The Love Lives of Horseshoe Crabs, Not Cannibals

Amidst the studies on Hurricane Sandy’s impacts on coastal communities—which affected the lives of people, most notably—some recent studies have examined the impacts on the lives of a strange ancient creature: horseshoe crabs. Distant relatives of scorpions and spiders, horseshoe crabs are not true crabs, or crustaceans. They’ve been around for over 1 billion years and lived alongside dinosaurs. See “The Life and Times of the Earliest Horseshoe Crabs,” (Rudkin, Royal Ontario Museum). Unlike a scorpion, crab or spider, horseshoe crabs don’t bite, sting or pinch. And unlike cannibalistic crustaceans, adult horseshoe crabs do not congregate (except to spawn seasonally), which is possibly a way to avoid large crabs attacking smaller horseshoe crabs—thus, avoiding cannibalistic behavior as a species. (Sekiguchi, Shuster, Jr., 1999) Their anatomy is interesting, as illustrated below.

Each spring, horseshoe crabs spawn along creek-mouth beaches and shoals. They like sandy beaches. Naturally, these coastal areas, rich in wetlands, peat bogs and saltwater marsh, were hit hard by Hurricane Sandy. What’s more, sea level rise has eroded certain coastal beaches where horseshoe crabs used to spawn, decreasing the habitat suitable for spawning. See Sea Level Rise and the Significance of Marginal Beaches for Horseshoe Crab Spawning (Botton and Loveland, 2011).

In the Delaware Bay, for example, the American Littoral Society, along with the Wetlands Institute, the New Jersey Department of Environment Protection and New Jersey Audubon, have assessed the impacts of Hurricane Sandy on horseshoe crab populations in the Bay. Watch this video of horseshoe crab spawning in Delaware. Read Hurricane Sandy Race To Restore Horseshoe Crab Spawning Grounds (March 2013). Videos depict horseshoe crabs spawning, swimming upside down and righting themselves.

In a joint report by the Wetlands Institute, NJ Audubon Society and NJ Division of Fish & Wildlife (“Damage from Superstorm Sandy to Horseshoe Crab Breeding and Shorebird Stopover Habitat on Delaware Bay,” December 2012), wetlands did well overall, despite some “wash over” during storm surges of Hurricane Sandy, according to Lenore Tedesco, Ph.D. Director of Research at the Wetlands Institute. Yet a major finding was a 70% decrease in suitable breeding habitat for horseshoe crabs. In addition, there was about the same amount of increase in unsuitable habitat for horseshoe crab spawning. Specifically, the scientists classified the types of habitat into five categories:

  1. Optimal: undisturbed sand beach;
  2. Suitable: sand beach with only small areas of peat and/or backed by development
  3. Less Suitable: exposed peat in lower/middle intertidal zone;
    sand present in upper intertidal;
  4. Avoided habitat:  exposed peat or active salt marsh fringing the shoreline;
    no sand present
  5. Disturbed due to beach fill, riprap or bulkheading.
    (Niles, Tedesco, Sellers, et. al. 2012)

In areas where the habitat is less suitable, with exposed peat, there is less sand for the horseshoe crabs to lay their eggs. The full report includes recommendations for habitat restoration. For more information about post-Sandy restoration recommendations, visit the Wetlands Institute’s website here.

Many years ago, I learned that horseshoe crabs (Limulus polyphemus) lay at the heart of some medical advances in immunology research. Apparently, horseshoe crab blood and immunology can serve scientists with a model to develop treatments for patients with HIV, AIDS or other immune deficiency disorders. Its “blue blood” contains Limulus Amebocyte Lysate, which allows medical researchers to detect bacterial toxins. In ecological projects, the spawning and genetic diversity of horseshoe crabs is the focus, or the relationship between horseshoe crabs and fisheries. For an overview of various research projects on horseshoe crabs happening in 18 states and two countries, see these project summaries. There’s some fascinating research underway.

The Wetland Institute has a number of publications on its website related to horseshoe crab research and conservation. There’s also an “Adopt a Horseshoe Crab” program and horseshoe crab census data available from 1999-2009. In May, the Institute holds festival activities, including teaching tank/aquarium talks on saltmarsh ecology, shorebirds and horseshoe crabs. For more information about the Horseshoe Crab Festival in May,click here.

More videos:

See horseshoe crab counting (Washington Post video, June 2012)
NATURE program on horseshoe crabs (PBS, 2008)
Horseshoe crab documentary (Nick Baker, Science Channel)

For more on horseshoe crab biology and ecology, see this National Park Service webpage and materials developed by the Mid-Atlantic Sea Grant and NOAA joint programs on horseshoe crab research. Finally, check outhttp://horseshoecrab.org/ which houses an online warehouse of information on the biology, conservation and research of horseshoe crabs.

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