BREWSTER, Mass. – How do you raise a horseshoe crab, a creature whose fossil record dates back 450 million years, whose ancestors weathered ice ages big and small, and who somehow survived predators willing to take a shot at something slow-moving and relatively defenceless?
“When we started this project I had no clue. I had never hatched them before,” said Marjorie Williams, senior aquarist at the Cape Cod Museum of Natural History. To help people connect with nature, Williams’ exhibits include the “baby” phase of many different species.
“People here really like babies,” she said. It seemed natural to hatch the eggs of the horseshoe crab, one of the Cape’s most visible and least-understood shoreline residents.
“Look at those animals. They’re so tiny,” she added, nodding at three tanks where horseshoe crabs, separated by age, scrambled over the bottom and each other searching for food. There’s no “puppy” phase for these crabs; a fingernail-sized youngster looks like a miniature version of a dinner-plate-sized adult.
After the eggs began to hatch, and survivors numbered over 1,200, the goal became keeping them long enough that they could be released into the wild at a stage when they are less prone to being eaten. But Williams found that shepherding horseshoe crabs from eggs through the first year has rarely been done, and she reached out to Stacy Epperson of the Maryland Department of Natural Resources, who oversees a program that helps teachers and students raise and study horseshoe crabs in the classroom.
A decade ago, Massachusetts Audubon Society’s Wellfleet Bay Wildlife Sanctuary had a horseshoe crab education program that hatched eggs and kept them in their display aquariums, according to sanctuary director Robert Prescott. The mortality was high, he said, but there was a lot of interest in the rarely seen juvenile stages of the crab. Over 150 people attended horseshoe crab symposiums hosted by the sanctuary, which continues to do research and annual population surveys.
“It’s kind of an iconic creature,” Prescott said. “People find leftover pieces on the beach and there’s a great deal of curiosity.”
Despite having survived the division and movement of the continents, horseshoe crabs are having a tough time in what some are referring to as the Anthropocene Era. Once caught by the millions for fertilizer, livestock feed and bait, the annual harvest dropped as low as 20,000 pounds in recent years. Concerned that a population crash could harm the millions of shorebirds that depend on horseshoe crab eggs to sustain them during migrations, states began regulating the harvest, shutting it down in key states around Delaware Bay, and Massachusetts imposed harvest limits on the bait fishery and stricter monitoring of a biomedical industry that bleeds captured crabs and returns them to the wild.
Because they didn’t go dormant for the winter, the crabs in Williams’ tanks are much bigger than wild ones at the same age. The 11-month-olds are already at a size where they are past the high mortality inflicted in the larval and initial stages.
The project has attracted the interest of New England Aquarium aquarists and crab expert Daniel Gibson of Worcester Polytechnic Institute. Gibson donated another batch of eggs for Williams and her staff to raise.
Under the microscope, those eggs contained what appeared to be a tailless version of the adult. In one of the tanks, green rice-sized eggs occasionally erupted, shooting a barely visible trilobite into the water. In the wild, the larvae would spend weeks buried 2 to 10 inches deep in the sand. When they are exposed to water, they swim for days before settling to the bottom to molt. They emerge as quarter-inch replicas of the adults, minus a distinct tail.
Horseshoe crabs are scavengers, eating whatever they turn up as they plow along the bottom. But this is a surprisingly sophisticated eating machine. It has multiple eyes with specialized tasks including a large pair that can magnify sunlight 10 times, and another pair to detect the moon’s ultraviolet light for navigation, particularly during spring spawning on moonlit beaches. Research has shown that these eyes are especially good at detecting the shape of another crab in murky shallow water and at night, important for mating. Eyes on the underside of the shell help with navigation as the crab swims a kind of backstroke.
The horseshoe crab also uses chemical receptors. It has two appendages with a million receptors each that test the chemical composition of the water flowing over its gills. The tips of the four pairs of walking legs are loaded with chemical receptors that can detect food. Specially adapted legs dig and can pry open shells like a clam knife. Tiny pincer claws grab morsels and put it into a mouth located between the pairs of legs. The action of the legs helps grind up the food.
“They are incredibly perceptive for such a primitive animal. It’s probably why they’ve lasted as long as they have, because they are so good at what they do,” Prescott said.
Williams found she had to match the size of the food to that of the animals and fed the tiny first molt crabs frozen brine shrimp. Dining on macerated dried clams, mummichaugs, shrimp and sea worms helped the crabs grow at a much faster rate than out in the wild.
“No one told us you couldn’t keep them and grow them to a larger than normal size,” Williams said. “They assumed what we were going to do was release them soon after we hatched them.”
She expects to release the crabs at Priscilla Landing in East Orleans, where the original eggs were harvested, in July. It may be symbolic, but releasing hundreds of a species that has suffered much at the hands of man gives her some satisfaction.
“It’s been a very exciting thing to do,” Williams said.
Information from: Cape Cod (Mass.) Times, http://www.capecodtimes.com