Dose-dependent effects of developmental mercury exposure on C-start escape responses of larval zebrafish Danio rerio

ABSTRACT

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; V_max] 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 V_max 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 V_max levels for longer intervals during the testing period than those exposed as embryos to either 50 or 75 ppb Hg²⁺.