Annual killishes exploit ephemeral and marginal habitats that experience a great deal of spatial and temporal variation. However, to describe their habitat as unpredictable would be a gross oversimplication. In reality, there are patterns of rather predictable changes in their environment that are mixed with stochastic events that are essentially unpredictable. Both the predictable and unpredictable aspects of their environment have likely played a critical role in the evolution of the unique suite of characters that dene annual killishes. Since most, if not all, of the adaptations used to survive dehydrating conditions have been invested in embryonic stages, the production of drought-tolerant diapausing embryos is of key importance to survival of these species and likely has been under intense selection pressures.