Functional Significance of the Sexual Dimorphism in the Cephalic Appendages of Euchaeta Rimana Bradford

Abstract

In Euchaeta rimana the cephalic appendages become profoundly sexually dimorphic at the final molt. The enlarged maxillipeds and robust maxillae, which are related to the predatory nature of this pelagic marine copepod are fully functional on the fifth copepodid (CV) males and adult females. The presumed prey detection sensory apparatus, a paired 4-point setal array on the antennules, is located within the capture volume of the feeding current of the CVs and adult females. SEMs reveal specialized basal articulations of the straight and curved setae of this array, that allow setal rotations that streamline the antennule during escape movements. In the final molt of the CV male to the adult stage, the maxillae are reduced to vestiges and the maxillipeds to half the size of the female's. The loss of functional prey capture appendages is accompanied by the loss of the prominent antennulary setal array, giving support for their postulated function as prey sensors. Instead, the male gains 19 more aesthetascs primarily in the proximal region of the antennules, where fluid velocities of the scanning current are greatest. Detection of pheromones transported within the feeding/scanning current is the hypothesized function of the chemosensory system of the adult male copepod. The parallel changes in the structure of the antennule as well as of the adjacent cephalic appendages suggest that a homeobox-like gene control system could be coordinating these morphological changes.