Just how do the various parts of a fish's anatomy work together to keep these fascinating creatures at home in their hazardous environment?
And what a risky life it is, with every fish species up to the Great White Shark himself being an item on someone else's lunch menu.
To survive a fish must me able to attack, defend, run (well, swim as fast as he can) or hide - and sometimes all of these in quick succession.
Fortunately, all fish have evolved to master at least one of these attributes to give itself a sporting chance of success.
Other than sharks, whose fins are more like the flippers we associate with dolphins, most fish have fins constructed of spines (or softer rays), supporting a webbed structure. These are the parts of a fish that enable it to swim, although some species have developed their fins for other purposes. Gurnards for example, can use their pectoral fins for poking around on the seabed.
This part of a fish is a its main propulsion unit. In most bony fishes the caudal fin is symmetrical, but in others the two lobes are of different shapes and sizes.
The shark is one such example where the upper lobe is the longer.
As you can see here, the thresher shark has taken this to the extreme.
Saltwater fish can have either one or two dorsal fins but some, like the pollack in the example above, have three. Their function is to keep the fish on an even keel when it's swimming, and to assist in making tight turns.
Often the 1st dorsal fin in supported by sharp spines which the fish uses to defend itself. In some cases the spines are poisonous and can cause a very painful injury, as anyone who has stood on a weaver fish will confirm. The second and third dorsal fins normally have soft rays rather than hard sharp spines - altogether more friendly.
Saltwater gamefish - marlin and sailfish for example - with their huge sail-like dorsal fin can fold it into a groove on their backs when it isn't needed, reducing drag and providing maximum hydrodynamic efficiency which allows the caudal fin to drive them forward at maximum speed.
Fish of the tuna and mackerel families have a number of small finlets between their final dorsal fin and the caudal fin, with a further set below.
Precisely what these are for I've no idea.
Fast-swimming fish species like the mako shark shown here often have one or two pairs of caudal keels which add stability to the caudal fin.
All bony fish have at least one anal fin, and others like our pollack above, have two. Whatever the quantity they're there to stabilise the fish when it's swimming. Gurnards use their pectoral fins for 'walking' along the bottom, a technique which mudskippers have developed even further.
The shark's pectoral fins are a more like a flipper than a fin, and their hydrodynamic design enable it to adjust its depth while swimming either slowly or at speed.
These give the fish the ability to turn sharply, stop quickly, dive and climb through the water.
This is a line of sensory receptors along the flanks of a fish which detects movement and vibrations in the water.
These are the two parts of the hard gill cover used to protect the delicate gills within. They're only present in bony fishes - cartilaginous fishes like sharks, skates and rays seem to manage perfectly well without them.
Fish don't use these for breathing of course, but they can detect scent in the water through them - sort of like smelling.
Downrigger ~ The cranelike device incorporating a line-counter reel often seen on the sterns of sport-fishing boats, which lowers a trolling weight on a wire line to a pre-determined depth. The trolling line is attached just above the weight, which gets the lure down to depths that would otherwise be unachievable.