Horseshoe crabs play a significant ecological role in the food web for migrating shorebirds, finfish, and Atlantic loggerhead turtles (Caretta caretta), a federally-listed threatened species that uses the Chesapeake Bay as a summer nursery area (Keinath et al. 1987).
The Delaware Estuary is the largest staging area for shorebirds in the Atlantic Flyway and is the second largest staging site in North America (New Jersey Division of Fish, Game and Wildlife, 1994). An estimated 425,000 to 1,000,000 migratory shorebirds converge on the Delaware Bay to feed and rebuild energy reserves prior to flying an additional 4,000 kilometers to complete their northward migration (Wander and Dunne, 1982; Dunne et al., 1982; Clark et al., 1993). Migratory shorebirds arrive in Delaware Bay and adjacent areas along the Atlantic coast at the peak of horseshoe crab mating from mid-May through early June, typically spending two weeks in the area. Clark (1996) states that the number of shorebirds coming to the Delaware Bay on spring migrations is between 900,000 and 1.5 million individuals representing six species. At least 11 species of migratory birds use horseshoe crab eggs to replenish their fat supply during their trip from South American wintering areas to Arctic breeding grounds (Myers, 1986).
Horseshoe crab eggs and larvae are a seasonal food item of invertebrates and finfish. In the Delaware River from May through August, striped bass (Morone saxatilis) and white perch (Morone americana) eat horseshoe crab eggs. American eel (Anguilla rostrata), killifish (Fundulus spp.), silver perch (Bairdiella chrysoura), weakfish (Cynoscion regalis), kingfish (Menticirrhus saxatilis), silversides (Menidia menidia), summer flounder (Paralichthys dentatus), and winter flounder (Pleuronectes americanus) also eat eggs and larvae (Shuster, 1982). All crab species and several gastropods, including whelks, feed on horseshoe crab eggs and larvae. Shuster (1982) reported a large leopard shark (Triakis semifasciatum) preying on adult horseshoe crabs in southern Florida.
Lutcavage and Musick (1985) examined the stomach contents or excreta from 527 loggerhead turtles from the Chesapeake Bay and nearby coastal waters and found that the most common prey was horseshoe crab. Musick et al. (1983) examined 27 loggerhead turtles and found horseshoe crabs were a common item in the stomach contents. Similarly, Lutcavage (1981) found that horseshoe crabs represented up to 42 percent of the diet of loggerhead turtles from the Chesapeake Bay (N=6), with an average of 22 percent. Data collected by the NMFS Sea Turtle Stranding and Salvage Network along the Atlantic Coast identified horseshoe crabs in 75 percent of loggerhead stomach contents in 1996 (N=8) and 55 percent in 1997 (N=11) (Evans, pers. comm., 1998). Morreale and Standora (1993) found no evidence of horseshoe crabs in loggerhead turtle diets in New York's Long Island Sound; however, diet largely depends on the relative abundance of prey species. Maintaining abundant stocks of adult horseshoe crabs may be an important component of ensuring the long-term survival of loggerhead sea turtles in the Chesapeake Bay area.
Gauvry, Glenn. "Horseshoe Crabs and Their Neighbors."
The blood of the horseshoe crab provides a valuable medical product critical to maintaining the safety of many drugs and devices used in medical care. A protein in the blood called Limulus Amebocyte Lysate (LAL) is used by pharmaceutical and medical device manufacturers to test their products for the presence of endotoxins, bacterial substances that can cause fevers and even be fatal to humans.