Sex or Sanctuary: How do Asexual Worms Survive the Winter?

Abstract

The common and geographically widespread freshwater worm Stylaria lacustris (Linnaeus, 1767) (Oligochaeta: Naididae) typically reproduces asexually through transverse paratomic fission during the spring, summer, and autumn. With the onset of shorter days and colder conditions, S. lacustris becomes a sexually mature simultaneous hermaphrodite and produces resting eggs that are capable of overwintering. However, like many naidid species, S. lacustris shows widespread variation in reproductive mode with some populations never attaining sexual maturity and others apparently exhibiting both sexual and obligately asexual genotypes. How then do obligately asexual genotypes and populations survive the harsh winter conditions? Extensive winter sampling of two, largely obligately, asexual populations of S. lacustris in Oxfordshire, UK, demonstrate that adult individuals can survive over the winter, but at densities way below that normally detected by standard sampling procedures. Laboratory experiments confirm that asexual individuals can survive cold water conditions but not freezing (unlike sexually produced cocoons). The proposed advantage of this seemingly risky reproductive strategy is that naidids like Stylaria, with their remarkably fast asexual reproductive rate, can respond instantly to favourable change in conditions.