Blue Blood


Yesterday’s post on red knots had me thinking of another coastal dweller, the horseshoe crab. Horseshoe crab eggs help nourish the red knot subspecies Calidris canutus rufa in the Delaware Bay and surrounding areas as the birds migrate from South America to their breeding grounds in the Arctic. In the spring, horseshoe crabs come ashore and mate on the sandy beaches of the US Mid-Atlantic region (they can be found along a large part of the Atlantic coast but the population is most dense in the Mid-Atlantic, particularly Delaware Bay). Along with them come the red knots and the feasting begins before the birds continue their trip to the Arctic and some birdy romance.

There are four species of horseshoe crab, one in North America and three in Southeast Asia. The species of interest to the rufa red knot is the North American species, Limulus polyphemus (also known as the Atlantic horseshoe crab). The horseshoe crab is not actually a crab but a close relative to spiders. It is often referred to as a “living fossil” because horseshoe crab fossils from 450 millions years ago look very much like their current form.

Horseshoe crabs are not just of interest to red knots, we humans have found a life saving use for these ancient sea creatures. Horseshoe crabs have amebocytes in their blood. These particular amebocytes causes blood to coagulate when it is in the presence (even in small amounts) of bacterial endotoxins. Horseshoe crab blood is therefore extracted to test intravenous drugs for contamination, the clinical product is called  Limulus amebocyte lysate. The impacts of bleeding horseshoe crabs is a topic of discussion with some evidence of reduced fitness in females after bleeding takes place.

Many moons ago I had the opportunity to see this process in action and it is something out of the Twilight Zone. It’s mind boggling to consider the role horseshoe crabs play in vaccine safety.

In recent years overfishing (they are popular bait for conch and eel fishing) and possibly clinical bleeding led to a major reduction in horseshoe crab populations which led to a reduction in the rufa red knot population. In addition, it appears that climate change may also be impacting this delicate balance between the two species. With rising water temperatures and increased storm activities, spring spawning for horseshoe crabs occurs earlier and earlier but unfortunately the migratory stopover of the red knot does not change. Thus the synchrony once found between these two events is fading. Without a horseshoe crab egg feast, red knots will not have the sustenance needed to endure their long journey north. For this reason (and others) the rufa red knot is listed as threatened under the US Endangered Species Act.

So that is the tale of the red knot and the horseshoe crab-a very, very, very short version. Oh and the title, horseshoe crab blood is copper-based (not iron like our own) which gives it a blue color. So horseshoe crabs are actually “blue blooded”. Ha!

About the Illustration: Simple watercolor and micron pens.

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