Introduction
''Hematodinium'' is a harmful parasitic dinoflagellate in marine decapod crustaceans, often dwelling in the hemolymph of host organisms . Drastic changes in the host’s vital organs, tissues, and hemolymph lead to the degeneration of the organism . Previous studies indicate that species in genus ''Hematodium'' affect many organisms in Order Decapoda, and may also affect Order Amphipoda and subclass Copepoda (Manier ''et al'', 1971). These crustacean taxa include crabs, lobsters, crayfish, prawns, and shrimp – all commercially important food sources for many countries. The effects of ''Hematodinium'' parasites are aggressive and can be lethal, thus capable of eradicating entire crab populations. Discoveries of ''Hematodinium'' pathogens in decapods span from western Atlantic Ocean to southern Australia . As a result of the prevalence of ''Hematodinium'' in many ocean waters, significant damage is done for many fishery industries around the globe.History of knowledge
''Hematodinium perezi'' is one of the first species to have been studied in genus ''Hematodinium'' . This organism is well-known and is consequently used as the main study subject for this genus. The first declaration of ''H. perezi'' was made in 1931 by Chatton and Poisson off the coasts of Normandy and France. They were discovered in swimming crabs ''Carcinus maenas'' and ''Liocarcinus depurator''. At that time, Chatton and Poisson’s studies were not successful in generating reliable data on ''Hematodinium.'' This was because only a small percentage of tested crabs showed indication that they carried the ''H. perezi'' parasite. Nonetheless, future discoveries of similar diseases in other crabs have been linked back to ''H. perezi'' since Chatton and Poisson’s first sighting. Later affected crabs include ''Cancer pagurus'' (Latrouite ''et al.'', 1988; Stentiford ''et al.'', 2002) and ''Necora puber'' (Wilhelm and Boulo, 1988; Wilhelm and Mialhe, 1996). In 1994, a second species ''Hematodinium australis'' found in Australia has been distinguished from ''H. perezi''. Hudson and Shields (1994) point out that besides its southern hemispheric location, the trophont size as well as the presence of rounded plasmodial stages differed between parasitic groups. Molecular research later confirmed the separation of ''H. australis'' from ''H. perezi'' . Up until now, there are only a few determining characteristics between ''Hematodinium'' species . All known species are found in crustacean hosts, have hemolymph-dwelling filamentous plasmodial stages, intrusive amoeboid trophont infections, and a dinokaryon. Species in speculation of belonging to genus ''Hematodinium'' are currently in need of further research. Lack of comparative work between the type species and the latest species discoveries prevent exploration of the diversity of genus ''Hematodinium'' . Difficulties arise due to the inaccessibility of representative type materials to use for comparative studies, and only a meager list of useful physical characteristics available between the type species and potential ''Hematodinium'' parasites . Therefore, newly discovered taxa are designated the general classification of ''Hematodinium'' sp. or are referred to being ''Hematodinium''-like. In some cases, new parasite discoveries are incorrectly identified as ''H. perezi'' due to close resemblance to the type species.Habitat and ecology
At the time of Small’s (2012) studies, 38 host crustacean species have been affected by ''Hematodinium'' parasites. Some of the economically-important infected hosts include tanner and snow crabs from the Northeast Pacific and Atlantic Oceans, blue crabs in the Atlantic and Gulf coasts of the United States, Norwegian lobsters, and Edible crabs from Europe. It is anticipated that the geographical spread of ''Hematodinium'' will grow and reach various other aquatic regions globally due to oceanic currents, host movements and habitat expansions, as well as transportation vessels. Furthermore, as new crustacean species are targeted and fished as commercial seafood, new species of pathogens may be discovered. This was the case in a recent finding in multiple Asian crustacean farm facilities. notes that only aquaculture species from southern and eastern China have been documented, which include swimming crab ''P. trituberculatus'', the mud crab ''S. serrata'', and the ridgetail white prawn ''Exopalaemon carinicauda''. In addition, a small new finding of ''Hematodinium'' parasites in crustaceans was also recently discovered in Russian waters. Recent findings from infected Chinese crustacean farms In 2004, swimming crabs ''P. trituberculatus'' from Zhoushan were diagnosed with “milky disease” . They showed signs of lethargy due to the development of white muscle tissues and milky hemolymph. In addition, these crabs had a deficiency in cellular hemolymph and developed discoloured shells. Over 3000 acres of coastal aquatic culture was affected, with death rates as high as 60%. This was the first noted outbreak of its kind in China. In 2005, mud crabs ''S. serrata'' were infected with “yellow water disease” in Zhejiang Province . ''Hematodinium'' trophonts, prespores, and