Hitchin' a ride: scuds, shrimps, and sponges are among the creatures that cling to the horseshoe crab

Natural History, June, 2002 by Dave Grant

From a bayside beach named for its most famous inhabitant, my dive partners and I stagger into the murky waters of New Jersey's Horseshoe Cove with scuba tanks strapped to our backs. Young, olive-colored horseshoe crabs glide away effortlessly as we approach, while the sluggish, dark, heavily encrusted older crabs hunker down in the sand a few yards offshore. As we watch them, the only sound I can hear is the gurgling of my exhaled breath as it bubbles up to the surface.

The role that Limulus polyphemus plays in my life has changed from nuisance to necessity over the course of two decades (coincidentally, the life span of a healthy horseshoe crab). My earliest encounters with Limulus on the beach were brief affairs; as a youngster, I gave the animal little thought except to poke one and then beat a hasty retreat if it moved. In college I learned that the species (not a true crab but a distant cousin of spiders) is a biological success story, having dodged whatever cosmic bullets exterminated the dinosaurs. Later, when I worked on research boats sampling fish in lower Sandy Hook Bay off the coast of New Jersey, I learned to avoid taking samples at the mouths of the Navesink and Shrewsbury Rivers in April, lest I fill the nets with migrating horseshoe crabs and incite a captain's tirade that could peel the paint off the wheelhouse walls.

In time, however, horseshoe crabs began to pique my interest instead of my ire. I started observing them to better understand their diet but soon became intrigued with the variety of hitchhikers that live on them. Since then, I have been keeping an inventory of organisms that can be found on Limulus, and these, rather than the crabs, have become the focus of my investigative efforts as well as my favorite tool for teaching marine biology.

Horseshoe crabs (sometimes referred to as soldier crabs) comprise four species. Three inhabit Indo-Pacific waters, and one populates the eastern shore of North America, from the Yucatan peninsula to northern Maine. The Atlantic species has survived for a quarter billion years, predating even the tectonic events that opened up the Atlantic Ocean. Although these crabs are sometimes dragged up in fishing nets from the continental shelf, in waters more than 650 feet deep, they seem to prefer spending the winter at depths of about 100 feet or less, where I've snagged them in the fall while trolling for bluefish. Every spring, vast numbers of horseshoe crabs travel twenty miles inshore to calm bays in search of sandy beaches, where they spawn in intertidal zones. The females lay up to 20,000 eggs in shallow nests dug at the high-water line during spring tides, which occur in conjunction with new and full moons. The young take a month or more to hatch, emerge from the sand, and drift among the plankton. Their vulnerable early years are spent burrowing, feeding on marine worms and shellfish, and apparently overwintering in the marshes and tidal flats of bays; I've accidentally uncovered them while digging for clams in Sandy Hook. There, as elsewhere along the eastern shore of North America, adolescents move into deeper bay waters and coastal waters for the rest of their first decade and are surprisingly hard to find until they mature and begin returning en masse to the beach to spawn each April.

Like a rock on the seabed, a horseshoe crab is gradually hidden over time by a mantle of encrusting marine life. First to colonize the shell are bacteria, which coat submerged surfaces with a slimy film that allows other creatures to attach. Soon a zoological "five o'clock shadow" develops. My ever growing list of organisms to which the crab plays host contains more than three dozen invertebrate species representing nine phyla. The first wave of multicellular organisms that cement themselves to the crab's shell typically includes barnacles, delicate encrusting bryozoans, and tube-forming polychaete worms.

Unique confederates of the horseshoe crab are Bdelloura flatworms. Commonly (and perhaps confusingly) known as Limulus leeches, they are members of Platyhelminthes, a phylum laden with parasites and commensals; the exact relationship between the three species of Bdelloura and their horseshoe crab hosts is unknown. Unlike true leeches, or the nineteenth-century bdellometer (a mechanical substitute for leeches), they don't draw blood. Instead they are obliged to live exclusively on the gills and legs of horseshoe crabs while feeding on bacteria and other microbial life. Flip an adult crab on its back, and you are sure to find some of these creatures.

Crabs arriving from offshore in late April often wear a crown of invertebrates that have attached as larvae and somehow avoided being worn off or smothered in the sand when their host crab burrowed. Other creatures are equally undeterred: the sand builder worm, which glues sand grains together for its home, establishes itself in thick patches along the sides of crabs as they wait, half buried in the sediment, for spring off our mid-Atlantic beaches. And high on the crabs' backs, I often find striking red beard sponges and fragile colonies of Bugula (a bushy bryozoan that is often dyed green and sold in florist shops as "Irish sea fern").