Hale, M.E.: Fast Start Behaviors of Fish Lacking Mauthner Neurons

The paired Mauthner neurons (M-cells) are used by fishes to initiate the C-start type of startle response. During C-starts, a fish turns rapidly away from an offending stimulus with a “C” shaped body bend. Mauthner neurons may also function in S-starts, in which the initial bend of the body is “S” shaped. While the large majority of fish taxa have M-cells, in a few species they have been secondarily lost. By examining the startle response of one of these species, the toadfish (Opsanus tau), this study investigates the role of M-cells and other large reticulospinal interneurons in fast-start behaviors. EMGs of axial, pectoral fin, and jaw muscle and high-speed video of responses to touch stimuli were recorded. Toadfish were found to perform both C-starts and S-starts. The C-start behavior was used by toadfish as part of several different behavioral responses. It could follow the stimulus directly or be preceded by fin movements and lunging and biting at the stimulus probe. This suggests that C-start behavior can function in many behaviors without the reflexive M-cell response to the stimulus. While the pattern of the toadfish C-start was like that of a M-cell elicited response, aspects of performance were decreased indicating that the alternative neural mechanisms by which the C-start behavior pattern is generated in toadfish do not compensate for the lack of large reticulospinal interneurons. S-starts were more consistent than C-starts with little variation in the response. Toadfish have a short latency, high performance S-start behavior similar to that of fishes with Mauthner neurons. This suggests that the M-cell may not be needed for S-start escape behavior. This work was funded by a Grass Foundation Fellowship and NRSA MH11861.