Copepods are arthropods (Phylum Arthropoda) because they have jointed legs and exoskeletons. The Phylum Arthropoda is divided into three large groups: terrestrial mandibulates (insects), terrestrial chelicerates (spiders) and aquatic mandibulates, the Crustaceans. Copepods are crustaceans because they are aquatic and have mandibles. What are mandibles? These are the mouthparts of insects and crustaceans and they form embryologically from different segments than the mouthparts called chelicerae in spiders and their relatives.
There are lots of different kinds of Crustaeans, with about 30,000 species known from both fresh and salt water bodies around the world, as well as a few terrestrial species. Crabs and lobsters, prawns and shrimp, pillbugs and barnacles are all well-known crustaceans, but there are many other smaller crustaceans as well. One such group is the subclass Copepoda.
Copepods are actually one of the most successful groups of Crustaceans, but they don't get much recognition because most are so small that they are barely visible. There are about 5000 species worldwide and they are common and abundant in both marine and aquatic ecosystems and are important members of their food chains. In fact in some food chains, the removal of copepods would have devastating effects on the numbers of higher organisms such as fish.
Copepods are typical looking crustaceans with a head that has five pairs of appendages (mouthparts, antennae) and a thorax with paired legs on each segment which can be adapted for either walking or swimming. The thorax is followed by a segmented abdomen with no legs. Their heads are covered by a dorsal carapace (hard shell) for protection. There is often a tail-like telson at the end of the abdomen.
There are 8 orders of copepods. Three orders, the Calanoida, the Cyclopoida and the Harpacticoida, consist of free-living species. The harpacticoids have short antennae and crawling legs for living in bottom sediments. The calanoids on the other hand have highly developed swimming setae and long antennae and are mostly found in the plankton in open waters. The cyclopods have not committed themselves to either way of life and are intermediate in character and found in both habitats. Swimming copepods are filter feeders while crawlers are herbivores or occasionally little predators.
The other five orders of copepods are parasitic or commensal in life style. This has led to a reduction in features or extreme modifications for a parasitic lifestyle so parasitic copepods are often hard to identify. So-called sea lice are actually parasitic copepods and not lice at all (true lice are insects). Sea lice can be found on the gills of many fish. Other parasitic copepods live as ectoparasites on their hosts or inside the sinuses and body cavities of fish, sharks and even whales.
Copepods, like all crustaceans, go through a complicated life cycle that begins with an egg and ends in a sexually mature adult. In order to grow, the young copepods must shed their shells by moulting. Copepods hatch as nauplii larvae and then go through several intermoult stages called instars. Eventually they reach a stage where they resemble small adults. When sexual maturity is reached, copepods mate. The male uses modified antennae to clasp the female and transfer a spermatophore to her, cementing it to her abdomen. The sperm move into seminal receptacles where they may wait for some time before fertilizing her eggs.
A look at pond water through a low powered microscope will almost always yield a glimpse into the lives of copepods. They are busy little creatures, darting here and there looking for food. Some have paired eyes while others have only one median eye. The females are often seen with egg sacs attached to their abdomens. Because they are transparent, one can often watch the inner workings of the digestive system and see what they have been eating. Copepods are fascinating little creatures and important in their own ways, even though they are so small.
Acknowledgment: Thanks, once again, to that mine of information: Paul Meglitsch's book, Invertebate Zoology, which introduced me t the fascinating world of animals without backbones so many years ago and still educates me whenever I open it.