Limulus and the Environment

Horseshoe crabs range from Maine to the Yucatan with the center of their population in the New Jersey, Delaware and Maryland coastal waters. They are typically found in waters close to shore and less than 20m (60 feet) deep.

Spawning - Limulus style

In the spring, horseshoe crabs move inshore. The larger females develop eggs in their ovaries while the males begin to seek out females. Pairing up begins in the early spring. Male Limulus use their eyes to locate females in the brightening spring waters (See more on Limulus vision). When a female is located, the male attaches himself to her abdomen with his first set of legs. The chelae (claws) of the male are modified into a grasping appendage reminiscent of a boxing glove; a shape perfect for hanging on. And hang on he must, sometimes for several weeks while waiting for the moon and sun to align.

Spawning takes place along sandy beaches, often in coves, marshes, and bays where there is some protection from surf. The unattached (literally) males arrive at these areas first and await the arrival of the females. They form a gauntlet through which females and their attached suitors must pass and by the time they reach the shore a female may find herself surrounded by 30 or more males eager to take part in the spawning. In some locations, the beaches may be crowded for miles with hundreds of thousands of spawning crabs shuffling and crawling over one another at the high water mark.

Limulus spawns on the spring tide which coincides with the full moon. On these nights the tide is higher than it will be until the spring tide two weeks later. Crabs make their way up the beach and the female digs a cavity in the sand to a depth of 6 inches of so where she deposits several thousand large, greenish eggs. As the female moves off the nest, she pulls the attached male over the spot where he fertilizes them. The remaining males crowd around her, trying to fertilize any eggs the attached male might have missed. Studies have shown that females average 3650 eggs per nest and may lay as many as 88,000 eggs in a season (Shuster, 1982)..

In areas where there is a high concentration of spawning crabs, burrowing females can unwittingly dig up the nests of females that nested earlier. In areas such as Delaware Bay, where hundreds of thousands, if not millions, of crabs spawn, these disturbed eggs accumulate on the surface and represent a significant food resource for animals that can reach them. Migrating shorebirds have come to depend on these eggs as an important food source. Most species of shorebirds cannot access undisturbed eggs as they are buried too deep for all but the longest bills but the exposed eggs are a free meal to any bird that happens upon them.

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Shuster, C.N., Jr. 1950. Observations on the natural history of the American horseshoe crab, Limulus polyphemus. Third report investigations of methods of improving the shellfish resources of Massachusetts, Woods Hole Oceanographic Institution, Control No. 564:18-23.