Trophic Polymorphism in the Pumpkinseed Sunfish (Lepomis gibbosus Linnaeus): Effects of Environment on Ontogeny

Abstract

The effects of diet on morphology of the feeding apparatus were explored in the pumpkinseed sunfish, Lepomis gibbosus Linnaeus. Adult pumpkinseeds feed primarily on gastropod molluscs that they crush between their pharyngeal jaws. Development of the muscles and bones of the pharyngeal jaws was contrasted in two Michigan (USA) lakes that differed both in snail density and in the numbers of snails in pumpkinseed diets. In least squares regressions of muscle masses, bone masses and bone shape variables on body mass, all structures examined showed isometry or positive allometry, with those structures most directly involved in snail crushing tending to scale with positive allometry. Of nine pharyngeal jaw muscles that were examined, analyses of covariance found six to be significantly larger in fish from snail-rich Three Lakes. The primary crushing muscle, the levator posterior, was more than twice as massive in Three Lakes fish than it was in fish from snail-poor Wintergreen lake. All three of the pharyngeal jaw bones used to exert crushing forces on snails were found to be significantly larger and of different shape in fish from Three Lakes. The masses of one muscle and two bones that do not function in prey crushing showed no differences between lakes. Teeth on the crushing surfaces of the jaw bones were worn down, and thus, shorter :tPresent address: P. C. Wainwright, Department of Biological Sciences, Florida International University, Miami, Florida 33199, USA. in Three Lakes fish. Ontogenetic variation in growth rates of two muscles, the levator posterior and the retractor dorsalis, occurred in fish from Three Lakes. These muscles exhibited strong positive allometry between 40 and 80mm standard length (SL), the size range over which fish diets shift from soft-bodied invertebrates to snails, but growth slowed to strong negative allometry in fish larger than 80 mm. These differences between lakes in muscle and bone growth appear to be a direct consequence of the physical regime created by repeatedly crushing snails during ontogeny. This plasticity indicates that transformations in the feeding mechanism may originate as specific responses to the demands of novel diets. Key-words: Allometry, Lepomis gibbosus, ontogeny, phenotypic plasticity