DISTAL SETAL MECHANORECEPTORS OF THE 1ST ANTENNAE OF MARINE COPEPODS

The first antennae (antennules) of many calanoid copepods can be divid ed into three sectors differing in setal arrangement, type, and number : the proximal sector where setae can be closely spaced; a middle sect or where setae tend to be sparse; and the distal tip where several set ae form a tuft. Each sector of the first antenna also lies within diff erent flow regimes of the field created by the copepod during normal s wimming and feeding activities (Fields and Yen, 1993): the distal tip experiences flow velocities that are much slower than the proximal sec tor. Extracellular recordings of mechanosensory discharges from the an tenna during controlled mechanical stimuli (Gassie et al., 1993) show large-spike (> 300 muV) neural responses from a small number (< 10) of units, as well as activity from numerous smaller units. When the dist al tip is removed, the large spiking units, but not all of the smaller ones, disappear. The large action potentials may be transmitted by th e large-diameter (> 5 mum) axons present in the distal region of the a ntenna. The large spiking units are sensitive to near-field displaceme nts and respond to high frequency stimuli. Threshold sensitivities can reach less than 10 nm at 1,000 Hz. The spike amplitudes and sensitivi ty 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 wou ld be to detect prey signals (movements, chemical exudates), while out in the quiet region near the distal tip, the setal receptors may be s ensing water displacements generated by more distant objects (e.g., ob stacles or predators).