“Wild fish did not come into this world just to be our food,” Paul Greenberg writes toward the end of his book Four Fish. “They came into this world to pursue their own individual destinies.”
Yet the way in which different fish species pursue those destinies – or, if you don’t like that quasi-mystical word, behave as their genes dictate – goes a long way in determining how likely a fish is to wind up pan-seared in butter and lemon juice.
Accordingly, one of Four Fish’s big questions is: Why do we eat certain species? Well, you might answer, because they’re delicious, or plentiful, or good for you. But what, specifically, about a fish’s anatomy makes it tasty? What specific behaviors make it easy to catch?
Take, for example, the European sea bass. Bass, according to Greenberg, owe their culinary popularity to one organ: their highly evolved swim bladder. (In Thailand, I discovered, bladder is often served in a dubious dish called “fish maw soup,” which even Thais don’t even seem to like very much.) Greenberg doesn’t go in-depth about what traits make bass’ swim bladders so evolutionarily excellent, so I’ll try to fill in what he leaves out.
A fish’s swim bladder has a basic function: it keeps the fish from sinking. Without the bladder, the fish would be heavier than its watery milieu, and would have to work very hard to stay afloat. (Some fish, including sharks and coelacanths, manage this trick. That’s why sharks have tails with longer upper lobes than bottom – heterocercal tails provide better lift).
By filling their bladders with air, fish remain neutrally buoyant – ie, they weigh as much as the water around them – and expend far less energy than they would otherwise. As fish move up and down, and the water pressure around them changes, they inflate and deflate their swim bladders correspondingly. Anybody who has ever dived has used a Buoyancy Control Device, and understands this intuitively.
Now for the cool stuff – how do fish inflate their swim bladders?
There are two kinds of swim bladder: physostomous, or open, bladders; and physoclistous, or closed, bladders.
Human lungs and swim bladders share an ancestral organ, and that homology is very apparent when you look at an open bladder: a duct, like our windpipe, connects the fish’s mouth to its bladder. The fish simply sticks its head out of the water and gulps some air when it wants to inflate and gain buoyancy; and burps when it wants to deflate. (Think about Charlie and his grandpa in the original Charlie and the Chocolate Factory.) Salmon, pike, goldfish, and many other species use this primitive system.
But sea bass, like all perciforms (perch-shaped fish), evolved the second, advanced kind of bladder: the closed bladder. Closed bladders are so-called because they have no duct connecting to the mouth, or to any outside sources of air*. Instead, perciforms regulate buoyancy by absorbing gas, usually oxygen, from the bloodstream into the bladder; and releasing that gas, when necessary, back into the blood. To accomplish this, closed-bladder fish have an organ called the gas gland which secretes lactic acid, causing the eventual diffusion of oxygen from the blood into the bladder. If you’re interested in exactly how this complicated process works, I suggest you consult either this Wikipedia page or a nephrologist.
*Closed-bladder fish have temporarily open bladders during their larval stage (approximately the first two weeks of their lives). A common cause of mortality among farmed fish is closed, or oil-covered, tanks, which prevent the larvae from taking gulps of air. The swim bladder forms around the first bubble of air that a fish swallows; without this bubble, larvae die, sink, or grow up deformed.
The upshot is that fish like sea bass have two big advantages over fish with open bladders: they don’t have to swim to the surface to freshen up their air bladders; and their buoyancy system is simply more efficient – they’re better at staying neutral. This means they expend less energy than other fish, and, says Greenberg, is why they’re so delicious:
“Without a need to fight gravity all the time, perciforms became more efficient swimmers and were able to trade in their heavy, energy-demanding “red muscle” tissue for lighter, more delicate flesh. Hence the white, light meat of many perciforms. Perciforms also evolved an efficient muscle structure that is principally attached only to the central spine column. The result: a smooth, mostly boneless fillet, very pleasant to eat.”
Not only is smooth, delicate, boneless bass flesh tasty to eat, it was once easy to obtain. Although a closed swim bladder allows its owner to remain below the surface, it also limits the depth to which the fish can dive – go too deep and the bladder may implode. In general, the fish that can go the deepest, and can change depths most rapidly, are the fish that have no swim bladders at all, like mackerel, sharks, and flounder. Because coastal sea bass are confined to shallow water by their large bladders, they’re much easier to spear and catch on a line than fish that lack bladders, or have small ones*.
*There are some perciforms, like the Patagonian toothfish (rebranded the Chilean sea bass, in a stroke of marketing genius) which live at great depths. The toothfish’s bladder has been replaced by glands which secrete lighter-than-water fat into the fish’s skin and allow it to maintain buoyancy effortlessly. The toothfish’s high fat content, incidentally, is what makes it so delicious, and thus so over-fished.
Europeans’ taste for sea bass, then, is the product of fish morphology and centuries of consequent exploitation. Humans developed and maintained a taste for sea bass centuries ago; began to farm them intensively in the 1980’s; and eat them ravenously today. All thanks to their remarkable closed-bladder buoyancy system that improves their energy efficiency and, in turn, makes them tastier, less bony, and comparatively easy to catch.
I’m sure I’ll return to both swim bladders and Paul Greenberg’s great book in future posts, but enough for now. Big hat tip to Davidson Biology for a great site on this subject.