The Monterrey platyfish (Xiphophorus couchianus) is a species

Species

In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

 of freshwater

Freshwater

Fresh water is naturally occurring water on the Earth's surface in ice sheets, ice caps, glaciers, bogs, ponds, lakes, rivers and streams, and underground as groundwater in aquifers and underground streams. Fresh water is generally characterized by having low concentrations of dissolved salts and...

 fish

Fish

Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...

 in family

Family (biology)

In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...

 Poecilidae of order

Order (biology)

In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...

 Cyprinodontiformes

Cyprinodontiformes

The Cyprinodontiformes is an order of ray-finned fish, comprising mostly small, fresh-water fish. Many popular aquarium fish, such as killifish and live-bearers, are included. They are closely related to the Atheriniformes and are occasionally included with them...

. A live-bearer

Live-bearing aquarium fish

Live-bearing aquarium fish, often simply called livebearers, are fish that retain the eggs inside the body and give birth to live, free-swimming young.-Common aquarium livebearers:...

, it is native to the vicinity of Monterrey

Monterrey

Monterrey , is the capital city of the northeastern state of Nuevo León in the country of Mexico. The city is anchor to the third-largest metropolitan area in Mexico and is ranked as the ninth-largest city in the nation. Monterrey serves as a commercial center in the north of the country and is the...

, Mexico

Mexico

The United Mexican States , commonly known as Mexico , is a federal constitutional republic in North America. It is bordered on the north by the United States; on the south and west by the Pacific Ocean; on the southeast by Guatemala, Belize, and the Caribbean Sea; and on the east by the Gulf of...

.

IUCN lists the Monterrey platyfish, formerly widespread in Mexico, as Critically Endangered, upgraded from Endangered in 1996. The fish's population, numbering fewer than 250 individuals, is falling rapidly and is confined to a very small area, occurring only in fragmented pockets of habitat populated by few individuals. It is additionally suffering from environmental degradation.

X. couchianus is harvested for the aquarium

Aquarium

An aquarium is a vivarium consisting of at least one transparent side in which water-dwelling plants or animals are kept. Fishkeepers use aquaria to keep fish, invertebrates, amphibians, marine mammals, turtles, and aquatic plants...

 fish trade. It has also been used for genetic

Genetics

Genetics , a discipline of biology, is the science of genes, heredity, and variation in living organisms....

 research.

Sexual dimorphism

Sexual dimorphism

Sexual dimorphism is a phenotypic difference between males and females of the same species. Examples of such differences include differences in morphology, ornamentation, and behavior.-Examples:-Ornamentation / coloration:...

is moderate, the male Monterrey platyfish growing to a maximum overall length of 4.0 cm (1.6 in) and the female 6.0 cm (2.4 in).