Distal Setal Mechanoreceptors of the First Antennae of Marine Copepods

The first antennae (antennules) of many calanoid copepods can be divided into three sectors differing in setal arrangement, type, and number: the proximal sector where setae can be closely spaced; a middle sector where setae tend to be sparse; and the distal tip where several setae form a tuft. Each sector of the first antenna also lies within different flow regimes of the field created by the copepod during normal swimming and feeding activities (Fields and Yen, 1993): the distal tip experiences flow velocities that are much slower than the proximal sector. Extracellular recordings of mechanosensory discharges from the antenna during controlled mechanical stimuli (Gassie et al., 1993) show large-spike (>300 μV) neural responses from a small number (<10) of units, as well as activity from numerous smaller units. When the distal tip is removed, the large spiking units, but not all of the smaller ones, disappear. The large aclion potentials may be transmitted by the large-diameter (>5 μm) axons present in the distal region of the antenna. The large spiking units are sensitive to near-field displacements and respond to high frequency stimuli. Threshold sensitivities can reach less than 10 nm at 1,000 Hz. The spike amplitudes and sensitivity of these distal receptors suggest that they may be used to trigger responses that require short reaction times such as in rapid escapes. Since prey entrained in the flow are captured closest to the proximal sector of first antennae, a possible role for setal receptors here would be to detect prey signals (movements, chemical exudates), while out in the quiet region near the distal tip, the setal receptors may be sensing water displacements generated by more distant objects (e.g., obstacles or predators).