Dose-dependent effects of developmental mercury exposure on C-start escape responses of larval zebrafish Danio rerio
Zebrafish Danio rerio embryos were exposed to 0, 25, 50 or 75 ppb Hg2+ from 0 to 24 h post-fertilization (hpf) then placed into Hg2+-free water. Inductively coupled plasma-mass spectrophotometer analysis of whole embryo Hg2+ content at 24 hpf showed a positive correlation with exposure regime (Pearson's one-tailed, r2= 0·698, P < 0·01); at 5 days post-hatch (dph), whole larval Hg2+ content was not detectable. Hg2+-induced behavioural deficits in larvae were, therefore, due to changes during embryogenesis and not to residual Hg2+ in the larvae. At 5 dph, larvae were tested for responses to different frequencies but equal intensities of vibrational stimuli generated by a remotely controlled plastic hammer. Data were recorded by high-speed videography and computer-analysed for latency of response (ms), amplitude of the response as measured by maximum initial velocity [normalized as body (standard) lengths s−1; Vmax] and duration of behaviour from initial head movement to cessation of caudal tail movement (ms). A single mechanical stimulus resulted in behavioural outcomes that were related to embryonic Hg2+ uptake. Response latency increased with exposure level and displayed an increase of ×1·5–2·5 over control values (ANOVA, P < 0·01). The Vmax decreased with exposure level to a low of 71% of control at the highest Hg2+ concentration (ANOVA, P < 0·01). Duration of behaviour displayed a biphasic response pattern in which exposure to 0, 50 or 75 ppb Hg2+ did not result in a significantly different response yet exposure to 25 ppb Hg2+ caused a significantly longer time of active response (ANOVA, P < 0·01). Repeated stimulation (1, 2 or 4 hits s−1) resulted in a concentration-dependent increase in response failures. Regardless of stimulation frequency, larvae exposed to 0 or 25 ppb Hg2+ as embryos maintained higher Vmax levels for longer intervals during the testing period than those exposed as embryos to either 50 or 75 ppb Hg2+.
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