Lab Times Summer Read (5) – Feel a Fish and Make it Happy
(July 25th, 2017) Digging deep into our archive, we found quite a few gems from the past, worth a second read. Here's a 2012 Research Letter from our corresponding author, Tocou Peixes, from Portugal.
In Portugal, we have a special relationship with fish. Since Roman times, our ships have navigated the oceans in their pursuit. But did we ever ask how fish feel? Lisbon University’s Marta Soares is now posing some profound questions on the matter: “We know that fish experience pain. Maybe fish have pleasure, too.”
In her latest report, Soares finds that ‘Tactile stimulation lowers stress in fish’. Her conclusion: fish like massages!
Soares sets the scene: “In humans, massage therapy reduces stress and has demonstrable health benefits.” To test her theory, she took a long, hard trip to Australia’s Lizard Island on the Great Barrier Reef. In this tough research environment, she bravely chose to look at stress reduction in the surgeonfish, Ctenochaetus striatus.
These coral reef fish regularly ‘visit’ cleaner fish, Labroides dimidiatus, at ‘cleaning stations’ where they have their parasites removed. Soares refers to this as a quasi-economic relationship between the cleaner fish and its ‘client’. But there’s a paradox in this cleaning mutualism because the cleaner fish would rather eat surgeonfish mucus than remove its parasites. This mucus is costly to produce, resulting in frequent conflicts between cleaners and their clients.
However, the cunning little cleaner fish can influence ‘client decisions’ by physically touching the surgeonfish. Soares equates this ‘tactile stimulation’ to a relaxing fish massage, “Cleaners often straddle the back of their clients and provide a massage with their pelvic and pectoral fins.”
Cleaners reputedly use their massage skills to “build relationships with new clients”, to “reconcile them” after they’ve stolen their client’s mucus and as a “pre-conflict management strategy with predators”.
Why do surgeonfish tolerate mucus theft? Couldn’t they have developed some strategy to deal with the cleaners’ tactics? Unless, that is, they actually like their massage!
Soares’ hypothesis is that fish massaging provides them with “direct benefits” analogous to the health benefits that humans get from physical contact, “Humans go to have massages when we feel sick or just to feel better, so maybe the reasons are basically the same.”
To test her theory, Soares simulated cleaner fish massages on captured surgeonfish and then tested them for signs of stress relief. Handmade, look-alike models of cleaner fish were used. In Figure 1C the model appears to be a converted domestic cleaning sponge on a plastic handle. Stuck on top is a fish-shaped bit of plastic with four tassels dangling off its left side.
The surgeonfish were transferred to individual plastic tanks (60 x 40 cm) with flow-through seawater. The fish were divided into two groups. For the control group (15 surgeonfish), the model stayed absolutely still throughout the experiment. Meanwhile, the treatment group (12 fish) saw their model move. As shown in the video (Supplementary Information), a machine rhythmically moved their model back and forth in a shallow curve along the side of the tank. This is apparently a “relatively realistic movement” for a cleaner fish.
For ten consecutive days, each group had 2 x 1 hour exposure to their respective models. On day ten, Soares filmed the fish and then quantified fish-model interactions from the video.
Her results showed that the fish only interacted with moving models. Basically, they weren’t very interested in the static piece of funny-looking plastic until it started moving!
On day 11, Soares looked to see how ‘relaxed’ the massaged fish had become. To do this, she stressed them again. Her reasoning was that a truly relaxed fish has much lower basal levels of stress hormones, like cortisol. Therefore, if she stressed her ‘massage-relaxed’ fish and measured their subsequent cortisol levels (= stress response), she predicted it would be lower than in the unmassaged, control group.
Half the fish from each group were given an ‘acute’ stress test. This involved sticking them for 30 minutes in a small bucket with little water (15 cm deep). Then she took a blood sample. Sure enough, the fish panicked and their cortisol stress levels tripled! But, says Soares, the cortisol levels were always lower in her massaged fish than the controls, both before and after the confinement stress test.
Therefore, Soares concludes grandly, “Our results show that physical contact alone, without a social aspect, is enough to produce fitness-enhancing benefits, a situation so far only demonstrated in humans.”
But do her results really show this? Look again at that experimental video in Supplementary Information: yes, the real fish does approach the slow-moving dummy but does it actually ‘touch’ it, let alone receive a ‘tactile stimulation’? In fact, the only assessment of physical contact between fish and dummy is visual. There are no electronic detectors on her models to confirm contact. Furthermore, is physical contact with a dummy really the same as ‘tactile stimulation’ by a live fish?
Instead, one could argue, the moving models provide ‘visual entertainment’. The fish have nothing else to look at in the plastic tank. Maybe they get bored by the absence of movement? Note, the fish showed no interest when the models were stationary. It is the fish’s ‘visual’ interest that is aroused. In which case, Soares has confirmed the relaxing benefits of visual entertainment – another clear analogy to humans!