Nephasoma (Nephasoma) abyssorum abyssorum (Koren & Danielssen, 1876)
This is a small species with the total body size up to 59 mm long. Introvert shorter than trunk. The general shape and appearance of this worm is smooth, white, and sometimes iridescent. Short but fully developed tentacles and hooks spirally arranged.
This species is distinguished by its short but fully developed digitiform tentacles and unique spiral arrangement of the hooks (Cutler, 1994).
N. lilljeborgi and N. minutum (Cutler & Cutler, 1986).
External anatomy
The introvert is about one-fourth the length of the trunk (Stephen & Edmonds, 1972).
Hooks are medium-size (50-150 µm long) spirally arranged at the anterior part of the introvert (Cutler, 1994). Characteristic shape with thick, cylindrical posterior border and flat anterior region.
Tentacle array consists of 20-24 short digitiform tentacles around the mouth (Stephen & Edmonds, 1972).
The cylindrical trunk is usually 10-30 mm long, with ill-defined elliptical skin-bodies. There are some dome-shape papillae at the posterior end (Cutler, 1994).
Internal anatomy
The trunk body wall has a continuous longitudinal muscle layer.
One pair of ventral retractor muscles, attached to the trunk wall on either side of the nerve cord in the second quarter of the trunk (Cutler& Cutler, 1994).
Two short nephridia unattached to the trunk wall open to the exterior anterior to the anus (Stephen & Edmonds, 1972).
Spindle muscle weakly developed within the gut coil (Cutle & Cutler, 1986).
According to Stephen and Edmonds (1972) this species is often associated with corals and Cerianthus. Some specimens were found with entoprocts attached to the tegument (Saiz Salinas, 1992).
This species is common in the northeastern Atlantic and Artic Ocean. Records from Pacific Ocean, Mediterranean, Red Sea, Japan and southwest coast of Africa are rare (Cutler, 1994; Saiz Salinas, 1992).
This species occurs frequently in bathyal to abyssal depth (500-5300 m) (Cutler, 1994), and has been reported occasionally in the sublittoral zone, in soft sediments such as sand, and mud (Saiz Salinas, 1992). Rarely found inside hard substratum, inside galleries made by the sponge Cliona abyssorum or inside the empty shell valves of Lima excavata (Saiz Salinas, 1992).