Latitudinal Body Size Pattern in a Marine Isopod Suggests Local Adaptation to Predator Risk
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Geographic patterns of body size are of particular interest to biologists because they suggest a correlation between local environmental conditions and phenotypic expression. Bergmann's rule is a widely observed latitudinal pattern of body size in which organisms from colder latitudes possess larger body sizes than their warmer latitude counterparts. Although widely studied, the evolutionary mechanisms and environmental driving forces dictating these latitudinal patterns remain relatively underexplored. Consistent with Bergmann's rule, I found that field-collected individuals of the marine isopod <italic>Idotea baltica</italic> from Massachusetts sites are larger than individuals from Virginia sites. I raised juvenile <italic>I. balthica</italic> from populations from both regions at a range of temperatures (6°, 12°, 18°, 24°, and 30°C), and measured growth and survivorship regularly for 15 weeks or until sexual maturity. I found that body size varied positively with temperature, with individuals from Massachusetts populations possessing an overall larger body size than those from Virginia populations (MA, 18°C = 9.21 mm, MA, 24°C = 10.12 mm, VA, 18°C = 7.77 mm, VA, 24°C = 8.65 mm, p < 0.0001). Time to maturity was greater in Massachusetts populations (MA, 18°C = 71.02 days, MA, 24°C = 51.14 days, VA 18°C = 52.52 days, VA 24°C = 45.64 days, p < 0.0001), while growth rate at a given temperature was consistent across all populations, indicating that higher predation rates in Virginia habitats may be an important driving force. Field tethering experiments support this argument, with higher isopod survival rates in Massachusetts sites than Virginia sites (MA = 57%, VA = 13%, p-value = < 0.0001). My data indicates that the latitudinal body size pattern in <italic>I. balthica</italic> is adaptive, and that predation may have been an important driving force in the development of this pattern.