The article by Dr. Day goes on giving numerous examples, among those known at the time (and often misinterpreted) to demonstrate the behavioural complexity exhibited by fish. As it often happens in Disney films, the author too made the mistake, forgivable in those years, of humanizing animals. In the following decades, however, many scientific studies have rigorously focused on fish ethology, demonstrating how this group of animals has evolved interesting behavioural strategies regarding, for example, reproduction.

Indeed, according to Breder & Rosen (1966), about a third of all families among living fish practice some kind of parental care. If we think that, considering the number of species, fish outnumber amphibians, reptiles, birds and mammals taken together, there are plenty other examples. The range of behaviours and strategies adopted is very wide: here we can just mention a few of the most peculiar examples. Let us start with one of the simplest forms of parental care, found in trout and salmons: the burial of eggs to hide them from potential predators. In this case, the new generations are then abandoned to their fate. In other cases, one or both parents are involved in the construction of a nest, more or less complex, which is then defended not only by predators, but also by potential competitors. Since most species of fish reproduce through external fertilization, there is often the possibility that a rival male will try, often succeeding, to sneak between the two lovers, managing to fertilize the newly laid eggs. Never as in this case mater semper certa est, pater numquam. The nest can simply be a stone or a leaf, which is carefully cleaned and onto which the mother sticks the eggs. Or, it can be a more elaborate construction, like the sand mound built by the African cichlid Cyathopharynx furcifer. This construction resembles a small volcano, and in its ‘crater’ eggs are laid and looked after. Another example of a nest, a ‘classic’ in ethology, is the tunnel of plant material built by the three-spined stickleback Gasterosteus aculeatus.
It is often the male who defends a territory, builds the nest and takes care of the fertilized eggs, which are oxygenated with the movement of fins, while the unfertilized eggs, which go mouldy, are carefully removed to avoid contamination of the fertile ones. The female, once the eggs are laid, can even be harshly driven away. There are, however, many examples of maternal and bi-parental care, in which both parents take care of their offspring.
Even in the Anabantids, a group of fish typical of the stagnant and oxygen-poor waters of Southeast Asia, it is usually the male who sets up home. In this family, however, it is a floating home. By breathing on the water surface, the future father swallows air and mixes it with the mucus in the oral cavity. The air bubbles produced will form a floating nest. Once the nest is ready, mating takes place: the male surrounds the female in an embrace that the naturalists of the XIX century would undoubtedly have called ‘passionate’. Then, the future father collects the fertilized eggs with his mouth, and places them between the bubbles, protecting the nest while waiting for the hatching.
In other situations, however, fish can resort to a pre-fabricate. The male of lagoon goby Knipowitschia panizzae, for example, uses as nest one of the two pieces of the shell from a bivalve mollusc. When it finds an adequate shell, it places it with the concavity downward, and with accurate blows of fin covers it with mud, to hide it, leaving only a small opening.
Evolution, however, has gone further, making the body of one of the parents a safe nest (our species, as mammals, knows about it well). The most famous case is within the Singnatids, a family of fish that includes pipefish and seahorses. In these fish, the female lays the eggs in a special pouch present on the male’s abdomen, which in this way incubates the eggs until hatching. However, it is no mere simple transportation, because they are also nourished.
Perhaps, the most curious example of parental care, however, could be oral incubation, which is widespread among the cichlids of the great African lakes of the Rift Valley. In this case, one of the two parents (usually the mother, more rarely the father or both) stops feeding for a couple of weeks, while housing the eggs in the mouth cavity, waiting for the development of the embryos and the hatching of the fry. Once the eggs hatch, their young begin to venture outside, swimming out from their mother’s mouth that can start feeding again at last. In the first few weeks of life, however, the fry continues to stay nearby. In case of danger, at a signal of the mother (usually particular movements of the fins), the fry swim again inside her gaping mouth.