Emersion and Terrestrial Locomotion of the Northern Snakehead (Channa argus) on Multiple Substrates

Abstract

Most fishes known for terrestrial locomotion are small and/or elongate. Northern snakeheads (Channa argus) are large, air-breathing piscivores anecdotally known for terrestrial behaviors. Our goals were to determine their environmental motivations for emersion, describe their terrestrial kinematics for fish 3.0-70.0 cm and compare kinematics among four substrates. For emersion experiments, C. argus was individually placed into aquatic containers with ramps extending through the surface of the water, and exposed to 15 ecologically-relevant environmental conditions. For kinematic experiments, fish were filmed moving on moist bench liner, grass, artificial turf, and a flat or tilted rubber boat deck. Videos were digitized for analysis in MATLAB and electromyography was used to measure muscular activity. Only the low pH (4.8), high salinity (30 ppt), and high dCO₂ (10% seltzer solution) treatments elicited emersion responses. While extreme, these conditions do occur in some of their native Asian swamps. Northern snakeheads >4.5 cm used a unique form of axial-appendage-based terrestrial locomotion involving cyclic oscillations of the axial body, paired with near-simultaneous movements of both pectoral fins. Individuals ≤3.5 cm used tail-flip jumps to travel on land. Northern snakeheads also moved more quickly on complex, three-dimensional substrates (e.g., grass) than on smooth substrates (e.g., bench liner), and when moving downslope. Release of snakeheads onto land by humans or accidentally by predators may be more common than voluntary emersion, but because northern snakeheads can respire air, it may be necessary to factor in the ability to spread overland into the management of this invasive species.

Fig. 1

Muscle activity pattern during the terrestrial forward crawl of northern snakeheads. (A) Each northern snakehead (n = 7) in the EMG experiments had six intramuscular fine-wire electrodes implanted on each side (blue = left, red = right) in the labeled muscles, as well as a ground electrode near the base of the dorsal fin (snakehead image by Susan Trammell). (B) Since their forward crawl is a cyclical behavior, the muscle activity pattern is shown from 0% to 100% for each stride, with 100% being 0% of the subsequent stride. The thick bars indicate the average start and end times of muscle activation, while the thin bars reflect the standard error (SE). The numbers on the thick bars show the number of strides recorded for each muscle, with two numbers present for the abductors and adductors because these muscles would activate up to two times per stride. Overall, there is a left–right activation pattern of axial muscles, while both pectoral fins are coordinated for each half of a stride. The RA Hyp was chosen as the starting frame of reference as it had the most strides recorded of any muscle. LA Epax, left anterior epaxials; LP Epax, left posterior epaxials; LA Hyp, left anterior hypaxials; LP Hyp, left posterior hypaxials; L Add, left adductor; L Ab, left abductor; RA Epax, right anterior epaxials; RP Epax, right posterior epaxials; RA Hyp, right anterior hypaxials; RP Hyp, right posterior hypaxials; R Add, right adductor; R Ab, right abductor.

Fig. 2

Northern snakehead forward crawl behavior. (A) A northern snakehead begins crawling by (B) moving its head toward the left and swinging the tip of its tail laterally toward the right. (C) The head and tail continue to move as the left pectoral fin begins retracting. (D) The right pectoral fin begins to retract, and the tail tip has achieved its approximate maximum lateral amplitude as it begins swinging toward the body’s midline. (E) Both pectoral fins are fully retracted as the head crosses the midline of the body. The fish rolls along its longitudinal axis partially onto its left pectoral fin, while the posterior axial body appears to push against the substrate. (F) The pectoral fins begin to protract as the tail crosses the body’s midline. (H) The fish rolls along its longitudinal axis toward the right side of its body as the tail tip rapidly increases in amplitude toward the left. (I) Both pectoral fins are fully protracted and the head has reached its maximum amplitude on the left side of the body. (J) The head begins moving back toward the right as the tail tip has reached its maximum amplitude on the left. The pectoral fins begin to retract. (K) The head and tail are both moving toward the right side of the body as both pectoral fins are fully retracted. The fish rolls partially onto its right pectoral fin, as the posterior axial body appears to push against the substrate. (L) The head has approximately reached its maximum amplitude as the fish rolls toward the left side of its body and rapidly increases the tail tip amplitude on the right side of the body. (M) The tail tip has reached its maximum amplitude on the right side of the body and the head begins moving back toward the left side of the body, starting a new stride. The three points tracked—tip of the snout, anterior insertion of the dorsal fin/COM, tip of the tail—are shown by yellow, red, and blue dots, respectively.

Fig. 3

Effects of substrate and length on kinematics and performance. Kinematic and performance parameters of the snakehead forward crawling behavior on each substrate (symbols in panel B) are plotted on the y-axes against length. The regression line includes all substrates, to show the overall effect of length. CC = Curvature Coefficient, DR = Distance Ratio, WA = Wave Amplitude, COM = Center of Mass.

Fig. 4

Juvenile northern snakehead tail-flip jump. (A) A juvenile northern snakehead (bottom right of the panel, ∼3.2 cm in TL) begins a tail-flip by lying on its lateral surface, and then (B) lifts its head above the substrate and bends it towards the tail, making a C-shape with the body. (C) Once the fish is maximally bent, it rapidly pushes against the substrate with its caudal peduncle, (D) launching itself into a ballistic flight path. (E) While the fish is airborne, it can travel several body lengths before landing (F) next to a stationary northern snakehead.