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RESOURCE ASSESSMENT & CONSERVATION ENGINEERING
(RACE) DIVISION

Shellfish Assessment Program

Fisheries Resource Pathobiology Team:  Bitter Crab Syndrome in North Pacific Chionoecetes spp.:  A Brief History and the Need for a Molecular Approach

Two commercially important crab species in Alaskan waters are the snow crab, Chionoecetes opilio, in the Bering Sea, and the Tanner crab, C. bairdi, in Southeast Alaska. Historically, these two crab fisheries have been very valuable. Since 1992, the snow crab fishery has been the largest crab fishery in Alaska with respect to commercial landings (Fig. 1) and the most valuable with respect to exvessel value.

Figure 1, see caption
Figure 1.  Historical Chionoecetes sp. commercial landings in the eastern Bering Sea.

A major decline in the abundance of legal-size snow crabs has been observed in the last 3 years. Although a major Tanner crab fishery still exists in Southeast Alaska, the fishery has been closed in the eastern Bering Sea (EBS) since 1996 due to low abundance. The EBS population trends are surprising considering that NMFS survey data have suggested the existence of strong upcoming prerecruit classes.

Figure 2, see caption
Figure 2.  Prevalence of BCS in male and female snow crab in relation to
crab size in the eastern Bering Sea (1988-98).

Poor recruitment and recovery of C. opilio and C. bairdi, respectively, suggest that other factors may be influencing their abundance and distribution patterns. One of these factors may be bitter crab syndrome (BCS). BCS was originally thought to only infect crabs larger than 70-mm carapace width at relatively low prevalence. This hypothesis was based upon crab pot fishery and survey data that excluded crabs less than 70-mm carapace width. However, data from NMFS trawl surveys that capture small crabs suggest that BCS affects crabs of all sizes and age classes, with a higher prevalence in small crabs: up to 10.5% in snow crab and 47% in Tanner crab (Figs. 2 and 3). These data suggest that BCS may play a major role in the recruitment of snow crab and recovery of Tanner crab by removing small crabs from the population before recruitment into the fishery.

Figure 3, see caption
Figure 3.  Prevalence of BCS in male and female Tanner crab in relation to
crab size in the eastern Bering Sea (1988-98).

BCS can be considered an emerging worldwide disease of marine decapods. Since 1987, there has been an increase in the numbers of species affected by the causative agent, a parasitic dinoflagellate of the genus Hematodinium. For example, Hematodinium infections range between 0.9%-59.7% in Norway lobsters, while in some embayments of Southeast Alaska, infection rates of up to 95% have been reported in Tanner crab populations.

BCS is characterized by radical changes in host hemolymph, lethargic behavior, and altered texture and flavor of cooked meat. Upon cooking, heavily diseased crabs possess a chalky texture and bitter, aspirin-like flavor. Although affected crabs are not a public health concern, the altered flavor and texture renders the meat completely unmarketable. High host mortalities are associated with Hematodinium sp. infections in some species; for example, BCS infection is nearly 100% fatal in Tanner crab of Southeast Alaska.

Figure 4, see caption
Figure 4.  Cells from hemolymph smears stained with Giemsa stain modified (Sigma) at 250X magnification.  Left: crab hemocytes.  Right: Hematodinium sp (center cell).

Detection of BCS can be accomplished by a number of methods: the simplest, but least accurate is by visual observation. Heavily infected crabs possess opaque shells and milky hemolymph, whereas healthy crabs possess slightly translucent shells and hemolymph that is clear or slightly cloudy. As a result, light infections may not be apparent to the visual observer, thus resulting in underestimates of disease prevalence. A more accurate but time-intensive method of detection uses a drop of hemolymph to make a smear which is stained in the laboratory to identify parasitic cells (Fig. 4). However, this method can also lead to underestimates of prevalence due to the small amount of hemolymph examined and morphological similarities between host hemocytes and parasitic cells. Clearly, there is a need for an improved method to detect BCS.

BCS is a disease of the circulatory system, meaning that the parasitic dinoflagellate can be found around all organs and tissues bathed by hemolymph. The parasite is not recognized by crab defense mechanisms and, over time, will proliferate and, either passively or actively, entirely remove hemocytes from circulation. Once a crab is infected, vegetative stages or trophonts first appear as single cells or as plasmodia. As the infection progresses, prespores are formed, frequently with multiple nuclei. In some hosts, dinospores develop and are released from the moribund host. Unfortunately, after 70 years of research, this is the extent of current knowledge of Hematodinium sp.-caused diseases: the method of infection is unknown, as is the role that dinospores play in the life history of the parasite. It is uncertain if host reservoirs exist, yet we do know that under natural conditions, not all Alaskan crabs are susceptible to Hematodinium sp. infections. The existence of free-living stages of the parasite is also unknown. As a result, to better understand parasite ecology and in an effort to better monitor the effects of BCS on affected crab populations, a more accurate and rapid diagnostic tool needs to be developed.

In an effort to begin addressing many of the above questions, the RACE Division’s Fisheries Resource Pathobiology team collected 16 sediment and 11 near-bottom water samples aboard the ADF&G research vessel Medeia in October 2003. Samples were collected from regions of Southeast Alaska with historically high prevalences of BCS, as well as regions that rarely, if ever, encounter the disease. Also from these regions, hemolymph was collected from 48 Tanner crabs exhibiting signs of BCS infection and from 48 visually healthy Tanner crabs. Hemolymph and whole crabs were also collected from the 2003 Bering Sea and Gulf of Alaska surveys.

We used the BCS positive hemolymph samples to sequence the DNA of the homologous 18S ribosomal regions of Hematodinium sp. using universal primers. Next we plan to identify the variable regions between samples with the goal of developing a species-specific Polymerase Chain Reaction-based Hematodinium sp. probe. If this technique proves to be successful and reliable, it will be useful for a number of applications, including rapid and accurate detection of BCS. This will hopefully allow us to determine if the same species of Hematodinium is responsible for disease in snow crab and Tanner crab, as well as other crustaceans worldwide. The Hematodinium-specific probe will also help us locate life history stages outside the host that may reside in the water or sediment samples collected in 2003 that were processed by combining a series of filters and density gradient centrifugation. If other organisms act as a reservoir for Hematodinium, we may be able to identify them using the developed probe.

By Vanessa Lowe

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