Ancistrus species show all the typical features of the Loricariidae.
This includes a body covered in bony plates and a ventral suckermouth.
The feature most commonly associated with the genus are the fleshy tentacles found on the head in adult males; females may possess tentacles along the snout margin but they are smaller and they lack tentacles on the head.
Tentacules, tentacles directly associated with odontodes, develop on the pectoral fin spine of the males of some species.
Males also have evertible cheek odontodes which are less developed or absent in females.
They also lack odontodes along the snout.
In comparison with a typical loricariid (pleco), a bristlenose is typically shorter (4–6 inches or 15 cm or less), more flattened and fatter with a comparatively wider head.
Colouration is typically mottled brown, grey or black.
Small white or yellow spots are common.
In contrast to many other loricariid 'plecos' (many of which regularly exceed a foot in captivity), the bristlenose plecos do not usually exceed six inches in length; they can thus be kept in relatively small tanks, contributing to their popularity in the aquarium hobby.

Ancistrus species are unusual among vertebrates in possessing an X0 sex-determination system, which is the prevailing method in many lineages of arthropod but is very rare elsewhere in the animal kingdom.
Ancistrus species show all the typical features of the Loricariidae. This includes a body covered in bony plates and a ventral suckermouth. The feature most commonly associated with the genus are the fleshy tentacles found on the head in adult males; females may possess tentacles along the snout margin but they are smaller and they lack tentacles on the head. Tentacules, tentacles directly associated with odontodes, develop on the pectoral fin spine of the males of some species. Males also have evertible cheek odontodes which are less developed or absent in females. They also lack odontodes along the snout. In comparison with a typical loricariid (pleco), a bristlenose is typically shorter (4–6 inches or 15 cm or less), more flattened and fatter with a comparatively wider head. Colouration is typically mottled brown, grey or black. Small white or yellow spots are common. In contrast to many other loricariid 'plecos' (many of which regularly exceed a foot in captivity), the bristlenose plecos do not usually exceed six inches in length; they can thus be kept in relatively small tanks, contributing to their popularity in the aquarium hobby.
Ancistrus species are unusual among vertebrates in possessing an X0 sex-determination system, which is the prevailing method in many lineages of arthropod but is very rare elsewhere in the animal kingdom.
Ancistrus