That's what I thought, too, as a kid. But I was wrong. The gills are actually the tissues behind that flap, and that flap is just a protective cover known as the operculum, which sorta helps control the rate of flow over the gills, thus matching the gas exchange rate to the fish's metabolic need. And another thing I didn't know about gills as a kid: non-fish creatures have gills, too!
Oh, I suppose I knew that bugs and other non-fish creatures in the water had to breathe somehow, but even with this knowledge, the gills I picture in my head are that flap of moving flesh, like Kevin Costner's neck in Water World, not the bloody, fibrous stuff underneath. The thing on the outside of a fish's head looks like it's helping the fish breathe water, right? Well, can you spot the gills on this little guy?
Does that look like breathing? 50 points if you guessed that this bug's gills are the "hairy" growths in the many armpits and joints on the little guy's underside. 50 more points if you can tell whether this is a stonefly, caddisfly, or mayfly. So yeah, that little jig he was dancing is him breathing. He's in a little display case, so the water stagnates a bit, and he has to shuffle side to side like that to get enough water passing over his gills. In his stream the water flows over him pretty consistently, and he can just walk to wherever is more comfortable, but not here. Don't worry, we returned him to his proper habitat not long after I took the video.
Gills are just the water-life equivalent of the alveoli in our lungs. They're extremely delicate, blood-rich tissues that allow for gas exchange with the surrounding environment. Blood gets brought so close to the outside surfaces of the tissues that Oxygen gets taken into the blood, Carbon Dioxide gets pushed out, and the blood just keeps moving right on through, refreshed for another go-round through the whole body. There's a great graphic and short explanation of how it works at this site here, if you're interested.
Can you imagine your breathing apparatus being practically outside your body? The sensitivity of these structures is incredible. In order to function properly, they need to be in the environment they're adapted for. This means the pH of the water (how acidic it is or isn't), how much bacteria and algae are in the water, how much mud or silt is suspended in the water, all have to be juuuust right in order for aquatic creatures to breathe. All these factors, even the amount of dissolved Oxygen that exists in the water, the excess that's floating about for the fish to breathe, can fluctuate dramatically based on temperature, which is why one of the biggest problem contaminants we introduce into streams is actually just heat. After a factory draws water from a stream, uses it, and discharges it back into the stream, even if they clean it perfectly and there's no contaminants or traces of their process left, it can still be a big problem for aquatic life if it gets returned 10 degrees warmer than it got taken out. More on stream temperatures in another post, another time.
Ok, so fish gills are under the operculum, fairly well protected, but what about a breathing apparatus ENTIRELY outside the body? Is it just bugs like our friend above? Nope. When I first started working with the Endangered California Tiger Salamander (CTS) in various regions of California, I was really surprised at what their larval form's gills looked like. A little background: They breed and are born in water, but spend most of their lives hiding in small mammal burrows. While they're still young, before they leave the vernal pools they hatched in, they still have gills. External gills, which I'd never really seen before.
I don't have copyright permission for any of my favorite photos of this, so here's a short description, followed by a couple links to some really nice shots, and I DID find a photo attached to a public newspaper article, so I think I can use that at the bottom. Picture a tadpole, but leggier; this salamander has all four legs for its whole life cycle, but while it lives in the water it also has a tadpole-like tail coming off of much of its body. Now add six little trees that sprout from the back of its head, three on either side. The trunk of that tree is structural support for all the feathery tissues (leaves?) that do the gas exchange, the gills. Now, go check out these three shots, taken by a very talented photographer and archived on the California Herps website . . . isn't that adorable?
When I have more time, maybe I'll do a post about the California Tiger Salamander's life cycle. It's really pretty interesting, and I loved learning about it. Let me know if you'd like to read it, and I'll get to work.
Seriously, ask questions, there is SO much more to write on any of these topics I touched on.