Date of Award

Fall 1997

Project Type

Dissertation

Program or Major

Zoology

Degree Name

Doctor of Philosophy

First Advisor

Winsor H Watson, III

Abstract

Melibe leonina is a subtidal nudibranch which is found in association with eelgrass and kelp. Melibe swims when disturbed, or knocked off of its substrate, by slow lateral undulations. Swimming is stereotyped in its form and execution, and can be reliably reproduced in the laboratory. Swimming consists of phases: (1) withdrawal, (2) flattening, (3) lateral flexions, (4) unfolding and swinging and (5) termination. Swimming can be reliably elicited using 1M KCl or contact with the tube feet of the predatory seastar Pycnopodia helianthoides. The duration of a single swim cycle and the amplitude of swimming flexions remain relatively constant, however, both the latency to swim, and the swimming duration, are more variable. Swimming moves the animal upward and anteriorly and the direction of travel is fairly predictable from one flexion to the next. Melibe swimming is a stereotyped fixed action pattern that is probably used, in part, to escape from predators.

The brain in Melibe is composed of four bilateral pairs of fused ganglia which surround the esophagus. The neural components for swimming make up a central pattern generator within the brain of Melibe. The nerves which innervate swimming structures arise from the pedal ganglion, and the motorneurons which drive swimming are also located within the pedal ganglion. Intracellular recordings of swimming can be reliably obtained in semi-intact and isolated brain preparations, however only if the pedal commisure is left intact and the ambient lighting is turned down. Melibe swimming is amenable for further neurophysiological studies to more rigorously look at the neural basis of swimming.

Light has a profound effect on the likelihood of swimming in Melibe. The initiation or termination of light cannot start or stop swimming, however, animals locomote and swim significantly more in the dark. This effect was also seen in isolated brain preparations, where fictive swimming was disrupted by, even low level, light. This effect could be reversed, however, if the eyes were removed. Thus Melibe is a model swimming system, that of a lateral bend swimmer, whose behavior is reliably modulated by natural inputs which can be methodically tested in the laboratory.

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