Ecological differentiation in cryptic rotifer species: what we can learn from the Brachionus plicatilis complex

In recent decades, the use of molecular techniques in rotifers has revealed the existence of many cryptic species. Although strong competition is expected among cryptic species, these species are often sympatric. Here, we present a review of sympatric cryptic rotifer species, focusing on those cases in which niche differentiation has been investigated. There are at least 42 cryptic rotifer species complexes, and species coexistence is commonly reported. Ecological differentiation among cryptic species has been detected in several complexes. However, the only available information regarding mechanisms that allow cryptic species coexistence is for several species of the Brachionus plicatilis complex: B. plicatilis, B. ibericus, B. rotundiformis and B. manjavacas. According to these studies, when species differ in body size, niche differentiation is related to abiotic and biotic factors (e.g. the differential use of resources and vulnerability to predation). In contrast, if species are almost identical in body size, their biotic niches and competitive abilities are very similar, and niche differentiation is facilitated by the differences in the species responses to fluctuating, physical environment in combination with the divergence in life-history traits related to diapause. Further studies of additional cryptic rotifer species are essential to know the generality of these conclusions.