Xiphophorus nigrensis (Rosen, 1960)

Etymology:

Xiphophorus: Greek, xiphos meaning ‘sword’; Greek, pherein meaning ‘to carry’

Nigrensis: Latin, nigri meaning ‘black’; Latin, ensis meaning ‘to come from’.

First description:

Donn E. Rosen 1960.

Middle-American poeciliid fishes of the genus Xiphophorus.
Bull. Fla. St. Mus. Biol. Sci 5 (4): p 57-242.

Synonyms:

Xiphophorus pygmaeus nigrensis Rosen, 1960

Common name:

El Abra Pygmy Swordtail

History:

Collected by D.E. Rosen, M.S. Gordon and M. Gordon on January 19^th 1957

Group:

Xiphophorus nigrensis is one of nine northern swordtails and belongs to the Pygmaeus clade consisting of consisting of X. pygmaeus, X. nigrensis and X. multilineatus.

Type Locality:

Nacimiento del Río Choy, San Luis Potosi, Mexico.

Distribution:

Only found in the Rio Choy.

Habitat:

Found along the length of the Rio Choy, more common at the mouth of the cave where the river originates from, the water at this point is clearer and the current stronger.

The Rio Choy is also known locally as the Rio Florido near the Cave. This river is the northern most tributary of the Rio Tampoan

The biotope is clear & fast flowing & originates from inside a cave under the El Abra Mountain. The biotope further downstream is reportedly slower flowing with dense aquatic plant growth especially near the banks.

Size:

Males 25 – 40 mm; females 40 mm.

Distinguishing characteristics:A species with a three known size variations within the male range, the largest variant is the one most often in the hobby. X. nigrensis in its larger form is a small high backed form of swordtail with a deep body and a medium sized sword of roughly ½ – ¾ total body length.

Colour/Pattern Variability:

Males
There are three size variants ranging from 25mm to 40mm. The larger males tend to be deeper bodied. The body colouration is greyish blue with a single bold zigzag midlateral line, (present from birth) but in fully mature males the colour becomes more of an intense metallic blue and the midlateral line will fade out.

The caudal spot is variable and when present really prominent on the large males. All other fins are generally clear. The sword length varies according to the size variant, the largest males can have a sword length equal to the body length and the smallest 1-2mm. The sword is distinctly upturned particularly when small, the sword has black edging and is clear except on some individuals it can be yellow.

The dorsal fin is generally clear, there is a black edge and a black band one third of the way up the fin.

As with all of the Pygmaeus clade a yellow form of male can be found.

Females:

A basic body colour of greyish blue with a single bold zigzag lateral line. All fins are clear, but the dorsal fin has a faint black edge and a faint black band one third of the way up the fin.

Behaviour:

Shy at first but will become bolder as the colony expands.

Husbandry:

The Aquarium should be well planted with filtration and aeration.

This species is very susceptible to P.H. swings so all water changes should be small but frequent.

Breeding Notes:

Females drop on a monthly cycle. Brood sizes are relatively small being only 2 for smaller females & up to 20 for more mature females. These females are thought to be fertile between 3 – 8 months of age.
Large morph males sex out at 5 months.
It is important to remove virgin females from the brood & mate with the larger form males as the small form will become dominant with the loss of the larger form. Females are not affected by this sex-linked gene which is only known in males.

References:

Donn E. Rosen: Middle-American poeciliid fishes of the genus Xiphophorus.       

Rauchenberger, Kallman & Morizot: Monophyly and Geography of the Rio P’anuco Basin Swordtails (Genus Xiphophorus) with Descriptions of Four New Species                                              Derek and Pat Lambert: Platies and swordtails