Differences in otolith morphologies between surface- and cave-dwelling populations of Poecilia mexicana (Teleostei, Poeciliidae) reflect adaptations to life in an extreme habitat

Questions: Do otolith morphologies differ between surface- and cave-dwelling forms of Poecilia mexicana? Do otolith contours show differences between populations and do they display a morphological gradient from the cave outflow to the innermost cave chamber? Organism: Surface- and cave-dwelling forms of a freshwater fish, the Atlantic molly (Poecilia mexicana). Field sites: Two sites in the sulphidic cave system Cueva del Azufre in Tabasco, Southern Mexico, and two sites outside the cave - the cave outflow and a freshwater brook. Methods: We analysed otolith morphology using scanning electron microscopy and otolith contours by Fourier shape analysis (N= 67). Results: The otoliths of the cave form of P. mexicana are heavier and have a deeper sulcus (= furrow on the inner otolith face) than those of surface dwellers. Otolith contours show a morphological gradient that concurs with the morphocline of other traits (e.g. eye size).