Bony fishesOsteichthyes

A bony fish does not usually bite its food. It inhales it.

In nearly every fish on the pages below this one, the upper jaw is not bolted to the skull. The premaxilla at the front of it rides on ligaments, so when the mouth opens the whole upper lip slides forward and the head briefly grows a short tube aimed at whatever is in front of it. At the same moment the floor of the mouth drops and the gill covers flare, the space inside the head expands faster than water can get out of the way, and the prey arrives on the inrush. The fish never crosses the last centimeter; the water does. That sliding jaw is the change most often credited with what followed, because it can be rebuilt into a beak, a scoop or a vacuum tube without altering anything else, and the group that has it holds roughly half of all vertebrate species alive.

The gill covers doing half of that work are the second piece. Each is a hinged bony plate with a skirt of membrane under it, and with the mouth it makes a two-stroke pump: the mouth draws water in, the opercular chamber pulls it through the gills and out the back, the strokes overlap, and the flow never stops or reverses. What the gill does with the water belongs to the page above. What the pump buys is stillness: a fish with an operculum breathes while wedged in a crack, buried in sand, or asleep under a ledge, and most of the animals in this catalogue live inside that permission.

Then there is the bag of gas. The swim bladder began as an outgrowth of the gut: filled to the right volume it makes the fish weigh nothing at that depth, so holding position costs nothing at all. Filling it against pressure is the hard part, and a gland backed with a knot of capillaries does it, forcing gas out of the blood. The bill comes due in the vertical: a fish weightless at thirty meters is not weightless at ten, and one hauled up fast arrives with its stomach pushed out of its mouth.

They also carry stones in their heads, three pairs in the inner ear, growing a layer at a time, thick in fed months and thin in lean ones. Slice the largest, count the rings, and you have the fish's age; read their chemistry and you have the water it grew up in. Almost everything known about how long reef fish live came out of those stones.