Beach Spawning under a June Moon

Two horseshoes headed back to the water after a busy night.

There has been quite a bit of full-moon horseshoe "crab" action this week on my stretch of the Delaware Bay. These curious critters have changed little since before the dinosaurs if you can imagine that, and every June full moon and the weeks surrounding it, they leave the water to spawn on the beach. I usually shy away from circle-of-life and copulation topics, but I think I can manage to tell the horseshoe story.

This critter burrows as s/he moves back to the water. Note the doggie footprints and the razor clam to the right.

They are not really crabs, but related to spiders and scorpions. Their scientific name is limulus polyphemus. They live for 17 to 18 years. They are found up and down the east coast, but are most plentiful in the Delaware Bay. They have six pairs of claws under their shells along with gills for breathing. Their blood is blue because it has copper in it and not the iron that makes human blood turn red. This blood is used by scientists to check medicines for endotoxins and bacteria (and the scientists do not harm the animals!). Male horseshoes have two extra claws called claspers that they use to hold on to females during mating. These claspers were considered good luck by whalers in days of yore. They wore them around their necks to ensure they would find their way back home no matter what weather they might encounter. The whalers called the claspers lucky bones (and this is also the name of a new favorite Cape May restaurant).

This guy's mate told him to make tracks.

So here's what has been happening this week on my beach: the males crawl out of the bay water and up onto the sand. They hang out in-between the high-tide and low-tide lines where the sand is flat. They wait around for the females to show up, and then attach themselves to the back of the female's shell with those claspers. Females are bigger than the males. The female drags the male around as she digs holes in the sand and deposits her pearly green eggs. (I've never noticed these eggs before, but I should look closer. They are smaller than peas but not microscopic.) She'll drag the male over the egg hole so he can fertilize the eggs and move on to another location. One female will deposit as many as 10,000 eggs in a season. This procedure repeats every night for several weeks, mostly at night, but there are still shenanigans to witness during the day.

That's the male in front holding onto the female who is not much bigger than he is. Cougar maybe?

Yum! Horseshoe eggs!

That would be a lot of horseshoes if all of those eggs matured into actual creatures. Alas, they don't, and this is where the circle of life comes in. Horseshoe eggs happen to be a delicacy for many birds who winter at the southern tip of South America and fly north to the Arctic to nest in summer. Wouldn't you know, Cape May is about halfway for them, so the birds stop to refuel on those tasty green eggs. This is one reason why Cape May is a fabulous bird watching spot: red-wing blackbirds, mourning doves, grackles (who also love my pre-ripe figs, laughing gulls, sanderlings, plovers, dowitchers, dunlins, willets, yellowlegs, and those famous red knots who are rarely seen elsewhere but in the vicinity of horseshoe eggs, all show up this time of year to feast. Bigger birds like hawks and herons eat the smaller birds that eat the green eggs, but also devour the insides of any unfortunate horseshoes that may have overturned on the beach.

The bay beach in June

Sometimes horseshoes can flip themselves by pushing their long, stiff tail called a telson into the sand. If they are unsuccessful they will bend at the hinge between their shell and abdomen to try and protect their insides. If their gills dry out in the sun, it is curtains for them. Here's a short video of a horseshoe heading back to the water. Notice how he changes direction when the water hits him and reminds him which way to go. In the background there is a less fortunate horseshoe "bent at the hinge" to protect his soft belly and gills from the sun.