2.1 Zebrafish husbandry
Adult (6-month-old) AB zebrafish (Danio rerio) were obtained from the Institute of Hydrobiology, Chinese Academy of Sciences. Before the test, the subjects were housed in groups of 100 to 120 fish/100 L aquarium at 22-26 °C with constant filtration and aeration of water. The aquarium was kept under a light/dark cycle of a 10:14 h ratio. The subjects were fed three times per day (Shelled brine shrimp eggs: ≥60% protein, ≥22% fat, ≤3% crude fiber, ≤8% moisture content), except for the 24 h before the commencement, and duration of the experiment. For the experiment, only healthy zebrafish with intact body shape were selected. The test time was from 8:00 a.m. to 8:00 p.m. every day. Test operation standards were in accordance with GB/T13267-91 Water quality—Determination of the acute toxicity of substance to a freshwater fish (Brachydanio rerio Hamilton-Buchanan).
2.2 Experimental apparatus
The device setup consisted of a 510×100×420 mm transparent test box (material: polymethyl methacrylate; Shenzhen, China), a pair of JOYO AC-20 (Shenzhen, China), sound power 40 W, and RS -217-4 mm high-definition CCD camera composition. Before starting the experiment, six zebrafish were put into the test chamber with a 1 L/min continuous flow rate, DO > 6 mg/L, residual chlorine < 0.03 mg/L, and the temperature at 22-26 ℃. A high-definition CCD camera connected to a computer (Lenovo, China) was used to capture the behavioral trajectories of zebrafish in the test chamber, and an EthoVision XT 15 (Nodas, Netherlands) was used for online analysis. The measurement point was the center of the test chamber’s front surface (Fig.1). For the acoustic stimulation, an audio signal program (SweepGenTM, China) was used to adjust the sound’s frequency, and then the sound pressure level (SPL) was adjusted by a Bada DC-811 power amplifier.
2.3 Measurement intervals and cadmium concentrations
Swimming activity is defined as the distance the object's center point moved per unit time, and is recognized as an effective index to evaluate the behavioral responses of zebrafish[12][23]. In this study, the swimming activity was calculated every 0.1 s to evaluate the changes caused by Cd exposure, and a total of 240 min of swimming behavior was recorded. For the first 60 min, pure water was delivered, then a Cd solution was introduced into the chamber. Based on our previous research, the 24 h median lethal concentration (LC50-24 h) of Cd to zebrafish was 7 mg/L[20]. Five concentration gradients (0.1 TU, 0.25 TU, 0.5 TU, 1 TU, and 2 TU) were set up to evaluate the effect of Cd on the swimming behavior of zebrafish. The average and standard deviation of swimming activities were calculated every 10 min.
2.4 Acoustic Stimulus thresholds and durations
Stimulus initial parameters were set to Sine Waveform, Log Sweep Mode, and Fast (smooth) Sweep Speed. The stimulus frequencies were 50, 100, 200, 400, 800 and 1000 Hz, and the acoustic behavioral response corresponding to each frequency was measured. The SPL range was determined as 75 dB to 115 dB— with its lower end limited by noise in the test environment, and its upper end by the available placement positions of the JOYO AC-20 loudspeaker. The SPL intervals were set at every 5 dB (HT-825 decibel meter - precision type). The SPL was the sensitivity threshold of the corresponding frequency until the increased rate of swimming activity was greater than 50% after stimulation. To test repeatability, the tests at all SPL and frequencies were measured in three separate trials. In order to reflect the sensitivity of zebrafish to the acoustic stimulus, the average rate of swimming activity (that is, the ratio of the increase in swimming activity in the acoustic stimulus within 1 min to 1 min before the stimulus) was used to record the experimental results.
To determine the stimulus duration and intervals, long-duration stimuli (1 min) at 100 Hz and 97 dB were delivered to the test chamber. The time required for the continuous increase of swimming activity (Sampling rate: 10 samples per second) was analyzed to determine the duration of stimulus; the time required for swimming activity (Sampling rate: 1 per minute) to return to the pre-stimulation level was analyzed to determine the interval of two stimuli. Finally, five concentration gradients (0.1 TU, 0.25 TU, 0.5 TU, 1 TU, 2 TU) were set up to evaluate the effects of Cd exposure on acoustic startle response.
2.5 Data analysis
EthoVision XT 15 (an automated video tracking and motion analysis system; Noldus, Netherlands) was used to smooth the pixels of the captured video[1] and to correct for frame drops[2]. EthoVision XT 15 video pixel acquisition rate is 10 fps. If the video frame rate is too high, the amount of data will be too vast for efficient data processing. Therefore, the original data was processed by MS-Excel with an average value per minute, and then the data were plotted by Origin (Origin 2022, USA). All experimental data were analyzed by one-way analysis of variance (ANOVA), followed by post hoc comparisons between the experimental groups. Significance was set at p<0.05. Data are presented as mean ± SEM.
[1] The noise will make the neighboring pixels’ gray scale values differ greatly from the actual value; EthoVision XT 15 can smooth the video pixels, thus reducing the influence of fine-grained noise on detection.