and Biecheler witnessed a two-by-two fusion of imperceptibly dissimilar spores from separate organisms ofC. mesnilii, resulting in a mobilezygote with two pairs of flagella.^[6] The zygotes form a hyaline cyst.^[2]

Parasitism in organisms usually implies ecological and human economic repercussions, but in the case ofCoccidinium, these repercussions carry relatively little importance in the ecology of host organisms from ananthropocentric point of view. Unless the host species is commercially significant, studies conducted on parasitic dinoflagellates are few and far between, therefore not much is known about the ecological importance ofCoccidinium specifically.^[7]

Coccidinium areendophytic, therefore live inside other organisms for the majority of their life. They tend to parasitize other dinoflagellates, thus are often found in aquatic environments, ranging from freshwater to marine.Coccidinium are able to inhabit environments with variable salinity levels as a result, though the exact range is not known due to insufficient research.^[4]

Coccidinium, while carrying little relevance to humans, contain species that have other marine dinoflagellates as hosts and therefore are relevant to theseprotists. There are often clear examples of sudden outbreaks of parasitism by specific dinoflagellates in a short time period, leading to a sudden decrease inzooplankton population density,^[8] implying that parasitic dinoflagellates can create fluctuations in marine plankton. There is insufficient information however regarding the parasitism process inCoccidinium, such as how thesporozoite enters the host and its specific effects on the dinoflagellate population. What is clear though is that the initial infection site is inside the nucleus of the host. Juvenile trophozoite generally lie close to the host’s nucleus, where it will undergo growth through the consumption of either the nucleus and chromosomes, or cytoplasm, depending on the species. The trophozoite will depress the host’s nucleus before infecting and destroying it, though the exact mechanism is yet to be known.^[2] Throughout this growth process, the host has been described as still motile, but the entire digestion of the nucleus and/or cytoplasm is relatively rapid, around 24 hours, therefore this motility can be described as short lived.^[4]