The great outdoors can produce baffling mysteries. MVTimes Wild Side columnist Matt Pelikan tries his best to solve them. Got a question for the Wild Side? Send it to onisland@mvtimes.com.
Dear Matt,
Found this on the beach behind my office. When I brought it in and showed it around, I was surprised how many people had no guesses as to what it is. An interesting find! My question is, are horseshoe crabs in trouble or what? I know their mating is monitored and also that they are chopped up for bait by the truckload at a Vineyard Haven business. What’s the deal?
Hi,
As menacing as this object looks, it’s a harmless piece of the anatomy of an equally harmless arthropod, the horseshoe crab. Not actually crabs, these odd-looking animals are more closely related to terrestrial spiders. There are four species worldwide; ours, the Atlantic horseshoe crab, occurs on the East Coast and in the Gulf of Mexico. Named for its overall body shape, the horseshoe crab is one of the most ancient creatures still extant, with a lineage dating back about 450 million years.
The aftermost body segment of these primitive sea creatures, the spike-like “tail,” or telson, guides, stabilizes, and helps right a horseshoe crab as it skims over the sea floor (often upside down!). The telson may also contain sensory organs to help the crab keep tabs on surrounding conditions. But the spike plays no role at all in self-defense, and indeed, except for its robust shell and a fringe of small, movable spines along its back edges, a horseshoe crab is defenseless.
Or you might say that being prolific is the defense strategy of the species. Horseshoe crab eggs are eaten in large quantities by a wide range of other animals, and young crabs are likewise tempting prey. But horseshoe crabs are highly prolific — they simply lay enough eggs, about 80,000 per female according to most sources — so that some are sure to escape being eaten. And by concentrating their egg-laying into narrow time windows, horseshoe crabs overwhelm the ability of predators to consume eggs. Mature crabs come ashore to spawn primarily around full and new moons (associated with particular high tides) in spring, mating and laying eggs near the wrack line.
Though horseshoe crabs have survived ice ages, mass extinctions, and the evolution of advanced predators, overharvest by humans (spawning crabs can simply be picked up off the beach) has threatened their survival in some regions. Horseshoe crab blood is a valuable resource for the biomedical industry, and for years crabs were harvested and drained of their blood. Current methods are less lethal — the crabs are generally caught, partially bled while still alive, and released; survival is said to be high, but the process must surely stress the crabs. On the Vineyard, horseshoe crabs are harvested mainly to be cut into bait for the conch pot-fishery, said to be the Island’s most valuable fishery.
Since other animals feed heavily on crabs and crab eggs, the decline in horseshoe crab numbers has echoed through the marine ecosystem. In particular, migrant shorebirds traveling along the East Coast of North America rely on crab eggs for fuel, timing and routing their migration to exploit the spawning cycle of the crabs (most notably along the shores of Delaware Bay). Without enough crab eggs to feed on, these birds may be unable to complete their northward migration, or, if they can reach their Arctic breeding grounds, they may have insufficient energy reserves for successful reproduction.
Horseshoe crab conservation measures, such as prohibiting harvest at the times of peak spawning or prohibiting the taking of females, have been put in place in several key states and may reverse the decline of these interesting and ancient animals, in turn helping to conserve shorebird populations. But horseshoe crabs mature slowly, not spawning until they’re nine or 10 years old, according to Island naturalist Suzy Bowman. So a meaningful rebound of the crab population may take some time.
Since 2008, numbers of spawning horseshoe crabs have been monitored at a couple of key Vineyard spawning sites as part of a larger effort coordinated by the state’s Division of Marine Fisheries. Ms. Bowman, who helped get the Vineyard effort up and running, says that the count of spawning crabs along the Lake Tashmoo and Sengekontacket shorelines may still be declining (though the data are not conclusive). While acknowledging the economic importance of harvesting (or bleeding) horseshoe crabs, the Wild Side perspective is that these ancient arthropods play critical roles in the inshore marine ecosystem, and management to ensure their recovery and survival is essential.