Beach Site Selection
The selection of spawning beaches by adult horseshoe
crabs is not well understood and may involve a number of physical
parameters. Along Delaware Bay, the horseshoe crab prefers
the middle beaches for spawning. This knowledge can be attributed
to information collected during the annual horseshoe crab census,
which over the course of a decade, clearly demonstrates a preference
for the middle beaches.
On the Delaware side of the bay, the highest
numbers of spawners are found from Pickering Beach to Slaughter
Beach. The top New Jersey spawning sites are found from Reeds
Beach to South Cape Shores Lab. New Jersey Fish & Game
also notes that there is additional spawning in rivers and
marshes associated with the bay. While these beaches will vary
in the number of spawning horseshoe crabs from year to year,
all of these beaches are found within a certain salinity range
greater than 15 but less than 30 parts per thousand. Thus,
salinity range appears to be a dominant parameter for beach
selection by horseshoe crabs.
Other physical parameters are thought to affect
or enhance beach selection as well. These include sand gain
size, beach slope, currents, cross-bay migration, and the presence
of peat. Because of the recent horseshoe crab population decline,
a number of studies are now under way to examine these parameters.
Researchers have learned that horseshoe crabs will not normally
lay eggs in exposed peat marshes or beaches. It appears that
the animals avoid the acidity, or associated hydrogen sulfide,
of the peat. It also appears from tagging studies that horseshoe
crabs do not routinely migrate across the bay, so cross-bay
migration for spawning or even food appears uncommon.
Horseshoe crabs have a relatively broad
tolerance for a range of salinity. Normal salinities for
juveniles and adults range from 8 parts per thousand (8 ppt)
to full seawater (36 ppt). In the Delaware Bay, that means
that horseshoe crabs are commonly found from Woodland Beach
to Cape Henlopen, Delaware, and out on the continental shelf.
Developing horseshoe crab embryos are affected
by salinity. At low salinities (10%), development time is
slowed. With higher salinity (25%), the speed of development
increases (Coslow, 1982). In fact, the higher the salinity,
the shorter the embryonic development time at all temperatures.
In the Delaware Bay, there is little doubt that spawning
adults have a preferred salinity range, around 18–25%,
for spawning. This is clearly documented through over 10
years of census data, where the beaches in the center portions
of the bay — from Pickering Beach to Slaughter Beach
in Delaware, and Reeds Beach to South Cape Shores Lab in
New Jersey — have consistently demonstrated that they
are the preferred beaches.
Studies to determine specific genetic
populations or variations in genetics of any harvestable,
fisheries species are rather recent, beginning only in the
last 20 years or so. Such studies tend to be opportunistic,
that is whether or not such genetic studies are completed
is often dependent on their importance as a commercial fishery.
But the population decline in horseshoe crabs, their biomedical
importance nationally and internationally, concerns for a
now-recognized commercial fishery, and the dependence of
migrating shorebirds on their eggs have a number of researchers
working to discern distinct populations along the East Coast
for management purposes.
Several scientists believed that there were genetic
differences in horseshoe crabs prior to DNA studies simply
because horseshoe crabs north and south of the Mid-Atlantic
region were typically smaller in size. Current studies, from
admittedly small samples, appear to confirm those beliefs.
The studies that have been completed indicate that there are
genetically distinct populations, in New England, the Mid-Atlantic,
and the South Atlantic/Gulf of Mexico. There appears to be
a New England population from Maine to Cape Cod Bay, a Mid-Atlantic
population from New Jersey to Chincoteague and the Carolinas,
and a southern population in Georgia and Florida. In sum, there
is but one species of Limulus; however, there are
at least three genetically distinct populations confirmed and
possibly one or two more. Some scientists and fisheries managers
believe that additional studies will show a number of isolated,
smaller, and discrete populations or sub-populations in bays
and estuaries from Maine to Florida and the Gulf of Mexico – Yucatan