Neural control of behavioural choice in juvenile crayfish

Abstract

Natural selection leads to behavioural choices that increase the animal's fitness. The neuronal mechanisms underlying behavioural choice are still elusive and empirical evidence connecting neural circuit activation to adaptive behavioural output is sparse. We exposed foraging juvenile crayfish to approaching shadows of different velocities and found that slow-moving shadows predominantly activated a pair of giant interneurons, which mediate tail-flips that thrust the animals backwards and away from the approaching threat. Tail-flips also moved the animals farther away from an expected food source, and crayfish defaulted to freezing behaviour when faced with fast-approaching shadows. Under these conditions, tail-flipping, an ineffective and costly escape strategy was suppressed in favour of freezing, a more beneficial choice. The decision to freeze also dominated in the presence of a more desirable resource; however, the increased incentive was less effective in suppressing tail-flipping when paired with slow-moving visual stimuli that reliably evoked tail-flips in most animals. Together this suggests that crayfish make value-based decisions by weighing the costs and benefits of different behavioural options, and they select adaptive behavioural output based on the activation patterns of identifiable neural circuits.

Figure 1.

Experimental set-up and signal recordings. (a) Top view of the experimental tank. Water containing the food odour flows into a tunnel on the left side and exits on the right. Animals enter the tunnel from the start compartment and approach the food odour release point. A pair of bath electrodes is attached to the tunnel walls, 8 cm from the tunnel entrance and 17.5 cm from the end. Shadows (indicated as the grey shaded area) always move from left to right over the tank. Photodiodes are placed on the front of the tank to measure shadow velocity and position. (b) Side view of the set-up. Animals inside the tank are filmed with a camera positioned above the tank. The camera is connected to a TV monitor. Bath electrodes and photodiodes are connected to an amplifier and digitizer and recorded signals are stored on a computer. The shadow is produced by moving a plastic rectangle through a light beam directed onto the front of the tank. The tank wall facing the light is covered with a white translucent paper. (c) Example of recorded signals from bath electrodes and photodiodes for a shadow moving at 2.5 m s⁻¹. (i) Recording traces of two photodiodes spaced 175 mm apart. The first photodiode (PD no. 1) was placed at the front edge of the tank and recorded the shadow when it first became visible. The second photodiode (PD no. 2) was placed at the position of the bath electrodes in the tank, i.e. the position of the animal when the shadow was released. Response latency was measured between the peak response of PD no. 1 and the beginning of the field potential that was generated by the tail-flip response and recorded by the bath electrodes (BE). (ii) Traces from PD no. 2 and BE at higher temporal resolution. The animal initiated a tail-flip response (arrow) 4 ms before the shadow produced the peak response in PD no. 2. The first small deflection in the BE trace is owing to the activation of the MG neurons (arrow), while the large phasic potential and the smaller more erratic potentials that follow are owing to muscular activity during tail-flips.

Figure 2.

Percentage of behaviours displayed in response to shadows of different velocities. Crayfish produce only one of two discrete defensive responses when exposed to approaching shadows: tail-flipping, which is always mediated by activity in the medial giant (MG) interneurons or freezing behaviour. Tail-flipping behaviour decreases and freezing behaviour increases as shadow velocities increase. The measured differences in displayed behavioural patterns are statistically significant. ** = p ≤ 0.01. Grey bars, freezing; black bars, tail-flipping.

Figure 3.

Latencies to initiate tail-flips in response to shadows of different velocities. Latencies are measured between the time when shadows first became visible and the time when the animals activated the MG neurons to produce a tail-flip. Response latencies are longer for slower shadows than faster shadows. The measured differences in response latencies are statistically significant. ** = p ≤ 0.01.

Figure 4.

Consequences of tail-flipping and freezing. (a) Approach times after tail-flipping and freezing measured between the time of contact with the presented shadow and the eventual time of arrival at the food odour release point. The measured differences in approach times are statistically significant. ** = p ≤ 0.01. (b) Recovery time measured between the time of contact with the presented shadow and the time when foraging activity resumed. The measured differences in recovery times are statistically significant. ** = p ≤ 0.01.

Figure 5.

Effect of food odour concentration on behavioural output. (a) Percentage of tail-flipping and freezing responses to shadows of high contrast that approached with a velocity of 2 m s⁻¹ and low or high (10×) food odour concentration in the tank. The measured differences in behaviour are statistically significant. * = p ≤ 0.05. (b) Percentage of tail-flipping and freezing responses to shadows of high contrast that approached with a velocity of 1 m s⁻¹ and low or high (10×) food odour concentration in the tank. The measured differences in behaviour are not statistically significant. (a,b) Grey bars, freezing; black bars, tail-flipping.