Sexing Aquarium Fish
How to tell male from female aquarium fish using sexual dimorphism, modified fins, behaviour and species-specific cues such as the guppy gonopodium.
Overview
Sexing is the identification of male and female fish, which is essential for setting up breeding pairs. Many fish display sexual dimorphism, in which the sexes differ in features beyond the reproductive organs, although in some species the difference is subtle and seen mainly in behaviour or only in breeding condition.
What sexual dimorphism is
Sexual dimorphism is the condition in which the sexes of one species show different morphological characteristics. Ray-finned fish display the widest range of sexual dimorphism of any class of animals, so the size and direction of the difference vary greatly between species.
Differences in size, colour and fins
- Size: in many fish females are larger, while in others, such as the cichlid Lamprologus callipterus, males are far larger.
- Colour: males are often more colourful, as in guppies where males show bright spots while females are mostly grey.
- Fins: males of some species have longer or modified fins, as seen in the dragonet where males are larger with longer fins.
Modified fins as a guide
In livebearers of the family Poeciliidae the male's anal fin is modified into a gonopodium, a copulatory organ used to transfer sperm. In male guppies it appears as a pointed, stick-like structure under the body, while females retain a fan-shaped anal fin. This makes the anal-fin shape a reliable sexing cue in these fish.
Behavioural cues
Behaviour can help confirm sex, especially in species with weak physical dimorphism. Males commonly display more colour or develop a breeding dress in the presence of rivals, court females and defend territory, while females may swell with eggs as they ripen. These cues are clearest as spawning approaches.
Reproductive-stage changes
Some differences appear only at maturity or in breeding condition. In sockeye salmon, for instance, males grow noticeably larger at maturity while females undergo a large increase in gonad size. Sexing may therefore be easier in conditioned, mature fish than in juveniles. Where physical and behavioural signs are weak, combining several cues at once, such as fin shape, colour and how the fish behaves toward others, gives a more reliable judgement than relying on any single feature.