This is a pair of tri-spine horseshoe crabs (Tachypleus tridentatus) walking across the ocean bottom in shallow water just prior to spawning. The smaller male uses modified legs (pedipalps) to grasp onto the female in front. The female has just started to burrow into the substrate, a process which involves digging deep enough to bury the front part of her body (Prosoma) and much of her abdominal area (Opisthosoma) as well. When she eventually reaches acceptable depth and deposits eggs, the male will fertilize them. Females often deposit several clutches of eggs in the same general area before moving on to do the same in different locations.
In the shallow intertidal area where this photograph was taken, pockets of air were trapped in the gravel and mud. Digging by female horseshoe crabs sometimes releases some of the air, as seen in this image. Note also the gastropod riding on the female’s head. Shells, barnacles and other passengers seem to be relatively common on these horseshoe crabs.
Although these animals are called crabs, they are not members of the Subphylum Crustacea. They belong to a separate Subphylum—Chelicerata—which also comprises sea spiders, arachnids, and several extinct lineages such as sea scorpions. The earliest known fossils of horseshoe crabs date back 450 million years ago, qualifying these animals as living fossils, as they have remained largely unchanged.
Tachypleus tridentatus is the largest of the four living species of these marine arthropods, all of which are endangered.