Functional Linkages Among Morphology, Feeding Performance, Diet, and Competitive Ability in Molluscivorous Sunfish

Pumpkinseed (Lepomis gibbosus) and redear (Lepomis microlophus) sunfish are phylogenetic sister species that are specialized for molluscivory. The native ranges of these two sunfishes are allopatric, except for a small region of overlap in the Carolinas. However, for purposes of sport fishery enhancement, redear have been introduced into many northern lakes where pumpkinseed populations were native. As a result, a large zone of artificial sympatry of pumpkinseed and redear populations has been created in the last decade in lakes of southern Michigan. In this study I examined the feeding performance and diets of co-occurring redear and pumpkinseed both in Michigan lakes and in laboratory studies of these sunfish species from those lakes. Previous research on these species suggests that redear are morphologically and behaviorally more specialized for molluscivory than pumpkinseed, and results from the present study showed that at a given size, the crushing capacity of redear was ∼ 2× greater than that of pumpkinseed. For at least the smaller size classes of sunfish, the extent of molluscivory was constrained by crushing strength. Therefore, greater crushing strength of redear allowed them to undergo an ontogenetic niche shift from a diet of predominantly soft-bodied invertebrates to a diet of snails at a smaller size than co-occurring pumpkinseed. Once molluscivorous, redear consumed larger snails than did pumpkinseed. Diet analysis also showed that following their earlier niche shift, redear were significantly more molluscivorous than sympatric pumpkinseed. Summing the effects of greater individual consumption of snails by redear and the effects of a greater proportion of a redear population being molluscivorous due to their earlier niche shift, redear were predicted to be more proficient competitors for snails. It appeared, however, that the superior feeding ability of redear on hard-bodied prey may have come at the cost of reduced efficiency at feeding on soft-bodied invertebrates.