The Something Wild team travelled to the Great Bay National Estuarine Research Reserve in Greenland, New Hampshire early in the morning in order to not miss out on one of nature's most amazing phenomena: spawning horseshoe crabs at high tide.
Horseshoe crabs are prehistoric "living fossils" that have remained virtually unchanged for over 450 million years; they pre-date the dinosaurs and flowering plants.
Our guide today is Beth Heckman with New Hampshire Fish and Game. We recommend you listen to this episode!
The high tide allows the horseshoe crabs to float close to shore for spawning. It also helps the females reach the high tide line on the beach, where they prefer to lay their eggs.
The Great Bay Discovery Center boat launch, which is about 8 feet of rocky shore between two outcroppings, was host to between 30 and 40 horseshoe crabs when the Something Wild team visited in early June.
The females, who are larger, usually have one male attached, with several males following them. They remain attached for the weeks of spawning season. This guarantees the male will have a female that's going to lay eggs and he'll be able to fertilize them for the duration of the season.
"Unpaired" or satellite males hover closely around the nesting pair. They crowd the mating pair and release sperm into the water as the eggs are laid.
The female digs a nest and lays her eggs near the high tide line, and the males fertilize them through the water. It's all external. Multiple males can fertilize the thousands of eggs that she lays.
The eggs are tiny, the size of a freckle.
“You can see a bunch of little fish called Mummichogs that will get right underneath the females as she's laying eggs,” says Heckman. “They pick off some of those eggs. You can imagine that thousands of eggs also provide food for many other Great Bay animals.”
It’s one example of the crucial role the horseshoe crabs play in the ecosystem.
Horseshoe crabs are not actually crabs. They are more closely related to spiders, scorpions and ticks. They lack the pinching claws of true crabs, and they don’t have a stinging tail like a scorpion, nor do they have jaws to bite.
Their stiff, spiky tail is used as a rudder and to flip themselves over if they get stuck upside down.
Horseshoe crabs eat by foraging on the ocean floor for worms, small clams, and crustaceans. Because they lack jaws or teeth, they pass food to their mouth using bristle-like appendages, which act almost like a conveyor belt to pull their food into their esophagus and down into their stomach.
Two of the horseshoe crab's eyes are readily apparent on the top of their shell. These are compound eyes and allow the crab to see in much the same way insects do. They also have 10 simple eyes around their body that perceive motion and light, which helps to detect mates.
Horseshoe crabs are incredibly resilient to injury, thanks to their copper-based, blue blood that contains a special clotting agent. Heckman says they sometimes see horseshoe crabs that have holes through their shells.
“If their shell is compromised, their blood goes right to that spot and walls it off to any bacteria with a gelling ability,” she says.
Pharmaceutical companies make use of horseshoe crab blood. The clotting agent ensures that vaccines, injectable drugs, and medical devices like prosthetics are free of dangerous bacterial contamination. A synthetic alternative to horseshoe crab blood is still being developed.
“We consider their blood to be very helpful to us, but it's helpful to them first,” says Heckman.
Something Wild is a collaboration between NH Audubon, the Forest Society and NHPR.
