Anatomy of finfish and shellfish

Unit 6 - Respiratory system in fishes

6.1.1. Gills
Gaseous Respiration in Fish
Just like you and me fish need a constant supply oxygen in the form of O₂ in order to run their metabolism. Without oxygen they can't turn their food into energy or make any new fish body. All the free oxygen on this planet on this planet, was, or is being, released into the air by plants, the atmosphere at the moment is about 21% O₂. However oxygen will dissolve in water, in a similar sort of way that the bubbles in your Coca Cola are dissolved into the liquid, which is mostly water, that makes up the drink.
Fish could of course breathe air like Seals and Whales, and some do, but if they wish to stay safe under the water for longer periods of time it would be much easier if they could get the oxygen they need from the water, and this is exactly what they do. In fact they were doing this long before any vertebrate animals learned how to breath the air.
So fish live in the water and they breathe the water, to do this they have special organs called gills. Gills are wonderfully well designed, and they have to be because although the water does hold some oxygen it never holds any where near as much as the air, and to make things more difficult the amount of oxygen a body of water can hold decreases the warmer it becomes, and also, salt water holds less oxygen than fresh water. If the air above a body of water is 21% O₂, this means that 210 parts per thousand are O₂, but if we do the math for one of the figures below, such as cold salt water we see that it contains only 7.58 parts per thousand O₂. What this means to the fish is that their gills need to be a lot more efficient at extracting O₂ from the water than our lungs need to be at extracting it from the air.

Fish solve the problems of extracting the O₂ they need from the water they live in in a variety of ways. Firstly they have different life styles, obviously a fish that spends most of its life resting on the bottom of the ocean waiting for its dinner to swim by needs less O₂ than a fish which actively chases smaller fish for its dinner. However most of the problem is solved in the design of the gills.
A fish's gills are situated one set on either side of the body and near the back of the head They are open to the gullet at the front, and open to the external environment behind. They are designed so that water can flow continually passed them, coming in through the mouth, and/or the spiracle in sharks and their allies, and passing out through the single external gill opening in fish or through one of the 5 to 7 gill clefts in sharks and rays. In fish there is a bony plate protecting the gills, this is called the operculum, and it is hinged and has muscles attached to it so it can be regularly opened and closed.
This ability to have water continually passing over the gills is one of the major factors making gills more efficient than lungs. With lungs the air comes in, fills the space, and then has to be expelled before any more O₂ rich air can be brought in. With gills there is no time wasted getting rid of the old air/water and no energy wasted reversing the direction of the flow.

In sharks and rays the number of gills is usually 5, but there are some species with 6 or 7 sets, in fish the number of gills is 4 on either side of the body. Each gill is supported by a gill arch and protected by gill rakers. Each gill arch supports one set of paired gill filaments. The gill rakers help make sure that no extraneous material gets into the gill filaments to clog them up. Each paired gill filament in turn supports numerous lamellae (sing. lamella), extending out from both sides of the filament body. It's here in the lamellae that the uptake of O₂ actually occurs.
The lamellae are very fine structures, however there exact dimensions depend on the normal activity levels of the fish in question. The more active the fish the thinner they are and the less distance there is between them. Also the absolute thickness of the individual lamellae walls varies, this is important in considering to facility with which O₂can diffuse from the water to the fish's blood, the thinner the membrane the more quickly. and easily the O₂ can pass across it.
Thus in sluggish fish like the American Brown Bullhead (Amerius nebulosus) the lamellae are 25 µ thick, 45 µ apart and the lamellae walls are 10 µ thick giving you 14 lamellae per mm, whereas in a highly active fish such as the Atlantic Herring (Clupea harengus) the lamellae are 7µ thick, 20 µ apart and the lamellae walls are >1µ in width, giving you 32 lamellae per mm.
The presence of all these lamellae greatly increases the surface area of the gills, meaning that a large amount of water is available for gaseous exchange at any particular moment of time. In active fish, such as the Atlantic Mackerel (Scomber scombrus), which has nearly the same gill dimensions as the Atlantic Herring, there may be as much as 1,000 square mm of lamellae surface for every gram of body weight. Such a fish weighing 1 kg will have approximately 1 square metre of gill surface area. Having such a large amount of gill area obviously helps the fish in its battle to extract enough O₂ from the water it lives in.