Sometimes we come across a thing so fantastically ugly that we are compelled to stop and gaze in admiration. If the object of our attention is found on a Dorchester beach, we might think it is just a bit of garbage, coughed up by the sea – until we notice that it is moving, barely. Then, it becomes apparent that this greenish-grey lump on the mud is not a grimy gimcrack low on batteries, either; it is Limulus Ployphemus, better known as the horseshoe crab, a marine animal that has been alive for twenty million years.

Today, horseshoe crabs are found only on eastern coasts of North America, Mexico, Asia, and India. Our type, the Atlantic horseshoe crab, is one of four species that are the only remaining members of the Xiphosura, one of the oldest classes of marine arthropods. The earliest known horseshoe crab fossils date back half a billion years. By studying the horseshoe crab, scientists have been able to discover things that are beneficial for all of us.

For example, horseshoe crab blood is the basis for LAL, or limulus amebocyte lysate (the copper containing protein present in its blood is an instant lightning rod for detecting the presence of minute amounts of bacteria). LAL is used by the biomedical industry to help make vaccines safe and effective. NASA uses LAL to test for bacteria aboard spacecraft, ensuring sterile environments for space travel. Considering these advantages, the horseshoe crab is a wonderful animal, even if it isn’t beautiful.

The crab resembles an ironclad stingray, or an army helmet with eyes. The body is attached under the top, and hangs there, like a clanger in a bell. To facilitate swimming, the shell is bisected on the lateral axis with a simple hinge; sticking out at the back is a long, rigid tail that is used for steering. It swims upside down at a 30-degree angle, and tools along at about one-quarter of a mile per hour.

The female horseshoe crab, which is larger than the male, can grow to be 24 inches in length, including the tail. The male likes to grab the back of the female, and hitch on for a ride. Actually, the horseshoe crab isn’t a crab at all; it is more like a huge underwater spider, with mouth and brain set within six sets of appendages, or legs, that are used for eating, locomotion, and reproduction.

We may think that this is repulsive, and that horseshoe crabs are primitive creatures that should have died out with the dinosaurs, but don’t bother trying to convince a male that this is true, especially during mating season. From late April to early June, horseshoe crabs come to spawn along the North Atlantic coast. The breeding season is short, only four weeks long, so time is precious. To the casual observer, their slow motion movements give no hint of the urgency that drives them on.

On nights of the new moon, and the full moon, horseshoe crabs arrive like college revelers on spring Break. Every year, they travel all the way from the floor of the continental shelf – the dreary place where they spend most of the year – and are hell-bent on having a good time. Then, they are gone as suddenly as they came. Dorchester residents of a certain age cannot forget the annual visitation of horseshoe crabs to our Dorchester beaches.

But they haven’t been coming around here as much, lately. Habitat erosion, and bait harvesting have taken a toll. And then there is biomedical harvesting: a routine of capture, bloodletting, and release; this practice continues even as the record of animal survival rate is still in question (it is said that horseshoe crab blood extract is valued at over $15,000 per quart). All these factors are impacting the horseshoe crab, resulting in a precipitous decline throughout the entire state. Researchers are now investigating whether it is possible to revive and increase the crab populations.

The lifespan of a horseshoe crab is thought to be 20 to 40 years; after that, there doesn’t seem to be a lot more that we know. Collection of data was begun only recently, which does not give scientists much to go on. To generate more data, and encourage community involvement, the Atlantic States Marine Fisheries Commission organizes volunteer census-takers to go to beaches and survey horseshoe crabs. The data they collect is vital for conservation efforts.

In Delaware, and in other mid-Atlantic states, strict laws govern horseshoe crab fisheries; New Jersey has imposed a total ban on harvesting. Unfortunately, in Massachusetts, the laws concerning horseshoe crabs are comparatively lax, and the situation is dire.

In collaboration with the Massachusetts Division of Marine Fisheries, Wellfleet Bay Wildlife Sanctuary, the National Park Service, and others, Mass Audubon has been surveying spawning horseshoe crabs on the Outer Cape. The Wellfleet sanctuary has released a summary of data collected from 2000 to 2012. There are no surveys being conducted north of Duxbury.

Though it is unlikely, it is not impossible that you will spot a horseshoe crab as you are walking the beach in Dorchester during a full moon in May. And if this happens, you can become a “citizen scientist,” and conduct your own survey. The US Fish and Wildlife Service tags horseshoe crabs to follow their migrations. If you should find one that is tagged, copy the number and send the info to fws.gov/crabtag.

Horseshoe crabs are still a mystery to us. Let’s hope that they have another surprise in store for us: that they will return, and flourish – if we can learn how to live with them.

A special thanks to Mark Faherty, Science Coordinator, Mass Audubon/ Wellfleet Bay Wildlife Sanctuary.