Culture of fish food organisms

Unit 3 - Cladocerans

3.2.1.4.Biology and life cycle of Daphnia
Daphnia is a frequently used food source in the freshwater larviculture (i.e. for different fish species) Daphnia are small crustaceans that are almost exclusively living in freshwater. The head projects ventrally and somewhat posteriorly in a beak-like snout. The trunk appendages (five or six pairs) are flattened, leaf-like structures that serve for suspension feeding (filter feeders) and for locomotion. Species of the genus Daphnia are found from the tropics to the arctic, in habitats varying in size from small ponds to large freshwater lakes. The adult size is subjected to large variations; when food is abundant, growth continues throughout life and large adults may have a carapace length twice that of newly-mature individuals . Normally there are 4 to 6 Instar stages; Daphnia growing from nauplius to maturation through a series of 4-5 molts, with the period depending primarily on temperature (11 days at 10°C to 2 days at 25°C) and the availability of food. Daphnia species reproduce either by cyclical or obligate parthenogenesis and populations are almost exclusively female. Eggs are produced in clutches with parthenogenetic eggs produced ameiotically and result in females, but in some cases males can appear. Factors, such as change in water temperature or food depreviation as a result of population increase, may induce the production of males. The fertilized eggs are large, and only two are produced in a single clutch (one from each ovary), and are thick-shelled: these resting or dormant eggs being enclosed by several protective membranes, the ephippium.