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Members of the family Loricariidae are commonly referred to as suckermouth armoured catfishes, armoured catfish, 'plecos' or simply 'plecs'; a shortened form of the species name Hypostomus plecostomus. Some loricarids are not normally considered plecostomus, such as farlowella catfish.
These names are used practically interchangeably when referring to the Loricariidae. The name "Plecostomus" and its shortened forms have become synonymous with the Loricariidae in general, since Plecostomus plecostomus (now called Hypostomus plecostomus) was one of the first species imported into the fish-keeping hobby. This can cause some confusion, as some unrelated fish may also be called plecostomus, such as the "Borneo plecostomus", which is actually a balitorid fish.
In their native range, these fish are known as cascudos or acarís.
Because of their highly specialized morphology, loricariids have been recognized as a monophyletic assemblage in even the earliest classifications of the Siluriformes, meaning it consists of a natural grouping with a common ancestor and all of its descendents. Loricariidae are one of seven families in the superfamily Loricarioidea, along with Amphiliidae, Trichomycteridae, Nematogenyidae, Callichthyidae, Scoloplacidae, and Astroblepidae. Some of these families also exhibit suckermouths or armor, although never together as in loricariids.
This is the largest catfish family, including about 684 species in around 92 genera, with new species being described each year. However, this family is in flux and revisions are likely. For example, the subfamily Ancistrinae is accepted in as late as the 2006 edition of Nelson's Fishes of the World; it later becomes grouped as a tribe because of its recognition as a sister group to the Pterygoplichthyini. Under Ambruster, six subfamilies are recognized: Delturinae, Hypoptopomatinae, Hypostominae, Lithogeneinae, Loricariinae, and Neoplecostominae.
Monophyly for the family is strongly supported, except, possibly, the inclusion of Lithogenes. Lithogenes is the only genus within the subfamily Lithogeneinae. This genus and subfamily, the most basal group in Loricariidae, is the sister group to the rest of the family. Neoplecostominae are the most basal group among the loricariids with the exception of Lithogeneinae. However, the genera of Neoplecostominae do not appear to form a monophyletic assemblage. The two subfamilies Loricariinae and Hypoptopomatinae appear to be generally regarded as monophyletic. However, the monophyly and composition of the other subfamilies are currently being examined and will likely be altered substantially in the future. The Hypostominae are the largest subfamily of Loricariidae. It is made up of five tribes. Four of the five tribes, Corymbophanini, Hypostomini, Pterygoplichthyini, and Rhinelepini, include about 24 genera. The fifth and largest tribe, Ancistrini (formerly recognized as its own subfamily), includes 30 genera.
Loricariid fossils are extremely rare. The fossil record of Loricariidae extends back to the upper Miocene. Within the superfamily Loricarioidea, the Loricariidae are the most derived; in this superfamily, the trend is toward increasingly complex jaw morphology, which may have allowed for the great diversification of the Loricariidae, which have the most advanced jaws.
The family Loricariidae is vastly distributed over both sides of the Andes; but most species are generally restricted to small geographic ranges. They are primarily found in freshwater habitats of South America, but several loricariines and hypostomines are native to Panama, and two species (Fonchiiichthys uracanthus and Hemiancistrus aspidolepis) are native to Costa Rica. Species occur in swift-flowing streams from the lowlands up to 3,000 m (9,800 ft) in elevation. They can also be found in a variety of other freshwater environments. They can be found in torrential mountain rivers, quiet brackish estuaries, black acidic waters, and even in subterranean habitats.
This family has extremely variable color patterns and body shapes. Loricariids are characterized by bony plates covering their bodies, similar to the bony plates in callichthyids (In Latin, lorica means corselet). These fish exhibit a ventral suckermouth with papillae (small projections) on the lips. When present, the adipose fin usually has a spine at the forward edge. These fish have, when they are present, a unique pair of maxillary barbels. These fish have relatively long intestines due to their usually herbivorous or detrivorous diets. The body is characteristically depressed in this family. Taste buds cover almost the entire surface of the body and fin spines. Their lengths can range from 2.22 cm (0.87 in) in Nannoplecostomus eleonorae to over 100 cm (39 in) in Panaque, Acanthicus, and Pterygoplichthys.
One of the most obvious characteristics of the loricariids is the suckermouth. The modified mouth and lips allow the fish to feed, breathe, and attach to the substrate through suction. The lips were once believed to be unable to function as a sucker while respiration continued, as the inflowing water would cause the system to fail; however, respiration and suction can function simultaneously. Inflowing water passing under the sucker is limited to a thin stream immediately behind each maxillary barbel; the maxillae in loricariids support only small maxillary barbels and are primarily used to mediate the lateral lip tissue in which they are embedded, preventing failure of suction during inspiration. To achieve suction, the fish presses its lips against the substrate and inflates its mouth, causing negative pressure.
Also, unlike most other catfishes, the premaxillae are highly mobile, and the lower jaws have evolved towards a medial position, with the teeth pointed rostroventrally; these are important evolutionary innovations. The fish rotates its lower and upper jaws to scrape the substrate. The lower jaws are more mobile.
Loricariid catfishes have evolved several modifications of their digestive tracts that function as accessory respiratory organs or hydrostatic organs. These complex structures would have been independently evolved a number of times. This includes an enlarged stomach in the Pterygoplichthyini, Hypostomus, and Lithoxus, a U-shaped diverticulum in Rhinelepini, and a ring-like diverticulum in Otocinclus. However, even loricariids with unmodified stomachs have a slight ability to breathe air.
Considerable sexual dimorphism occurs in this family, most pronounced during the breeding season. For example, in Loricariichthys, the male has a large expansion of its lower lip, which it uses to hold a clutch of eggs. Ancistrus males have snouts with fleshy tentacles. In loricariids, odontodes develop almost anywhere on the external surface of the body and first appear soon after hatching; odontodes appear in a variety of shapes and sizes and are often sexually dimorphic, being larger in breeding males. In most Ancistrini species, sharp evertible cheek spines (elongated odontodes) are often more developed in males and are used in intraspecific displays and combat.