Evolution of swimming behavior inTritonia and its neurophysiological correlates

Summary

We have studied the escape-swimming behavior and neurophysiology ofTritonia hombergi and compared it with another species,Tritonia diomedia in an effort to determine some of the neurophysiological correlates of the modifications in the behavior brought upon by evolution. The general form of the responses in the two species is similar. However, there are well-defined differences in the facility with which the response can be elicited, and in the parameters of response duration over repeated trials (Fig. 2).T. hombergi has many neurons that are homologous in terms of relative size, pigmentation, location and function when compared toT. diomedia (Fig. 3). We attribute the reduced probability of initiation of escape-swimming inT. hombergi to specific alterations in the condition of the central neurons (TGN's) responsible for triggering it. It was found that; (i) these neurons produce weaker responses to appropriate stimuli (Fig. 4), (ii) there are substantially fewer such neurons and, (iii) there are neurons on the margins of the group with reduced regenerative firing capabilities (Fig. 5). Since the TGN's are electrically coupled to these latter neurons, it is suggested that the regenerative excitability of the TGN system as a whole is reduced by the loading effect of these less excitable neurons. It is proposed that the TGN system and consequently the escape-swimming response ofT. hombergi are in a transitional state in evolutionary terms. The small size of the TGN group and its interactions with nearby neurons are consistent with the idea that it is either recruiting or losing efficacy as the adaptive value of the behavior changes.