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

Synopsis 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.

Determining the safety range of electrical pulses for two benthic invertebrates: brown shrimp (Crangon crangon L.) and ragworm (Alitta virens S.)

Pulse trawling is currently the most promising alternative for conventional beam trawls targeting sole and shrimp, meeting both the fisher's aspirations and the need for more environmentally friendly fishing techniques. Before electrotrawling can be further developed and implemented on a wider scale, however, more information is needed about the effects of electrical pulses on marine organisms. The organisms used in the present experiments were brown shrimp (Crangon crangon L.) and king ragworm (Alita virens S.) as model species for crustaceans and polychaetes, respectively. These animals were exposed to a homogeneously distributed electrical field with varying values of the following parameters: frequency (5–200 Hz), electrical field strength (150–200 V m−1), pulse polarity, pulse shape, pulse duration (0.25–1 ms), and exposure time (1–5 s). The goal of this study was to determine the range of safe pulses and thereby also to evaluate the effect of the pulses already being used on commercial electrotrawls. Behaviour during and shortly after exposure, 14-d mortality rates, and gross and histological examination were used to evaluate possible effects. The vast majority of shrimp demonstrated a tail flip response when exposed to electric pulses depending on the frequency, whereas ragworm demonstrated a squirming reaction, independent of the frequency. No significant increase in mortality or injuries was encountered for either species within the range of pulse parameters tested. Examination of the hepatopancreas of shrimp exposed to 200 V m−1 revealed a significantly higher severity of an intranuclear baculoform virus infection. These data reveal a lack of irreversible lesions in ragworm and shrimp as a direct consequence of exposure to electric pulses administered in the laboratory. Despite these promising results, other indirect effects cannot be ruled out and further research hence is warranted.

Sensory Activation and Receptive Field Organization of the Lateral Giant Escape Neurons in Crayfish

Crayfish ( Procambarus clarkii ) have bilateral pairs of giant interneurons that control rapid escape movements in response to predatory threats. The medial giant neurons (MGs) can be made to fire an action potential by visual or tactile stimuli directed to the front of the animal and this leads to an escape tail-flip that thrusts the animal directly backward. The lateral giant neurons (LGs) can be made to fire an action potential by strong tactile stimuli directed to the rear of the animal, and this produces flexions of the abdomen that propel the crayfish upward and forward. These observations have led to the notion that the receptive fields of the giant neurons are locally restricted and do not overlap with each other. Using extra- and intracellular electrophysiology in whole animal preparations of juvenile crayfish, we found that the receptive fields of the LGs are far more extensive than previously assumed. The LGs receive excitatory inputs from descending interneurons originating in the brain; these interneurons can be activated by stimulation of the antenna II nerve or the protocerebral tract. In our experiments, descending inputs alone could not cause action potentials in the LGs, but when paired with excitatory postsynaptic potentials elicited by stimulation of tail afferents, the inputs summed to yield firing. Thus the LG escape neurons integrate sensory information received through both rostral and caudal receptive fields, and excitatory inputs that are activated rostrally can bring the LGs' membrane potential closer to threshold. This enhances the animal's sensitivity to an approaching predator, a finding that may generalize to other species with similarly organized escape systems.

Mechanisms of Serotonergic Facilitation of a Command Neuron

The lateral giant (LG) command neuron of crayfish responds to an attack directed at the abdomen by triggering a single highly stereotyped escape tail flip. Experimentally applied serotonin (5-hydroxytrptamine, 5-HT) can increase or decrease LG's excitability, depending on the concentration, rate, and duration of 5-HT application. Here we describe three physiological mechanisms that mediate serotonergic facilitation of LG. Two processes strengthen electrical coupling between the primary mechanosensory afferent neurons and LG: first, an early increase in the conductance of electrical synapses between primary afferent neurons and LG dendrites and second, an early increase in the membrane resistance of LG dendrites. The increased coupling facilitates LG's synaptic response and it promotes recruitment of weakly excited afferent neurons to contribute to the response. Third, a delayed increase in the membrane resistance of proximal regions of LG increases the cell's input resistance near the initial segment. Together these mechanisms contribute to serotonergic facilitation of LG's response.