Today we have photos from Jan Malik taken at Cape May, New Jersey emphasizing the bizarre horseshoe crabs, which are not crustaceans but chelicerates, more closely related to spiders and scorpions than to real crabs. Jan’s captions and IDs are indented, and you can click on the photos to enlarge them.
People visit Cape May, NJ, in spring, mostly to see the migrating birds, but what makes the high density of animal migrants and residents possible in that area is in large part hidden in the water. Delaware Bay is home to a large population of the Atlantic horseshoe crab (Limulus polyphemus) which spawn around the time the migrating birds pass through. Horseshoe crabs’ eggs, along with crabs themselves, are an important link in the food chain that fuels the spring migration.
Early morning visitors to the Delaware Bay side of Cape May are welcomed by the ruckus made by Laughing Gulls (Leucophaeus atricilla). These birds nest locally but also depend on the crabs as a major food source:
Walking onto the beach in the morning after a high tide, a visitor will see these animals stranded helplessly on their backs, flipped by the waves:
Here is a larger female with a male still attached. All three parts of a crab are visible: the main body (prosoma) with mouth and legs, an abdomen (opisthosoma) with book gills and a tail (telson). What can be also seen here are male’s pincers, modified forelegs used to grasp the carapace of the female:
The crabs are not blind thanks to their pair of compound eyes. They also have 8 other simple eyes and ocelli distributed on top of their carapace and additional photosensors underneath and on the telson. These organs are sensitive to UV and visible light and are used to detect phases of the lunar cycle and determine when to come out of water onto the sand to mate and lay eggs:
The primary function of a telson is to help the animal steer and to right itself in water. On dry land, however, it is of little help and a flipped crab, if left on its own, usually succumbs to desiccation or is preyed upon by other animals:
A human visitor to the beach has to decide whether to save the crab or leave it to die and let birds have their fill. In my case I usually flip them back on, giving them a chance to live another day. They are used by the medical industry to develop a test for the presence of bacteria in medical devices, which involves catching them, drawing about a quarter of their blood and then releasing them, but such a crab is then greatly weakened and mortality is high:
A Sanderling (Calidris alba) eating a horseshoe crab egg. For some evolutionary reason, these marine arthropods must leave the water in order to lay eggs. They prefer to do the laying at high tide, hence the lunar phase detection. The eggs may be then uncovered by waves and spread far and wide on the beach:
The waves may also uncover a whole cluster of eggs which is then found by shorebirds patrolling the ecotone between the sea and the land:
This is what the washed up eggs look like; eggs are 1.5 mm to 2 mm in diameter and they grow as the embryos inside them grow. This is the main fuel for the thousands of plovers, sanderlings, turnstones, red knots and others on their way toward the Arctic:
Using a macro lens, one can make out an embryo inside, complete with legs, telson and tiny eyes:
I think it was only at the beginning of this century that the significance of these “crabs” (which have a common ancestor with spiders and scorpions) for migrating birds was properly recognized and some harvesting bans were put in place in NJ and Delaware. Before, they would be harvested in excess as crab bait or just for fertilizer. This picture shows how they can be a host to barnacles and limpets:
Even though they can and do come onshore during daytime, they prefer nighttime at high tide, at new or full moon. In the Delaware Bay, many thousands of them come out then, males crowding around the females to claim the best spot and be first to fertilize eggs. They must have been doing similar things for many millions of years – earliest fossils with similar body plans date back to Paleozoic era, 450 mya, which was well before the dinosaurs. Fossils quite similar to the Atlantic crab date back 200 mya. There were many species but now only four are left, this site presents a neat diagram illustrating their evolutionary history:
Just to give a sense of scale, here is a “scrum” of crabs around human feet:
Spring migration attracts many visitors to Cape May, benefiting local businesses. I think it is just to also count humans as dependent on these “crabs”, to an extent. Note that the forefront crab has a tag from the US Fish and Wildlife Services, allowing it to track the animal:
I grew up on Staten Island, NYC. You could often find hundreds of Horseshoe Crabs dead along the shore. An artist named Nicky Schneider (no relation) would make lamps out of them. He made a desk lamp with the telson upright holding the body with the light at right angle. They were attractive and sold in the stores in the West village. I made a ceiling light out of one, it looked great.
We owe much of our knowledge about vision and brain circuitry to Limulus. Though the mechanisms of invertebrate and vertebrate vision are different, many of the basic principles are the same. The similarities of molecular factors and cellular network circuitry are, IMO, some of the strongest evidence for the unifying principle of life. Luckily, we have a Theory to explain the mechanisms of that principle. Otherwise, to paraphrase Dobzhanzky, nothing would make sense.
What a fantastic set! I would love to see a spawning event. On a different web page, an acquaintance has pictures showing that they fluoresce under UV light, like scorpions. I didn’t know that!