Morphological and growth rate differences between bluegill males of alternative reproductive life histories

We characterized the morphology of mature cuckolder and mature parental male bluegills Lepomis macrochirus from Lake Opinicon, Ontario. By rearing bluegill larvae to maturity in experimental ponds in Illinois we also examined whether morphological differences between males of the two reproductive life histories were conserved across widely varying environmental conditions. Cuckolder and parental males collected in Lake Opinicon differed in their size-adjusted body shape. They also differed in their pattern of mass allocation to testis versus somatic tissue. Cuckolder males allocated significantly more of their total body mass to testes compared to parental males. Body shape and testis:somatic ratio differences between male types reared in experimental ponds in Illinois were nearly identical to those observed in bluegills from Lake Opinicon. This divergence between male types remained consistent even when bluegills were reared over a 10-fold variation in growth rate. Average growth rate within ponds was strongly associated with the probability that a male would mature as a parental male within the first year. However, the proportion of males maturing as cuckolders was relatively constant among ponds and was not influenced by growth rate differences. These findings have important implications for management of bluegills and other Lepomis species where growth differences or reproductive divergence occur within single populations.