Fish Versus Mammal

Mammals nurse their young; this is one way they continue caring for their progeny after birth. Although fish don't nurse, discus fish do care for their young well: by feeding them mucous produced by the parents' skin.
Discus fish originate from South America. These tranquil fish are often kept in aquariums. Specific breeding has led to a myriad of variations in colour.

After the eggs have hatched, both parents take good care of their offspring. The skin of both parents sports a heavy mucous layer. During the first few weeks of their lives, the young stay near their parents in order to eat this mucous. When one of the parent fish wants to eat or rest, the young are turned over to the other parent. The mucous contains several antibodies to strengthen the young fishes’ immune system; this qualifies the mucous as an important source of nutrition. In the course of the third week after the hatch, the parents’ care changes. They swim away with increasing frequency, leaving the young to learn to take care of themselves.

A research team from the University of Plymouth (UK) and the University of Manaus (Brazil) has studied the behaviour of the discus fish. They compared the behaviour to that of mammals: nursing first, then weaning. They commented, however, that there is actually very little known about the complexity of fish behaviour.

The importance of clean water was once again affirmed by this study. The waters of South America are often polluted through sources such as intensive mining. The investigators suspect that toxic wastes can find their way to the young fish through the mucous they feed on.