ABSTRACT
The escape response is the main defense mechanism used by most fish species when facing a predator attack. Escape responses have been studied in a variety of species, revealing a high variability in kinematics and behavior. Typically, fish respond to a startle stimulation with a unilateral contraction of their axial muscle (called a “C-start”) in a direction away from the threat (Domenici and Blake, 1997). This contraction (stage 1), can be followed by a further contraction on the opposite side of the body (stage 2) (Fig. 5.1). During stages 1 and 2, high accelerations and turning rates are produced. Little is known about the fish’s swimming behavior beyond these two stages. Weihs (1973) and Webb (1976) defined this period as “stage 3”, during which fish may show variable behavior, i.e., continue swimming, glide or brake. Escape responses are usually triggered by one of a pair of large reticulospinal cells (the Mauthner cells, M-cells) and several other related neurons found in the hindbrain (Eaton and Hackett, 1984). The Mauthner cells receive stimulation from a number of ipsilateral sensory inputs, typically visual and/or mechanoacoustic. As a result of the 124firing of a M-cell, the contralateral axial musculature as well as other motor components (e.g., pectoral and dorsal fins, opercula) are activated, while the opposite M-cell is inhibited (Eaton and Hackett, 1984). A large body of evidence now demonstrates the existence of many “variants” of the typical escape response described above. For example, escape responses may consist of stage 1 only (Domenici and Blake, 1997), certain species may exhibit an S-start rather than a C-start, where contractions on both sides of the body occur during stage 1 (Hale, 2002), and responses in which Mauthner cells are not activated have been observed (Eaton et al., 1984; Hale, 2000 and Kohashi and Oda, 2008).
