Reproductive Behaviour of Fishes

Did you know that fishes embark upon courtship behaviour that leads to pairing and spawning? Many people are totally unaware of this. The courtship of fishes in not as complex nor as varied as that of men and women, and the courtship of fishes is not complicated by innuendos, hidden meanings and cryptic signals as in Homo sapiens. Rather, courtship in fishes, as with birds, mammals and other animals, comprises sets of recognisable signals, following set sequences which for fishes, provided the partners are physiologically ready, culminate in spawning. Courtship behaviour of fishes might be a simple set of signals and answers or it might be very elaborate and complex, depending on the type of fish. However, the signals are always easy for potential mates to read. Indeed, it is easy for biologists to read and interpret these signals.

Among the most interesting and best known to aquarists is the elaborate courtship of cichlid fishes. Typically a male develops a territory which he defends from all intruders. Many species also build nests, which might be elaborate sand castles, or excavations with tunnels or burrows, or may simply be a cleaned part of the rocky or sandy area in which the territory is held. Once the tasks of developing a territory and building a nest are complete the male is ready to invite females to his nest to spawn. Eager males may attempt to do this even before the nest is complete, but they are nearly always refused as the female does not get all the correct messages if the nest is not there. One of the reasons for this is that part of the courtship ritual includes the nest, and each nest type is species-specific; it is a characteristic of the species that is important to mate recognition and the acceptance of the correct partner.

The initial invitation is ‘side-shake’ during which the male approaches a female, extends his fins and flexes so that the anal fin, which usually has attractive spots and colours on it, towards the female. If the female is interested the male then turns away and facing the spawning site beats its tail in a movement inviting the female to follow (‘follow-shake’). If the female is still interested, then with exaggerated, undulating movements the male leads the female to the spawning site (lead-swim). Once in the nest or spawning site, the couple circle each other, perform a variety of shakes (nest shake, vertical nest shake etc) and gently nudge each other till the female lays eggs. The female then picks up the eggs in her mouth, sometimes doing so before the male has had time to fertilise them. This would be a waste of eggs, so to solve the problem the males of many species have dummy eggs on the anal fin. This fin is spread out in front of the female to display the egg dummies and curled slightly to hold spermatozoa that he will release. The female tries to pick the egg dummies from the male anal fin and in doing so collects spermatozoa, that fertilise the eggs in her mouth. The need to collect the eggs before they are fertilised arose from the fact that eggs are often stolen almost as soon as they appear by other fishes which feed upon them. Another problem is that males that do not have territories, and were not chosen as mates by the female, try to sneak spawnings by darting down between the spawning couple and fertilising eggs that are exposed. By collecting the eggs promptly, the efforts of robbers and sneakers are largely circumvented.

Once in the mouth the eggs develop into young fish that remain there until they are ready to feed for themselves. Most cichlids will then look after their young for sometime (a few days to four or more weeks), taking them into their mouths when there is a threat from predators. The fry are also taken into the mouth at night for safekeeping. Once the fry are independent they leave the care of the mother. Some cichlids behave as cuckoos; they give their fry to another species to care for. It seems that the foster parent can recognise the foreign young, but she is unable to chase them away or swallow them as both activities will endanger her own brood. Chasing foreign fry that are mixed tightly in a school with ones own fry is not possible without chasing away the own fry too. Similarly, how does the mother swallow foreign fry and not her own?

Submitted by: Dr Tony Ribbink

Email:

A.Ribbink@ru.ac.za

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