The mangrove horseshoe crab is the smallest of the four living species of horseshoe crabs. Like the other species, females grow larger than males. On average in Peninsular Malaysia, females are about long, including a tail that is about , and their carapace (prosoma) 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. Those from West Bengal in India average somewhat smaller than those from Peninsular Malaysia, with a carapace width of about and in females and males respectively. Elsewhere it averages even smaller, with the smallest reported from the Balikpapan and Belawan regions in Indonesia where the carapace width of females is about and in males . The largest females of the species may reach up to in length, including the tail.