Differences in physiological tolerance between coexisting taxa of the Madeiran land snail genus Heterostoma under controlled humidity and simulated rainfall

The Madeiran land snail genus Heterostoma contains two taxa distinguished by differences in genital anatomy: one has full hermaphroditic genitalia (euphallic) while the other lacks the distal male organs (hemiphallic). Snails from the Madeiran islands of Porto Santo and IlheÂu de Cima were kept under controlled conditions differing in humidity and simulated rainfall. There were four treatments: (1) low humidity, no rainfall; (2) low humidity, rainfall; (3) high humidity, no rainfall; (4) high humidity, rainfall. For snails from Porto Santo there were viability differences between taxa under all treatments such that hemiphallics were most viable under treatment 1 and euphallics were more viable under all other treatments. There were no signi®cant differences between taxa taken from IlheÂu de Cima. This may be a real effect or the result of smaller sample size or a combination of both. Under all conditions, both taxa from both islands show a time-dependent reduction in viability in response to rainfall. Taxa from Porto Santo also respond differently to humidity with hemiphallics surviving for less time than euphallics at high humidity. These results are interpreted as showing a difference in physiological tolerance between taxa from Porto Santo with respect to humidity. It is suggested that this ecological difference may have been a factor in the divergence of the taxa, perhaps through interaction with genital anatomy variation. No ®rm conclusions can yet be drawn regarding samples taken from IlheÂu de Cima.