Plastic sex for a brittle star

The different potential fitness outcomes between reproduction by cloning or by sex have puzzled researchers for years. But whatever the reason for the two quite different strategies, most species have evolved to use one or the other, or a fixed pattern of both. But new work on a brittle star, a marine invertebrate, suggests that at least this one species may be able to choose one or the other reproductive strategy as environmental conditions vary. The brittle star, Ophiactis savignyi, lives throughout the tropics but inhabits an unusual environment: it lives in sponges. Within each sponge colonies of more than 300 individuals can be found. It may have neighbours just centimetres away but the next colony may be many metres distant. Work by Tamara McGovern at Florida State University reported in the Proceedings of the Royal Society of London, series B, published online, suggests that this brittle star, which each individual is of a single sex but can reproduce both sexually and by cloning, may choose the best reproductive strategy depending on its situation. The species reproduces asexually dividing into two and each half then regenerated the missing parts. It reproduces sexually by releasing sperm or eggs into the water. The first strategy is reliable, but not very productive, leading to just a single offspring. The sexual strategy, in contrast, when there is a member of the opposite sex in the vicinity, can lead to far higher numbers of offspring. But in the absence of a sexual partner, this strategy is obviously unproductive. McGovern’s work in the field and the laboratory used randomly amplified DNA analysis to look at the clonal relationships between individuals within a colony and also involved the transfer of individuals between natural colonies. Her results suggest that in colonies of mostly a single sex, which comprised three quarters of the colonies she looked at, cloning predominates but in colonies of mixed sexes, sexual reproduction is favoured. Animals transferred to colonies with potential mates divided less frequently than animals without a potential mate. Such a potentially plastic reproductive strategy is remarkable but one question remains: how do individuals recognise the sex of their neighbours? Plastic sex for a brittle star