Abstract

Bitter crab syndrome (BCS) is a fatal disease of crustaceans that is caused by a parasitic dinoflagellate of the genus Hematodinium. The causative agent has been reported in crustacean hosts from Australia, Europe, eastern Canada and the United States, western United States and Canada, and Alaska. Upon gaining entry into a crustacean host, the parasitic dinoflagellate proliferates, eventually replacing all host blood cells in the hemolymph. The mechanism of death is not completely understood, but it is clear that just prior to death, remarkable hemolymph changes do occur. Death resulting from secondary invaders is also likely.

Because infections are fatal and prevalences in some regions have approached 100%, the disease is believed to have significant impact on host populations. In southeast Alaska, areas of high BCS prevalence have been avoided in attempts to minimize spread of the pathogen and because commercial crabbing in these high prevalence areas is not economically profitable; product from these areas is rejected by the processors because of poor quality.

Between 1988 and 1996, hemolymph samples from 14,359 Eastern Bering Sea (EBS) Tanner crabs, Chionoecetes bairdi (n=5180) and C. opilio (n=9184) were examined for the presence of a parasitic dinoflagellate, Hematodinium sp., the causative agent of bitter crab syndrome (BCS). For this time period, total prevalences of BCS in C. bairdi and C. opilio were 1.87% and 3.57%, respectively. In C. bairdi, prevalences from yearly random samples ranged from 0% in 1989 and 1994 to 5.68% in 1996. Infections in both males and females were highest in 1996, reaching 9.93% in females and 2.74% in males; however, overall Hematodinium prevalences were only slightly more elevated in females (1.93%) than males (1.65%). For C. opilio, yearly random sample prevalences ranged from 0.30% in 1994 to 8.45% in 1988. Highest Hematodinium prevalences in C. opilio were observed early in the survey; in 1988 during which male and female infection prevalences were 7.62% and 10.00%, respectively. Overall parasitic prevalences in C. opilio were more elevated in females (4.23%) than males (3.23%).

BCS infections in both C. opilio and C. bairdi were most common in the Bering Sea at latitudes above 57N. In general, infection prevalences in C. opilio increased with increase in latitude with prevalences of 50–80% common in Norton Sound and west of St. Lawrence Island. Despite the fact that prevalences were generally lower in the Chukchi Sea than in Norton Sound and west of St. Lawrence Island, a greater percentage of sampled stations were positive for BCS in the Chukchi Sea. For C. bairdi, infections were rare in the Eastern Bering Sea, and increased only slightly along the shelf edge west and north of the Pribilof Islands.

Infections of the parasitic dinoflagellate in C. opilio were more common in shell condition-1 crabs (e.g., recently molted but soft-shelled crabs) with females (15%) more frequently infected than males (6.7%). In contrast, infections in C. bairdi were generally much lower and most frequent in shell condition-2 crabs (e.g., recently molted but hard-shelled crabs); overall, male and female prevalences were 1.7% and 2.6%, respectively.

For both C. opilio and C. bairdi, infections were more common in small crab less than 60 mm; after which, prevalences remained low. In C. bairdi, the highest infection rates were observed in 20 mm crab attaining levels of 62.5% in males and 65% in females. In C. opilio, highest prevalences were observed at 35 mm with little difference in prevalence between males and females.

In summary, our results suggest that: 1) juvenile crabs are more frequently infected than large mature crabs, 2) infections are more prevalent in crabs collected from warm water than cold, 3) infections are more prevalent in recently molted crabs, 4) infection prevalences in C. opilio increase with increase in latitude, 5) infections in C. bairdi are sporadic and rare, and 6) two species of Hematodinium may exist in the Bering Sea. At the present time, sufficient information on the life history of the parasitic dinoflagellate is not available to address the annual differences that the data have presented.