This is a pair of endangered tri-spine horseshoe crabs (Tachypleus tridentatus) moving across the ocean bottom, with the larger female in front and the male clasped onto the rear. The tips of some of the animals’ legs are visible.
The female is searching for a location to spawn. Once she finds a place that she likes, she burrows partially into the substrate and deposits a clutch of eggs, which the male fertilizes. Such spawning occurs repetitively. When searching in this manner, horseshoe crabs can travel at a relatively rapid pace. Otherwise, they tend to move slowly, as evidenced by the gastropod that has crawled on top of the male’s head.
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.
Though habitat loss and overharvesting of these animals for food are primary contributors to the population decline of horseshoe crabs, the biomedical industry is also a major factor. Horseshoe crabs are bled for their amoebocytes (akin to white blood cells), which are used to derive an extract that reacts in the presence endotoxin lipopolysaccharide, which is found in the membranes of gram-negative bacteria. Estimates suggest that between three and 30% of the animals die as a result. There have also been suggestions that taking up to a third of each animal's blood adversely affects their ability to undertake vital functions, such as procreation, even if the animals survive.