Atlantic silverside

Abstract: Sex determination in an atherinid fish, the Atlantic silverside (Menidia menidia), is under the control of both genotype and temperature during a specific period of larval development. The sex ratios of the progeny of different females are variable and differ in their responsiveness to temperature. This demonstrates that sex ratio in fishes that normally have separate sexes can be influenced by the environment.

Abstract: In ectotherms, lower mean temperatures and shorter growing seasons at higher latitudes would be expected to cause a reduction in the annual growth rate of an individual. If slower growth reduces fitness, then organisms at higher latitudes may evolve compensatory responses for these climatic effects. Two such forms of local adaptation with increasing latitude are possible: (1) the capacity for growth may shift to a lower range of temperatures (i.e., temperature adaptation) or (2) maximum growth rate may evolve inversely with length of the growing season (i.e., countergradient variation). A third alternative is a mixed strategy involving both of the above. We hypothesized that the form of local adaptation may be affected by constraints that vary within vs. among species. We used common-environment experiments to compare reaction norms for growth in response to temperature among local populations of two contiguous, closely related fish species, the Atlantic silverside, Menidia menidia (L.), and the tidewater silverside, M. peninsulae (Goode and Bean), which together have a range spanning much of the North American Atlantic coast. The common-environment experiments revealed countergradient variation: maximum growth rate increased with latitude both within and among species. However, growth reaction norms of the northern species were shifted to a lower range of temperatures than those of the southern species, indicating adaptation to temperature at the interspecific level. Hence, adaptation to temperature contributes to the interspecific variation, while countergradient variation contributes to both the intra- and interspecific differences.

Abstract: Evidence suggests that the capacity for growth (i.e., maximum growth potential) within a species may vary inversely with the length of the growing season across a latitudinal gradient. I evaluated this hypothesis with data on three species—American shad Alosa sapidissima, striped bass Morone saxatilis, and mummichog Fundulus heteroclitus—having wide latitudinal ranges (≈29–46°N) along the east coast of North America. For each of these species, the length of the first growing season decreases by a factor of about 2.5 with increasing latitude within the species' range, yet body size at the end of the first growing season is independent of latitude. Northern fish must, therefore, grow substantially faster within the growing season than do southern fish. This “countergradient variation” in growth rate may be more widespread than has been recognized. A similar latitudinal pattern in growth rate has a genetic basis in the Atlantic silverside Menidia menidia, and data on Atlantic salmon Salmo salar, lar...

Abstract: Genetic differences among populations of Atlantic silverside ( Menidia menidia) are hypothesized to be evolutionary responses to intense, size-selective winter mortality at high latitudes. Three experiments were conducted to test features of w inter mortality. In the first experiment, we varied size and whether food was provided or withheld; temperatures were permitted to follow ambient (New York) wintertime fluctuations. Mortality and depletion of energy reserves were more rapid in the units receiving no food. Small fish died before larger fish in these units, but not in the units receiving food. Energy depletion of fish in the no-food treatment resembled that of fish in the wild. In the second experiment, we varied size and population of origin, representing high-latitude (Nova Scotia), midlatitude (New York), and low-latitude (South Carolina) populations. These fish were provided food and showed minimal depletion of energy reserves, but mortality rates were high when water temperatures were low. Mortality did not vary with size in New York and South Carolina fish, but was highest in intermediate-size fish from Nova Scotia. There was a pronounced population difference in survival rate (Nova Scotia > New York > South Carolina). In the third experiment, food was withheld and extreme low temperatures were moderated. Energy depletion was rapid and small fish died before large fish. We conclude that populations in seasonal environments are likely to be subject to size-selective winter mortality when energy reserves are depleted and that juvenile growth rates have evolved in response to this selection pressure. In addition, high-latitude populations have evolved greater tolerance to other winter stresses associated with low temperatures.