Female leg 4 development in Laophontidae (Harpacticoida): a juvenile adaptation to precopulatory behaviour

Development of postmaxillipedal appendages in harpacticoid copepods is in general identical in both sexes. Exceptions are minor and result merely in secondary sexually dimorphic structures of the endopodites. From the third copepodid stage on, females of several laophontid species develop an entirely different leg 4 exopodite than in males. Whereas the male leg has a normal morphology, the female leg appears as a large flattened and sturdily sclerified structure. This structure remains present until copepodid V but is drastically reshaped during the terminal moult resulting in a normally ornamented and segmented adult leg. Thus far, the family Laophontidae is the only harpacticoid taxon of which it is known that adult males clasp the 4th or 5th leg of subadult females during mate guarding. The curious shape of leg 4 in subadult females is considered as a juvenile adaptation of the original developmental pattern to facilitate clasping by the adult males. Within the family this juvenile adaptation evolved from a simple anchorage for the male antennulae to a highly advanced morphological complex indicating that it functions as a specific mate recognition system (SMRS).