Copepods are important little animals in many freshwater and marine ecosystems. They are arthropods so have the usual jointed legs and hard carapace that is good for protection but which must be shed in order to grow. They are crustaceans and so have mandibles and gills and are adapted for an aquatic existence. Some people call them the insects of the sea. They are small, being only one to a few millimeters long, but they are important in food chains, for they feed on smaller organisms and in turn are fed on by larger carnivores. There are about 5000 species world wide and many larger animals would suffer if this group of organisms were to disappear.
The average copepod has a head with five pairs of appendages, a thorax with paired appendages on each segment and an abdomen without appendages. They have a dorsal carapace that protects the head and the first two thoracic segments. The head usually has one median eye.
There are nine orders of copepods. Three of these are the free-living forms while the other six are adapted for a parasitic lifestyle. The largest order are the Calanoid copepods, with thousands of species. They are usually pelagic and have very long antennules. They form an important part of the plankton and are reasonably good swimmers. The other two orders of free living copepods are the Cyclopoids and the Harpacticoids. Cyclopoids are generally pelagic, while the harpacticoids are usually bottom dwellers. They are found in both marine and freshwater littoral habitats.
I often see cyclopoid copepods in pond water samples. They have one bright red eye, long branched antennae, a fat body and a long abdomen to which egg sacs are often attached. These little animals dart here and there and lead very busy little lives, looking for food and avoiding predators while rearing their young. Their bodies are transparent and so one can see what they have eaten and watch their muscles move.
The rest of the orders are parasites. Most live on the bodies or in the gills of fish and are called 'fish lice'. The Caligidea are ectoparasites on marine and fresh-water fish. The Lerneopodidea are also ectoparasitic on fish. the males are dwarfed and attached to the females. as are the males of another order, Andreinidea. The Philichthyidea live in the sinuses of marine fishes and are highly modified for this life style. Another order of parasitic copepods are the Sarcotacidea. The males and females live together in a gall produced in the host. Monstrilloid copepods are an exception to the fish parasite rule, being parasites of polychaete worms in their larval stages and free living as adults. They have no mouth parts and the gut is greatly reduced.
The importance of copepods in marine food webs cannot be overstated as they are probably the biggest source of protein in the ocean. Fish and baleen whales are dependent on these little animals at the base of the food chain. In addition, the waste products of copepods rain down on the bottom dwellers, providing them with an important food source. If you want to know more, check out the website in the references.
References: Megllitsch, P. 1972. Invertebrate Zoology. Oxford University Press. http://www.uni-oldenburg.de/zoomorphology/Biology.html