Horseshoe Crabs Lay Thousands of Eggs on Coastal Beaches

Along the Mid-Atlantic Coast and Delaware Bay, horseshoe crabs return to sandy beaches to reproduce, following a breeding cycle that has persisted for hundreds of millions of years. According to The Dodo, females can lay as many as 100,000 eggs in one season, and the developing embryos become transparent before hatching, revealing the young animals inside.

The U.S. Fish and Wildlife Service explains that these marine animals typically spend most of their lives in deeper ocean waters. When reproduction begins, they move toward shore during the unusually high tides associated with a full or new moon.

Sheila Eyler, project leader of the Mid-Atlantic Fish and Wildlife Conservation Office for the U.S. Fish and Wildlife Service, described the mass movement that brings the animals to their breeding grounds.

"When it's time to spawn, they all make this mass movement toward beaches and they spawn on sandy beaches — that's where they like to lay their eggs," Eyler told The Dodo.

Once ashore, female horseshoe crabs dig into the sand and deposit eggs in nests. Over several weeks, each female may return to the beach multiple times and create several nests, with each nest containing as many as 4,000 eggs.

Eyler said the total number of eggs produced by a single female during the breeding season can be substantial.

"[S]he can lay up to 100,000 eggs over the course of one season," Eyler said.

The eggs initially resemble small green pellets. As the embryos develop, however, the eggs gradually become more translucent, allowing the growing animals to become visible through their shells.

The incubation period lasts two to four weeks. By the end of that period, the eggs become completely transparent, and observers can see the baby horseshoe crabs moving inside before they hatch.

After hatching, the young crabs receive no parental care and must survive independently. According to the report, horseshoe crabs take about 10 years to reach reproductive maturity, when they can begin the breeding cycle themselves.

The species faces conservation challenges beyond its unusual reproductive process. Four horseshoe crab species exist worldwide, and three are considered endangered, highlighting concerns about the future of these animals and their role in natural ecosystems.

Eyler emphasized the animals' extraordinary evolutionary history, noting that their appearance has remained largely unchanged across an immense span of time.

"They are an ancient critter," Eyler said. "They haven't really changed … for 350 million years. So they were around during the time of the dinosaurs, basically like they look today."