The Yucatán freshwater ecoregion, FEOW ecoregion 175, is a tropical and subtropical coastal-river and karstic groundwater ecoregion of southeastern Mexico. It includes most of the Yucatán Peninsula, especially the states of Yucatán, Campeche, and parts of Quintana Roo, and is strongly shaped by limestone geology. Unlike many Mexican ecoregions, it has few large surface rivers. Much of its freshwater occurs underground or in cenotes, sinkholes, caves, springs, coastal lagoons, mangroves, marshes, and brackish wetlands.

The ecoregion’s hydrology is dominated by karst. Rainwater rapidly infiltrates the porous limestone and moves through underground channels, aquifers, caves, and cenote systems before reaching coastal lagoons, wetlands, mangroves, or the sea. Surface drainage is limited or discontinuous in many areas, so aquatic habitats may appear as isolated cenotes, aguadas, ponds, cave waters, or coastal wetland systems. In reality, many of these waters may be connected underground. This makes the Yucatán fish fauna unusual: some fishes live in open cenotes and lagoons, while others inhabit subterranean waters where light, food, and dispersal are very different from ordinary rivers.

The native fish fauna is moderately species-rich but highly distinctive due to the combination of karstic groundwater, coastal wetlands, brackish habitats, and isolated lakes and lagoons. FEOW lists several strict endemic fishes, including Fundulus grandissimus, Fundulus persimilis, Floridichthys polyommus, Cyprinodon suavium, and the Mexican blind brotula, Typhliasina pearsei. The fauna also includes poeciliids, cyprinodontids, cichlids, characids, catfishes, atherinopsids, gobies, sleepers, mullets, eels, snooks, gars, and other peripheral or estuarine-associated fishes.

Cyprinodontids and fundulids are among the defining non-cichlid fishes of the ecoregion. Fundulus grandissimus, Fundulus persimilis, and Floridichthys polyommus are endemic or strongly characteristic killifishes of the Yucatán region. These fishes are associated with coastal lagoons, marshes, mangrove edges, brackish waters, and sometimes inland or groundwater-influenced habitats. Cyprinodon suavium is a particularly important endemic pupfish restricted to Laguna Chichancanab, a lake system that has played an important role in the evolution of localized pupfish diversity.

Cave and groundwater fishes are among the most remarkable elements of the Yucatán fauna. Typhliasina pearsei, the Mexican blind brotula, is an endemic troglobitic fish of Yucatán aquifers, caves, and cenotes. It is blind, scaleless, and adapted to subterranean life. The blind swamp eel Ophisternon infernale is another highly specialized subterranean fish known from Yucatán caves and cenotes, where it may occur together with Typhliasina pearsei. These fishes show that the ecoregion’s biodiversity is not confined to visible surface waters; some of its most unusual species live in the dark aquifer itself.

Poeciliids are also important throughout the Yucatán ecoregion. Species of Poecilia, Gambusia, Heterandria, Belonesox, and related genera occur in cenotes, springs, lagoons, marshes, mangroves, ponds, and disturbed lowland waters, depending on locality. The sailfin molly Poecilia velifera is especially characteristic of the Yucatán region and is associated with coastal, brackish, and freshwater habitats. Livebearers are often abundant in warm, vegetated, shallow, or variable waters and form an important part of the native fish community.

Characids and catfishes add another important freshwater component. Astyanax species occur in cenotes, ponds, and connected waters, while catfishes such as Rhamdia may occur in cenotes, cave systems, and open freshwater habitats. Work on Yucatán cenote fishes has specifically compared surface-dwelling Rhamdia guatemalensis with the troglobitic Typhliasina pearsei, emphasizing how different levels of subterranean adaptation may occur within the same regional aquifer system. Other fishes, including atherinopsids and peripheral coastal taxa, occur in lagoons, wetlands, and brackish transitions.

