The immediate relatives of the mangrove horseshoe crab (Carcinoscorpius rotundicauda) are not crabs, but spiders and scorpions. Besides, studies suggest they are more of Arachnids, yet are still classified under the family Limulidae retaining their primitive and prehistoric looks. Other communities call it the round-tailed horseshoe crab and unlike most crabs, it holds on one partner for life. These benthic feeders are the smallest species of the horseshoe crab, possessing a tail for uprighting itself or locomotion and not for self-defense as was once thought. Look for it in the marine and brackish waters of Southeast Asia, where they are caught for their roe.