The living fossil

StoryImage( ‘/Images/Story//Auto-img-11382206167964.jpg’, ‘Photo y Dr. Robert A. Hedeen’, ‘Horseshoe crabs that don't make it back to the sea present a problem for beach-goers. Special details are required to remove dead crabs.’);

During the Paleozoic era, some 500 million years ago, of the Earth’s history, thousands of animal species evolved. It is interesting to note that only one has endured to the present day in an unchanged condition—Limulus, the horseshoe crab.

Those of us who have visited or lived by the sea are familiar with this odd-shaped creature. A horseshoe-shaped shell covers the animal’s fused head and thorax, and offers protection against possible predators. Its smaller abdomen is encased by a heart-shaped shell that bears numerous spines and projections. Trailing to the rear from the abdomen is a long, fearful-looking pointed spine called the telson, which is not an offensive weapon but aids in navigation in the water and on the beach.

On the upper surface is a pair of unblinking, primitive eyes and on the underside are numerous appendages to assist in the capture of food, crawling about, and burrowing on the ocean floor. The breathing organs, constructed like the pages of a book, are also on the undersurface. These crabs may reach an overall length of about 2 feet and are dark brown in color.

Each year, beachgoers in various areas may witness to a portion of the bizarre mating ritual of this animal. In the Mid-Atlantic region, this usually occurs during the first two weeks of June when these ancient mariners come ashore in great numbers during the high tide of the full moon to spawn and continue the perpetuation of the species.

Males, usually about half the size of females, gather at the water’s edge to await the arrival of the egg-loaded females. A test of fitness to become fathers occurs as the amorous males compete to determine which one shall ride a female, piggyback fashion, onto the beach. Only the strongest and most robust win the contests and acquire the right to sire the next generation.

As the animals move slowly up the beach, the female pauses every few feet to dig a nest and deposit up to 20,000 greenish eggs, each about the size of bird-shot. The male cooperates in the endeavor by fertilizing the eggs as he is dragged over them. When the female has exhausted her supply of eggs, the two attempt to make their way back to the sea, and incoming waves wash sand over the nests, thus covering the eggs. Many of the parents do not make it back to the water and are stranded to die on the beach. Their numerous bodies sometimes create problems for fastidious beachgoers. The sand keeps the eggs moist and permits oxygen to reach the developing embryos. The next flood tide (in some two weeks to a month) disrupts the nest and ruptures the membrane surrounding the embryos, and they are washed back into the sea, where only a very few will survive to adulthood.

Two popular misconceptions about this creature need to be addressed: First, they are not true crabs at all, about are more closely related to such creatures as spiders, scorpions, and ticks; second; horseshoe “crabs” are not dangerous. The long, pointed telson at the rear of the body is not a weapon. In addition to its use in navigation, it is used as a lever by a horseshoe to right itself in the event it becomes turned upside down.

In spite of the nuisance they may cause during the spawning season, horseshoes are beneficial to man and other animals. The eggs are an important source of food for numerous species of shore birds, some of which time their migration to the time when the horseshoes come ashore. Years ago, horseshoes were ground up to make a nitrogen-rich fertilizer, which was also used as chicken feed. Clam dredges catch considerable horseshoes, which are then sold to eel fishermen. Eelers have found there is no better bait to place in their eel pots than a portion of a female horseshoe, especially one loaded with eggs.

In recent years, it has been determined that the horseshoe crab is truly a blueblood. Their copper-based blood was found to be useful in tests to determine substances called endotoxins, which are poisons found in the cell calls of certain bacteria and cause serious infections in humans. There is a small knot of muscle tissue at the base of the telson that some say is good to eat.

Whatever this ancient creature has been doing for the past 400 million years must be right, because all of its brethren that evolved at the same time are now extinct. Maybe the horseshoe—shaped shell is a lucky charm for this interesting creature.

Dr. Robert Hedeen is a former resident of Maryland’s eastern shore and resided in the Chicago area from 1960 to 1971. He is a retired professor emeritus of biological sciences in the University of Maryland system. He has published more than 30 scientific papers, has written numerous magazine articles, and is the author of two books on the natural history of the Chesapeake Bay.

From the Jan. 25-31, 2006, issue

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