Glinting beneath tropical balmy waters, the Great Barrier Reef is one of the brightest shows on earth. With shimmering shoals of iridescent fish ceaselessly darting between glimmering coral edifices, the spectacle is enchanting. ‘The complexity of interactions between animals out on the reef is really amazing’, says Lauren Nadler from James Cook University, Australia, who is intrigued by the factors that drive fish to form shoals. ‘I had heard whispers and read some anecdotal evidence on this idea of a calming effect of living in a group. The rationale is that group-living may make individuals less fearful of threats, due to having “many eyes” to look for predators. This reduction in stress may allow fish to reduce their overall basic energetic needs’, says Nadler. However, no one had succeeded in measuring the metabolic rates of individual fish in the midst of a shoal. ‘There have been studies that looked at how the group as a whole is changing in terms of their overall energy demands, but they have not been able to get at that individual change’, says Nadler.
After discussing the challenge of how to simulate the conditions experienced by individual damselfish without letting them directly interact with the other members of the shoal with Philip Munday, Mark McCormick and Shaun Killen, Nadler began developing a new respirometry approach. She started by placing a fish in a glass tube – which allowed her to measure their oxygen consumption – while immersing it in a tank surrounded by shoal-mates. The isolated fish remained calm so long as it could see and smell its chums; however, the fish's serenity vanished as soon the shoal-mates swam away. ‘When the fish in the chamber couldn't follow, it would thrash around and make its metabolic rate increase’, she recalls. The next time, she introduced a second chamber to ensure that the shoal remained in close proximity, allowing her to measure the oxygen consumption of an individual fish while surrounded by its shoal.
Having refined the details of the technique, Nadler tested the method on damselfish collected at the Lizard Island Research Station on the Barrier Reef. ‘One of the things I wanted to do was make sure I was using fish that had already been living together in shoals, to ensure that I wasn't freaking them out by exposing them to strangers as opposed to friends’, says Nadler. After using a barrier net to herd shoals together with Eva McClure and transporting the fish back to the lab, Nadler measured the metabolic rates of individuals in isolation and when surrounded by their shoal-mates, and saw an extraordinary 26% drop in metabolic rate, ‘which is really quite a lot’, she says understatedly. The fish were certainly soothed when surrounded by friends in a shoal. And when Nadler kept individuals in solitary confinement for several weeks, the strain was evident as they initially lost weight. However, after a fortnight, their condition stabilised, and when Nadler checked the metabolic rates of these isolated fish, she found the same impressive drop in metabolic rate when they were reintroduced to their shoal.
Converting the enormous energy savings gained from living in a shoal into practical benefits, Nadler explains, ‘They have extra energy for all sorts of things; they can go out and find mates and reproduce and they have more energy to grow, and these are all processes that are going to help them to survive and do really well out on the reef and pass their genes on to the next generation of fish’.