Horseshoe Crabs and their Neighbors
What species depend on the horseshoe for food?
The Delaware Estuary is the largest staging area for shorebirds in the Atlantic Flyway and is the second largest staging site in North America, with an estimated 425,000 to 1,000,000 migratory shorebirds converging on the Delaware Bay to feed and rebuild energy reserves prior to completing their northward migration.
At least 11 species of migratory birds use horseshoe crab eggs as their primary food supply during their 2 to 3 week stopover. The eggs replenish their fat supply during their trip from South American wintering areas to Arctic breeding grounds. Most of the eggs consumed are from nests that have been disrupted by waves and storms, so the feeding of the shorebirds does not have an adverse affect on the breeding success of the horseshoe.
Finfish and Sea Turtles
Horseshoe crab eggs and larvae are a seasonal food item of invertebrates and fish. In the Delaware River from May through August, striped bass and white perch eat horseshoe crab eggs. In addition, American eel, killifish, silver perch, weakfish, kingfish, silversides, summer flounder and winter flounder also eat eggs and larvae.
All crab species and several gastropods, including whelks, feed on horseshoe crab eggs and larvae. They are also the most common prey of sea turtles. In fact, maintaining abundant stocks of adult horseshoe crabs may be an important component of ensuring the long-term survival of loggerhead sea turtles in the Delaware and Chesapeake Bays.
Living off the Limulus
Illustrations by Dave Grant
As a crab ages and its growth rate slows, it sheds less frequently and begins to display a striking variety of hitchhikers. An abundance of smaller creatures live on (and off) the horseshoe crab in a symbiotic relationship.
Sponges and other fouling Porifera occasionally become established on the posterior of horseshoe crabs; probably when the water is cool and the crab is half buried in a dormant stage.
The Limulus leech (Bdelloura) is a flatworm that is found around the book gills and leg joints of crabs, especially on older females that have not shed for a long time. The leech lays its eggs in the "pages" of the crab's book gills and these are visible as little dark spots. It may also use the cuticle of the gills as a substrate for chemical activity.
Several crustaceans are regular companions of the horseshoe crab, including mud crabs and sand shrimp. Tiny juvenile spider and rock crabs also find a home in the crevices inside Limulus molts.
The mollusks are better represented on the horseshoe than any other phylum. Several species of bivalves become attached to the crabs, and a number of snails are also regularly found on them. Mussels usually attach themselves near the hinge where water is circulated to the gills by the resting crab. Three species of slipper shell--the common, convex and flat-- are regularly found attached to the underside of the crab.
Snails are well represented on horseshoe crabs, including periwinkles, basket and mud snails and the drill. Snails also lay eggs on the back of the crab and, in the spring, horseshoes can be carpeted from head to tail with drill and mud snail egg capsules.