Females are about 20–30% larger than males. The smallest species is "C. rotundicauda" and the largest is "T. tridentatus". On average, males of "C. rotundicauda" are about long, including a tail (telson) that is about , and their carapace (prosoma) is about wide. Some southern populations (in the Yucatán Peninsula) of "L. polyphemus" are somewhat smaller, but otherwise this species is larger. In the largest species, "T. tridentatus", females can reach as much as long, including their tail, and up to in weight. This is only about longer than the largest females of "L. polyphemus" and "T. gigas", but roughly twice the weight. The juveniles grow about 33% larger with every molt until reaching adult size. Atlantic horseshoe crabs molt in late July.