Decline may affect at-risk shorebirds
A distinct decline in horseshoe crab numbers has
occurred that parallels climate change associated with the end of the
last Ice Age, according to a study that used genomics to assess
historical trends in population sizes.
The new research also indicates that horseshoe crabs numbers may
continue to decline in the future because of predicted climate change,
said Tim King, a scientist with the U.S. Geological Survey and a lead
author on the new study published in Molecular Ecology.
While the current decline in horseshoe crabs is attributed in great
part to overharvest for fishing bait and for the pharmaceutical
industry, the new research indicates that climate change also appears to
have historically played a role in altering the numbers of successfully
reproducing horseshoe crabs. More importantly, said King, predicted
future climate change, with its accompanying sea-level rise and water
temperature fluctuations, may well limit horseshoe crab distribution and
interbreeding, resulting in distributional changes and localized and
regional population declines, such as happened after the last Ice Age.
"Using genetic variation, we determined the trends between past and
present population sizes of horseshoe crabs and found that a clear
decline in the number of horseshoe crabs has occurred that parallels
climate change associated with the end of the last Ice Age," said King.
The research substantiated recent significant declines in all areas
where horseshoe crabs occur along the West Atlantic Coast from Maine to
Florida and the eastern Gulf of Mexico, with the possible exception of a
distinct population along the Yucatan Peninsula of Mexico
These findings, combined with the results of a 2005 study by King
and colleagues, have important implications for the welfare of wildlife
that rely on nutrient-rich horseshoe crab eggs for food each spring.
For example, Atlantic loggerhead sea turtles, which used to feed
mainly on adult horseshoe crabs 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, horseshoe 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 uses horseshoe crab eggs at Delaware Bay
to refuel during its marathon migration of some 10,000 miles. Since
the late 1990s, both horseshoe crabs and red knot populations in the
Delaware Bay area have declined, although census numbers for horseshoe
crabs have increased incrementally recently.
"Population size decreases of these ancient mariners have
implications beyond the obvious," King said. "Genetic diversity is the
most fundamental level of biodiversity, providing the raw material for
evolutionary processes to act upon and affording populations the
opportunity to adapt to their surroundings. For this reason, the low
effective population sizes indicated in the new study give one pause."
These studies should help conservation managers make better-informed
decisions about protecting horseshoe crabs and other species with a
similar evolutionary history. For example, the 2005 study indicated
males moved between bays but females did not, suggesting management
efforts may best be targeted at local populations instead of regional
ones since an absence of enough females may result in local extinctions.
"Consequently, harvest limitations on females in populations with
low numbers may be a useful management strategy, as well as relocating
females from adjacent bays to help restore certain populations," King
said. "Both studies highlight the importance of considering both
climatic change and other human-caused factors such as overharvest in
understanding the population dynamics of this and other species."
Horseshoe crabs are not crabs at all – in fact, they are more
closely related to spiders, ticks and scorpions. While historically
horseshoe crabs have been used in fertilizer, most horseshoe crab
harvest today comes from the fishing industry, which uses the crab as
bait, and the pharmaceutical industry, which collects their blood for
its clotting properties. While the crabs are returned after their blood
is taken, the estimated mortality rate for bled horseshoe crabs can be
as high as 30 percent.
The research, Population dynamics of American horseshoe
crabs—historic climatic events and recent anthropogenic pressures, was
published in the June issue of Molecular Ecology and was authored
by Søren Faurby (Aarhus University, Denmark), Tim King, Matthias Obst
(University of Gothenburg, Sweden) and others.
The 2005 study, Regional differentiation and sex-biased dispersal
among populations of the horseshoe crab (Limulus polyphemus), was
published in the Transactions of the American Fisheries Society and
authored by Tim King, Mike Eackles Adrian Spidle (USGS) and Jane
Brockman (University of Florida).
Contact: Catherine Puckett
[email protected]
352-264-3532
United States Geological Survey