Daphnia: development of model organism in ecology and evolution

Few freshwater organisms are better recognised by aquatic biologists, or are more symbolically representative of freshwater habitats, than the cladoceran, Daphnia. Several species share the similar body-plan, featuring a short, segmented body and a compressed carapace that part-encloses several pairs of flattened limbs, called phyllopods. Co-ordinated rhythmic beating of the phyllopods generates a current within the carapace chamber, from which food particles are strained by the marginal filtering setae and then channelled back to the animal’s mouth. Partly because this turns out to be a highly efficient means of removing and concentrating appropriately-sized foods from the water and partly because the animals are able to grow rapidly and recruit subsequent generations, Daphnia can be a major consumer of phytoplankton (algae and bacteria). At the same time, however, individual animals are not inconspicuous to predatory young fish or, on occasions, to older fish of those species that remain planktivorous specialists for much of their lives. As a consequence, populations of Daphnia are frequently pivotal to energy transfer through aquatic food webs. In turn, the impacts on species selection in freshwaters, on system function and upon perceived water quality, are known to be far reaching