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, Mexico, prefer Nerita (Ritena) scabricosta (Lanark 1822 ) over other species of shells. Nerita scabricosta, the lightest speci es of shell, has the highest internal volume/weight ratio (IV/W ratio) of all species available at the island. Heavier shells (low IV/W rati o) are more costly to carry and may restrain growth. We hypothesized t hat crabs will search for shells with high IV/W ratios to save energy, and predicted that in every individual shell-exchange crabs will pref er a shell with a higher IV/W ratio. Observations of spontaneous aggre ssive shell-exchange interactions in nature between crabs carrying pre ferred shells (with high IV/W ratio) and crabs using less preferred sp ecies of shells (low IV/W ratio) support this idea. By inducing indivi dual shell exchanges in the field, we experimentally confirmed the pre ference 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 mal es apparently have more access to females, and large females produce l arger clutches, thus indicating the possible reproductive benefits for preferred light and large shells. (C) 1998 Elsevier Science B.V.