Hematodinium is a genus of dinoflagellates. Species in this genus, such as Hematodinium perezi, the type species, are internal parasites of the hemolymph of crustaceans such as the Atlantic blue crab (Callinectes sapidus) and Norway lobster (Nephrops norvegicus). Species in the genus are economically damaging to commercial crab fisheries, including causing bitter crab disease in the large Tanner or snow crab fisheries of the Bering Sea.

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 the genus Hematodium affect many organisms in Order Decapoda, and may also affect Order Amphipoda, and subclass Copepoda. 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 the western Atlantic Ocean to southern Australia. As a result of the prevalence of Hematodinium in many ocean waters, significant damage is done to many fishery industries around the globe.

Hematodinium perezi is one of the first species to have been studied in the 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 indications 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, and Necora puber.

A second species Hematodinium australis found in Australia was distinguished from H. perezi. Besides its southern hemispheric location, the trophont size as well as the presence of rounded plasmodial stages differed between parasitic groups. Molecular research 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 speculated to belong to the 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 their close resemblance to the type species.

At the time of Small's (2012) studies, 38 host crustacean species had 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 the 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.

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 discolored shells. Over 3000 acres of coastal aquatic culture were affected, with death rates as high as 60%. This was the first noted outbreak of its kind in China.