THE DISTRIBUTION OF THE MAUTHNER AXONS IN FISH AND AMPHIBIANS AND ITSRELATION TO THE FRIGHT REACTION IN OSTARIOPHYSI AND ANURA

Mauthner neurons were found in 165 of the 200 families of fish studied . They are absent in most Elasmobranchii, all Anguilliformes, several marine bottom-dwelling fishes, and many others. Mauthner axons during phylogeny have been lost independently in more than a dozen groups of fish. While Loricariidae possess the Mauthner axons and the Mauthner-i nitiated startle response, both are absent in Aspredinidae, although t hese are also bottom-dwelling, night-active catfish. No Mauthner axons could be found in Gymnotus carapo and Carapus acus. This indicates th at Mauthner neurons are absent in species in which the caudal motoric system is reduced or lacking. The Mauthner-initiated startle response has been investigated in Ancistrus dolichopterus (Loricariidae). There is no difference between a startle response during the day under ligh t conditions and a startle response during the night in complete darkn ess. The Mauthner-initiated startle response differs fundamentally fro m a normal turning movement by its enormous speed: in the Mauthner-ini tiated startle response, the angular velocity is 5300 degrees/s and th e displacement speed 20 body lengths/s; in the normal turning movement 700 degrees/s and 3 body lengths/s. All ostariophysean species having epidermal alarm substance cells possess Mauthner axons as well. This is in accordance with the fact that the fright reaction elicited by th e alarm substance in Ostariophysi always begins with a Mauthner-initia ted startle response. In amphibians the situation is completely contra ry. The tadpoles of Bufonidae are the only amphibians possessing a fri ght reaction elicited by the alarm substance from conspecifics, and th ey are the only ones lacking Mauthner axons.