Performance Implications of Divergent Shell Size Preferences and Exoskeleton Mass of Two Closely Related Hermit Crabs

TL;DR

Two hermit crab species use different shell sizes, leading to trade-offs between protection and mobility.

Contribution

The study reveals how divergent shell preferences correlate with morphological adaptations and performance trade-offs in hermit crabs.

Findings

P. hirsutiusculus has a 15%–90% heavier carapace relative to body mass compared to P. granosimanus.

P. hirsutiusculus moves faster in small shells but survives longer in large shells under desiccation.

P. granosimanus uses larger shells for better protection but likely incurs higher energetic costs.

Abstract

Hermit crabs use gastropod shells for protection from abiotic stressors and predators. However, two sympatric species of hermit crab,
Pagurus hirsutiusculus
and
Pagurus granosimanus
, have divergent shell size preferences. Differences in shell size use were evident in the field: for a given body mass,
P. granosimanus
used shells that were 136%–300% larger than
P. hirsutiusculus
. The present study examined the possible morphological adaptations associated with the shell size preferences of
P. hirsutiusculus
and
P. granosimanus
as well as the costs and benefits associated with the preference of
P. hirsutiusculus
for shells that are too small to enclose and protect the entire body of the crab. When exposed to desiccation conditions commonly encountered during low tide emersion,
P. hirsutiusculus
using large shells survived much longer than individuals using small shells. And in motility trials,
P. hirsutiusculus
moved significantly faster when using a small shell than when using a large shell. It was therefore hypothesized that
P. hirsutiusculus
might produce a heavier exoskeleton than
P. granosimanus
to compensate for the reduced protection obtained by
P. hirsutiusculus
from small shells. Our findings support this hypothesis: relative to body mass, the carapace was 15%–90% heavier in
P. hirsutiusculus
than in
P. granosimanus
, a difference further confirmed by the claw mass, which was 59%–81% heavier in
P. hirsutiusculus
than in
P. granosimanus
. The use of larger shells in
P. granosimanus
provides enhanced protection, allowing for the production of a lighter exoskeleton, but likely imposes reduced motility and increased energetic cost.
Pagurus hirsutiusculus
, on the other hand, uses smaller, lighter shells that impose lesser energetic costs and allow greater motility but leave the animal more vulnerable to stressors;
P. hirsutiusculus
compensates for this increased vulnerability by producing a heavier carapace and claws. The differences in shell size and exoskeleton mass suggest ecological implications for these species, particularly with regard to microhabitat use.

Two co‐occurring species of hermit crabs use very different shell sizes, one species using shells that are too small to accommodate all the carapace or claws. The two crab species were found to have diverged in their adaptations to their environment: one species adopting a suite of traits, including the use of large shells, that enhance protection from environmental stressors at the cost of reduced motility and increased energetic expenses, the other species adopting traits maximizing motility and low energetic expenses at the cost of reduced structural protection.

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