FishesPisces

Fish is not one group of animals, and it never has been. The split between the sharks and rays on one side and the bony fishes on the other happened before there were trees on land, and it is older and deeper than the split between you and a bird. Everything in this part of the catalogue, from a goby shorter than a fingernail to a whale shark, sits somewhere inside that gap.

What they do share is the hard problem: pulling oxygen out of water. A given volume of water holds a small fraction of the oxygen the same volume of air does, and it is heavy and slow to move. The solution is to run the water one way across the gill filaments and the blood the other way through them. Because the two flows oppose, blood meets water that is always slightly richer in oxygen than the blood is, the whole length of the exchange, and a fish strips far more oxygen out of a mouthful of water than your lungs take from a breath of air. Lungs work by tidal in and out, and leave most of it behind.

They also have a sense you do not. A line of pores runs down each flank and branches over the head, opening onto hair cells that read water movement. It amounts to touch at a distance: the pressure wave bouncing off a wall, the wake of something approaching from behind, the exact position of the fish alongside. It is what lets a school of hundreds turn as one body with no leader and no lag, and it works in total darkness.

Nearly all of them run at the temperature of the water around them, which in a sea this warm means animals that are fast and hungry all year. The 🌊 Red Sea holds well over a thousand species, and the striking thing about the list is how much of it is local: a large share of the reef fishes here live nowhere else, the legacy of a basin that has been repeatedly cut off at its southern end during ice ages and then refilled.