‘Fish’ is one of my favourite words. Having only a single syllable means it can be dropped into conversation without a second thought, thus enabling one to cause maximum confusion with minimal time spent considering one’s move, which often rather spoils the moment. The very… forward nature of the word also suits this function- the very bluntness of it, its definitive end and beginning with little in the way of middle to get distracting, almost forces it to take centre stage in any statement, whether alone or accompanied by other words, demanding it be said loud and proud without a trace of fear or embarrassment. It also helps that the word is very rarely an appropriate response to anything, enhancing its inherent weirdness.
Ahem. Sorry about that.
However, fish themselves are very interesting in their own right; and yes, I am about to attempt an overall summary of one of the largest groups in the animal kingdom in less than 1000 words. For one thing, every single vertebrate on the planet is descended from them; in 1999 a fossil less than 3cm long and 524 million years old was discovered in China with a single ‘stick’ of rigid material, probably cartilage, running down the length of its body. It may be the only example ever discovered of Myllokunmingia fengjiaoa (awesome name), but that tiny little fossil has proved to be among the most significant ever found. Although not proven, that little bit of cartilage is thought to be the first ever backbone, making Myllokunmingia the world’s first fish and the direct ancestor of everything from you to the pigeon outside your window. It’s quite a humbling thought.
This incredible age of fish as a group, which in turn means there are very few specimens of early fish, has meant that piscine evolution is not studied as a single science; the three different classes of fish (bony, cartilaginous and jawless, representing the likes of cod, sharks and hagfish respectively- a fourth class of armoured fish died out some 360 million years ago) all split into separate entities long before any other group of vertebrates began to evolve, and all modern land-based vertebrates (tetrapods, meaning four-limbed) are direct descendants of the bony fish, the most successful of the three groups. This has two interesting side-effects; firstly that a salmon is more closely related to you than to a shark, and secondly (for precisely this reason) that some argue there is no such thing as a fish. The term ‘fish’ was introduced as a coverall term to everything whose lack of weight-bearing limbs confines them to the water before evolutionary biology had really got going, and technically the like of sharks and lamprey should each get a name to themselves- but it appears we’re stuck with fish, so any grumpy biologists are just going to have to suck it.
The reason for this early designation of fish in our language is almost certainly culinary in origin, for this is the main reason we ever came, and indeed continue to come, into contact with them at all. Fish have been an available, nutritious and relatively simple to catch food source for humans for many a millennia, but a mixture of their somewhat limited size, the fact that they can’t be farmed and the fact that bacon tastes damn good meant they are considered by some, particularly in the west (fish has always enjoyed far greater popularity in far eastern cultures), to the poor cousins to ‘proper meat’ like pork or beef. Indeed, many vegetarians (including me; it’s how I was brought up) will eschew meat but quite happily eat fish in large quantities, usually using the logic that fish are so damn stupid they’re almost vegetables anyway. Vegetarians were not, however, the main reason for fish’s survival as a common food for everyone, including those living far inland, in Europe- for that we can thank the Church. Somewhere in the dim and distant past, the Catholic Church decreed that one should not eat red meat on the Sabbath day- but that fish was permitted. This kept fish a common dish throughout Europe, as well as encouraging the rampant rule bending that always accompanies any inconvenient law; beaver were hunted almost to extinction in Europe by being classed as fish under this rule. It was also this ruling that lead to lamprey (a type of jawless fish that looks like a cross between a sea snake and a leech) becoming a delicacy among the crowned heads of Europe, and Henry I of England (third son of William the Conqueror, in case you wanted to know) is reported to have died from eating too many of the things.
The feature most characteristic of fish is, of course, gills, even though not all fish have them and many other aquatic species do (albeit less obviously). To many, how gills work is an absolute mystery, but then again how many of you can say, when it comes right down to the science of the gas exchange process, how your lungs work? In both systems, the basic principle is the same; very small, thin blood vessels within the structure concerned are small and permeable enough to allow gas molecules to move across the gap from one side of the blood vessel’s wall to the other, allowing carbon dioxide built up from moving and generally being alive to move out of the bloodstream and fresh oxygen to move in. The only real difference concerns structure; the lungs consist of a complex, intertwining labyrinth of air spaces of various size with blood vessels spread over the surface and designed to filter oxygen from the air, whilst gills basically string the blood vessels up along a series of sticks and hold them in the path of flowing water to absorb the oxygen dissolved within it- gills are usually located such that water flows through the mouth and out via the gills as the fish swims forward. In order to ensure a constant supply of oxygen-rich water is flowing over the gills, most fish must keep swimming constantly or else the water beside their gills would begin to stagnate- but some species’, such as nurse sharks, are able to pump water over their gills manually, allowing them to lie still and allow them to do… sharky things. Interestingly, the reason gills won’t work on land isn’t simply that they aren’t designed to filter oxygen from the air; a major contributory factor is the fact that, without the surrounding water to support them, the structure of the gills is prone to collapse, causing parts of it cease to be able to function as a gas exchange mechanism.
Well, that was a nice ramble. What’s up next time, I wonder…