While plecostomus are well-known fish among aquarists, with relatively simple husbandry requirements, even experienced keepers sometimes struggle to sex them. While many pleco species have broadly similar habitat needs, they often exhibit different gender traits. This means the characteristics that distinguish males from females in one species may differ markedly in other species. In general, body proportions, anatomy, behavior and size are all characteristics to examine when trying to determine your pleco's gender.
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Behavior can provide clues to the gender of some pleco species. The males of some species are antagonistic towards other males; they will battle for reproductive rights and territory. However, many adult plecos fight with any plecos with whom they are housed, so social interactions are not completely reliable for determining gender.
Males and females of some pleco species exhibit different body forms. For example, blue-eyed pleco (Panaque spp.) females are thought to have larger abdomens relative to similarly sized males. Males of this and several other species also may display elongated dorsal fins; male common plecos often feature red dorsal fins. Additionally, females often grow larger than males do, but this is hard to use as a standalone criterion unless the fish are the same age and receive similar care.
Secondary Sexual Characteristics
Sailfin plecos (Pterygoplichthys gibbiceps) are one of the few species for which definitive sexual characteristics are plainly visible. Mature male sailfin plecos possess a single genital organ that is thick and stump-like; the genital papilla of females is much smaller than that of males. Other species have similar visible characteristics. For example, the bristlecone catfish (Ancistrus dolichopterus) -- a type of pleco -- is easy to sex. Males of this species develop the namesake bristles on their face and heads, which females lack.
Part of the reason plecos are sometimes called "armored catfish" is the presence of bony growths on their scales, called odontodes. Many species can be sexed by noting where the odontodes are placed on various portions of their body. The exact differences between the patterns of males and females vary from one species to the next. For example, in the small species Harttia longipinna, males possess odontodes on the spines of the pectoral fins and near the margins of their snouts. As with most other gender-determining traits, this only holds true for mature animals.