Sipunculans are non-segmented marine worms with an anterior retractable introvert, which are commonly included in Annelida based on molecular phylogenetic and phylogenomic analyses. They generally burrow in the soft sediments or live inside the crevices of hard substrata (e.g. calcareous/coralline rocks). However, members of some sipunculan genera (mainly Phascolion and Aspidosiphon) are known to have a peculiar habit of dwelling in vacant shells of gastropods or scaphopods. In this study, we investigated the shell utilization and preference pattern of the species of Phascolion and Aspidosiphon in Japan. We collected 302 sipunculans, comprising 273 and 29 individuals in Phascolion and Aspidosiphon, respectively, from 57–800 m depth of three study sites in the Pacific coast of Honshu Island, Japan. The species of Phascolion were found in vacant shells of 38 genera of 27 families of gastropods and six genera of four families of scaphopods, whereas the species of Aspidosiphon were found in 11 genera of 11 families of gastropods and one genus of scaphopod. These results suggest that members of each genus use a wide range of gastropod and scaphopod shells. The body size of the sipunculans was positively correlated with the shell size, suggesting that they change the shells as they grow. Furthermore, we investigated the shell preference of Phascolion species by comparing morphological characteristics of shells occupied and unoccupied by sipunculans. Generalized linear mixed model (GLMM) analyses suggest that the species of Phascolion tend to use long and narrow shells. Such shells likely fit well the elongated trunk of sipunculans.

Terrestrial hermit crabs use mollusc shells to protect their soft bodies from predation and desiccation, but their use is costly. The energetic short-term cost of using shells has been demonstrated, yet this could theoretically be overcome by increasing feeding rate or food quality. In the long term, shells have the potential to constrain growth, which may in turn have a negative effect on fitness. Thus crabs should choose the lightest (least expensive to carry) shell amongst those of a size that permit growth. Since in nature lighter shells are also thinner-walled, these may provide insufficient protection from predation or desiccation, facing the crabs with a trade-off between protection and growth. This potential trade-off was investigated in the terrestrial hermit crab Coenobita compressus. Crabs were individually maintained in captivity for a variable period of between 1 and 7 months, during which light or heavy replicates of the preferred shell were available. In a second experiment, crabs were given a choice between a thick (and heavy) or a thin (and light) shell. In addition, the resistance and water retention capacity of thin and thick shells was compared. As predicted, crab growth was negatively correlated with the weight of the shell used, but crabs rejected the thin (light) shells. Thin shells were more likely to break when crabs used the normal ‘rolling’ behaviour in response to perceived threats. In addition, thin shells lost water at a faster rate than thick ones. Since two putative functions of shell use in terrestrial hermit crabs are protection against predation and against desiccation, it is inferred that the preference for shells of intermediate weight in C. compressus results from a balance between the need to grow and the cost of carrying a brittle shell that is not sufficiently water tight.