Natural History

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.

Salinity Preferences

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.

Population Genetics

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 region.

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