SurgeonfishesAcanthuridae

Algae are a poor living and a hard one to digest. Plant material is bulky, low in energy and locked up in molecules almost no animal makes the enzymes for, so a surgeonfish farms the work out. It crops turf all day, and in most of the family the front of the gut is a thick-walled grinding chamber that works the mouthful over against grit the fish swallows with its food. Behind that sits a fermentation gut full of microbes that do the actual digestion and hand back the products. The fish is a small ruminant with fins.

One of those microbes was found here. In the gut of a brown surgeonfish from the Gulf of Aqaba, biologists described a bacterium so large you can see it without a microscope, bigger than many single-celled animals and thousands of times the volume of an ordinary bacterium. It does not divide in two the way bacteria are supposed to: it grows daughter cells inside itself and releases them alive. For years it was the largest bacterium known to science, and it has never been found anywhere except in the guts of surgeonfishes.

The blade at the base of the tail is where the name and the reputation come from, a scalpel that folds into a groove and swings out sideways when the fish is cornered, and it shapes the family's social arrangements as much as its defense. Some species hold a feeding territory and fight for it. Others have abandoned territories entirely and roam in schools, which is how a crowd of grazers gets into ground a single fish would be driven out of.

Seven species are listed here, and the unicornfishes are surgeonfishes too, listed separately here because they solved the same problem with different equipment. What the family has in common is the gut. The blade, the territory, the schooling and the endless grazing all follow from being an animal that made a living on food nobody else wanted.