The outlook for a newly hatched or newborn fish is bleak. The probability of the young surviving to adulthood is tiny; most will suffer the fate of being eaten. Their predators lurk everywhere, above and below the water, and come in every guise.
Fry are at risk from the deadly tentacles of the hydra and the jaws of a beetle larva, while a host of other fish, birds, and even animals such as snakes threaten them in later life. The dangers posed by birds to many fish is one reason why fish are so super-sensitive to any kind of movement passing over the top of the water. Most fish are defenseless -- a soft parcel of valuable and tasty proteins and fats -- which puts them at the top of the menu for many animals. The pressures they face are, therefore, very high.
How does predation influence evolution in fishes?
Only fish that are successful in dodging predators have the chance to breed and to pass on their genes. Those characteristics that enabled the parent fish to survive are then passed on through their genes to their offspring and thus the process of evolution advances in tiny incremental steps. But while successive generations of prey fish become fractionally better able to avoid their predators, the predators respond in kind, producing an arms race of adaptation and counter-adaptation. And there are limits to how far the evolutionary process can take them; it can only work with the materials at hand. For instance, a neon tetra could not simply evolve large toxic spines. Even if it could, the costs of growing them and swimming while so encumbered would be disastrous.
How do fish defend themselves against predation?
Fishes' defenses against predation can occur at any stage in the so-called predation cycle. Some try to avoid detection by predators by using camouflage or by hiding. Others try to deter the predator from attacking by bluff, by being poisonous, or by their behavior. Some species have false eyespots that direct predators' attacks away from critical areas of the body. Most fish remain highly alert in the presence of predators, ready to dart away at speed. And if the worst comes to the worst, others have evolved armor and spines so that a predator is unlikely to be able to devour them.
What has predation got to do with aquarium fish?
This is of course a site about aquarium fish and if all runs to plan, predation ought not to be a concern for domestic fish, should it? While this is true, fish in the home aquarium still display the behavior and appearance of their wild ancestors -- what may be called the "ghost of predator past." Armored catfish are still armored and tetras still school. Although we do sometimes see a reduction in anti-predator behavior patterns in the home aquarium as each generation of fish raised in captivity become bolder, it often only takes a sudden fright to see their natural response come to the fore. Predation has had a huge influence on fish for millions of years and it will be a considerable while before the ghost of its effect disappears.