Sweet Potato Sea Cucumber, Molpadia arenicola

Sea Cucumbers of the Molpadiidae Family, like Starfish and Sea Urchins are in the Phylum Echinodermata. Echinoderms, indicative of spiny skin, are characterized by a larval stage with bilateral symmetry and an adult stage with radial (5-rayed) symmetry. They are unsegmented and their disc may be cucumber, sphere or star shaped. They have a water vascular system, tube feet, and a complete digestive system, but they lack eyes, a head, nervous system, or excretory system. They are found only in marine environments.

Sea Cucumbers are in the Class Holothuroidae, meaning that they lack arms, and they have a soft body wall and branched tentacles around their mouth utilized to capture food. Sea Cucumber in the Order Molpadiida, have tentacles that are unbranched, or pinnate. They are primarily burrowers, so they have an internal madreporite (perforated plate covering entry port of water vascular system) to keep it out of the sediment. They lack the tube feet of other echinoderms. Also unlike other Sea Cucumbers, their bodies narrow at the hind end, giving them a tail. The common name Rat Tail Sea Cucumber refers to this tail, which may be up to one third the length of the body. Sea Cucumbers in the Molpadiidae Family generally have smooth, or slimy, skin. They reach a maximum length of 45 cm (18 inches).

Sea Cucumbers in the Molpadiidae Family are elongate and their skin has fine to large papillae (projections). They vary in color and pattern, but most are black, brown, or greenish. They also vary in size, from small, to over 60 cm (2 feet 0 inches) in length. They are found on, under and within rocks, rubble, and sand from the intertidal zone to depths that exceed 300 m (984 feet). They are active nocturnally. Some species are suspension feeders, filtering food from the surrounding water, and others are depositional feeders, gathering food from the surface of the substrate. Some filter sand through their digestive tracts, removing food from the sand as it passes through. They primarily eat dentris and diatoms. In turn they are preyed upon by crabs, crustaceans, fish, starfish, and sea turtles. Molpadiidae larvae are planktonic, but the adults are benthic. These Sea Cucumbers are either male or female, and fertilization takes place externally. Their larvae is planktonic and the adults are benthic.

Molpadiidae Sea Cucumbers are found subtidally, and to depths exceeding 5,200 m (17,056 feet). Most species are found at depths greater than 100 m (328 feet). They spend the majority of their life cycle in their burrow. They are perhaps the most sedentary of all the sea cucumbers. They are surface and subsurface depositional detritivores. They feed by ingesting large amounts of the sand/mud substrate and filtering out the organic matter. The sand/mud is then expelled. They are prey for fish, sea stars, crabs, gastropods, and marine mammals. They may also play host to several species of commensal, or parasitic, crustaceans and mollusks. Molpadiidae Sea Cucumbers reproduce sexually, with all individuals being either male or female. Fertilization takes place externally. The embryos develop in to planktonic larvae and then doliolaria (barrel-shaped stage) before becoming juvenile sea cucumbers.

Some species of Molpadiidae Sea Cucumbers are the subject of commercial fisheries and aquaculture operations. They are considered a food source and a source of pharmacological agents.

The Molpadidae are found worldwide in temperate tropical and polar seas. There are currently 65 known members of the Molpadidae Family of which 3 are found along the Pacific coast of Mexico.