Dinoflagellated parasite of the genus Hematodinium. (Photo: Dr. Grant Stentiford)
Further findings on crustaceans' problematic parasites
Friday, September 07, 2012, 23:50 (GMT + 9)
A group of scientists found out that parasitic dinoflagellates of the genus Hematodinium, representing a serious issue for crab, prawn and shrimp fisheries across the world, have bacteria-like endosymbionts in wild European brown shrimp (Crangon crangon).
The study, which was published in BioMed Central’s open access journal Aquatic Biosystems, points out the existence of these endosymbionts reveals a previously unknown side to the lifecycle of this parasite.
Hematodinium sp. and its sister species H. Perezi cause ‘bitter crab’ disease and are thus a nuisance for blue crab fishers. In fact, these parasites may be responsible for the decline of blue crabs in Chesapeake Bay; more than 40 species of crustaceans are known to be the targets.
Researchers at the European Union Reference Laboratory for Crustacean Diseases (CEFAS) and the Virginia Institute of Marine Sciences (VIMS) worked together to identify the parasite responsible for causing sickness in wild European brown shrimp collected from the North Sea. These shrimp had lost carapace transparency and their blood was no longer able to clot.
The invading Hematodinium had also infiltrated the animals’ muscles, destroyed internal organs, and the infection had damaged the shrimp’s ovaries, impairing the females’ ability to reproduce. Moreover, the shrimp were infected with Crangon crangon bacilliform virus (CcBV) as well.
Additional investigation revealed that two of the lifestages of the parasite were present in these shrimp: trophont (the adult, mobile stage) and dinospore (the infectious stage). However, the dinospores were themselves unprecedentedly seen to be infected with bacteria-like cells in the cytoplasm as well as inside the nucleus.
“The symbionts inside Hematodinium sp. appeared to make no difference to the ability of the parasite to infect shrimp,” Dr Grant Stentiford from Cefas explained. “However, for these relationships to survive the endosymbiont must supply an evolutionary advantage.”
“It seems most probable that the endosymbiont in some way increases the chance of the dinoflagellate to survive outside the shrimp, and successfully transfer to a new host. One of the problems with Hematodinium infection is that we do not yet fully understand their lifecycles. The role of this endosymbiont to its survival may be the key to controlling infections in species of farmed crustaceans,” he added.
Outbreaks of these parasites have damaged commercial stocks of Norway lobster (Nephrops norvegicus), snow crab (Chionoecetes opilio), Tanner crab (Chionoecetes bairdi), American blue crab (Callinectes sepidus) and velvet swimming crab (Necora puber), according to VIMS.
Species of Hematodinium can reach high enough levels to regulate their host populations, but mortalities are also based on the un-fished juveniles and females, which are hosts not normally sampled by fisheries, meaning that impacts are often underreported. Seasonal prevalences of up to 85 per cent occur annually in many host populations.
By Natalia Real