(Montagu, 1804)
Description:
A medium-sized species, up to about 50 mm in length. The form, shape and diameter are very variable depending on the shelter inhabited. The oral disk carries tentacles, which surround the mouth; small specimens (<25 mm long) have a single circle of 8 to 16 tentacles but with growth, further tentacles develop to form short, radial/longitudinal rows and large specimens have 40-50 tentacles. Simple, spine-like hooks are arranged irregularly on the anterior introvert. The whole body surface is covered with papillae of varying sizes; over the base of the introvert and anterior trunk, the papillae are large and broadly conical in shape, whilst over the middle trunk region they are modified to form chitinous "holdfasts" of rather variable appearance. A single nephridiopore is posterior to the prominent anus on the anterior trunk. The longitudinal muscle layer of the body wall is continuous, not collected in bands.
Internally, two retractor muscles, of unequal size, are inserted at the posterior end of the trunk (P. strombus-internal); the dorsal retractor has a wide, undivided base whilst the ventral retractor has two roots attached on either side of the nerve cord. The intestine is arranged in a series of longitudinal loops fastened by a number of fixing muscles; its middle section is usually weakly coiled in a double spiral without a spindle muscle. The contractile vessel is simple and relatively large, often about the same diameter of the adjacent oesophagus. A rectal caecum is present. The right nephridium only is developed; this is fastened to the body wall for most of its length.
The external features of this species are subject to great variations, which appear to be influenced by environmental conditions. Many forms, now considered to be conspecific, have been described as separate species or as distinct varieties.
In Swedish waters this species has a prolonged spawning season perhaps extending throughout the year.
Habitat:
Found in mud and sand, in depths of 4-3800 m. It usually inhabits a wide variety of shelters. Typically found living in gastropod and scaphopod shells. In European seas, the shells of Turritella, Aporrhais, Nassarius and Dentalium are commonly utilized. Shells inhabited by P. strombus are readily recognisable since the aperture is partially blocked by a plug of cemented sediment particles with a central hole through which the introvert is protruded. The shelter is often inhabited by a variety of other animals, notably the polychaete Syllis cornuta, the parasitic gastropod Menestho diaphana and, more rarely, the bivalve Tellimya phascolionis . Entoprocts of the genus Loxosomella are commonly attached to the trunk surface, especially the holdfasts.
Distribution:
It is one of the most common species in European waters and is widely distributed throughout the Atlantic Ocean. Also reported off southern Chile and in the Red Sea.