The north-east Pacific intertidal barnacle Pollicipes polymerus in India? A biogeographical enigma elucidated by ¹⁸O fractionation in barnacle calcite

Abstract

Pollicipes polymerus Sowerby (1833) is an abundant and well-known intertidal barnacle of the north-east Pacific. However, a few decades ago a subspecies, P. polymerus madrasensis Daniel (1953), was reported from south-eastern India. This created a biogeographical enigma because, of the two species-groups of Pollicipes, that of P. polymerus would be considered the least likely to have a tropical representative. To test the validity of this surmise, a determination of oxygen isotope fractionation in the calcitic shell of the holotype of P. p. madrasensis was made. The ensuing analysis revealed that the specimen grew under cold temperate conditions that could not have been encountered on the south-east coast of India. It is concluded, therefore, that the specimen is not native to that region but rather was imported, one way or another, from the north-east Pacific.