These are endangered tri-spine horseshoe crabs (Tachypleus tridentatus) engaged in spawning. The male is visible in his entirety, attached to the rear of the female. The female (much larger than the male) has burrowed into the substrate, with her head and most of her torso buried. She is in the process of depositing eggs, with the male fertilizing them.
In the intertidal location where this pair were photographed, the mud and gravel substrate contained pockets of air. Digging and other activity by the female releases the trapped air, which bubbles up as seen in this image.
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.
Synthetic substitute tests have been available since 2003. The biomedical industry has however been reluctant to discontinue the practice of bleeding live animals.