GEOGRAPHIC-VARIATION IN DEVELOPMENT RATE BETWEEN POPULATIONS OF THE TELEOST FUNDULUS-HETEROCLITUS

The teleost Fundulus heteroclitus (Linnaeus) employs a semilunar spawn ing strategy, whereby eggs deposited on a spring tide optimally hatch on the following spring tide. This spawning strategy constrains the de velopment period to similar or equal to 2 wk, regardless of the mean w ater temperature, which varies throughout the species' range (Nova Sco tia, Canada, to Florida, USA). We hypothesized that F. heteroclitus em bryos would exhibit development-rate compensation among populations to ensure appropriate hatching during the spring tide. Development rates of embryos from Massachusetts and Florida were examined. Northern emb ryos had faster temperature-specific development rates than southern e mbryos, while data from the literature showed that embryos from Delawa re have an intermediate development rate. Results from reciprocal hybr id crosses indicated that there may be a genetic basis for these diffe rences. In addition, northern embryos were more cold-tolerant and sout hern embryos more heat-tolerant. Field studies showed that embryos in their local environments develop at similar rates despite large temper ature differences. Embryos seldom, if ever, experience lethal temperat ures in their native habitats, but would do so if exposed to the tempe rature regimes at the extreme ends of the species' range. Thus, F. het eroclitus populations along the Atlantic coast have specifically adapt ed their development rates and thermal tolerances to the local thermal regime.