Chinese horseshoe crab

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 (the prosoma) and a smaller, spine-edged rear one (the opisthosoma). 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 name tridentatus refer to the three small spiny processes on the rear part of the opisthosoma (one spine in the middle above the tail and one on either side), while other species only have a single spine (in the middle). 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 79.5 cm (31.3 in) long, including their tail. On average in Sabah, Malaysia, females are about 66.5 cm (26.2 in) long, including a tail that is about 34.5 cm (13.6 in), and their carapace (prosoma) is about 31 cm (12 in) wide. In comparison, the average for males is about 54 cm (21 in) long, including a tail that is about 28.5 cm (11.2 in), and their carapace is about 25.5 cm (10.0 in) wide. There are significant geographic variations in the size, but this does not follow a clear north–south or east–west pattern.