by Erik Baard
Horseshoe crabs having been laying and fertilizing eggs along our beaches, and the beaches of the continents as they were once assembled, for at least 350 million years and through several global mass extinctions. Somehow they’ve done all this mating without the help of MP3 players stocked with Barry White. A couple of years ago I was impelled to visit Plumb Beach, Brooklyn to investigate. I was lucky enough to have my extraordinarily talented photographer friend, Klaus Schoenwiese, with me. I hope you take the chance this Sunday, with the American Littoral Society. Check our WildWire, below, for details.
It was a magical night, set to the subtler music of crickets and lapping small waves. A yellow squash-colored full moon rose over eastern Rockaway Inlet, Jupiter shone brightly further south, and Ursa Major reeled across the northern sky. And Klaus observed another beautiful sight often glared out by the riotously incandescent city: bioluminescence. A tiny spot of sand glowed blue-green when nudged. Perhaps it was a juvenile jelly fish, since it was a tight source, not the mind of nebulous expanse one would expect from a patch of dinoflagellates saturating the sand.
In Klaus’ photo of horseshoe crabs under the moon you’ll note the slipper shells and barnacles that are among the many species that make their homes on all surfaces these long-lived creatures (sometimes over 30 years, though they cast shells in youthful molting). Contrary to mythology, the pointed tail isn’t a stinger, but rather an ultraviolet light-sensitive periscope of sorts and a tool to right horseshoe crab when waves flip it on shore, leaving it vulnerable (despite a nifty ability to regrow limbs). Gleefully interfering with nature, we lifted and returned distressed specimens to the water.
Every May and June you can spot horseshoe crabs swimming and crawling ashore in prime places like Jamaica Bay, Staten Island, and Plumb Beach. At the last location, the pattern was disrupted for a few years in the ’90s when sterile sand was dumped over the rich mud flats, depriving birds and horseshoe crabs of small snails and such to eat.
The females are significantly larger than the males, who desperately cling to their would-be mates (needy is the new black?). The female will haul herself to the intertidal zone’s highpoint and then partially bury herself into the wet sand to lay thousands of tiny, greenish eggs. At least one male will be in tow, and often several will descend upon her when she’s tucked in.
The males’ approach might be a terrible case study in the art of seduction, but it’s an efficient means of winning in what biologists call “sperm competition.” Besides, give the horseshoe crabs credit for one romantic touch, which human voyeurs can appreciate: they prefer to do it under a full moon.
The advantage of a full moon is that by laying eggs in the highest tidal point, subsequent high tides won’t wash away the eggs before they’re hatched and juveniles are ready to take their places in the deep bays or continental shelves a few weeks later. It will be ten years before you see them again, matured for reproduction. The lunar cycle and strictly defined mating ritual is so central to the horseshoe crab’s life that it has ten eyes (yes, TEN!) on its body. Additionally, there’s a row of optic sensors on its spike-like tail. The eyes and sensors are geared for measuring ambient ultraviolet light levels and for spotting mates. Mind you, this didn’t stop three of four amorous males from trying to mate with Klaus’ feet!
As alien as their mating habits might be (or creepily familiar), horseshoe crabs are more a part of your daily life than you might expect. Each time you see pretty shorebirds and migrating birds in these areas, soaring over a strand, think of the humble horseshoe crab. This creature’s eggs are a critical food stock for the birds you’re admiring — so much so that they time their own annual cycles to it. You might also owe thanks to horseshoe crabs for your very ability to see: chitin from its shell and exoskeleton are a key ingredient in many contact lenses (as well hairspray and skin cream), and horseshoe crabs compound eyes and simple optic nerves have been a mainstay of medical optical studies for decades. Indeed, horseshoe crabs, which are not crabs at all, were the center of research that earned the Nobel Prize for medicine in 1967.
Another aspect of horseshoe crab physiology for humans is their blue-green blood (they use copper to carry oxygen, not iron), which sells for about $15,000 a quart. It’s chief value is a compound called Limulus Amebocyte Lysate (LAL). LAL is a fast, reliable indicator of bacterial toxins in medicines (so good that the Food and Drug Administration requires use of it) and in the human body. That’s right: there’s a chance you’ve been injected with horsehoe crab blood and didn’t even know it. A hot field of medical and evolutionary research is the commonality of human and horseshoe crab complementary immune responses.
But as you can imagine, the more uses humans find for this slow-maturing species, the fewer remain in the waters. Populations have markedly declined in recent years. Bird advocates like the Audubon Society have taken up the cause of horseshoe crab protection. If the spirit moves you, get involved in the important work of protecting this ancient cousin.
Nature Calendar 