Are hermit crabs looking for light and large shells?: evidence from natural and field induced shell exchanges

Terrestrial hermit crabs ( Coenobita compressus (H. Milne Edwards)) on Isabel island, México, prefer Nerita (Ritena) scabricosta (Lanark 1822) over other species of shells. Nerita scabricosta, the lightest species of shell, has the highest internal volume/weight ratio ( IV/ W ratio) of all species available at the island. Heavier shells (low IV/ W ratio) are more costly to carry and may restrain growth. We hypothesized that crabs will search for shells with high IV/ W ratios to save energy, and predicted that in every individual shell-exchange crabs will prefer a shell with a higher IV/ W ratio. Observations of spontaneous aggressive shell-exchange interactions in nature between crabs carrying preferred shells (with high IV/ W ratio) and crabs using less preferred species of shells (low IV/ W ratio) support this idea. By inducing individual shell exchanges in the field, we experimentally confirmed the preference for shells with high IV/ W ratio, and we also showed that crabs prefer larger shells over those they were bearing. Moving to a larger but not heavier shell may facilitate growth though saving energy that would otherwise be spent in locomotion. High growth rates seem to be advantageous because size is a correlate of fertility, since large males apparently have more access to females, and large females produce larger clutches, thus indicating the possible reproductive benefits for preferred light and large shells.