Anguillicoloides crassus is a parasitic nematode worm that lives in the swimbladders of eels (Anguilla spp.) and appears to spread easily among eel populations after introduction to a body of water. It is considered to be one of the threats to the sustainability of populations of European eel (Anguilla anguilla). It was introduced to the European continent in the 1980s, where it was reported independently from Germany and Italy in 1982, having probably been introduced from Taiwan. It is thought to have reached England in 1987 from continental Europe. It is a natural parasite of the Japanese eel in its native range. The life cycle of Anguillicoloides crassus begins when the adult nematode releases thousands of eggs in the eel's swimbladder. The eggs pass through the eel's digestive tract and the larvae emerge in the water and settle onto the substrate. They are ingested by their intermediate host, which is often a copepod or other crustacean but may also be a fish. The nematode larva reaches its infective stage within this intermediate host. The host is eaten by an eel, and the nematode finds its way from the eel's digestive tract to its swimbladder. An eel with an advanced parasite load shows symptoms such as bleeding lesions and swimbladder collapse. The eel becomes more susceptible to disease, its rate of growth slows, and if the infestation is severe enough, it may die. Since the swimbladder is the buoyant organ which allows the eel to swim, a severe parasite infestation can hamper its ability to reach its spawning grounds. The state of being colonized by Anguillicola nematodes is termed anguillicolosis. Adaptation to the European eel Three significant changes were made to the life cycle of A. crassus which facilitated its success in colonizing A. anguila. First, it utilizes paratenic hosts in its transmission, such as a number of freshwater fish, amphibians, snails and aquatic insects. Despite there being no record of the use of paratenic hosts in A. crassus transmission cycles in Asia, this possibility has not been rejected. The suitability of paratenic hosts in facilitating transmission differs according to species, with physostome (open swimbladder) fish being less suitable than physoclist (closed swimbladder) fish. The latter allow further development of A. crassus larvae into the fourth stage and have lower rates of encapsulation, thereby permitting longer rates of survival. Infected copepods tend to inhabit epibenthic regions due to their sluggish movement.