CADMIUM HITS TROUT IN THE SNOUT

Abstract

When a predator strikes, it's too late for the victim, but not for the rest of the crowd if the victim can sound the alarm in time. Some fish have evolved a chemical alarm system that sends a school fleeing for cover when an attack is launched. But if there are traces of cadmium in the water, the alarm seems to fail, leaving the predator free to feast, which could have significant consequences for fish ecology. Chris Wood was intrigued how cadmium threatens a fish's reaction to the life saving alarm. Working with Graham Scott and Katherine Sloman, he began testing how the metal affected the trout's reaction to the chemical alarm signal, and discovered that it only takes a small dose of the toxic metal to disable the fish's defences by knocking out its sense of smell (p. 1779).But before they could test cadmium's effect on the fish's reactions, the team needed to produce an alarm-essence, extracting the alarm's active component from fish skin. Sure enough, when they released the extract into water, the trout went scurrying for cover. Then they tested how doses of cadmium at similar concentrations to the levels fish might encounter in polluted waters affected the trout's responses. After returning the fish to fresh water they released the alarm, and after seven days of exposure to 2μg cadmium per litre of water the trout stopped reacting to the warning,and became easy pickings for a passing predator.How was the cadmium blunting the fish's reaction? Had it disrupted their fight-or-flight response, or had it hit the fish somewhere else: in the snout?Scott knew that once cadmium got into the fish's system, it could not pass through the blood brain-barrier, either remaining in the fish's olfactory system or in its body. So if fish that had been fed cadmium lost their ability to flee, the metal was disabling their bodies' hormonal flight response. However if fish that had been exposed to cadmium in the water lost the alarm response, the metal must have destroyed their sense of smell. Scott exposed fish to equal doses of cadmium, either in their diet or from the water, and released the skin extract into the water. Again the cadmium-bathed trout lost their ability to flee for safety. The metal had destroyed the trout's sense of smell.But the team needed physiological evidence that the metal was targeting the fish's olfactory system. Working with Claude Roulaeu, Scott and Sloman exposed trout to radioactive cadmium in their water, radiographed the fish's bodies and looked for tissues where the toxin accumulated. The disabled olfactory nerve lit up with high cadmium levels. By contaminating one nerve, the cadmium had destroyed the fish's sense of smell and sole protection from attack.Scott speculates that the cadmium disrupts the fish's reaction to the alarm signal by interfering with the calcium ion channels in the olfactory nerve. But he is also concerned; `what is important is that these concentrations are close to the levels that the environmental regulations thought were safe' says Scott. Even if the metal doesn't affect the fish's health directly, it could have significant consequences for their ecology by robbing them of the chance to protect themselves from attack.