Fish Predation: A Preliminary Study of its Role in the Zoogeography and Evolution of Shallow Water Idoteid Isopods (Crustacea: Isopoda: Idoteidae)

Abstract

Behavioural, morphological and distributional patterns of idoteid isopods (family Idoteidae) from the northeast Pacific are analysed with respect to the hypothesis of increasing predation pressure from fishes in the lower latitudes. Minimum reproductive size and maximum body length of idoteids both decrease with decreasing latitudes; body ornamentation increases with decreasing latitudes. Members of the tropical and subtropical genera Parasymmerus, Eusymmerus, Colidotea and Erichsonella are smaller and more ornamented than species of the temperate genus Idotea. The subtropical isopod Colidotea findleyi is a 'swimmer', whereas the larger, more conspicuous, temperate species Idotea urotoma shows no preference between 'swimming' and 'nonswimming' behaviour. In laboratory predation experiments, the tropical wrasse Thalassoma lucasanum selected larger isopod prey items, as well as preferring Idotea urotoma over Colidotea findleyi of equal length. The geological evolution of the region is discussed in regards to vicariant events, shifting faunal waves, and modern zoogeographic patterns of idoteid isopods. Data obtained in this study support the hypothesis that fish predation may play a major role in controlling the southern range limits of temperate rocky-shore isopods, and that this same pressure may have affected the evolution of idoteid isopod body size and morphology in the northeast Pacific.