Peripheral and estuarine-associated fishes are central to the ecoregion’s identity. Gobies, sleepers, mullets, eels, snooks, gars, and other coastal or brackish-tolerant fishes may use lagoons, mangroves, freshwater springs, cenote outlets, estuaries, and coastal wetlands. In the Yucatán Peninsula, the boundary between freshwater and marine influence is often gradual, especially where groundwater discharges into coastal lagoons or mangrove systems. For FFM, these fishes should be treated as part of the ecoregion’s ecological character, not as mere visitors from the sea.

Cichlids are native and important in the Yucatán ecoregion. They do not form a lake-type species flock, but they are central members of the peninsula’s cenote, lagoon, marsh, mangrove, wetland, and lowland freshwater assemblages. The most important native cichlid is Mayaheros urophthalmus, the Mayan cichlid or mojarra castarrica. It is native to the Atlantic slope of Mexico and northern Central America, including the Yucatán Peninsula, and is one of the most widespread freshwater and brackish-tolerant cichlids of the region. It occurs in cenotes, springs, rivers, marshes, mangrove swamps, coastal lagoons, and brackish waters, and can tolerate high salinity.

Mayaheros urophthalmus is especially important for FFM because it links the Yucatán’s freshwater, brackish, and coastal habitats. FishBase describes adults as inhabiting freshwater marshes and mangrove swamps, preferring coastal lagoons and rivers, and tolerating marine conditions. It feeds on small fishes and macroinvertebrates and spawns on the bottom in both fresh and brackish water. In the Yucatán Peninsula, studies describe it as occupying freshwater and brackish cenotes, springs, and coastal lagoons, making it one of the best examples of a native cichlid adapted to the peninsula’s karstic and coastal environment.

Other cichlids may occur in the broader Yucatán and adjacent southeastern Mexican context, but locality and native status must be handled carefully. Petenia splendida, Rocio octofasciata, Trichromis salvini, Thorichthys meeki, and other Middle American cichlids may be relevant in connected or neighboring drainages, lagoon systems, or adjacent ecoregions, but they should not automatically be assigned to every Yucatán locality. Many older records use broad Cichlasoma names, so current taxonomy should be checked carefully.

Taxonomic caution is especially important because Yucatán fishes are often recorded from cenotes, caves, lagoons, or coastal wetlands rather than from conventional river systems. A record from a cenote, cave, coastal lagoon, mangrove channel, Laguna Chichancanab, Campeche wetland, or Quintana Roo coastal system may represent very different ecological and biogeographic contexts. Names such as Cichlasoma urophthalmum, Heros urophthalmus, and Mayaheros urophthalmus may refer to the same Mayan cichlid under different historical combinations, but other old Cichlasoma records may represent different species.

Introduced fishes should be treated separately from the native fauna. Tilapiines such as Oreochromis niloticus, Oreochromis mossambicus, and related aquaculture species may occur in ponds, canals, reservoirs, modified wetlands, and disturbed lowland waters, but they are not part of the native fauna of Yucatán. Other non-native fishes may arrive through aquaculture, aquarium releases, mosquito-control introductions, or accidental movement between water bodies. Introductions are especially risky in cenotes, caves, isolated lagoons, and small wetlands, where native species may have limited opportunities to escape or recolonize.

Conservation priorities include protecting cenotes, caves, aquifers, springs, Laguna Chichancanab, coastal lagoons, mangroves, wetlands, marshes, groundwater quality, and freshwater-to-brackish ecological connections. Major threats include groundwater pollution, sewage, agricultural runoff, tourism development, urban expansion, solid waste, aquifer contamination, salinization, mangrove loss, wetland drainage, invasive fishes, and climate-driven changes in rainfall, drought, hurricanes, and sea level. Because much of the freshwater system is underground, pollution can move invisibly through the aquifer and affect cenotes and coastal discharges far from the original source.

This account is based in part on information from Freshwater Ecoregions of the World (FEOW), © WWF/TNC, used as a reference for educational and scientific purposes. See: http://www.feow.org.