By land or by sea: a modified C-start motor pattern drives the terrestrial tail-flip

Aquatic C-start escape responses in teleost fishes are driven by a well-studied network of reticulospinal neurons that produce a motor pattern of simultaneous contraction of axial muscle on the side of the body opposite the threatening stimulus, bending the fish into the characteristic C shape, followed by a traveling wave of muscle contraction on the contralateral side that moves the fish away from the threat. Superficially, the kinematics of the terrestrial tail-flip resemble the C-start, with the anterior body rolling up and over the tail into a tight C shape, followed by straightening as the fish launches off of the caudal peduncle into ballistic flight. We asked whether similar motor control is used for both behaviors in the amphibious mangrove rivulus, Kryptolebias marmoratus. Fine-wire bipolar electrodes were percutaneously inserted into repeatable paired axial locations in five individual fish. Electromyograms synchronized with high-speed video were made of aquatic C-starts, immediately followed by terrestrial tail-flips. Tail-flips took longer to complete than aquatic escapes; correspondingly, muscles were activated for longer durations on land. In the tail-flip, activity was seen in contralateral posterior axial muscle for an extended period of time during the formation of the C shape, likely to press the caudal peduncle against the ground in preparation for launch. Tail-flips thus appear to be produced by modification of the motor pattern driving the aquatic C-start, with differences consistent with the additional requirement of overcoming gravity.