Geoscience Reference
In-Depth Information
a protective serrated edge, and underneath are housed so-called topic gills, the breathing organ. The crab sports
a tail-spike, or telson, which it uses for righting itself if it is flipped over by waves while spawning, and which
native Americans employed as a spear point.
Horseshoe crabs reach sexual maturity at nine to ten years, when they make their spawning migration from
deep waters of the bays or continental shelf to the beaches. They time their coming ashore to the highest lunar
tides in May and June, at the new and full moons, to reach the higher intertidal portions of the beach. Often
they hit the beaches on the night tides or in the low light of dawn or dusk, and they seek out beaches where
there is protection from heavy surf.
Males arrive first and attach themselves to the female by locking onto her spiky opisthosoma with a pair of
“boxing glove” pincers. Dragging the male— or sometimes more than one—the female works her way up the
beach and excavates a nest 15 to 20 centimeters (6 to 8 inches) deep, where she deposits approximately four
thousand tapioca-sized, greenish-gray eggs. When she turns to make her way back to the sea, the male fertil-
izes the eggs externally as he passes over the nest. The female will return to the beaches to spawn up to twenty
times, depositing as many as eighty thousand eggs in a season. The return trips often churn up eggs that
already have been laid, thus making them more readily available to the shorebirds. Horseshoe crabs live to be
sixteen to seventeen years old. Their longevity, combined with the fact that they lay such an abundance of
eggs, ensures their survival—provided they are not harvested too heavily, as has unfortunately been the case in
recent history.
For a century, beginning in the early 1800s, crabs were harvested for fertilizer. They were first dried, then
ground up in factories and spread on farmers' fields. The crabs were also fed to hogs and their eggs scooped
up for chicken feed. Inevitably, the crab population collapsed in the early 20th century, and with it the unsus-
tainable fertilizer industry. At the same time, shorebird populations had been drastically reduced by relentless
gunning and market hunting.
Their exploitation of the crab eggs, which individually supply a minuscule caloric benefit, depends on a su-
perabundance of eggs and therefore of crabs. Because shorebirds are both long-lived and opportunistic in their
feeding habits, those that survived this onslaught on themselves and their prey most likely went elsewhere,
perhaps utilizing the mudflats and sounds of New Jersey, which still attract as many as twenty species of
shorebirds in spring. These include the short-billed dowitcher, dunlin, yellowlegs, black-bellied plover, semip-
almated plover, and semipalmated sandpiper.
The crab harvest was revived in the 1980s, this time as a lucrative source of bait for conch and eel fishers.
Again, horseshoe crab numbers collapsed by as much as 75 percent, and with them the number of the East
Coast rufa race of red knots, which have declined by more than 85 percent on their spring migration to
Delaware Bay—plummeting from 100,000 in the early 1980s to as few as 15,000 in 2005—and by 64 percent
on their wintering grounds near Tierra del Fuego.
Shorebirds can accumulate fat quickly to fuel their remarkable intercontinental flights. In the 1980s, when
crab numbers had rebounded to healthy levels, the red knots were gaining 8 grams of fat per day, but by 2002
they were gaining a quarter as much. The accumulation of fat is critical for the red knots to complete the last
leg of their epic migration to their breeding grounds in Canada's High Arctic. To do so, they need to gain 6.5
grams per day for twelve to fourteen days, which means consuming approximately eighteen thousand horse-
shoe crab eggs every day. Otherwise, they will arrive in the Arctic without sufficient reserves to lay eggs and
rear their young, or they will simply fall out of the sky from starvation en route. Those that do complete the
journey often arrive before the weather has warmed enough to provide a reliable food supply, and so they must
subsist on their stored fat reserves. Either way, accumulating enough fat is critical not only to reproductive
success but to survival.
Delaware Bay has the largest concentration of spawning horseshoe crabs along the Atlantic coast. It is no
coincidence that, historically, 90 percent of the Western Hemisphere's population of red knots stopped at
