Alternative startle motor patterns and behaviors in the larval zebrafish (Danio rerio).

In fishes, the C-start behavior, initiated with a C-shaped body bend, is a taxonomically common and widely studied escape response. Its simple neural circuit has made this behavior a model for examining neural control of movement. The S-start, initiated with an S-shaped body bend, is a physiologically distinct escape that occurs in esocid fishes. Here we examine whether zebrafish larvae perform S-starts in order to better understand startle diversity and to attempt to identify the S-start in a system that is tractable for neurobiological studies. We found that larval zebrafish startles varied in the extent of their caudal bending, resulting in C, S and intermediate-shaped responses. We recorded two distinct motor patterns: nearly simultaneous initial activity along one side of the body, characteristic of C-starts, and nearly simultaneous activity rostrally on one side and caudally on the other, characteristic of S-starts. Head stimulation generally elicited C-starts while tail stimulation elicited C- and S-starts. These results demonstrate that the S-start is more common than previously documented and occurs in early developmental stages. We suggest that the S-start may be a fundamental escape behavior in fishes and may provide a comparative model to the C-start for understanding simple neural circuits.