SIZE-SELECTIVE PREDATION AND SEXUAL SELECTION ON BODY SIZE VARIATION AMONG KARST SPRING POPULATIONS OF THE AMPHIPOD GAMMARUS MINUS

Body size of sexually mature individuals varies widely across karst spring populations of the amphipod Gammarus minus. The mean size of Gammarus minus in eight karst springs in the presence of the Banded Sculpin (Cottus carolinae) (male: 37.1 to 45.3, female: 27.1 to 33.8 {head capsule length in ocular micrometer units]), is significantly smaller than those in nine springs without sculpins (male: 49.4 to 55.8, females: 36.3 to 42.8). Within populations of sculpin-less springs the positive correlation between male and female size is significantly higher (Pearson’s r = 0.36 to 0.70) than in populations with sculpins (0.18 to 0.61) (p=0.036). Bigger females have larger brood sizes in all populations, but the slope of the regression of brood size on female size is steeper in populations without sculpins (p=0.008). In populations with sculpins, the magnitude of the correlations between male and female size is not significantly correlated with the slopes of the regression of brood size on female size (p=0.566) whereas in populations without sculpins the magnitude of this correlation is almost significant (p=0.082). We interpret our results to mean that in populations with sculpins, the intensity of sexual selection to increase body size is countered by size-selective predation by sculpins, while in populations without sculpins the intensity of sexual selection is not countered by this predation pressure, partially accounting for the variation in body size among springs.