Reading passage
For most fish, reproduction ends at the moment eggs are laid and fertilised; the parents simply swim away, leaving their offspring to chance. A significant minority of species, however, have evolved a striking alternative: they carry their eggs inside the mouth. This behaviour, known as mouthbrooding or oral incubation, has arisen independently in several fish families and is most famous among the cichlids of the African Great Lakes.
The sequence begins at spawning. In the typical cichlid pattern, the female lays a batch of eggs on a cleaned rock or in a shallow pit. As she releases them, the male sheds his milt over the clutch, and the female immediately takes the fertilised eggs into her mouth. Some species have even evolved markings that assist this transfer: the male's anal fin carries bright spots that resemble eggs, and when the female tries to add these false eggs to her mouth, she draws in milt that fertilises the real clutch she already holds.
Once collected, the eggs rest in the buccal cavity, the space inside the mouth, where they are gently tumbled by movements of the parent's head and jaws. This circulation keeps the eggs supplied with oxygen-rich water and removes waste, while the surrounding tissues protect them from predators, currents and fungal infection. Incubation lasts from about ten days to three weeks depending on the species and the water temperature, with clutches ranging from fewer than ten eggs in some species to several hundred in others.
The costs to the parent are severe. A brooding fish cannot feed properly while its mouth is full, and most females eat nothing at all during incubation. Studies of Lake Tanganyika cichlids show that brooding females can lose up to a quarter of their body weight, and their ovaries stop producing new eggs until the current brood is released. Breathing is also harder work, because water must be pumped past a mouthful of eggs to reach the gills. To compensate, brooding fish hold their jaws slightly open and fan water almost continuously.
The payoff for this sacrifice is a dramatic improvement in survival. In an open nest on the lake floor, a clutch may lose most of its eggs to predators within days; inside a parent's mouth, nearly all survive to hatching. Even after the fry emerge and are released, many species continue to guard them. At the first sign of danger, the young fish dash back into the parent's mouth, a refuge they may use for another week or two until they grow too large to fit.
Mouthbrooding is not exclusive to cichlids, nor to females. Among sea catfishes and cardinalfishes, it is the male who incubates the eggs, and he may fast for weeks with his head visibly swollen by the clutch. In some cichlid species both parents share the duty, passing the eggs between them. Evolutionary biologists see these variations as experiments in parental investment: whichever sex carries the eggs guarantees the survival of its own genes, but pays for the guarantee in lost feeding and growth.
The strategy reaches its most spectacular expression in the African Great Lakes. Lake Malawi alone is home to more than 500 species of cichlid, the great majority of them maternal mouthbrooders, and biologists believe mouthbrooding helped drive this explosive diversification. Because a brooding female is mobile, she can shelter in a safe patch of habitat while her eggs develop, and small isolated populations quickly accumulate genetic differences. Over time this has produced hundreds of species packed into a single lake, each adapted to a narrow strip of rock or sand.
Today these fish face pressures their evolutionary ingenuity cannot easily solve. Overfishing with fine-meshed nets removes brooding females along with the next generation they carry, and sediment from deforested shores clogs the rocky crevices where many species spawn. Aquarists now breed many endangered cichlids in captivity, and conservationists argue that protecting the lakes' remaining clear water habitats is the only way to keep this remarkable nursery system functioning in the wild.