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Living Sanibel: Old as time

By Staff | Jun 17, 2015

Horseshoe crab. BLAKE SOBCZAK

A living fossil, the horseshoe crab has remained essentially unchanged for the past 250 million years. The present-day species is little changed from an earlier relative that dates back to the Silurian period, more than 400 million years ago. It is the closest living relative to one of the oldest known fossil finds on earth, the trilobite.

The horseshoe crab is actually far more closely related to spiders, ticks, and scorpions than to any other species. It is sometimes called a “living museum” because of the wide array of species it carries, including algae, flatworms, mollusks, barnacles, and byozoa. As adults they are heavily preyed upon by raccoons and otters, and as juveniles they are taken by birds, fish and larger crabs.

The horseshoe crab is unique in that it has blue blood, based on copper rather than iron, making it valuable to scientists. The pharmaceutical industry, for example, uses horseshoe crab blood derivatives to detect bacterial endotoxins in drugs. A single horseshoe crab can be worth $2,500 over its lifetime in blood extractions alone.

The animal is also studied for its unusual vision. It has four eyes and a complex method of sight that is still not fully understood. Lacking jaws, it grinds its food-bits of fish, mollusks, annelid worms, and other invertebrates-with bristles on its legs and a primitive gizzard containing sand and gravel.

The horseshoe is a very important part of the estuary system, and because of its recent declining numbers, scientists are concerned about its future. A decade ago thousands of these living fossils were found along either side of Wildlife Drive in “Ding” Darling. Today finding a single living horseshoe crab is rare.

-This is an excerpt from The Living Gulf Coast – A Nature Guide to Southwest Florida by Charles Sobczak. The book is available at all the Island bookstores, Baileys, Jerry’s and your favorite online sites.