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News and Resources

Survey counts horseshoe crabs - April 26, 2012

The 11th annual horseshoe crab spawning survey is about to begin. The survey continues the local assessment of population abundance and critical habitat availability in Maryland's coastal bays.

Thanks to the generosity of volunteers who provided their time and efforts last year, 58 surveys were collected from five sites, and revealed a total of 23,105 crabs. This is just slightly lower than the total number observed in 2010.

Last year, more than 21,000 horseshoe crabs were spawning on Skimmer Island, the focus of beach renourishment efforts over the past two years. The second-highest spawning location was on the bayside of Assateague Island, where 1,007 crabs were observed. The small beach behind the Inlet Oceanic Motel came in third with 824 crabs counted.

In the coastal bays, horseshoe crab spawning peaks in May and June, with most spawning occurring on evening high tides near the full and new moons.

The compound eye structure and photoreceptors on the tail enable the crabs to detect the ultraviolet light of the moon. The number of crabs spawning around each lunar tide varies from year to year, but will peak in the low light around dawn or dusk while taking advantage of the higher than usual tides. Eggs are deposited in the sand between the low and high tide marks on the beach. The closer the nests are to the high tide line, the more heat from the sun can be counted on to incubate the eggs.

Spawning usually occurs along sandy beaches in bays and coves, where there is protection from waves. Weather conditions, water temperature and lack of habitat can affect when and where crabs gather. Heavy surf will prevent crabs from spawning, as well as wash away egg masses and juveniles.

During the spawning season, males arrive first and wait in the near shore shallows for the arrival of females. Females emit chemical attractants, pheromones, which alert the males to attach themselves to her abdomen with a front pair of "boxing glove" pincers. Males are about 20 percent smaller than females, and as many as six will try to clasp onto the female as she works her way up the beach. The female will burrow into the sand about 6-8 inches and deposit 200-3,500 pale green eggs. Several nests may be laid during a single beach trip.

Studies in Delaware found that females laid, on average, more than 3,000 eggs per nest and can lay as many as 88,000 eggs per season. If the numbers of spawning crabs and nests are high enough, it is possible the crabs will inadvertently dig up existing nests while preparing new ones.

This is significant to shorebirds since the exposed eggs accumulate on the surface of the beach and are readily available as food.

At least 11 species of migratory birds use these eggs as their primary food supply during their brief spring stopover, mid-migration.

Flocks of migrating birds will time their arrival at certain beaches to correspond with egg supply.

The eggs replenish their fat supply during their trip from South American wintering areas to Artic breeding grounds. Additionally, fish, crabs, snails and sea turtles rely on horseshoe crab eggs and larvae as food.

The time eggs take to develop in the nest is dependent on temperature, moisture and oxygen. Most larvae emerge from their eggs after a month or more of development, in relatively calm water when the moon is full.

Receding waves will carry the small (1 cm) crabs into the intertidal shallows, where they swim around for about six days before settling to the bottom and molting. Juveniles tend to stay in the intertidal flats near where they hatched for the first two years of life.

The crabs continue to grow and molt until maturity is reached at age 9-10. Once they reach maturity, they will migrate back to shore each year to spawn. Horseshoe crabs have a life span of approximately 19 years.

The horseshoe crab, Limulus polyphemus, can be found from the Gulf of Maine to the Gulf of Mexico.

Ninety percent of the population lives along the mid-Atlantic coast, with the largest concentration along the Delaware Bay. It is unknown if the Maryland coastal bays have their own sub-population of crabs which return yearly or if these crabs entered our inlet mistaking it for Delaware Bay.

Future surveys could help to answer this question (via tagging crabs) and remind us all of the role and importance of these creatures to the ecosystem.

One of the biggest threats to horseshoe crabs along our coasts is development. Hardened shorelines prevent horseshoe crabs from reaching critical nesting habitat.

To date, the purpose of the annual survey was to gain an understanding of where crabs were spawning locally and at what abundance.

These hard numbers regarding population dynamics and spawning densities allow resource managers to make informed decisions that will protect the species, ensuring its continued contribution to the region's ecosystem and economy.

If you would like to learn more about local wildlife monitoring efforts, visit

Carol Cain is the technical coordinator for the Maryland Coastal Bays Program.

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