This is a pair of endangered tri-spine horseshoe crabs (Tachypleus tridentatus) engaged in spawning. They are walking along the ocean bottom in shallow water, with the larger female in front searching for a place to deposit eggs, which will be fertilized by the smaller male attached to the rear.
The tips of the horseshoe crabs’ legs are visible in this image. These animals have five pairs of legs used for locomotion—the pedipalps first, followed by three pairs of legs in the middle, and a pair of pusher legs at the end. The male’s pedipalps are modified into a pair of claspers used to grasp the female as seen here.
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.