Being bullied reduces brain cell birth in harried weakly electric fish

Abstract

South American knifefish are remarkably adaptable. While the loss of a tail would be a major concern for most fish, some knifefish can replace the lost appendage entirely in only 3 weeks – which is just as well in an environment where hungry catfish continually nibble at them. Despite the knifefish's extraordinary ability to recover from the most extreme injury, Kent Dunlap from Trinity College, USA, noticed that Brachyhypopomus occidentalis survivors that had lost their tails naturally to predators in the streams of Panama also had lower rates of cell birth in the part of the brain that controls responses to predators: the telencephalic region. Could this brain deficit be increasing the fish's vulnerability to a catfish attack, raising the number of wounded fish in the predator's vicinity? Or had living in the constant shadow of a predator affected cell birth in the region of the brain that could protect the fish from assault? And, if catfish attacks were the cause of the reduced brain growth, were the knifefish investing more of their resources in tail regrowth at the expense of their own brains?Intrigued by the possibility that living under the constant threat of predation was affecting the brains of knifefish, Dunlap and his colleagues decided to simulate the impact of the catfish's attentions in the laboratory. Dunlap admits, ‘Coming up with an experimental “chase” protocol for the lab that might mimic the experience of living among dense predators was hard’; it was not clear how closely tapping the fish lightly on the tail and amputating the rear portion of the body under anaesthetic reproduces the experience of a harried B. gauderio. However, when Vielka Salazar from Cape Breton University, Canada, measured the levels of the stress hormone cortisol in the blood of B. gauderio, it was apparent that losing the tail was stressful, although the fish didn't seem too bothered when they were harassed with a plastic rod as their cortisol levels did not rise.Next, Geoffrey Keane, Elise Lasky and Michael Ragazzi checked the amount of brain cell birth in the fish that had been disturbed with a rod but kept their tails, and found it was reduced to half of that of the undisturbed fish. Living under the constant threat of attack had altered cell birth in the agitated fish's brains. And when the team checked how losing a tail affected brain cell birth after 17 days, it was clear that the fish suffered a similar reduction. In addition, when they tested whether the fish had reduced brain cell birth in favour of regrowing their tails, it was clear that there was no trade-off. ‘The reduction of brain cell proliferation following tail amputation was… greatest even before the fish were adding a lot of tail cells and lower during the period of fastest tail generation’, says Dunlap, adding that brain cell birth plummeted by 80% within 24 h of the loss of the tail, which is before regrowth has began in earnest.So, B. gauderio fish reduce cell birth in the telencephalic region of the brain when they are bullied, and when Dunlap and his team tried to terrorise brown ghost knifefish (Apteronotus leptorhynchus), which are popular in the aquarium trade and more easily available, the fish suffered the same brain impairment. Speculating about possible reasons for the fish to scale back the region of the brain that might provide them with the best protection from lurking predators, Dunlap says, ‘We suspect that fish with a lower cell birth rate might be less exploratory and more timid. This could reduce their opportunity to forage and find mates, but might also reduce the probability of falling victim to a predator’.