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Shellfish Assessment Program:  Fisheries Resource Pathobiology

New Diagnostic Tool Tested for Monitoring Bitter Crab Syndrome

Vanessa Lowe of the Fisheries Resource Pathobiology team has participated in the annual Alaska Department of Fish & Game (ADF&G) Southeast Alaska Tanner Crab Survey for the past 3 years to monitor and contrast Bitter Crab Syndrome (BCS) between historically low (eastern Bering Sea) and high (Southeast Alaska) prevalence areas. For example, overall prevalence in both Chionoecetes bairdi and C. opilio from the EBS remain 3%-5% although prevalences in crab less than 50 mm carapace width may exceed 10%, while overall prevalence in C. bairdi from Southeast Alaska may approach 100% in certain bays.

From 2003 to 2004, bloodsmears were prepared and hemolymph was preserved in 100% ethanol from random (2004 and 2005) and nonrandomly (2003) selected crabs. Water and sediment samples were also collected from Stephens Passage (west of Douglas Island) and Icy Straits, two sites within Southeast Alaska where BCS is present at relatively high and low prevalences, respectively. The team was interested in developing and testing a DNA-based diagnostic tool for monitoring BCS that could also be used to search for free-living life history stages of the pathogen in both water and sediment. We also wanted to contrast the traditional method of detecting BCS (i.e., examination of bloodsmears) with the DNA-based diagnostic tool.

The DNA-based method has proven to be more accurate and rapid in detecting BCS in diseased crabs. However, detection of free-living stages of the parasite in water and sediment samples is not unequivocal. All bloodsmears have been examined and progress is being made toward applying the DNA-based diagnostic tool to all ethanol preserved samples. A report on the results of the bloodsmear examinations follows.

Crabs were collected by crab pot which limited our access to crabs less than 70-mm carapace width (Fig. 1 below). Regardless, BCS in male crabs greater than 75-mm carapace width ranged between 0% to 33% for each 5-mm size group. For females, prevalence ranged between 0% and 17% for each size group. No trend with respect to size and prevalence was observed. The 65-mm group is represented by one positive crab.

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Figure 1.  Prevalence of Bitter Crab Syndrome in Tanner Crabs by size.
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Figure 2.  Prevalence of Bitter Crab Syndrome in Tanner Crabs by sex.

  see caption
Figure 3.  Prevalence of Bitter Crab Syndrome in Tanner Crabs by shell condition.

In general, many more male (N = 579) than female (N = 103) crab were randomly collected. Despite this disparity, there was little difference in BCS prevalence by sex (Fig. 2). Prevalence for both sexes hovered around 10%.

Shell condition determination is a traditional method to estimate time from a decapod’s most recent molt. Our data indicate that shell “Condition-2” crabs are more frequently encountered with disease than other shell conditions (Fig. 3). The data indicate that while older shell condition crabs may be infected, prevalences diminish over time.

The data were analyzed by logistic regression. The original model included the following variables: sample site, sex, size, shell condition, and depth. Initial analysis indicated that the variable size was not a good predictor of disease. As a result, the variable size was removed from the model and the data were again analyzed. This second analysis indicated that sample site, shell condition and depth are good predictors of disease. As a result, it might be expected that for this project, crabs collected from Stephens Passage, at shallow depths and of shell Condition 2 are more likely to be infected than crabs collected under other conditions.

By Frank Morado

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