Unlike humans, many fish grab their food by sucking up the water that surrounds it. Once sucked into the fish's mouth, the food then gets swallowed up. What of the water that carried it there? The simplest way for water to exit is across the gills, but if food just followed suit, then fish would be inhaling and spewing it out through their gill slits. Pauline Provini, from Département Adaptations du Vivant, Paris, France, wasn't convinced that this would be the most elegant option and wondered whether fish independently filter their food away from the water. Together with Alexandre Brunet, Andrea Filippo and Sam Van Wassenbergh, they set out to see what goes on inside a fish's mouth while eating.As if it's not challenging enough to see into a fish's mouth, Provini and colleagues needed to see how food and water swirl around inside; it's hard to persuade a fish to say ‘ah’ and take a peek. As the next best thing, the team used fluoroscopy – moving x-rays – to see what happens inside the mouths of two kinds of fish, carp and tilapia, from multiple angles. It's hard to see how the food and water swirl about using fluoroscopy alone, so they used metal markers, which show up really well on x-ray images. First, they implanted fish food pellets with small metal beads. Next, to see the flow of water, they crafted an armada of miniscule floats, by encasing 0.4 mm long metal rods in tiny foam life jackets. These drifted about in the water, waiting to be taken in by the hungry fish when they sucked up their food.The researchers found that the fish used their expandable mouths to create a spout of water, a fast-flowing fluid funnel that shifts water down the centre of the mouth, from the front to the back. Provini and colleagues had wondered before whether this was the case, but they had now seen it first-hand. Pieces of food hitch a ride on this water jet and, instead of being belched out of the fish's gill slits, are taken to the entrance of the oesophagus, the muscular tube that leads to the stomach.But the story doesn't end there. The fish then synchronised the movements of their jaws and throat to waft the food back and forth a little in their mouth – a form of rhythmical gargling. Provini and colleagues suggest that the fish's ability to extend the consideration they gave to their food was a way to sample and sift the tastiest morsels, before rejecting unpalatable scraps.In addition, Provini saw that the tiny floats then swirled around the edges of the fish’s mouths and out of their gill slits, without a single one ever getting swallowed. Astonishingly, the fish managed to avoid swallowing any water by mistake. Far from being messy eaters, it seems that these fish have impeccable and quite particular table manners.
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May 29, 2023
Kathryn Elizabeth Mcgoldrick 
Open Access