Hermit crabs have an odd housing situation. Rather than produce their own shells like other crustaceans, they must find an empty shell made by a completely different species, marine snails, in order to protect their delicate abdomen. Hermit crabs that find the perfect shells, research has shown, have a greater chance of surviving than those who get stuck with a non-ideal house, so finding a shell with a good fit is a pretty big deal for these little guys.
As they grow, hermit crabs must abandon their old shell and find a new one they can comfortably squeeze into. If there aren’t enough shells to go around, hermit crab populations will actually shrink. Despite the gravity of shell availability for the crabs’ survival, research don’t really understand much about how hermit crabs select their shells. For example, they don’t know whether hermit crabs change their shell preferences over time—much as a person may change her fashion style as she grows up—or whether they stick to one shell type throughout their lifetime. They also puzzle over whether different species of hermit crabs compete for the same shells.
In a new study, researchers trapped dozens of hermit crabs of four different sizes and ages, and from two different species. They gently removed the crabs from their shells, and presently them individually with a choice of six empty, size-matched shells from six different species of snails common to the study site, in Vancouver. They also carried out field surveys to see how hermit crabs were handling shell decisions in nature.
The animals’ preferences, they found, do indeed change throughout their life. As the crabs got older, their tolerance for shell diversity decreased, and they honed in on a single shell type they liked best. “In addition, similar-sized hermit crabs of the 2 species mostly ignored the shell types used by the other species, a resource partitioning that would facilitate coexistence,” the researchers report. In other words, hermit crabs avoid making waves with cousins of different species by establishing their own unique shell preferences. Whether the crabs would break that shell-sharing truce if their ideal abode suddenly went into short supply, however, is a matter left to future research.
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