The bodies of copepods can be extremely different during their different stages of life as they change from nauplius to copepodid to adult. The body structure of copepods also has large variation between species, but in general they share physical traits that unite them as a group and visually mark them as copepods.
The copepod body is separated into two major segmented regions, the prosome which contains the head and legs and the anterior urosome which is an abdominal region containing the genitalia and ends in forked appendages called the rami. Different species have differing amounts of segments that make up the prosome and urosome.
At first glance the most notable feature on a copepod are its antennae which range in size from the short (10 segments or less) antennae seen in the Harpacticoid order to the very long ones (16 to 26 segments) seen in the Calanoid order. On top of length, copepod antennae also vary in the amount of hairlike sensory structures they possess called setae, which can make them look quite bristly. Antennal structure is also different between males and females, with the male’s being modified to grasp the female during mating. This can be used to visually differentiate the sexes.
Copepods are generally cyclopian, possessing just one compound eye on the center of the top of their heads. The World Association of Copepodologists’ website, monoculus.org, (monoculus, meaning ‘one eye’) is named after this particular feature. Like the other body parts found on copepods, there is a large degree of variation between species; for example copepods of the Corycaeus genus have paired lensed eyes which they use to help them capture prey.
The variability seen in the copepod form allows them as a group to live in and exploit different environments, from the open ocean, the surface of reefs, and even on damp mass found on the mainland.
Gophen, M., Harris, R.P., 1981.Visual Predation By A Marine Cyclopoid Copepod, Corycaeus anglicus. Journal of the Marine Biological Association of the United Kingdom.61:391-399