These are endangered tri-spine horseshoe crabs (Tachypleus tridentatus) engaged in spawning. The larger female is in front, the smaller male attached to the rear.
During spring high tide, pairs like this move to shallow water. They remain largely motionless. When the time comes for spawning, the female digs into the substrate and deposits a clutch of eggs, which the male fertilizes. Spawning occurs repetitively.
In the intertidal location where this pair were photographed, the mud and gravel substrate contained pockets of air, which were sometimes released when the horseshoe crabs became active. The escaped gas bubbled up and emerged from the muck, sometimes passing through the carapace of the female, resulting in the illusion of exhalation, as pictured 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.