Chronic exposure to low concentrations of waterborne cadmium during embryonic and larval development results in the long-term hindrance of antipredator behavior in zebrafish.

Cadmium has been recognized for some time as a potent environmental pollutant with the capability of disrupting olfactory-mediated behaviors. Failing to respond to chemical cues in the environment could adversely affect foraging, reproduction and predator avoidance. Recognizing this impaired perception as a serious ecological problem has been undermined by the fact that the damage is often reversible; short depuration periods of 5 d may allow for the re-establishment of responses to chemical cues. In this experiment, early life stage zebrafish were continuously exposed for 50 d at 0, 0.2, 2.0, and 20 microg Cd/L. The subjects were depurated for 14 d and then subjected to behavioral testing where antipredator responses to chemical alarm cues were observed. Our data show that continuous exposure during rearing to a concentration as low as 20 microg Cd/L is sufficient at eliminating antipredator behavior in zebrafish (Danio rerio) even after the source of the cadmium had been removed for 14 d. Furthermore, subjects raised under a 10-fold lower concentration also showed alteration in their behavioral responses, taking significantly longer to respond to the predation threat. Exposure to low levels of cadmium throughout development may alter neurogenesis, subsequently resulting in long-term impairment of chemical cue perception.

Brief embryonic cadmium exposure induces a stress response and cell death in the developing olfactory system followed by long-term olfactory deficits in juvenile zebrafish.

The toxic effects of cadmium and other metals have been well established. A primary target of these metals is known to be the olfactory system, and fish exposed to a number of different waterborne metals display deficiencies in olfaction. Importantly, exposure over embryonic/larval development periods can cause deficits in chemosensory function in juvenile fish, but the specific cell types affected are unknown. We have previously characterized a transgenic zebrafish strain expressing the green fluorescent protein (eGFP) gene linked to the hsp70 gene promoter, and shown it to be a useful tool for examining cell-specific toxicity in living embryos and larvae. Here we show that the hsp70/eGFP transgene is strongly and specifically upregulated within the olfactory sensory neurons (OSNs) of transgenic zebrafish larvae following a brief 3-h exposure to water-borne cadmium. This molecular response was closely correlated to an endpoint for tissue damage within the olfactory placode, namely cell death. Furthermore, cadmium-induced olfactory cytotoxicity in zebrafish larvae gives rise to more permanent effects. Juvenile zebrafish briefly exposed to cadmium during early larval development display deficits in olfactory-dependent predator avoidance behaviors 4-6 weeks after a return to clean water. Lateral line neuromasts of exposed zebrafish larvae also activate both the endogenous hsp70 gene and the hsp70/eGFP transgene. The data reveal that even a very brief exposure period that gives rise to cell death within the developing olfactory placode results in long-term deficits in olfaction, and that hsp70/eGFP may serve as an effective indicator of sublethal cadmium exposure in sensory cells.

Epidermal 'alarm substance' cells of fishes maintained by non-alarm functions: possible defence against pathogens, parasites and UVB radiation.

Many fishes possess specialized epidermal cells that are ruptured by the teeth of predators, thus reliably indicating the presence of an actively foraging predator. Understanding the evolution of these cells has intrigued evolutionary ecologists because the release of these alarm chemicals is not voluntary. Here, we show that predation pressure does not influence alarm cell production in fishes. Alarm cell production is stimulated by exposure to skin-penetrating pathogens (water moulds: Saprolegnia ferax and Saprolegnia parasitica), skin-penetrating parasites (larval trematodes: Teleorchis sp. and Uvulifer sp.) and correlated with exposure to UV radiation. Suppression of the immune system with environmentally relevant levels of Cd inhibits alarm cell production of fishes challenged with Saprolegnia. These data are the first evidence that alarm substance cells have an immune function against ubiquitous environmental challenges to epidermal integrity. Our results indicate that these specialized cells arose and are maintained by natural selection owing to selfish benefits unrelated to predator-prey interactions. Cell contents released when these cells are damaged in predator attacks have secondarily acquired an ecological role as alarm cues because selection favours receivers to detect and respond adaptively to public information about predation.