SWANSON, B.O.; GIBB, A.C.; Northern Arizona University; Northern Arizona University: Ontogeny of fast starts: What are the biomechanical consequences of development in teleost fish?

Fast start escape responses are stereotyped behaviors that are used to escape predators throughout the life histories of fishes. Here, we examine the biomechanical basis of the C-start, focusing on ontogenetic changes to the axial muscle and skeleton using data from rainbow trout and razorback sucker escape responses collected across development. We note that the axial skeleton develops from a notochord, which thickens, then segments and ossifies into vertebral centra separated by intervertebral disks, as the fish become juveniles. The increase in notochord radius leads to an increase in stiffness due to the larger second moment of area. These transitions appear to affect the passive stiffness of the system, and length specific curvature during the escape response decreases through ontogeny. The axial muscles also undergo a morphological transformation: the myomeres change from vertical, non-overlapping bands to concentric, overlapping cones. This should allow improved conversion of intramuscular pressure to longitudinal force. We hypothesize that these changes in morphology enable increased energy recovery from elastic recoil during the propulsive phase of the escape response. Interestingly, for fish larvae, performance (e.g. decreased time to maximum velocity) improves as the above changes occur and adult morphology is acquired. However, we also note that the architecture of these locomotor structures changes in concert with allometric scaling of the whole organism. Thus, juvenile escape performance typically decreases with growth, presumably because muscle cross-sectional area (generating force) decreases relative to body mass (which must be accelerated). Therefore, fish performance actually peaks at the larvae to juvenile transition.