(full paper is archived in the Miller Library)
Title: Changes associated with the development of swimming activity in larval Pacific bonito: behavior, anatomy of mononeurons, and electrophysiology
Student Author(s): Nevin, Linda
Faculty Advisor(s): Thompson, Stuart
Location: Final Papers Biology 175H
Date: June 1998
Abstract: Developmental changes in the swimming behavior and the underlying muscular function and motoneural anatomy in the Pacific Bonito, Sarda chiliensis, were investigated in the course of the first twelve days of post-hatch development of the fish. Quantitative observations of behavior revealed peaks in activity at days 8 and 12 of development. Analysis of meximum body angle while turning showed no trend towards decreasing or increasing flexibility of the fish (f-value=18.4,r^2=.26), even though Bonito use different amounts of flexion while swimming in different stages of development. Motoneurons in the spinal cord from days 2 through 7 are organized into tiers: one tier was seen on days 2 and 3, two tiers on days 5 and 6, and three tiers on day 7. Tier distinction decreases as the fish grow older, and tiers are no longer distinguishable on day 11. Spinal cord cells are also divided into two categories - cells in pattern A tend to be larger and extend their axons ventrally to a fiber tract which travels parallel to the spinal cord and then veers to form the ventral root, while pattern B cells are smaller and extend their axons directly to their ventral root. Finally, neuritic connections between cells were seen in early days and multiplied with age. In later fish, extensive swellings were seen in the ventral roots and neurites of the spinal cord. Electrophysiology studies revealed that on day 7, the motor units of the fish began to fire in sustained low frequency series' reminiscent of the tail beat in continuous swimming. Also on day 7, variable pattern of firing was observed for the first time. These data correlate acquisition of swimming behavior with increased complexity of motoneurons and greater sophistication of muscle activity. Understanding Bonito's striking development is a step towards developing a well-characterized model system for the study of the acquisition of swimming in teleosts.