Hopkins Marine Station Student Paper

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Title: Changes associated with the development of swimming activity in larval Pacific bonito: behavior, anatomy of motoneurons and muscle activation
Student Author(s): Hsu, Elizabeth
Faculty Advisor(s): Thompson, Stuart
Pages: 36
Location: Final Papers Biology 175H
Date: June 1998
Abstract: As the first step in investigating the Pacific bonito (Sarda chiliensis) as a model system for studying the acquisition of swimming behavior and underlying neural activity in teleost fish, we studied the changes in the swimming behavior, anatomy of motoneurons, and muscle activation throughout the larval developmental period. The larval period lasts just 12 days, during which there is rapid development of locomotion. At 12 days post fertilization (dpf), bonito undergo a rapid metamorphosis to the juvenile stage. The short, 12-day developmental period suggests rapid development and organization of the motoneuron and motor system. Swimming behavior was videotaped for five fish in each day, and analyzed by determining a time budget for active time and determining maximum body flexion exhibited by each fish. Motoneurons were fluorescently labeled with DiI and muscle activity was recorded using electromyography. Locomotion is initially produced using the trunk and develops to include use of the full tail and tip of the tail. Early in development, larval bonito spend the majority of their time inactive but sustained swimming periods become more common as the fish develops. Maximum body flexion did not appear to change with development. The number of motoneurons increased from less than 10 to over 30 cell bodies per segment. We also observed two types of motoneurons which differed in morphology. Cell bodies also appeared to develop in tiers until they were of the same size. Motoneuron development first took place in more rostral segments of the fish with the number and complexity of the motoneurons decreasing toward caudal segments. Signals obtained from EMG recordings appeared to correspond to the basic swimming modes of rapid bursts and longer duration swimming using tail beats. We cannot draw a direct correlation between swimming behavior, motoneuron anatomy, and muscle activity based on the data obtained thus far, however, we have shown that development of bonito swimming behavior is associated with rapid development of the motoneuron system and muscle activity which correlates to swimming mode.