FINE-STRUCTURE OF THE DISTAL SENSORY SETAE ON THE FIRST ANTENNAE OF PLEUROMAMMA-XIPHIAS GIESBRECHT (COPEPODA)

An external (SEM, light microscopy) and internal (TEM, serial section) study of the distal segments and the setae of the first antenna in Pleuromamma xiphias, a deep-ocean calanoid copepod, is presented here. The morphological results indicate the presence of 9 identifiable setae on the distal tip of the first antenna. These include 5 dually innervated and 2 singly innervated mechanoreceptive setae. A single thin-walled chemoreceptive seta, innervated by multiple dendrites, was identified. A presumed bimodal mechano-/chemoreceptive seta has an apical pore and is innervated by 1 mechano- and 2 chemosensory dendrites. Two pores were found on the ventral surface of the distal segments of the first antenna: 1 near the base of the chemosensory seta, and the other proximal to the feathered seta. Internally, the mechanoreceptors are characterized by well-developed scolopale tubes and an extraordinary proliferation of dendritic microtubules (up to 3,000/dendrite). Externally, the setae vary in length (0.01-1 mm) and type (smooth versus plumose, stiff versus flexible, round versus laterally flattened). The setae are positioned in a single plane, pointing either anteriorly or posteriorly. The diversity in form and position suggests sensory specialization of individual setae.