Temperature, pH, and photoperiod effects upon gemmule hatching in the freshwater sponge, Ephydatia mülleri (Porifera, Spongillidae)

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Abstract
Mature gemmules were isolated from three individuals of the freshwater sponge, Ephydatia mülleri (Lieberkühn), collected near the termination of the Ottawa River near Montreal, Canada. Following a 4‐month period of vernalization, the gemmule populations were incubated to determine the effects of major environmental variables, temperature, pH, and, photoperiod upon hatching rate and hatchability. The three gemmule populations, each assumed to represent genetically identical clones, exhibited significant variation in hatching rate or hatchability under all incubation conditions, with the exception of total absence of hatching at 5°C. Under a 15L/9D photoperiod, individual differences in hatching rate were detected at all pH levels, and individuals were found to differ in the degree of inhibition of hatching (slowing) at low pH (5.3). Exposure to low pH (6.5‐5.8) at low temperature (5°C) for 1 week resulted in depressed hatching rate and hatchability for all three gemmule populations, with clear differences between populations. The shift of photoperiod to 9L/15D resulted in reduced hatching rate in one population and a slightly increased rate in another. These results demonstrate high genetic and physiological variation within a small local population of E. mülleri, and suggest that members of this population are differentially sensitive to environmental change‐particularly to relatively minor decrease of pH level.