Copepods are an extremely diverse group with more than 10,000 species. In many ecosystems copepods are the most common multicellular animals in the plankton. Many copepods live associated with other animals, often as parasites.
The body of calanoid and cyclopoid copepods can be divided into a carapace (prosome) and abdomen (urosome). In harpactacoid copepods the body seems to consist of one part. On the head there are two antennas. Copepods can have several eyes.
Copepods have several sets of legs of which usually the front set is used for collecting food and the back set for swimming. Many species can also jump away quickly by flipping their tail. They can do this so fast that they are (relative to body length) the fastest jumpers in the world, comparable to a human jumping at over 6000 km/h!
The life cycle of copepods starts with an egg. Many species carry the eggs for a while in egg sacs on the urosome, but some release the fertilized eggs directly into the water. Out of the egg comes a nauplius larva that looks like a small spider (see film below). The nauplius larva eats plankton, sheds its skin six times and turns into an immature copepod (copepodite). It sheds five more times until it eventually becomes a male or female copepod. Copepods reproduce sexually, and often have complicated mating rituals. Males often have specially adapted antennae or legs to grab the female.
Copepods eat anything. The smallest species eat mainly single-celled plankton, larger species eat mainly algae and the largest species are often predators that eat other zooplankton, including other copepods. copepods themselves are an important food source for many aquatic animals and an extremely important link in the food chain. Many species of copepods store lipids (fats) from their food in their bodies as food supplies and for buoyancy. These lipids can often be observed as round oil globules within the animal. When copepods are eaten by fish such as herring these fish absorb the fats, which is why herring and the like are rich in fatty acids such as omega-3.
Three different groups of copepods are discussed below, the calanoids, cyclopoids and harpactacoids.