This is a top-down view of a pair of endangered tri-spine horseshoe crabs (Tachypleus tridentatus) engaged in spawning. The male is clasped onto the rear of the female, preparing to fertilize eggs that she will deposit. Seen here, the female is still burrowing into the substrate. She will dig deeper, until her primary compound eyes and most of her prosoma is buried. The bubbles are the result of pockets of air trapped in the mud and gravel being released due to the female’s excavation.
Spawning takes place in the intertidal zone, which is the reason that substantial quantities of air are present in the sediment. It is also the reason that the water is murky and somewhat hazy, as there is mixing of fresh water flowing in from land.
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.