MIDDLE TOWNSHIP — Researchers and students released about 10,000 3-month-old horseshoe crabs into the Delaware Bay on Wednesday, in the hopes a few will survive.
The odds against the young arthropods’ survival are so great, Rutgers raised 250,000 of them this summer in tanks at the Rutgers Aquaculture Innovation Center in Cape May.
Most have been set free, with Wednesday’s group entering the bay off the Rutgers Cape Shore Laboratory in Green Creek.
But some are being kept for further study on their feeding preferences and habits, and on what type of tracking system might allow researchers to follow them in nature.
In their first few months, they are ideal prey for other larger critters or predators like striped bass, bluefish, and blue crabs, said Mike DeLuca, director of the Aquaculture Innovation Center.
Giving them a head start for three months will probably result in about a 90 percent greater chance of survival than those that hatch in nature, DeLuca said.
But even after three months’ growth, they are only a few millimeters long, and the vast majority will be eaten by some predator, said Thomas Grothues, associate research professor at the Rutgers Department of Marine and Coastal Sciences.
Humans have long harvested horseshoe crabs for commercial fishing bait and taken their blood for medical research, while migrating shorebirds eat their eggs to fuel their flights each spring.
New Jersey has had a moratorium on harvesting horseshoe crabs since 2008, after a period of overharvesting had many worried about declining numbers.
While the moratorium has helped stabilize the population, it hasn’t started rebuilding it yet, Grothues said. And even releasing a quarter of a million young horseshoe crabs a season won’t do it, he said.
“At this point, the efforts to rebuild the population aren’t there yet,” Grothues said. “What we are doing now is learning how to do that.”
Grothues’ goal is to eventually be able to raise the horseshoe crabs for two to three years, so more of those raised at the aquaculture center will survive.
But scientists must first understand where they go when they are 2 or 3 years old, so they can release them into the right place.
“The small hatchlings are secretive and live just under the sediment or sand surface,” DeLuca said. But we know little about where they go from there.
Grothues is working on finding a way to tag and follow the small creatures, which molt frequently.
Horseshoe crabs are evolutionary survivors that have remained relatively unchanged for 350 million years, according to the U.S. Fish and Wildlife Service. Not a true crab, the horseshoe crab is an ancient arthropod, related to spiders and scorpions.
When they first hatch, they are microscopic, DeLuca said. Under a microscope they look just like the adults — but translucent.
Researchers still don’t know what the young horseshoe crabs eat in the wild. In the lab they ate nothing for the first couple of months, instead relying on an egg sack for nutrients. Then they started eating brine shrimp, DeLuca said.
“But obviously they are eating other things in nature,” he said.