ARE FOUR SEASONS ENOUGH

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The living World

In addition to blooming flowers, May brings us the Season of the Horseshoe Crab, when the crabs make their big coming out. This "living fossil," which looks just like a walking helmet, has virtually no living relatives in the animal kingdom. From Maine to Mexico, you can see them traveling the Atlantic beaches. (The only other place the horseshoe crab exists in the entire world is Southeast Asia.) Perhaps it's strangest characteristic—aside from its blood, which is a rich copper blue—is its extra set of eyes, which can see in ultraviolet light. As for its most useful characteristic, it would have to be the strange chemical, limulus lysate, which is found in the horseshoe crab's blood. This life-saving chemical is used to test whether or not medicines have become contaminated with bacterial toxins.

This strange animal also provides an essential feast to several species of migrating birds in May. The primary location of this event is in New Jersey and the Delaware shores of Delaware Bay for a brief period in late May. Horseshoe crabs are then seen in enormous numbers on the beaches, because this is when the females males lay eggs for the males to fertilize. Enter hundreds of thousands of birds, most in the midst of immensely long migrations, all looking for horseshoe crab eggs and overturned horseshoe crabs. One type of sanderling (a small type of shore bird), called the Red Knot, travels 9,000 miles, from as far south as Tierra Del Fuego in Argentina on its way up to northern Canada.

Scientists believe that a significant part of the world's total population of bird species may show up in this one place for overturned horseshoe crabs. If for some reason, the horseshoe crab did not keep his appointment one year, most of these birds would perish, and entire species could actually be threatened. Only recently has the magnitude of this migration feast and its significance been recognized.

Diagrams for dusk and dawn show scenes about 45 minutes after sunset or before sunrise, as viewed from 40° north latitude (approximately correct for the United States or southern Canada). "Age" of the moon is amount of time since New Moon.—Adapted from Sky Calendar, Abrams Planetarium, Michigan State University, East Lansing, Michigan 48824.

SKY CALENDAR OF SPECIAL EVENTS FOR 1993

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