A prehistoric creature, from 500 million years ago, is expected to come to the New Jersey shore again this year. Related closely to spiders, the ancient Horseshoe Crab may soon be searching the beaches and mud flats for good places to lay their tiny green eggs. However, when I looked at the tide chart for 2007 and the schedle of the full moon, I wondered how the Horseshoe Crab would have enough time to reach the shore, lay eggs and get back into the water before the hot summer sun came with its scalding glare.
The tide chart for 2007 for the New Jersey area indicated that the highest tides would occur early in the morning, right before the sun came up. The moon would be setting early also, giving only a short time for the nocturnal Horseshoe Crabs to safely lay their eggs. Does the element of time determine the nesting sites of the Horseshoe Crabs? These prehistoric creatures that have been roaming the earth for centuries, may sense the need for better nesting sites elsewhere. Would they be going to the Caribbean islands or the Gulf of Mexico? It’s a mystery that needs to be solved.
Horseshoe Crabs come up across the continental shelf from the cold ocean’s depth to the shoreline in spring and early summer. Estuarial waters and mud flats are the perfect locations for nesting. The crabs push their way along the sand and mud then wallow round impressions into the sand where the female lays eggs and the male fertilizes them. There are four species of Horseshoe Crabs found in the waters of Asia, North America and the Caribbean. The Latin name for the North American species is Limulus polyphemus. It grows to a length of about 2 feet. To get to this size the crab sheds it skin as many as sixteen times in twelve years. The abandon shells are often seen on the beaches.
The construction of the crab is peculiar compared to most living things. The mouth is situated in the middle of the body with the feet surrounding it. The feet are used like hands to grab worms from the ocean floor and pull food into the mouth of the crab. The Horseshoe Crab has four eyes that are sensitive to ultraviolet light. The crab breaths through gills located under the body, behind the legs.
The best nesting times for the crab are during the highest tides of spring and summer. They like to nest just a little beyond the tide line, usually after sunset or during a full moon. But in 2007, in New Jersey, the highest tides with a full moon were in the morning just before sunrise and in the evening before sunset. The highest tides occurred right before the sun comes up or in the evening during daylight. Would this cause a dilemma that would send the Horseshoe Crabs to some other shore?
At Wilson’s Beach, Campobello Island, New Brunswick, Canada the relationship between tides and sunrise was different. The schedule of tide and moon could be more favorable for Horseshoe Crab nesting. In New Brunswick, Canada on May 2, 2007, a nearly full moon was shining at 8:12 p.m. and the sun was setting at 8:32 p.m. The high tide of 21.39 feet occurred at 12:28 a.m. The next high tide at night on May 3, 2007 was at 1:01 a.m. rising to 21.46 feet. This time schedule in Canada could be better for the Horseshoe Crabs. They would not have to contend with the bright rising sun and its scalding heat which would kill them.
Could this explain why the crab population appears to be diminishing? The crab population could be altering its course according to the tide and moon. Maybe the bird populations, such as the Rudy Turnstone and Red Knot that feed on Horseshoe Crab eggs during migration, suffer population declines during the times when the Horseshoe Crab is farther north. Is this a natural mystery that needs to be solved?
Copyright 2007 Patricia Hilliard
