Sexing Fish

Fish reproduce sexually so it is a handy thing to be able to identify males and females when setting out to breed. This is not always easy to do as differentiation of the sexes is not always easy to spot in some species.

Every species has what is known as primary sex characteristics which are the parts of the body required for reproduction, such as ovaries and testicles. Unfortunately these can’t be seen easily and we usually need to rely on secondary sex characteristics to distinguish between males and females. In fish these characteristics include size, body shape, coloration, markings, fin shape and behavior.

Fortunately some species are dimorphic which means that it is easy to identify them by using secondary sex characteristics. For example many of the livebearers can be told apart by the shape of their anal fins. In females and juveniles the anal fin is fan shaped, but in males the anal fins thicken and fuse as they mature to form a gonopodium – a tube-like organ used for internal fertilization of the female. With most livebearers, males are usually more brilliantly colored and/or have longer fins.

Some examples of easy to identify secondary sex characteristics in other species include the male Pearl Gourami which has a red breast and long pointed dorsal fins, while the female has shorter, more rounded dorsal fins and a silver breast. With the bristlenose catfish, males have much larger bristles than females.

If we use behavior as an indicator then males will often chase females around the tank and where males are involved in care of the eggs and fry, they are often quite aggressive near their spawning sites. Sometimes the drive to reproduce is so strong that two females may court one another through the lack of a male tank-mate, and may even lay eggs.

Unfortunately in some species males and females resemble each other so closely that even seasoned aquarists cannot differentiate them by sight alone and use what is known as the Venting Technique. This requires removing the fish from the water and turning it upside down to examine the shape of the genital opening or vent, located between the anus and tail, where the fish excretes either eggs or sperm. In females the vent or ovipositor (egg laying tube) is shorter and wider than the male’s genital papilla, but these differences can be quite subtle and do require a lot of practice and experience. It can be quite stressful for the fish when being removed from the water, so the process needs to be carried out quickly and efficiently and is best usually left to the skilled breeders.

Sometimes the drive to reproduce is so strong that a pair of female fish will court each other through lack of a male tank-mate. This is common with Anglefish which may even spawn and lay infertile eggs.

Indeed, sexing fish can be a little tricky and probably the best way to overcome this situation is to start out with a group of about half a dozen juveniles and with this system there is an excellent chance of having a good mix of both sexes.