Upon collection, the body of the deceased hermit crab was almost completely obscured by the foot of the still living gastropod (Fig. 1A). Upon closer in situ inspection, the dactylus and the propodus of the second and third pereopods of the crab were found to be sticking out between the shell aperture and the gastropod foot (this species of gastropod does not have an operculum). Although still alive, the foot of the gastropod showed signs of a failed predation attempt, most likely from a species of large swimming crab known to use a tactic called ‘peeling’ that can lead to non-lethal injuries. Figure 1(B-C) presents the extraction sequence for the hermit crab in the laboratory. The shield length (a most common measure of size) of the hermit crab was 5.8 mm, while the shell internal volume was 10 mL, making it a large and particularly desirable home if not occupied by the body of the gastropod.
Isocheles sawayai is a filter-feeding hermit crab that lives partially buried in sandy substrates within shallow coastal waters. Although has been shown to inhabit the empty shells of Olivancillaria vesica it is not the preferred domicile shell species in sheltered environments (i.e., 5.6% of observations). In the surf zone, however, as was the case in the study area, these shells can act as a ballast for hermit crabs preventing burial and overturning by waves.
This is the first observation of a hermit crab is found dead inside the shell of a still-living gastropod that we could find in the published literature. This is surprising given that there have been decades of targeted work investigating the ecology of a range of common benthic intertidal species (including Isocheles sawayai). While there is no way to know exactly how this curious incident came about, there are two plausible explanations. First, the crab searching for a replacement shell, encountered the live gastropod and preyed upon it in an attempt to secure its shell. Although our literature review revealed some species of hermit crabs have been shown to attack live gastropods in the laboratory, this has never been reported for this species or indeed any other group in nature. The second explanation is that like many other hermit crabs, this individual was attracted by the smell of the injured gastropod (almost certainly the result of a failed predation event) and climbed into the mantle cavity of the gastropod – as hermit crabs typically do when evaluating or exchanging shells. Subsequently, and while attempting to remove the remaining tissue, the individual became trapped by the contracting foot and was subsequently suffocated.
Although we will never know definitively how this curious incident occurred, it highlights the pitfalls of acquiring resources in limiting environments and is interesting from a perspective of marine ethology and evolutionary biology. Greater sampling within the study area may provide evidence on whether this is an isolated instance or whether it might be a more common phenomenon exhibited by this and perhaps other species of the hermit crab. It would also be interesting to evaluate if other hermit crabs (some of which live in extremely shell limiting environments) may engage in similar extreme behavior to be the first to access limited resources. Globally, it will become increasingly important to understand the responses of animals to decreasing resource availability within marine and coastal environments.