What the devil?

A friend of mine living in London alerted me to this story: thousands of velvet swimming crabs (Necora puber) have washed up on the Thanet shore for the third year in a row! One Thanet Coast local said that the arrival of dead velvet swimming crabs along the shore in winter is not that shocking; it is simply the number of crabs that is alarming for beach combers. Estimates range from 25,000 to 40,000 dead crabs. Also among the mix are sea stars and anemones.

Thanet Coast starred

velvet swimming crab carnage

Nicknamed the devil crabs because of their red eyes, these little guys may have succumbed to Britain’s coldest December in 120 years. The velvet swimming crab is the largest swimming crab in British waters with a carapace width reaching 8 cm (3.15 in). In comparison, blue crabs Callinectes sapidus grow to about 9.06 in. Velvet swimming crabs are blue in color, but appear brownish because of their brown hairs, leading to their velvety texture.

two "devil crabs" in happier times

At first, scientists were testing for possible viruses affecting the crabs. Declines in their French population were linked to possible Hematodinium infections – Hematodinium is a parasitic dinoflagellate responsible for bitter crab disease in Chionoecetes bairdi and C. opilio in Alaska (as well as several other crustaceans). There were massive mortalities of the devil crabs linked to these dinoflagellate infections in Mor-Braz, France.

Mor-Braz (south Brittany, France): sight of more crab deaths

The British scientists concluded that it was more likely the crabs off the Thanet Coast were dying from hypothermia due to sudden drops in temperature. In a quick search I had trouble finding in what optimal temperature range juvenile and adult N. puber thrive, but in some lab experiments they’ve been held at 17 ± 1° C. Lowest lethal temperature noted for embryonic development was measured from 2 to 4° C. Does anyone know the sea surface temps for that coast in December?

coastline of crabs

In any case, I’d just like to extend a “Good Luck” to Necora puber!

Velvet swimming crab literature:

Valdes, L., M. T. Alvarez-Ossorio, and E. Gonzalez-Gurriaran. 1991. Influence of temperature on embryonic and larval development in Necora puber (Brachyura, Portunidae). Journal of the Marine Biological Association of the United Kingdom 71: 787-789.

Wilhelm, G., and E. Mialhe. 1996. Dinoflagellate infection associated with the decline of Necora puber crab populations in France. Diseases of Aquatic Organisms 26: 213-219.