As part of the in vivo microscopic observation of the moulting stages of Penaeus vannamei, it is planned to develop a method for conferring a tranquillizing effect on handled shrimps. This could facilitate handling by temporarily slowing their activity. The study focuses on the tranquillizing potential of a sudden and short-term transfer of Penaeus vannamei shrimp to low temperatures. The objective is to identify the ideal temperature range and acclimatization time that would slow down shrimp activity in the short term, while ensuring that the process does not affect the moulting cycle in the long term.
The study is divided into three trials. First, the effect of temperature on shrimp behaviour was highlighted to identify an ideal range. 20 shrimp were used. Each day, a temperature range (18-20°C, 20-22°C, 22-24°C, 24-26°C, 28-30°C) was tested on each batch of 5 shrimp in a test tank. The following parameters were measured during 5 minutes : average swimming speed, proportion of time spent in motion, travelled distance, proportion of pool explored and amount of food consumed during the test. Secondly, the effect of the acclimatization time was tested and the definition of a reliable criterion to confirm the tranquillization of the shrimps was carried out. Three temperature ranges were tested (18-20°C, 23-25°C and 28-30°C), for four acclimatization times (20, 40, 60 and 90 minutes). Five shrimp were used. At the end of acclimatization, shrimp were held by hand in the open air for two minutes. The time elapsed before the first tail beat and the number of tail beats were evaluated. The third trial is currently underway. It consists in evaluating the potential impact of the tested method on the moulting cycle of Penaeus vannamei by dividing 12 shrimps into 3 batches : no handling, handling twice a day (protocol of the second test) and handling twice a day with exposure to low temperatures. Each day, moults are counted to assess the duration of moulting cycles. Growth of each batch is assessed as well as individual feed consumption. A rotation of the batches will allow 36 repetitions. The analysis of the videos was performed via AnimalTA software and the statistical analysis of the data via RStudio.
Regarding the first test, there are significant differences (p < 0.05) for each parameter studied between the two extreme temperature ranges (18-20°C and 28-30°C). This validates a calming effect of low temperatures on the behavior of Penaeus vannamei. For the second test, we observed a significant difference (p < 0.05) between the 18-20°C and 28-30°C ranges for the acclimatization time of 60 minutes for the two parameters observed (time before the first tail beat and number of tail beats). This combination of modalities would then be the best among those tested and would allow a significant decrease in the activity of shrimps handled during microscopic observation of moult stages. The combination of the temperature range 18-20°C with the acclimatization time of 20 minutes also seems interesting in that it allows a great reduction of the acclimatization time. The implementation of the method of tranquillization at low temperatures will thus be possible if the results of the last test allow us to affirm that it has no long-term effect on the moulting cycle of Penaeus vannamei.