Mexican Cenote and Livebearer Spring Biotope

Hard, alkaline, transparent karst water of Yucatán cenotes and Mexican springs, the Poecilia and Xiphophorus livebearers that live there, and limestone, plants, light and flow for the aquarium.

Karst springs and cenotes: where the water comes from

The Yucatán Peninsula is a flat platform of porous limestone, dolomite and evaporites. Rain absorbs carbon dioxide from the air and soil, forming weak carbonic acid that dissolves the calcium carbonate of the bedrock into calcium and bicarbonate ions; this is the process that makes karst groundwater hard and alkaline and that hollows out caves beneath the surface. In the northern half of the peninsula surface rivers are virtually absent, and the only access to fresh water is through cenotes, natural pits formed when the roof of a dissolved cavity collapses and exposes the aquifer. An estimated 10,000 cenotes exist on the peninsula, at least 6,000 of them documented, including the Ring of Cenotes that traces the buried rim of the Chicxulub impact crater.

Cenote water is transparent because it is rainwater that has filtered slowly through rock and carries almost no suspended particles. Near the coast the fresh lens floats on denser saline water intruding from the sea, separated by a halocline. Wikipedia distinguishes four shapes: jug-like cenotes with a narrow opening, open vertical-walled pools, shallow basin-like aguadas, and cave entrances. Fish, eels and frogs live in them, and bats depend on them for water in a landscape without rivers. Annual rainfall ranges from under 800 mm in the dry north-west to about 2,000 mm in the south, peaking in August and September.

Water chemistry: hard, alkaline, clear

A peer-reviewed study of four cenotes in the Ring of Cenotes (X'batun, X'Canche, Kankirixche and Yokdzonot) measured temperatures of 25.2–27.7 °C, pH 6.73–7.49, conductivity of 902–2,320 µS/cm and total dissolved solids of 676–1,440 mg/L, with dissolved oxygen of roughly 3–5 mg/L and nitrate up to 7.52 mg/L at the most visited site. These values are far above those of Amazonian rivers and reflect dissolved limestone plus, at some sites, saline influence. Practical Fishkeeping's cenote-inspired layout used hard, alkaline tap water at 26 °C and kept the pH at or above neutral even while injecting carbon dioxide for plants.

The livebearers themselves confirm the pattern. FishBase lists Poecilia velifera at pH 8.0, 13–19 dH and 25–28 °C, and Xiphophorus hellerii at pH 7.0–8.0, 9–19 dH and 22–28 °C. Seriously Fish gives pH 7.0–8.5 and 15–35 dGH for Poecilia velifera, pH 7.0–8.5 and 15–30 dGH for Poecilia sphenops, pH 7.0–8.0 and 10–25 dGH for Xiphophorus hellerii and pH 7.0–8.2 with 10–30 dGH for Xiphophorus maculatus and Xiphophorus variatus.

Livebearers of Mexican springs and cenotes

Poecilia

Poecilia velifera is confined to the Yucatán Peninsula below 20 m elevation, from Tulum to the Campeche coast and including Isla Mujeres and Cozumel. It lives in lagoons, estuaries, salt marshes and freshwater cenotes, in still or slowly moving water at 25–30 °C, usually less than 1 m deep; although it tolerates a broad range of salinities, it grows faster and larger and matures earlier in fresh water. FishBase gives a maximum of 15 cm TL and an IUCN category of Vulnerable (2018; verify the current entry), and Wikipedia attributes the decline to habitat loss from urbanisation and road building, with about two of 28 known sub-populations already gone. Most fish sold as sailfin mollies are hybrids with Poecilia latipinna; Seriously Fish notes the pure species has 18–19 dorsal-fin rays against 14. Poecilia sphenops ranges from Mexico south to Colombia and Venezuela and reaches 8 cm SL, feeding on benthic invertebrates and detritus; Wikipedia also records cave-dwelling and hydrogen-sulfide-tolerant populations of Poecilia mexicana in the genus.

Xiphophorus

The genus Xiphophorus contains 28 recognised species native to Mexico and northern Central America, from the southern Rio Grande basin along the Atlantic slope to Guatemala, Belize and northern Honduras; nine species are restricted to the Pánuco basin and three to the Coatzacoalcos. Xiphophorus hellerii, the green swordtail, occurs from the Río Nautla in Veracruz to the Usumacinta drainage, from sea level to about 1,500 m, in fast rocky streams, springs, ditches, ponds and rivers with clear to turbid water. Most of these waters are less than 1.5 m deep with little aquatic vegetation; adults favour stronger current while juveniles stay in quiet margins. It reaches 14 cm SL and is listed as Least Concern (2018; verify the current entry). Xiphophorus maculatus lives on the Atlantic coastal plain from Veracruz to Belize in slow canals, ditches and warm springs and reaches 5–6 cm; the IUCN lists it as Data Deficient. Xiphophorus variatus inhabits shallow, heavily vegetated streams, ponds, swamps, canals and ditches of the Mexican Atlantic slope. Four Xiphophorus species are threatened, two Endangered and two Extinct in the Wild.

Practical Fishkeeping's two Mexican layouts illustrate typical companions: a cenote tank with wild-type mollies of the Poecilia cf. salvatoris group, and a Usumacinta river tank pairing Poecilia velifera with the cichlid Thorichthys meeki, which share that drainage.

Limestone hardscape

The natural setting is dissolved and collapsed limestone, so calcareous rock is the appropriate hardscape. The same karst chemistry works in the aquarium: carbonate rock in contact with carbon-dioxide-rich water releases calcium and bicarbonate, which helps hold hardness and pH in the alkaline range these fish need. Practical Fishkeeping's cenote layout combined pale sand, flat stone slabs and pre-soaked driftwood; its river layout followed the observation that most Central American rivers share rounded rocks, driftwood and patches of leaf litter. Pale sand suits the transparent water of a cenote, where light reaches the bottom, while rounded stones and a few larger boulders suit a swordtail stream.

Plants, light and flow

Practical Fishkeeping notes that Chara, a calcium-loving green alga, grows in the natural cenote habitat and substituted Riccia fluitans tied to stone in the aquarium, lit for 8 hours a day. Seriously Fish describes Xiphophorus maculatus and Xiphophorus variatus as appreciating a planted aquarium, whereas swordtail streams carry little vegetation; the choice depends on the species. Vegetable matter belongs in the diet: Seriously Fish recommends blanched spinach or vegetable-based dried food for Poecilia velifera and Poecilia sphenops, and FishBase lists plant matter alongside worms, crustaceans and insects for both Poecilia velifera and Xiphophorus hellerii. Flow is a design choice as well: cenotes are still, while adult Xiphophorus hellerii seek current, so a cenote tank should be gently filtered and a spring-stream tank given stronger circulation. Males of all these species pursue females persistently, and Seriously Fish advises keeping more females than males.