Horseshoe crab in decline

horseshoe crab
Horseshoe crab
Credit: Wikimedia

horseshoe crabs
Horseshoe crabs gathering on the shore of Delaware Bay.

red knots
Red Knots.

A decline in horseshoe crab numbers has occurred, paralleling changes that took place at the end of the last ice age, according to a study published in the journal Molecular Ecology.

The new research, which used genomics to assess trends in population sizes, also indicates these crabs may continue to decline because of predicted climate change, said Tim King, a scientist with the U.S. Geological Survey and a lead author of the study.

While the current decline in horseshoe crabs (Limulus polyphemus) is attributed in great part to excessive harvesting, the new research suggests climate change also has played a role in reducing the number of crabs. More importantly, said King, predicted future climate change, with its accompanying sea-level rise and water temperature fluctuations, may well limit the distribution and interbreeding of these crabs, resulting in localized and regional population declines, such as happened after the last Ice Age.

The research substantiated recent significant declines all along the Atlantic Coast from Maine to Florida and in the eastern Gulf of Mexico.

These findings, combined with the results of a 2005 study by King and colleagues, have implications for the welfare of wildlife that feed on nutrient-rich eggs these crabs produce each spring.

For example, Atlantic loggerhead sea turtles, which used to feed mainly on adult horseshoes and blue crabs in Chesapeake Bay, already have been forced to find other less suitable sources of food, perhaps contributing to declines in Virginia's sea turtle abundance. Additionally, crab eggs are an important source of food for millions of migrating shorebirds. This is particularly true for the Red Knot, an at-risk shorebird that eats this crab's eggs during migration. These birds and a wide variety other organisms will be adversely affected if Limulus continues to decline.

(Adapted from materials obtained from the USGS)

Horseshoe Crab Facts

  • In taxonomic classifications Limulus is grouped with spiders and scorpions, and not with ordinary crabs.
  • While historically these crabs have been used in fertilizer, most of the crab harvest today is by the fishing industry, which uses them as bait.
  • The pharmaceutical industry also collects large quantities horseshoe crab blood, which has unique clotting properties. Although the crabs are returned after their blood is taken, the estimated mortality rate for bled crabs can be as high as 30 percent.
  • From an evolutionary standpoint, Limulus polyphemus is what is known as a living fossil. Judging from fossils, it has not changed in the last 300 million years.

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