There are many different species of plecos available. Some keep them due to their ability to eat algae and help keep the tank presentable. Other species are beautiful enough to make the centerpiece of an aquarium in their own right. In order to breed them, the first step is to determine their sex.
Some species of pleco have bristles. In many species, the male will grow these protrusions from the front of the pectoral (side) fins and along the edge of the head and mouth. Some species only grow these in the breeding season. In the bushy-nosed or bristlenose plecos, both males and females have these. However, they are much more pronounced in the males. In males of these species, they cover the head and are longer. In females, these only cover the edge of the mouth.
In plecos, the females are generally larger than the males. However, this assumes that all of the fish in question are the same age and have received similar care. A fishkeeper can’t be sure of this unless the pleco were purchased and raised together.
The body shape of a pleco can help a fishkeeper determine the fish’s sex. This works best from a top view. Females generally have a more rounded body, while males are generally skinnier. When viewed from the side, females have a more rounded abdomen, which is longer in relation to the rest of the body. This is most obvious if the plecos are in breeding condition.
In some species of pleco, males are much more aggressive than their female counterparts. This varies widely depending on the specific species of pleco. The royal pleco (Panque spp), males are very aggressive, even towards other fish. However, this does not work in many plecos. The common pleco (Hypostomus plecostomus) and the dwarf pleco (ottocinclus sp) are both so mellow that this is not a reliable way to spot males.