A stable temperature may favour continuous reproduction by Theodoxus fluviatilis and explain its high densities in some karstic springs

Theodoxus fluviatilis is a common gastropod in many karstic springs in central Portugal. We investigated the possible reasons for the near-total restriction of this species to these springs. We first determined the spatial distribution of the species within a spring (Anços) and related the densities at sampling-patch scales to selected physical and chemical variables. We then determined the densities at several locations downstream from the spring and related these densities to selected physical and chemical variables. Finally, we assessed the population dynamics of the gastropod in the spring. In the spring, T. fluviatilis was more abundant in shallow areas with a rapid current and cobble-boulder substrates. In June-July 2006, the mean densities of T. fluviatilis in the spring varied from ∼ 10 to ∼ 9000 individuals m^–2 but decreased to zero 3800 m downstream. The physical and chemical changes along the stretch studied were minor; no significant correlations (Spearman rank correlation; p > 0.05) were observed between the gastropod abundances and the measured environmental variables or the PCA axes. In the spring, juveniles and egg masses were observed year-round, suggesting continuous reproduction. The temperature at the spring was fairly constant, ranging from 15.3 to 16.6 ºC during the year (daily variation < 0.1 ºC). Nine hundred metres farther downstream, the daily temperature variation reached 5 ºC. We propose that the high densities of T. fluviatilis in the Anços spring are maintained by continuous recruitment mediated by the constant high temperature.