Reciprocal transfer study of north temperate and subtropical populations of Mesocyclops edax (Copepoda: Cyclopoida)

The examination of local and geographically based life history variation is one approach to examine how different selective forces may have molded life histories. Michigan and Florida populations of the warm-water copepod Mesocyclops edax exhibit genetically based differences in maturation time, body size, clutch size, and egg volume as determined by common rearing experiments in the laboratory. Florida females mature more rapidly, yet achieve larger body sizes and clutch sizes relative to Michigan females. In an effort to relate the different life histories to the specific environments, in the present study I reared both populations in enclosures suspended in the Michigan and Florida lakes. Under the high temperature and food conditions of the Florida lake, the Michigan population retained the relatively longer female maturation times and smaller clutch sizes, providing further evidence that the Michigan population is genetically constrained to divert relatively less energy to reproduction than growth compared to the Florida population. The higher survivorship of the Florida population at temperatures above 31°C relative to the Michigan population suggests the Florida animals are better adapted to extreme, high temperatures. Under the low food conditions of the Michigan lake relative to the Florida lake, the Michigan females matured more rapidly and achieved the same body size as the Florida population, but the Michigan clutches were still smaller and had larger eggs. The relative performances of the Michigan and Florida females in the in situ enclosures provided additional support for the conclusion that the Michigan population is adapted to low food conditions and the Florida population to high food and high temperature conditions.