A review of Mauthner-initiated escape behavior and its possible role in hatching in the immature zebrafish,Brachydanio rerio

Synopsis In certain fish rapid escape responses have been observed to occur from early stages of embryogenesis. It is the purpose of this review to consider the development of one escape pattern, the C-type fast-start. During this behavior the animal initially coils its body into the shape of a letter C, and then rapidly uncoils and propels itself through the water. Relative to body size, the speed of the embryonic and larval C-start is comparable with that of the adult. These responses in the zebrafish, Brachydanio rerio, are utilized in the escape from predators, such as the protozoan Coleps sp. C-starts are triggered by the firing of one of the Mauthner (M-) cells, a single pair of large neurons in the brain stem. These neurons receive a rich supply of connections from sensory and integrative centers in the brain. The M-cells activate the escape movement by driving motor and relay neurons controlling the various muscular contractions associated with the behavior. During hatching, the rupturing of the egg envelope appears to be triggered by strong tail contractions following dissolution of the envelope by hatching enzymes. These contractions are similar to those known to be driven by the M-cell. The M-cell fires spontaneously up to the normal time of emergence from the egg, but is quiescent afterwards. This spontaneous activity of the M-cell may result in behavior that helps to break the egg envelope. The M-cell also reliably fires to repeated stimulation up to about the normal time of hatching, but habituates rapidly thereafter. We suggest that the M-cell may be utilized in escaping from the egg when it is under attack by a small predator.