Aspidosiphon (Paraspidosiphon) steenstrupii Diesing, 1859
Many specimens of A. steenstrupii have a characteristic calcium carbonate deposit covering the anal shield, which easily separates this species from the congeners. Other characters that help to define the species are the presence on the introvert of distal bidentate hooks followed by many dark scattered pyramidal hooks, a well-defined caudal shield and the division of longitudinal muscles into anastomosing bands.
The dark scattered pyramidal hooks, on the proximal introvert, are reminiscent of A. elegans, but A. steenstrupii is easily distinguished by the presence of a calcium carbonate deposit on the anal shield of most specimens and the presence of a well defined caudal shield (Cutler, 1994).
(From Migotto & Ditadi, 1988; Cutler & Cutler, 1989 and Cutler, 1994)
Introvert commonly approximately as long as the trunk, but sometimes longer.
Tentacles number 10 to 11 and are white, short, and fused laterally.
On the distal introvertl bidentate hooks are arranged in rings and measure 30-60 µm in height in specimens 10-15 mm long, but can measure up to 90 µm in worms 25-40 mm long. The internal clear streak of the hooks usually has a tongue-like extension. Most specimens lack compressed unidentate hooks. On the proximal introvert there are many dark, scattered pyramidal hooks, measuring 30-60 µm in height.
The anal shield is ungrooved and has small, compact, uniform underlying granular units, which may be covered by a calcium carbonate deposit. The color of the anal shield may vary from nearly white to dark.
The caudal shield is conical, grooved (15-30 radial grooves), and, similar to the anal shield, may vary in color.
Internal anatomy
The trunk wall has 14-22 longitudinal muscles bands anteriorly and 20-28 posteriorly. The number of bands is independent of the trunk size and the the bands appear anastomosed.
Intestine attached anteriorly and posteriorly by spindle muscle.
Rectal caecum present in most worms.
Nephridia are usually 50-80% of the trunk length.
A pair of retractor muscles originates about 70-85% of the distance to the posterior end of the trunk.
(From Cutler, 1994)
This is a widespread species found throughout the western and northern Indian Ocean, from northern Australia through Indonesia, Vietnam, and South China Sea, to southern tropical Japan and through the western Pacific islands to Hawaii. In the Caribbean it has been collected in many locations and in the eastern Atlantic it has been reported from Cape Verde Island and the Gulf of Guinea.
This species lives in shallow water burrowed in coral rubble. Rice & MacIntyre (1979) reported a preliminary study of sipunculan burrows in rock thin sections. Based on one species of Phascolosoma and two Aspidosiphon, including A. steenstrupii, they provided evidencefor both chemical action and mechanical abrasion as possible mechanisms for the formation of burrows. Rice (1975) recorded the presence of A. steenstrupii in rocks of varying texture from the coarse-grained biocalcarenite of the beachrock of Barbados to the dense coralline limestone from other localities throughout the Caribbean. The same author noted a thickened rim surrounding the anal shield of A. steenstrupii as well as a frequent deposit of calcareous material and an occasional attached growth of filamentous algae. The author suggested that the anal shield, in a position at the mouth of the burrow, functions as an operculum device to close the opening of the burrow when the worm’s introvert is retracted.