Since we've been on an endangered-species kick around these parts, here's a nice piece in The New York Times today on horsehoe crabs, whose numbers may be rapidly dwindling down to dangerously low levels after being over-harvested for fish bait. This part, in particular, nicely explains the crab's role in sustaining the larger coastal ecosystem—and why it's invaluable to humans, too:

The loss of the horseshoe crab would be tragic, researchers said, not only because the creatures are fascinating and cute and predate the dinosaurs by 200 million years, but also because so many contemporary life forms depend on them. Their annual spawns draw hundreds of species of migratory birds, predatory fish, reptiles, amphibians and various other alimentary canals eager to brunch on the freshly deposited Limulus eggs. "Horseshoe crab eggs are like filet mignon around here," Dr. Mattei said. "They’re a very popular item on the menu." ...

We, too, are multiply tethered to the ancient mariners. From their blood we extract a protein that is exquisitely sensitive to bacterial toxins and is used to test surgical instruments and intravenous drugs to ensure they are safe. The relatively simple visual circuitry of the horseshoe crab has proved an ideal model system for decoding the basis of sight. "Only with the horseshoe crab eye is it possible to predict what each nerve fiber in the retina will send to the brain as it sees," said Robert B. Barlow, a professor of ophthalmology at the State University of New York Upstate Medical University.

Another bit of trivia: Earlier this year, scientists found a few horseshoe-crab fossils in Manitoba that dated back 445 million (!) years. The biggest surprise was that the crab's basic structure has hardly changed a bit since then. Not bad for an old man...

--Bradford Plumer