Hierarchical Sensory Guidance of Mauthner-Mediated Escape Responses in Goldfish (<i>Carassius auratus</i>) and Cichlids (<i>Haplochromis burtoni</i>) (Part 2 of 2)

Abstract

Acoustically-evoked escape behaviors were compared between goldfish <i>(Carassius auratus), </i>a hearing specialist, and the cichlid <i>Haplochromis burtoni, </i>a hearing nonspecialist. Fish were startled with compressive and rarefying, stimuli presented alone or together, and with compressive pulses preceded by a visual cue or after exposure to cobalt, an inhibitor of lateral line-innervated neuromast hair cells. These acoustic startle stimuli can evoke Mauthner neuron firing and are similar to but weaker than those produced by a largemouth bass <i>(Micropterus salmoides) </i>feeding on guppies. When sound stimuli were presented alone, both species avoided the direction of either the compressive or rarefying stimulus. If a light preceded and was contralateral to the compressive sound pulse, goldish continued to avoid the sound source; cichlids avoided the visual cue and turned toward the sound. Goldfish performance improved significantly when the visual cue was in the same direction as the sound source. Goldfish performance also improved significantly after exposure to 0.1 mmol I^-1 cobalt solution for 24 hours before testing, but cichlids would not startle after cobalt acclimation. A compressive pulse presented to one side of a fish simultaneously with a rarefying pulse on the other side causes the entire fish to accelerate with the water current. This strongly and directly accelerates the ear but tends to reduce both the pressure changes transduced by the swimbladder and activation of the mechanosensory lateral line. In this test, goldfish reliably avoided the compressive pulse. Cichlids, however, randomly avoided either speaker polarity but significantly avoided the speaker which had a faster onset. With more closely matched speakers, cichlids also preferentially avoided the compressive stimulus. Thus, the primitive sensory condition for auditory activation and guidance of Mauthner-neuron-initiated escape responses may have evolved to detect the initially compressive sounds associated with ram-type predators.