Relict Splitfin

Relict Splitfin
Xenoophorus captivus

(Hubbs, 1924)

Also known as: Relict Goodeid, Captive Splitfin, Mexclapique viejo

Max Size: 5 cm TL (males) / up to 6 cm TL (females; SL to about 66 mm)

The only species in its genus — a compact, moderately deep-bodied goodeid with a quiet metallic glow. The body is grey to brown with a lighter throat and belly. Flank scales flash green, greenish-blue, or coppery iridescence, especially along the sides and caudal peduncle. A dusky midlateral band is often present but usually hidden under that sheen. The caudal fin has a thin white to yellowish trailing edge, most visible on dominant males. Sexes are similar in color; males are told apart by the classic goodeid splitfin (notched anal fin) and a slightly larger dorsal fin.

The name captivus (“captive” or “prisoner”) refers to an ancient stream-capture event that isolated these fish from the Lerma fauna and left them stranded in the Pánuco and two nearby endorheic basins. Former names erro (“wanderer”) and exsul (“exile”) tell the same story.

Origin & Habitat
Endemic to San Luis Potosí, Mexico, in three isolated groups:

• Xenca1 — upper Río Santa María / Pánuco drainage (type area around Jesús María, plus Bledos and Santa María del Río)
• Xenca2 — Illescas springs (near the Zacatecas border) — extinct in the wild
• Xenca3 — springs at Venado, Moctezuma, and Agua de Enmedio

It lives in shallow springs, spring brooks, small streams, and ditches (usually under 1 m), with slight to moderate current, over silt, mud, sand, gravel, or rock. Water is clear to murky and often heavy with algae. Plants include green algae, Lemna, Potamogeton, Ceratophyllum, Nasturtium, and water hyacinth. Temperatures commonly range from the mid-teens into the mid-20s °C.

Diet
Primarily a herbivorous grazer. A long, coiled gut and incisor-like outer teeth point to algae and aufwuchs as the main foods, with some detritus and occasional small invertebrates. In the aquarium it takes high-quality flakes, granules, tablets, freeze-dried foods, and green matter; strong lighting that grows algae is a plus.

Behavior & Temperament
Peaceful and not especially shy. It does well in groups in a well-structured tank with moderate current, rocks, roots, and patches of dense plants plus open swimming space. Fry are rarely eaten, so colonies can build quickly.

Reproduction
Viviparous livebearer. Breeding typically runs from February through July; in warm springs it may continue year-round. Gestation is about 55–60 days, with small broods of relatively large young. Males possess the goodeid andropodium.

Conservation Status
IUCN: Endangered (EN).
Mexican NOM-059: En Peligro de Extinción.

Range and numbers have shrunk sharply since the 1970s–80s because of groundwater pumping, spring diversion for agriculture, drought, pollution, and invasive fishes. Xenca1 survives only in a few small Santa María sites; Xenca3 persists in small spring populations; Xenca2 (Illescas) vanished when its springs dried in the mid-1990s and now exists only in captivity. Locality lines should be kept separate. Captive breeding in Mexico, the United States, and Europe is now the main insurance for this isolated lineage.

A metallic highland relict — grey-brown with a copper-green flash — left behind by an ancient river capture and now clinging to a handful of springs in San Luis Potosí.