If there was a monstrous night-time thunderstorm and the power went out in your home, all during a zombie apocalypse, you would probably freak out at the slightest unusual noise and flee to your panic room. However, if you’d just got back from a relaxing hot stone massage at the spa, had cozied up in a fuzzy bathrobe with a chilled glass of buttery chardonnay and were watching a soothing nature documentary when you heard an unusual noise, you would probably ignore it and attribute it to pipes, the wind or something benign. Your condition can affect how you perceive threats, but is it the same for fish? To figure this out, a team of scientists led by Vinicius Giglio of the Federal University of São Paulo, Brazil, devised a cleverly simple experiment.

Fish do not drink wine to relax and they cannot wear fuzzy bathrobes, but they do go to spas – of sorts. At many coral reefs, there are cleaning stations where little fish – often teeny tiny wrasses with a black stripe – pick parasites and dead skin from the skin of larger fish, sea turtles, or whoever wants to be cleaned. The client gets to feel clean, healthy and smooth while the cleaner gets a delicious snack. Everyone wins, or at least seems to win. But is there a negative tradeoff for the client?

To test this, Giglio had snorkelers go to coral reefs in a marine reserve off the coast of Brazil to chase parrotfish and squirrelfish – some of which were out and about, while others were getting pampered at cleaning stations. The snorkelers tried to see how close they could get to the fish before the animals swam away, recording that distance as the ‘flight initiation distance’. Then, the team compared the behaviors of the two groups to see whether spa treatments affect fish behavior.

It turned out that the fish at cleaning stations would allow snorkelers to approach closer than those just doing average fish stuff. While respectful snorkelers are not likely a serious threat to reef fish, they do simulate predators. Therefore, reef fish at cleaning stations may allow Bruce the shark – of Nemo fame – to get too close for comfort, but why might these fish lower their guard?

Giglio has three potential answers. One is that fish at cleaning stations need to weigh the benefits of being picked clean of parasites against the danger of being eaten; leaving the spa early means missing out on the full treatment, so fish may take a risk and allow predators to get closer before fleeing to get the most out of their cleanup. Another reason could be that cleaning stations may be governed by a peace treaty, which allows them to act as safe havens where fish do not eat other fish, because aggressive behaviors would ruin the spa for everyone, including predatory fish that also wish to use them. The last reason may be that the gentle grooming of cleaners is soothing to reef fish and may lower levels of the stress hormone cortisol; because these fish are less stressed, they may be less likely to perceive a threat as such.

Regardless of the reason, client fish at cleaning stations perceive threats differently. However, feeling safe and being safe are two very different things. Therefore, getting groomed at a fish spa may be a double-edged sword that leaves the client more vulnerable to being eaten. Next time you are relaxing, make sure not to let your guard down, or else Giglio's snorkelers may get the drop on you!