omega iris. The dorsal segment of the iris expands downward over the pupil to form a loop which can expand and contract, called an iris operculum; when light levels are high, the pupil reduces in diameter and the loop expands to cover the center of the pupil, giving rise to a crescent-shaped, light-transmitting partial pupil. This feature gets its name from its similarity to an upside-down uppercase Greek letter omega (Ω). The origins of this structure are unknown, but breaking up the outline of the highly visible eye has been suggested to aid camouflage in what are often highly mottled animals. Species in the tribe Rhinelepini are an exceptional group among loricariids, having a circular iris. The presence or absence of the iris operculum can be used for identification of species in the subfamily Loricariinae. Genetics As of 2000, only 56 loricariid species have been cytogenically investigated. The basal diploid number of chromosomes is 2n = 54 in this family, but with a wide variation in the chromosome number in this fish group, ranging from 2n = 36 in the Loricariinae, Rineloricaria latirostris, to 2n = 96 in a species of Upsilodus (Hemipsilichthys). Most members of the Ancistrini and Pterygoplichthyini have 52 chromosomes. Karyotypic evolution by means of centric fusions and centric fissions seems to be a common feature among loricariids; this is demonstrated by a higher number of biarmed chromosomes in species with lower diploid number and many uniarmed chromosomes in species with higher diploid numbers. Studies conducted with representatives of some genera of Hypostominae showed, within this group, the diploid number ranges from 2n = 52 to 2n = 80. However, the supposed wide karyotypic diversity the family Loricariidae or the subfamily Hypostominae would present is almost exclusively restricted to the genus Hypostomus, and the species from the other genera had a conserved diploid number. In some species, there is a ZZ/ZW sex-determination system. Ecology The suckermouth exhibited by these catfish allows them to adhere to objects in their habitats, even in fast-flowing waters. The mouth and teeth also are adapted to feed on a variety of foods, such as algae, invertebrates, and detritus. Some species, notably the Panaque, are known for xylophagy, or the ability to digest wood. Most species of loricariids are nocturnal animals. Some species are territorial, while others, such as Otocinclus, prefer to live in groups. Air-breathing is well known among many loricariids; this