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Why These Fish Bully Other Fish–Into Breathing Air

[Two Clarias gariepinus catfish caught near Bogor, West Java in Indonesia. Image by W.A. Djatmiko via Wikimedia Commons and CC-BY-SA-3.0] 

There’s one in every school. These aggressive individuals go around pushing and biting their neighbors, goading them into risky behaviors–like breathing.

Several species of catfish, including the African sharptooth catfish (Clarias gariepinus), have both gills that take in oxygen from water and an organ called the “labyrinth” or “dendritic organ” that allows the fish to “breathe” air.

These catfish evolved to survive in warm, humid rivers–where oxygen levels in the water can drop dangerously low at night.  So while they breathe through their gills most of the time, the fish also have the option of swimming to the surface and taking a big gulp of air.

But, oddly enough, low oxygen levels don’t fully explain catfish’s breathing choices. For one thing, they tend to surface in groups.

Behavioral ecologist Shaun Killen of the University of Glasgow  wondered whether something in the social lives of catfish might be driving them to the river surface, so he teamed with catfish researchers at the Federal University of Sāo Carlos in Brazil to find out more.  

In the experiment, the scientists assigned the catfish to groups of four per tank and counted the number of air-breaths. They repeated the test at several different dissolved oxygen levels and then compared the group counts to how often the fish air-breathed while alone.

They found that fish breathed air more frequently when other fish are around, even when there was plenty of oxygen in the water.

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