Aspidosiphon (Aspidosiphon) elegans (Chamisso & Eysenhardt, 1821)
This species is the most common and widespread Aspidosiphon in tropical regions, inhabiting coral rubble and soft rocks in shallow water. It is distinguished from it congeners by the presence of dark conical hooks on the posterior portion of the introvert. Asexual reproduction, rare among sipunculans, occurs by transverse fission into two unequal parts. A posterior constriction is commonly observed in specimens collected from the natural habitat.
(From Cutler, 1994, Migotto and Ditadi, 1988, and Stephen and Edmonds, 1972)
Introvert nearly as long as the trunk.
Six to 12 stubby tentacles surround the nuchal organ.
On the distal part of the introvert bidentate compressed hooks are arranged in rings. Scattered on the posterior portion of the introvert are dark conical hooks, with a nearly circular cross section and a gentle posterior curve (conical hooks type C according Cutler, 1994).
The anal shield is ungrooved.
The caudal shield is not well developed, and in many worms is barely discernible.
The longitudinal muscle layer on the trunk wall is often split in the area under the anal shield.
The intestine is normally in a helical coil, whichmay appear loosely formed in some specimens. A spindle muscle fixes the intestine anteriorly and posteriorly.
Rectal caecum can be present in some worms.
Nephridia are reddish-brown, about as long as the trunk and attached for their whole length.
A pair of retractor muscles originates about 85-95% of the distance to the posterior end of the trunk.
The common size for this species is between 10-15 mm long, but specimens up to 57 mm have been recorded (Cutler 1994).
Ecology and Distribution
(From Cutler, 1994)
This species is considered to be the most widespread Aspidosiphon in tropical waters, being present in the Indian Ocean and Western Pacific oceans, from south central Japan to northern Australia to Hawaii. It is present in the Red Sea and Israelian waters. In the western Atlantic it occurs from northern Brazil to the Caribbean, Florida and Bermuda.
Habitat and Ecology
Often occurring in great densities, it is found in borings in coral rubble and soft rocks in shallow water. As a rock-boring species it contributes to the erosion of reefs by weakening the supporting structures of the corals, consequently increasing their susceptibility to breakage and destruction by current and wave action (Rice, 1979).
In 1970, Rice described asexual reproduction in this species, which occurs by a transverse fission of the body into two unequal parts. Before the fission each part regenerates the structures essential to the formation of a worm. The fission appears near the end of the trunk and the daughter part (the smaller portion of the trunk) regenerates an anterior body, including introvert, anterior gut, retractor muscle and nephridia, while the parent part (large portion of the trunk) regenerates only the posterior part of the trunk.