Although some marine fish will breed and grow quite readily in captivity, others are more problematic. One of the major difficulties in breeding these fish is the supply of suitable food for the larvae. In nature, many marine fish depend on copepods as their initial first food, but very few species of marine copepods have been successfully cultivated on a scale that is suitable for use in home aquaculture. Hopefully, this column will provide a basic background into copepod biology and life cycle, as well as provide a simple procedure for growing your own copepods. The procedures described below are taken from a basic understanding of the life requirements of the animal and a practical understanding of what a home hobbyist has access to. In my opinion, copepod cultivation may make an important contribution in home aquaculture, when fish fry are too diminutive to eat rotifers as a first food.
It is well known that for many species of fish fry live food is essential at the first critical stages of first feedings. In the oceans, potential food items most likely to be encountered by fish larvae are the nauplius stages of copepods. Copepods have probably been important in the diet of many fish during their evolution and effective predation strategies have evolved for capturing copepods as primary foods. Marine copepods are particularly suitable as food for fish fry. The size range (~100uM nauplii to ~1000uM adult) fit into the mouth of many larval fish. Copepod nauplii elicit a strong feeding response from many fish larvae, and Copepods have naturally higher levels of essentially fatty acids.
Currently the easiest way to supply copepods for home aquaculture is to capture them via netting in the wild; however, since our column focuses on home culture we will explain a simple how-to home culture copepods.
Copepods are a class of animals within the larger group Crustacea. The group is diverse, with more than 10,000 different species in many different ecological niches. Copepods occur in most bodies of marine and freshwater. Many copepod species are parasitic, others swim freely as part of the plankton, while still others are benthic (bottom dwelling) or live on or around other organisms. Few free-living copepods exceed 2 mm in length as adults. Three major groups of free-living copepods have been identified: the Calanoida, primarily free swimming planktonic animals, The Cyclopoida, which may be planktonic or demersal, and the Harpacticoida, which are entirely benthic.
Copepods pass through very distinct life stages. They emerge from an egg as a nauplius, usually 100-150uM in length. After six nauplius stages (referred to as stages N1 to N6), with growth between each stage, the body shape changes and a series of usually six copepodid stages follow (referred to as stages C1 to C6). The last of these stages is the adult in which different sexes can be identified. Reproduction is sexual in nature, and in parts of the sea the nauplius larvae of calanoid copepods are the most abundant metazoan animals.
Life history and development:
Fertilized eggs are held in a sac against the urosome of the female. When first released the eggs appear dark brown. As embryos develop the color and shade lightens until the mature embryos appear light brown with a dark eye spot just visible in each. Nauplius larvae emerge from the egg sac and swim freely. Newly released nauplii have up to four or five small lipid droplets regularly arranged in their body cavity. The first nauplius stage (N1) is very brief (a few hours) before the animals metamorphose to N2, then a rapid growth to N6. Following N6, the first copepodid stage (C1) occurs. By this stage the overall body form has changed from the