This is a juvenile of an endangered tri-spine horseshoe crab (Tachypleus tridentatus), juxtaposed with a five Yen coin for scale. The animal’s carapace measured about 37mm, which suggests that this juvenile was at the fifth instar of development, about a year since hatching. This species takes 15 years to mature, molting 18 times during that period. The estimated lifespan for this species is 25 years.
These horseshoe crabs are generally only active when the water temperature exceeds 18 degrees Celsius. As such, they are dormant for much of the year, resting in deeper water until the ocean warms again.
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.