Smallest horseshoe crab

The world's smallest species of horseshoe crab is the mangrove horseshoe crab Carcinoscorpius rotundicauda. Native to India, Indonesia, Malaysia, the Philippines, Singapore, Thailand and Hong Kong, it has a circular shell or carapace whose diameter measures up to approximately 15 cm. It inhabits mangroves, mudflats and shores near to them. Only four species of horseshoe crab exist today. The last surviving members of an ancient arthropod lineage known as the xiphosurans, they are related to arachnids and sea spiders but most closely to the prehistoric sea scorpions or eurypterids. All of these taxonomic groups are referred to collectively as the chelicerates, constituting the arthropod subphylum Chelicerata.