Parasitic dinoflagellates infect algae, protozoans, annelids, crustaceans, molluscs, salps, ascidians, rotifers, and fishes (Chatton 1920, Cachon & Cachon 1987, Shields 1994, Coats 1999). At present, approximately 35 genera within four botanical orders, the Phytodiniales, Gymonodiniales, Blastodiniales, and Syndiniales, contain species that are known to be parasitic, with the latter two orders consisting entirely of parasitic taxa.

Interestingly, the dinoflagellates share certain affinities with the Ciliophora and the Sporozoa (Apicomplexa) placing them collectively in the superphylum Alveolata (Gajadhar et al. 1991). Morphologically, they all possess a trilaminar alveolate pellicle (Cavalier-Smith 1993) and have distinct molecular affinities (Gajadhar et al. 1991). Recently, micropores, organelles for endocytosis, were observed in the filamentous trophont (cf. vermiform plasmodium) and amoeboid trophont stages of Hematodinium parasites from N. norvegicus  (Appleton & Vickerman 1996). Micropores are an important morphological feature of the Apicomplexa and are also common to the Alveolata. Nonetheless, details of mitosis are quite different between these phyla and morphologically there are few other similarities.

Systematically, the genus Hematodinium belongs in the family Syndiniceae, Order Syndinida (Syndiniales to botanists).  Members of the Syndinida have life cycles consisting of at least three phases: a multinucleate plasmodial stage, a vegetative phase (trophont, produced via merogony) and an asexual reproductive phase (sporont produced via sporogony). In the Syndinida sporogony leads to the formation of two dissimilar forms of biflagellate dinospores (‘swarmers’) that arise from different parent infections and ensure dispersal and new infection (Cachon & Cachon 1987). Meiosis and gamogony have not been described for members of the genus Hematodinium; the macrodinospores and microdinospores appear to be products of sporogony and have similar DNA content as other stages (Eaton et al. 1991).

Virtually all of the Syndinida are parasitic in the haemocoels of invertebrate hosts. They occur primarily as plasmodial forms that rapidly divide and grow until they undergo sporogony to produce a motile spore stage. The plasmodial stage has no chloroplasts and obtains nutrition via osmotrophy during the trophic phase, where lipid and polysaccharide inclusions suggest active feeding at the expense of the host. Sporogenesis is simple, with multiplication of the nuclei, plasmodial and cytoplasmic divisions occurring to produce sporocysts, following which the biflagellate zoospores are produced and liberated (Chatton 1920, Cachon & Cachon 1987). Four genera of syndinids are parasitic in crustaceans; Actinodinium, Trypanodinium, Syndinium and Hematodinium. Of these, Actinodinium in copepods and Trypanodinium on copepod eggs have not been well documented, while Syndinium and especially Hematodinium have been better studied (see Shields 1994 for review). Infections by Syndinium turbo, S. corycaei and S. gammari have been described in copepods and amphipods, where infection presumably occurs via ingestion of dinospores by the host (Chatton 1920, Manier et al. 1971).