Limiting factors on the distribution of shell/tube-dwelling sipunculans

Abstract

Many sipunculan species use empty gastropod shells or polychaete tubes as a resource to keep them safe from predation. Nonetheless, little is known about the extent to which the availability of this resource is involved in the distribution of these species. We examined shelter availability and sediment granulometry as factors to explain the distribution of shell-dwelling and/or tube-dwelling sipunculans. For this purpose, artificial sampling units with different sediment granulometry and different shelter availability were deployed at two localities. Four species have been reported, being three of them usually dwellers of vacant gastropod shells or polychaete tubes: Aspidosiphon (Aspidosiphon) muelleri, Phascolion (Phascolion) strombus strombus and Phascolion (Phascolion) caupo. Shelter availability was the main factor explaining the distribution and abundance of shell/tube-dwelling sipunculans. Samples in which shelter were added had an abundance of A. (A.) muelleri ten times higher than samples in which shelters were removed (p < 0.01). Moreover, sipunculans selected preferably as shelter Ditrupa arietina tubes over gastropod shells. A strong linear correlation between body width and shelter internal diameter was found for A. (A.) muelleri (r = 0.796, p < 0.001). On the other hand, differences between localities were detected. However, these differences are not linked with the studied variables (shelter availability and granulometry), and have to be explained by other factors.