Tuesday, May 24, 2011

So bitter!

What makes a crab bitter? Is it the lack of contact with its parents after hatching? Is it the constant diet of polychaete worms (I mean, really, who would enjoy that?)? Oh no, it’s something much worse:

Hematodinium sp.
aka the parasitic dinoflagellate that causes bitter crab disease (BCD).

Hematodinium perezi in blue crab (Callinectes sapidus) hemolymph

This nasty phytoplankton infects crabs, destroying the hemocytes (blood cells) in the host's hemolymph which leads to respiratory and organ failure, and finally death. You can tell if you have a bitter crab by their cooked appearance: Chionoecetes bairdi look bright red and C. opilio look milky whitish yellow. The name “bitter crab” (referred to as both disease and syndrome) comes from the bitter flavor of the infected crab’s meat (Taylor and Khan, 1995). The mechanism behind the spread of BCD is unknown, but may happen during molting, cannibalism, or even just physical contact with other diseased hosts.

comparing C. bairdi:
top was infected with BCD (note the milky-colored hemolymph)
while the bottom was healthy

Recently, Mullowney et al. (2011) investigated possible factors of what may be regulating BCD transmission in Newfoundland and Labrador opies. What they found is that BCD may be density-regulated for snow crabs, meaning that the number of crabs in a given area correlated with the prevalence of BCD in that population. In this study’s case, the density-dependence was influenced by small to medium new-shell (recently molted) crabs. Does the newly molted status mean infection happens during molting? They didn’t say. We’ll have to stay tuned on that one.

comparing C. opilio:
the top crab was healthy while
the bottom was infected with BCD (note the opaque color)

Mullowney et al. (2011) did note that the idea of measuring prevalence of BCD in a population is a tricky thing. It is possible to underestimate prevalence due to inefficient fishing techniques and the ability to only diagnose chronic cases of infection. Overestimation of prevalence can occur through those infected crabs seeking out pots (and the accompanying bait) more so than healthy crabs due to the extra nutritional demands the infection is putting on their little crab bodies.

an eyestalk-ablated C. bairdi from the Tamone lab
potentially infected with BCD (from this article)

BCD is on the rise (remember the velvet swimming crab deaths in France, mentioned here?), and the infection’s resulting mortality is affecting commercial stocks’ reproductive potentiontial. Siddeek et al. (2010) observed BCD and the way it may alter harvest strategies for C. bairdi in Alaska (Sherry Tamone presented on the proportion of BCD in Tanners from southeast Alaska here). Hopefully biologists can learn more about how this disease is transmitted and how it’s affecting populations to better manage our crab stocks!

Read more:
Mullowney, D. R., E. G. Dawe, J. F. Morado, and R. J. Cawthorn. 2011. Sources of variability in prevalence and distribution of bitter crab disease in snow crab (Chionoecetes opilio) along the northeast coast of Newfoundland. ICES Journal of Marine Science 68: 463-471.

Siddeek, M. S. M., J. Zheng, J. F. Morado, G. H. Kruse, and W. R. Bechtol. 2010. Effect of bitter crab disease on rebuilding in Alaska Tanner crab stocks. ICES Journal of Marine Science 67: 2027-2032.

Taylor, D. M., and R. A. Khan. 1995. Observations on the occurence of Hematodinium sp. (Dinoflagellata: Syndinidae), the causative agent of bitter crab disease in Newfoundland snow crab (Chionoecetes opilio). Journal of Invertebrate Pathology 65: 283-288.

1 comment:

  1. You have some really good ideas in this article. I am glad I read this. I agree with much of what you state in this article. Your information is thought-provoking, interesting and well-written. Thank you.