Profitable Wonders
‘Possessing a gasfilled sac of sufficient volume renders them essentially weightless’
Fish have an easy ride in their buoyant, watery habitat. Because water is 800 times denser than air, it neutralises the pull of gravity. They thus have no need of those bones and muscles that we, and all terrestrial creatures, require to keep us upright.
And so they can devote all their energy to the joys of swimming. Movement incarnate, fish deploy practically every muscle of their bodies in generating the thrust that propels them forwards.
‘At one extreme is the eel,’ wrote Sir James Gray in his authoritative monograph The Muscular Movements of Fishes, ‘its distinct waves of curvature passing alternately down each side of the body from head to tail.’
Contrasting with this sinusoidal progression, the powerful muscles of the long-distance-cruising tuna exert their pull on its large tail, fin moving it
from side to side with
quick vigorous strokes. There are numerous further variations on the same theme.
There is more than enough space for fish to enjoy their easy ride. Almost ubiquitous, nearly numberless, they have diverged over the past 400 million years into more species (30,000) than all other vertebrates combined.
This complete mastery of their domain is the more remarkable – extraordinary indeed – because water should, by rights, be profoundly inimical to their flourishing on three counts. It is oxygen-impoverished. Its composition of salts is either too high (seawater) or too low (fresh water) to be readily compatible with life. Though water neutralises the pull of gravity, fish should nonetheless sink under their own weight.
The ingenuity with which they circumvent this threefold threat to their existence is the triumph of fishiology.
Let’s start with that oxygen deficit. The challenge for fish is how to absorb enough of this vital gas at a concentration 30 times lower than in air. First, two natural pumps maximise the flow of water across the site of gas exchange – the long, blood-rich, finger-like filaments of their gills. Contraction of the mouth acts as a positive pump, forcing the water forward while, simultaneously, expansion of the cavity between the gills acts as a negative pump, sucking it inwards.
The real genius of this anatomical arrangement is a counter-current mechanism, where the flow of blood through the filaments is in the opposite direction to the inflowing water – ensuring that every last molecule of oxygen is absorbed into the circulation.
Next is the threat posed by the amount of salt in water being much higher or much lower than in the body fluids of, respectively, marine and freshwater fish.
Salt is essential for many physiological processes. Its presence within body fluids must thus be held constant within very narrow limits. It’s constrained by the universal law of osmosis, where water will move across a cell wall to equalise its concentration on either side.
Thus the tissues of marine fishes are constantly prone to dehydration, as osmosis dictates that their body fluids, being half as salty as the seawater, should leak outwards. That is compensated for by their drinking copiously while excreting only small amounts of highly concentrated urine.
For the freshwater trout, the situation is reversed. The concentration of salts in its body fluids is greater than in rivers and lakes. Here, osmosis dictates that water should surge into its tissues, which would rapidly become waterlogged – were it not for the compensatory mechanisms of its drinking practically nothing while excreting large volumes of dilute urine. The practicalities of these two contrasting forms of osmo-regulation are of the utmost complexity.
The third profound challenge is to the freedom of movement within fishes’ domain. Their bones and muscles, being denser than water, should cause them to sink. Here they exploit the principle of gas being lighter than water. Possessing a gas-filled sac (or swim bladder) of sufficient volume renders them essentially weightless.
When they’re swimming downwards, the increasing pressure of the surrounding water compresses the volume of this bladder, causing it to shrink. This necessarily compromises their ‘natural buoyancy’.
That is compensated for by the fishes’ inflating the bladder (like blowing up a balloon) with oxygen and carbon dioxide absorbed from the bloodstream.
When they swim back upwards, this situation is reversed. The reduced pressure of the surrounding water would cause the bladder to become overinflated. That’s adjusted for by the reabsorption of those gases back into the circulation.
It took scientists the best part of 100 years to work out the complex physics of this finely tuned hydrostatic mechanism. Without it – and the simultaneous ingenious solutions to those two other threats to their existence – fish would never have happened.