Some evolutionary trends in caligid copepods

The genera Caligus and Lepeophtheirus can be used to illustrate some evolutionary trends. The most striking example of these trends is a gradual reduction of the fourth thoracopod, now largely functionless. All stages of this reduction can be found in Caligidae. Other, apparently correlated, changes are also present. For example, reduction in the armature of the first thoracopod occurs mainly among species in which the fourth exopod has become two-segmented, though still retaining the seta associated with the original three-segmented structure.