The Redtail Splitfin, or Redtail Goodeid, (Xenotoca eiseni) is a species of goodeid fish from the family Goodeidae

Goodeidae

Splitfins, are a family, Goodeidae, of teleost fish endemic to Mexico and some areas of the United States. This family contains 40 species within 18 genera. The family is named after ichthyologist George Brown Goode.-Range and Geographic Distribution:...

 and subfamily Goodeinae

Goodeinae

Goodeinae is a taxon of splitfins. It is a small fish which mostly lives in fresh water in Mexico, especially around Mesa Central, west of Mexico City. It is also found in brackish water on both the east and west coasts....

. Like other members of Goodeinae, the Redtail Splitfin is a livebearer. However, the goodeid mating system differs in several ways from the more common livebearing fish from the family Poeciliidae

Poeciliidae

Poeciliidae is a family of fresh-water fish which are live-bearing aquarium fish . They belong to the order Cyprinodontiformes, tooth-carps, and include well-known aquarium fish such as the guppy, molly, platy, and swordtail...

 that includes guppies and swordtails. While no goodeid species is a very popular aquarium fish, the Redtail Splitfin is one of most popular.

Range and habitat

The Redtail Splitfin comes from several rivers in the highlands of 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...

, including the Rio San Leonel and Rio Grande de Santiago

Río Grande de Santiago

The Río Grande de Santiago is one of the longest rivers in Mexico, measuring up long. The river begins at Lake Chapala and continues roughly north-west through the Sierra Madre Occidental, receiving the Verde, Juchipila, Bolaños, and other tributaries...

 in Nayarit

Nayarit

Nayarit officially Estado Libre y Soberano de Nayarit is one of the 31 states which, with the Federal District, comprise the 32 Federal Entities of Mexico. It is divided in 20 municipalities and its capital city is Tepic.It is located in Western Mexico...

 and the Rio Tamazula in Jalisco

Jalisco

Jalisco officially Estado Libre y Soberano de Jalisco is one of the 31 states which, with the Federal District, comprise the 32 Federal Entities of Mexico. It is located in Western Mexico and divided in 125 municipalities and its capital city is Guadalajara.It is one of the more important states...

. They live in clear mountain streams as well as roadside ditches.

Description

The Redtail Splitfin is a deep-bodied fish, and males have a hump behind the head. The hump gets larger as the males get older. Males' body color is blue, and the caudal fin (tail) and penduncular area are red or orange. There are a number of color variations, and fish from Nayarit have a bluer body, while fish from San Marcos, Jalisco

San Marcos, Jalisco

San Marcos is a town and municipality, in Jalisco in central-western Mexico. The municipality covers an area of 292.85 km².As of 2005, the municipality had a total population of 3,533....

 are blotched. Females are more plainly colored Females are generally slightly larger than males. Females grow to 7 centimeters (2.8 inches) while males grow to 6 centimeters (2.4 inches).

Breeding

Male goodeids like the Redtail Splitfin have a notched anal fin that gives the fish the name "splitfin", instead of the gonopodium of poeciliids

Poeciliidae

Poeciliidae is a family of fresh-water fish which are live-bearing aquarium fish . They belong to the order Cyprinodontiformes, tooth-carps, and include well-known aquarium fish such as the guppy, molly, platy, and swordtail...

. This notched anal fin, or andropodium, is used to transmit sperm to the female. The female nourishes the unborn young via trophotaeniae, which function like an umbilical cord

Umbilical cord

In placental mammals, the umbilical cord is the connecting cord from the developing embryo or fetus to the placenta...

in mammals.

Females give birth about every 2 months to 10 to 50 fry. The newborn Redtail Splitfin fry are relatively large compared to most newborn livebearer fry, at about 15 millimeters (2/3 of an inch) long.

In the wild, both male and female Redtail Splitfins prefer to mate with fish of similar size. This limits the sexual advantage to males that grow especially large. This minimizes the sexual dimorphism in the size of the fish between males and females.

Aquarium keeping

The Redtail Splitfin is not a very common aquarium fish, but it is one of the most common goodeids to be kept in the aquarium. They are not very demanding in the aquarium, although they prefer hard, alkaline water. They can tolerate a wide range of water temperatures, from 15 to 30 degrees Celsius (59 to 86 degrees Fahrenheit). They accept a wide range of foods, but do require some vegetable matter in their diets. Although they are generally peaceful, they are known to occasionally nip the fins of fellow aquarium residents. This behavior can be reduced by keeping the Redtail Splitfin in schools.