Fish face. It's one of the first impressions we do of an animal, and the second you say those words almost everyone knows what you're talking about. Hands at the side of your head, cheeks sucked in, lips puckered, and moving your lips open and closed.
Yeah that's the one! Not really sure how the top hat fits in, but whatever.
Courtesy: Mark Norman Francis via Flickr
Opening and closing their mouth over and over is characteristic of many of the bony fish that we see most often, especially gold fish. Why are they doing that? Well they're breathing, and we're just seeing them getting water into their bodies. It's a bit like your stomach or chest going up and down as you breathe. When you make a fish face your hands on the side of your head moving with your breathing are mimicking an important part of the bony fish breathing system. It's not the fins (although I know that's what of lot of people are mimicking), and it's not technically the gills, it's the gill cover. It might seem like a silly distinction, but in fish the gill cover is as different from the gills as your nostrils are from your lungs. If it helps you can call the gill cover by its more scientific name, the operculum (oh-perk-you-lum).
To breathe, the opercula (plural) close and the fish creates negative pressure to pull water into its mouth. Then the mouth closes and the opercula open pushing water across the gills and out the fish's body. The gills themselves though, are the coolest part of this system.
This is either a fish gill or someone scalped Ronald McDonald.
Courtesy: Allan Reyes via Flickr
Gills are really amazing because they take huge amount of surface area and bundle it into a tiny package. One study found that Atlantic horse mackerel (Trachurus trachurus) can have 14 square meters of surface on their gills! That's a greater area than the floor plan of this guy's house, all packed into a 70cm long fish. How is that possible? Each of those red filaments coming off of that yellow arch are covered in microscopic sheets called lamellae (lam-ell-ay). There are a ton of filaments on each arch and even more lamellae on each filament. It's just another great example of how tiny things can really add up in surprising ways.
Scanning electron micrographs of filaments on the left and lamellae on the right
Courtesy: Evans et al. 2005
All of that surface area is very important to the fish's ability to breathe underwater. Fish don't actually breathe the water itself. It would take a huge amount of energy to break water molecules apart to get at the oxygen, so instead animals with gills breathe tiny bubbles of gas dissolved in the water. If you can't picture that, think about a soda bottle. None of the bubbles are visible until you take the cap off the bottle. Just like the horrifying amount of sugar in there; the bubbles are really small and surrounded by water. When the cap comes off the tiny bubbles expand, slam into each other, make bigger bubbles, and escape.
The problem is that there's only a fraction of the oxygen available in air, in the water. So gills actually need to be more efficient than lungs at removing oxygen from their surroundings All of those lamellae are in contact with the water and have deoxygenated blood running just under their surface (that's why they're dark red). The blood has less oxygen in it than the water nearby so the oxygen moves from the water into the blood. Nature doesn't care for an uneven distribution.
Communist implications of nature aside, it's a really efficient system for getting an essential chemical into the body. It's also great for getting waste out of the fish's body. Not only can CO2 leave through the gills, but other waste as well, nitrogen based wastes like ammonia especially. Even though fish do have kidneys that process waste; the gills are just so damn efficient at exchanging chemicals with the water that they actually excrete most of the fish's nitrogenous waste. In some species up to 80% of the ammonia and urea are excreted through the gills. Imagine if we only had to pee one fifth as often because we breathed our waste out. Netflix binges would be so much easier.
"Oh god! Is that what all these warm spots are?"
Courtesy: Alex Derr via Flickr
There's one more question I want to pose to you all this week. If fish breathe gas inside the water, why do they suffocate in dry air? It could be that their gills need to be wet to work. Which makes sense since our lungs are wet and protected from outside. But there's more to it than that. It could also be that it's harder for gasses and materials to move between two different media, in this case gas and liquid, which is also true. But one of the most significant reasons is that when a fish comes out of water the filaments and lamellae aren't buoyed up by the water and they lay flat on one another. When they bunch up, the surface area on the gills decreases, and the efficiency benefit is lost. So if you're doing catch and release fishing and you take your fish out of the water for a minute, slowly swish them back and forth a little in the water a little to help get their gills unstuck. That way he/she will stay healthy and have lots of babies so you can keep coming back every year.
Evans et al., "The Multifunctional Fish Gill: Dominant Site of Gas Exchange, Osmoregulation, Acid-Base Regulation, and Excretion of Nitrogenous Waste.", Physiological Reviews, Vol. 85 No. 1, 2005, DOI: 10.1152/physrev.00050.2003
Accessed via: http://physrev.physiology.org/content/85/1/97
Hughes, G.M., "The Dimensions of Fish Gills in Relation to Their Function.", Journal of Experimental Biology, 45, 177-195, 1966
Accessed via: http://jeb.biologists.org/content/45/1/177.full.pdf
"Trachurus trachurus, Horse Mackerel"
Encyclopedia of Life, Accessed via: http://eol.org/pages/206048/details