Thelepus japonicus Marenzeller 1884

As with all terebellids, this species is benthic, few if any of the segments are longer than wide, the dorsal surface has no paleae, elytra, or felt, it has no cross-barred capillary setae, it has several long, threadlike, extensile, unbranched filamentlike tentacles closely associated with the mouth and are used in feeding but cannot be retracted into the mouth, the notosetae do not form transverse rows that extend nearly to the dorsal midline, the prostomium does not extend posteriorly as an elongated caruncle, and does not have a distinct caudal region which lacks setae, nor form an operculum. Terebellids have soft, light-colored pads on the ventral surface of all or most thoracic segments; these secrete the mucus which builds the tube. The anterior, "thoracic" portion of the body is often larger than the posterior portion. Thelepus japonicus has no proboscislike extension from the peristomium, uncini in the thoracic region in a single row which does not form a loop, 3 pair of slender, unbranched red gills, capillarynotosetae beginning on the second or third gill-bearing segment, and the notosetae are absent from only about the last 10 segments. The gill filaments are usually coiled.

How to Distinguish from Similar Species: Thelepus crispus has uncini in an oval loop on some segments. Thelepus setosus is missing notosetae from about the last 40 segments and the gill filaments are not coiled.

Biology/Natural History: Terebellids build thin, membranous, sand-encrusted tubes in soft mud and sand. They extend their feeding tentacles from the entrance of the tube like spaghetti over the surface of the nearby sediment. The tentacles are extended out from the burrow by ciliary creeping and they can be retraced by muscular action. Mucus and ciliary action on the feeding tentacles transports small organic particles from the sediment to the mouth, especially along a groove along one side. Mucus and ciliary action on the feeding tentacles transports small organic particles from the sediment to the mouth. There are several genera of terebellids that are difficult to tell apart in the field. The feeding tentacles of Thelepus has threadlike, unbranched tentacles, and gills of similar length. Neoamphitrite and Terebella have dark, branching tentacles, which are longer than the gills which are white. Neoamphitrite has 17 thoracic segments while Terebella has 23 to 28. Members of Family Terebellidae have special pumping vessels at the base of the gills to actively pump blood through them.