Delayed effects of embryonic exposure of zebrafish (Danio rerio) to methylmercury (MeHg)

Since previous short-term bioassays of methylmercury (MeHg) indicated no mo rphological effects in zebrafish (Danio rerio) after embryonic exposures be low 20 mug/l MeHg, studies were done to determine whether embryonic exposur e to MeHg at lower concentrations would induce behavioral effects. Newly fe rtilized embryos were exposed to 0, 5, 10 or 15 mug MeHg/l for selected exp osure durations: single day, multiple day or continuous exposure from ferti lization through hatching. Larvae were maintained in an essential salt solu tion after hatching. Spontaneous swimming performance and prey capture expe riments were conducted. Continuous embryonic exposure to 15 mug/l caused de layed mortality syndrome (DMS). These larvae hatched normally and appeared normal, but beginning at Day 3 post-hatch (ph), general activity was severe ly reduced and by Day 5 ph, larvae were completely moribund; many had faint heartbeats, severely enlarged body cavities and upward flexures of the spi nal cord. Most of these larvae were dead by Day 6 ph. Multi- and single-day embryonic exposures to 15 mug/l caused reduced swimming activity and prey capture ability, and by Day 4 ph, these larvae also began to show signs of DMS. Continuous embryonic exposure to 10 mug/l significantly reduced sponta neous swimming activity, which did not improve after 5 days in clean water. Similar results were seen in larvae exposed during the last 24 h of embryo nic development. Prey capture ability was also impaired in larvae exposed c ontinuously to 10 mug/l, even after 4 days in clean water. Single-day expos ures to 10 mug/l did not affect prey capture ability. Larvae from the 5-mug /l exposures were not significantly different from controls for either para meter. This study reinforces the idea that functional impairment is a more subtle response to developmental toxicants than mortality or the production of morphological defects. (C) 2001 Elsevier Science B.V. All rights reserv ed.