Findings: Evolution's fishy shortcut

August 1, 2003

A scientist in the US has unravelled one of evolution's maladaptive mysteries, writes Natasha Gilbert.

Philip Stoddard of the department of biological sciences at Florida International University has discovered why the electric knife fish emits an enhanced low-frequency electrical signal when in a stressful environment - in essence offering itself to predators on a plate.

This nocturnal fish uses the signal to communicate with others and to see in the dark. The signal is always present but it follows a 24-hour physiological cycle, increasing at night when needed most and attenuating during the day when the fish is resting. But scientists have been puzzled for years as to why the fish makes itself more conspicuous and uses valuable reserves of energy by increasing this signal when under attack from predators such as the electric eel.

Dr Stoddard said: "This is the worst possible thing to do when stressed."

He studied the regulation system of the signal's rhythm and found that it was connected to changes in the levels of the stress hormone ATCH (adrenocorticotrophic hormone). ATCH levels also fluctuate by night and day and escalate in response to social challenges and other stressors.

According to Dr Stoddard's findings, as ATCH levels start to rise at dusk the electrical signal follows suit. When the fish becomes stressed, the ATCH regulation machinery kicks off, stimulating the signal.

Dr Stoddard said he thought that the ATCH system presented evolution with a convenient solution to the problem of regulating the fish's electrical signal.

He said that evolution took a shortcut by hitching the signal to the hormone's cyclical machinery that was already in place. This achieved the goal in a single mutation rather than multiple mutations.

"While making the fish more obvious to predators is clearly the wrong thing to do, evolution had a good reason to do it," Dr Stoddard said. "We have to assume that the advantage of an increased night-time signal outweighs the cost of being more obvious to predators."

These findings were announced at the American Animal Behaviour Society Meeting.

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