Horseshoe crabs are not crabs at all, but are most closely related to spiders and scorpions, and may even be arachnids themselves. The cephalothorax is protected by this single large, horseshoe-shaped plate, and neither it nor the abdomen is visibly segmented. The tail bears a long spike, known as the telson. Like other horseshoe crabs, the carapace of T. tridentatus consists of a larger frontal one and a smaller, spine-edged rear one. There are six pairs of prosomal appendages/legs, consisting of a small frontal pair in front of the mouth and five larger walking/pushing legs on either side of the mouth. The book gills are located on the underside of the opisthosoma. Both the common name tri-spine horseshoe crab and the scientific nametridentatusrefer to the three small spiny processes on the rear part of the opisthosoma, while other species only have a single spine. The tri-spine horseshoe crab is the largest of the living horseshoe crab species. Like the other species, females grow larger than males. The largest females of the tri-spine horseshoe crab can be as much as long, including their tail. On average in Sabah, Malaysia, females are about long, including a tail that is about, and their carapace is about wide. In comparison, the average for males is about long, including a tail that is about, and their carapace is about wide. There are significant geographic variations in the size, but this does not follow a clear north–south or east–west pattern. The largest are from the Kota Kinabalu region in Malaysia where the average carapace width is about and in females and males respectively. The smallest are from Zhoushan region in China where the average carapace width is about and in females and males respectively. Intermediate average sizes, ranging from in females and in males, have been reported from Imari in Japan, Xiamen in China, the Philippines, and Manado, Tarakan, Padang and Sibolga in Indonesia. Females typically reach maturity at a carapace width of about and males at. Females weigh and males. In addition to the difference in size, males' two front pairs of walking legs, prosomal appendages two and three, have hooks and they have six long spines on either side of the rear carapace. Juveniles also have six long spines on either side, similar to adult males. From the eggs hatch to adulthood takes 4 years and involves 15 instar phases.
Ecology
Like other species of horseshoe crabs, T. tridentatus is an omnivore and feeds on molluscs, worms, other benthic invertebrates and algae. Large batches of eggs are laid in holes dug in sandy beaches in special nursery areas off the coast. On hatching, the larvae remain in the nest over winter, feeding on the yolks of their eggs for several months, remaining in this nursery area during the next spring and summer. As juveniles they remain buried in the sediment during high tide, emerging at low tide to feed on the exposedsurface. They have limited opportunities to disperse widely. Adults move offshore for the winter, hibernating buried in the seabed at depths of around, coming inshore again when the water warms up the following year. As a poikilotherm, this horseshoe crab is much affected by rising seawater temperatures, reacting by burying itself deeply in the sediment and sometimes going into diapause.
Status
The tri-spine horseshoe crab are at risk from over-fishing, pollution and the loss of their breeding grounds. Populations have declined for decades, and the species was granted protection in Japan in 1928. In China it has become less common and, from being once abundant in Taiwan, it is now seldom seen inshore there. As of 2019, the International Union for Conservation of Nature classifies the species as "Endangered" based on recent population and habitat declines.
