Introduction
Litopenaeus vannamei is one o f the most bred crustaceans worldwide. In farms feed is, by far, the main expenditure. Thus, the objective of feed and premix manufacturers is to find affordable but attractive formula. Currently the methods for measuring the attractivity of a feed are poorly precise and reproductible. One option is to conceive a device and a procedure that will be able to track shrimp during a feeding process and evaluate their behaviour in relation with pellet attractivity. Light is an important environmental factor for animals living in water. It has significant effects on behaviour, fee d intake and growth of aquatic animals
. Light can also be a source of stress that can bias behavioural experiments. Thus, to determine which colour and intensity of light cause less stress for this species, three experiments were conducted . The first experiment aimed to find the less stressful colour. The second one aimed to find the less stressful light intensity. The last experiment aimed to determine which colours L. vannamei is able to perceive.
General protocol
For these three experiments, L. vannamei shrimp were purchased from a local Bulgarian farm and brought to the station Halieutica (France) during a 24h journey.
Experiment 1 – colour preferences. Protocol
Thirty PL20 L. vannamei shrimp were acclimatised for four weeks. After acclimatisation, six waiting tanks of 10 L with five shrimp were prepared .
Shrimp were tested in a random order in a closed structure and were enlightened and filmed once with each following colour: red, orange, yellow, green, blue, or purple for 15 min. Following
), four motion behaviours were recorded to assess stress: swimming, freezing, time spent along the walls and time spent in the centre of the tank. In addition, the number of loops, tail flips, and eye beats were counted, as they a re considered as stress markers in shrimp
. Median speed was also analysed. We hypothesise that non-stressed shrimp will slowly explore their environment, leading to a low speed.
Experiment 1 – colour preferences. Results
Median speed and the number of loops and tail flips were significantly lower for red and orange light (p<0.05) suggesting that these colours are less stressful. We decided to choose orange for Experiment 2.
Experiment 2 – light intensity preferences. Protocol
Thirty PL20 L. vannamei shrimp were acclimatis ed for twelve weeks. Procedure and tested behaviour were the same as in Experiment 1.
For this experiment, shrimp were enlightened and filmed once with every following orange light intensity: 17, 64, 170 or 430 Lux for 15 min.
Experiment 2 – light intensity preferences. Results
No effect of the light intensity was shown on any stress markers , meaning that orange is not stressful for shrimp, whatever the light intensity. One possible explanation is that shrimp do not perceive this colour. Thus, we studied L. vannamei’s colour vision in a third experiment.
Experiment 3 – L. vannamei’s colour vision. Protocol
Ten PL20 L. vannamei shrimp were acclimatised for twenty-two weeks.
Shrimp were individually tested in an optomotor device. This device consists of a rotating cylinder (6 rpm) , which inside walls were covered with a filter made of vertical stripes (6,28 cm) of grey, and one of the following colours: red, orange, yellow, green, blue, purple, and white (positive control). We also used a negative control (uniform grey) . The shrimp was placed on a still platform in the middle of the cylinder. Each shrimp was tested once for each colour. The device turned 1min clockwise, and 1min counter clockwise , the percentage of time the shrimp spent turning in the same direction and at the same speed as the device was measured, this is called the optomotor response . We considered that the shrimp perceived the colour (and hence discriminated the stripes) when the percentages of rotation in the same direction as the cylinder was at least superior to 50%.
Experiment 3 – L. vannamei ’s colour vision. Results
S hrimp showed an optomotor response to the green, blue, purple striped filters and to the positive control. Thus, L. vannamei might perceive green, blue, and purple, and not red, orange, and yellow.
Discussion
This study underlines that the less stressful colours for L. vannamei are red and orange, probably because they cannot perceive these colours. These results are in accordance with other studies held in crustaceans (crabs and crayfish mainly)
. High intensity orange light can thus be used in a tracking device, as it will not stress shrimp and allow a good recognition of the animals by the tracking software . More generally, these colour and light intensity can be used for any experiment led in this shrimp species .
Bibliography