Sensory specialization along the first antenna of a calanoid copepod, Pleuromamma xiphias (crustacea)

A morphological map of the sensory setae on the first antennae of Pleuromamamma xiphias was made utilizing light and electron microscopy. Our results indicate that: 1) purely mechanoreceptive setae are located only on the most distal segments; and 2) chemoreceptive and mixed-modality mechano-/chemoreceptive setae are located along the entire antenna with the greatest abundance in the proximal third of the antenna. The aesthetasc-like chemoreceptive setae are thin-walled and are innervated by many small dendrites. The mixed-modality setae, which comprised the most abundant type, are characterized by an apical pore, and they are innervated by large mechanosensory dendrites as well as smaller chemosensory dendrites. The purely mechanoreceptive setae lack both apical pores and the smaller sensory dendrites. We hypothesize that the function of the mixed-modality setae is ''gustatory'', that is, they may be sensing particles by tasting (chemoreception) and mechanical contact (mechanoreception) combined. These setae may be involved in the acceptance and/or rejection of potential food particles which come into physical contact with the first antenna.