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Plants and Wildlife

Creature Feature: Horseshoe Crab

July 25, 2017

by Brooklyn Bridge Park Conservancy

When it comes to horseshoe crabs, looks as well as name can be deceiving. A long dagger-like “tail” and a shield-shaped carapace give the ancient invertebrates a ferocious otherworldly appearance. Far from being a weapon, the pointy dagger, technically known as a telson, serves for steering and for righting the animal when it gets flipped over.

A crab in name only, the invertebrate is more closely related to spiders, ticks, and scorpions as its underside reveals. In addition to the telson, horseshoe crab bodies have two sections. The head, or prosoma, features brain, heart, and nervous system as well as nine eyes, the mouth, a pair of feeding pincers and five sets of walking legs, for a total of twelve appendages. The abdomen, or opisthosoma, contains six sets of book gills, which flap like the pages of a book when the animal breathes underwater. The book gills also serve for swimming upside down—a crowd-pleasing quirk of evolution occasionally demonstrated by the horseshoe crab that resides in the 250-gallon saltwater tank at Brooklyn Bridge Park Conservancy’s Environmental Education Center.

When it arrived at its temporary home, that crab dove straight down into the tank’s gravelly substrate and then went several months without being seen, let alone being seen eating. That it didn’t go hungry can be attributed to the way a horseshoe crab gets its food: as it makes its way across the ocean floor, the animal constantly shovels mud containing small mollusks, crustaceans, and worms into its mouth located in the center of its legs. Presumably, the Ed Center’s horseshoe crab did much the same, feeding on bits of food left over by its finned tank mates. These days, whenever the horseshoe crab does emerge and eat the event makes for an appealing spectator sport. As one young visitor recently commented, “I loved watching how slowly it ate.”

A second smaller horseshoe crab currently makes its home in the touch tank, where it readily demonstrates its tail flip technique when gently turned over by a BBPC educator and cradled in both hands much like a soup bowl. While observing one of the animals up close, try spotting some of their many eyes or sensory organs. Two are on the underside, near its mouth; seven more are on the top. Two compound lateral eyes are easily spotted on each side of the first section, rudimentary lateral eyes are right next to them. Two median eyes are near the center of the first section and another eye is right behind those. The rounded edge of the carapace also features light sensors, as does the telson. The different photo-sensitive organs serve various purposes such as orienting while swimming, finding mates, detecting circadian rhythms, seasons, and tides.

Horseshoe Crab swimming in a tank

The two Ed Center horseshoe crabs are juveniles at different stages of development. They will eventually be released to the local beach where they were captured. Horseshoe crabs have never been observed to molt into mature adult form in captivity. Meeting adult horseshoe crabs takes a well-timed visit to a beach frequented by them. While they spend most of the year in deep water off shore, they move close to shore for mating season in late spring and early summer. Twice a month during the highest high tides around full and new moons, they come to spawn along sheltered beaches from Maine to the Yucatán Peninsula. Between May and July, they can be found along NYC beaches such as Brooklyn’s Plumb Beach.

At about 14 to 15 inches from head to tail, adult males are noticeably smaller than their female counterparts, which reach 18 to 19 inches. The male’s first claws behind the feeding pincers look like boxing gloves and its carapace has a bow-shaped indent in the front. The modifications help the male slide onto a female and hold fast. With the smaller male riding piggyback, the larger female scratches a slight indent in the sand at peak tide. As she releases her eggs (up to 90,000 per season), the male releases a plume of sperm. If all goes well, horseshoe crab larvae will hatch at the next spring tide two weeks later and make a beeline for the water. Wave action commonly uncovers a bounty of horseshoe crab eggs, which are an important nutrient-dense food source for many shorebirds. The chance of a horseshoe crab making it from egg to adult is estimated to be about one in a million, in a process that takes fifteen or more molts over the course of seven to ten years.


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