Populations of the European freshwater pulmonate Physa acutaare not reproductively isolated from American Physa heterostrophaor Physa integra

Abstract. It has long been speculated that Physa acuta, a pulmonate snail widespread and invasive in fresh waters of the old world, may have originated in North America. But the identification of a new‐world cognate has been complicated by the confused systematics and taxonomy of the Physidae in America. More than 40 species of physids are currently recognized in the United States, many with variable and overlapping morphology. We have previously established that premating reproductive isolation is negligible among physid snails. Here we report the results from no‐choice crosses each involving 2 populations of the widespread American species Physa heterostrophaand Physa integra, both with each other and with P. acuta, designed to compare measures of reproductive success between species and between populations within species. Samples of P. acutawere collected from France and Ireland, P. heterostrophafrom eastern Pennsylvania and South Carolina, and P. integrafrom southern Indiana and northern Michigan. The 6 intrapopulation controls varied quite significantly in their survival, age at first reproduction, parental fecundity, F1viability, and F1fertility under our culture conditions. Measures of survival and reproduction in the 6 interpopulation crosses were generally intermediate, but in no case significantly worse than the more poorly performing control. Thus we were unable to detect evidence of reproductive isolation among our 6 populations of snails from 2 continents. All should be referred to the oldest available nomen, P. acuta.