Friday, July 22, 2011

Crabday: Horseshoe Crabs

This Crabday is dedicated to:

Horseshoe Crabs!


Wait, say what now??

OK, so a horseshoe crab isn't really a crab, per say, but more closely related to spiders (phylum Arthropoda, but subphylum Chelicerata like spiders instead of Crustacea like snow crabs). The common name "horseshoe crab" covers four living species within the family Limulidae: the Atlantic horseshoe crab Limulus polyphemus, the mangrove horseshoe crab Carcinoscorpius rotundicauda, and the congeners Tachypleus gigas and T. tridentatus.

a simple family tree showing the relationship between
horseshoe crabs, spiders, and true crabs like our snow crab

Despite not being true crabs, when I was working in the crustacean department for the NY State Department of Environmental Conservation I monitored the horseshoe crab harvest along with the lobster population. Fishermen harvest both males and females, and target the females more so because the females are so much larger than the males. (I never feel good about that kind of arrangement, but perhaps that's just me.) Fishermen use horseshoe crabs as bait for whelks (most commonly the channeled whelk Busycotypus canaliculatus), which they can then sell as scungilli. (This also explains why, when I see large snails up in the Bering Sea, I identify them as whelks... and then get made fun of. But now you know!) They are also harvested in some areas for their blood, which is then used in medical research.

These animals are obviously a hot commodity, and as such suffer from potential overharvesting. This becomes an even bigger problem because horseshoe crab eggs are the primary meal for red knots, who make an annual stop in Delaware Bay, Maryland for this treat. This may not be a sad tale, however, because at least in 2009 the red knots appeared to be gaining enough weight during their stop in Delaware Bay to make the flight north to the Arctic.

a red knot amid horseshoe crab carnage

Anywho, I want to end this Crabday with a lesson on how to properly hold horseshoe crabs. Pick them up by their "heads" and not their "tails"! The "tail spine" is actually a telson which helps them turn over if they've flipped on their backs, so it is important to not damage it. It is NOT a stinger like a sting ray: it will not hurt you. So be nice and friendly and only handle them by the front of their carapaces.

this little guy is perfectly demonstrating
how NOT to hold a horseshoe crab


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