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Horseshoe Crabs Pulled Three Ways, Declining

Beaches along Delaware Bay are hotspots for horseshoe crab spawning each spring.
Delaware National Estuarine Research Reserve

You can't be everything to everybody. The decline of horseshoe crabs is an example of why not.

Horseshoe crabs are bizarre-looking and slow-moving, sometimes called living fossils. In fact, these ancient animals aren't really crabs, at all. More like horseshoe scorpions or horseshoe ticks, if you want to be evolutionarily correct.

In the past, horseshoe crabs have been considered pests, killed to keep them from eating lucrative shellfish. Today, fishermen and biomedical scientists consider them essential and irreplaceable, in their own right. So do the endangered species that rely on horseshoe crabs for food. Here's the scoop:

  1. Horseshoe crabs save lives. Their blood contains cells that react to certain bacteria, called gram-negative, by releasing a clotting factor. This phenomenon has been turned into the widely-used test for bacterial contamination of drugs and medical devices, and there's no substitute for the horseshoe crab's blood. Biomedical "harvesting" isn't intended to kill the crabs; they're released after the blood-letting. Manufacturers of the test have said that only a few percent of bled crabs die after being released, but recent studies have suggested the number may be closer to a third.
  2. Horseshoe crabs make yummy bait. Horseshoe crabs are used as bait in commercial fisheries, including conch and eel. For reasons not entirely clear, horseshoe crabs appear to be unparalleled in their tastiness (at least as far as conch and eel are concerned). Attempts to create synthetic bait (think protein bars for fish) are ongoing, but even those efforts still rely on some smaller amount of horseshoe crabs. The bait fishery is the primary threat to horseshoe crab populations.
  3. Horseshoe crabs feed endangered species. The endangered loggerhead sea turtle is known to eat horseshoe crabs at times, but it's the crabs' eggs that are really key. When migrating shorebirds arrive on the shores of Delaware Bay each spring, having flown thousands of miles already, they are desperately in need of food. But their digestive systems can't handle solid food. The soft, rich eggs of horseshoe crabs are what help them start to rebuild their strength for the next leg of their journey. Falling horseshoe crab numbers are thought to be one of the primary factors in the decline of the red knot, which was recently added to the federal Endangered Species List.

Horseshoe crab numbers have plummeted in recent years, leading to calls for - and, in some cases, adoption of - restrictions or bans on their harvest. But more research is needed to enable managers to set useful and realistic limits. That's the goal of a horseshoe crab tagging projectrun by Mass Audubon's Wellfleet Bay Wildlife Sanctuary, and they need volunteers.

Learn more at an upcoming conference about horseshoe crabs.

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