First study on the male inducing signal in Keratella cochlearis: crowding is the key

Most monogonont rotifers usually reproduce by cyclical parthenogenesis. For many rotifer species the factor inducing sexual reproduction is high population density (crowding). While Keratella cochlearis is one of the most ubiquitous monogonont freshwater rotifers, little is known about its reproduction. We provide experimental evidence that the switch from parthenogenesis to production of males in K. cochlearis is density-dependent. Single females (F1 generation) were kept in containers of varying volume representing various levels of crowding. The percentage of sexual daughters (F2 generation) producing males was higher in smaller volume treatments (higher crowding) than in larger volume treatments (lower crowding). The lowest population density at which males appeared was 38.5 females L-1 of K. cochlearis as estimated by beta regression. We also present a detailed photograph of a K. cochlearis male which has previously been known only from drawings. Surprisingly, we never observed mating and resting egg production.