Allometric shell scaling drives body size-dependent shifts in preference and performance in a hermit crab

As animals develop and increase in body size, they often shift the value they place on specific resources. Such shifts can also arise because resource traits change allometrically with size. With increasing body size, the hermit crab Calcinus californiensis shifts from assigning similar value to several shell species to exhibiting a strong preference for conical shells. In this study, we evaluated how allometric changes in shell mass and internal volume influence the functional costs of using a conical versus a globose shell species, and how these costs relate to shifts in shell preference. We found that key shell traits differ as they increase in size; conical shells gain mass at a higher rate than globose shells relative to their internal volume. As such, small hermit crabs carry similar shell mass in both shell types, while larger individuals must carry much heavier conical shells to obtain the same internal volume as globose shells. Consequently, metabolic rate and muscular strength show shallower scaling with body size in hermit crabs occupying conical shells than in those using globose shells. Therefore, the costs and benefits of using each shell type are similar at small sizes but progressively diverge as hermit crabs grow, leading to size-dependent shifts in shell preference. Our findings suggest that preference for shell species can be a potentially adaptive plastic process shaped by the allometric scaling of both hermit crab physiology and gastropod shell morphology.