Water fleas: Daphnia

Water flea, Daphnia spp. likely feeding while anchored to glass.

Water fleas are crustaceans of the order Cladocera. In general, these creatures range in sizes of less than 0.5 mm to more than 6 mm with males being smaller. Within Cladocera are the planktonic crustaceans of the genus Daphnia which are characterized by their flattened leaf-like legs. Their bodies are enclosed by an uncalcified shell with either five or six limbs on the trunk. In the water, they swim in a jumping-like behavior that earned the nickname of water fleas. There are more than 100 known species within Daphnia that have been studied for more than 250 years.

Transverse perspective of Daphnia spp. swimming.

Daphnia are mostly freshwater microcrustaceans that can be found in most types of freshwater including permanent and temporary ponds, lakes, and slow moving streams. While mostly pelagic, some are found clinging to plants or browse in the bottom of shallow ponds. They are filter feeder, their prey the small, suspended particles in the water, mostly consuming planktonic algae. Their leaf-like legs are used to produce a water current to collect the particles. They mature fast and serve as an important source of food for fish and other aquatic organisms.

Transparency of Daphnia has been useful in various research projects.

There are numerous reasons why Daphnia has been subject to intense biological investigations for more than a century. Their transparency allows for the conduction of bioassays using endpoints other than death such as heart rate or changes in appetite. The wide distribution of Daphnia, from freshwater lakes to saline ponds, means the manifestation of extensive diversity allowing for the study of gene function and genome by environmental interactions. They are often used as a model for ecotoxicological studies. In addition, they are a valuable outgroup for comparative genomic studies, particular due to their close relationship to insects which have provided for study of genome function and evolution. The fast maturity of Daphnia, as well as asexual reproduction that can be induced environmentally to be sexual, adds to their appeal for study. We have learned much from Daphnia, from genetics and epigenetics to immunology and toxicology, all that can be applied to humans, but there is plenty more left to learn.


Edit: Another version of this post with more focus on Daphnia in research was written this past Sunday and can be found here along with three photographs not on the blog.