Phylum Chordata
Class Osteichthyes
Order Synbranchiformes
Family Synbranchidae

Ophisternon infernale (Hubbs, 1938)

	Ophisternon infernale (Hubbs, 1938)

Synonyms: Pluto infernale Hubbs, 1938

Taxonomic Characterization: Total lack of external eyes and of melanin pigment. Muzzle spatulate. Lack of scales. Head is long and bulb shaped with sensitive pores. Caudal region (37-54%) longer than in other synbranchids (Navarro y Valdés, 1990).

Ecological Classification: Stygobitic

Size: Up to at least 325 mm SL

Number of Species in Genus: Six (Rosen & Greenwood, 1976)

Genus Range: India, Ceylon, Philippines, Northern Australia, Western Australia, Portuguese Guinea, South America, Guatemala, Cuba and Mexico (Campeche, Tabasco, Chiapas, Oaxaca, Veracruz, Yucatan and Quintana Roo).

Species Range: Cenotes and groundwater on the Yucatan Peninsula, Mexico. O. infernale has been reported from caves in the Reserva de la Biósfera de Sian Ka'an in Quintana Roo. This species coexists with Typhliasina pearsei - 100% of the records where Ophisternon was found - and the catfish Rhamdia guatemalensis - 57% of them. Very few specimens (maximum two at the same time) have been observed in very few localities. It occurs in only about 7% of the caves recorded. Underwater exploration should increase the number of recorded sites.

Closest Related Species: This species is closely related to O. aenigmaticum and probably invaded freshwater caves from wetlands during the Pliocene (Rosen & Greenwood, 1976; Schmitter-Soto, 1998b).

Habitat: Freshwater limestone caves

Ecology: Builds mucus-lined burrows in organic rich sediments within the freshwater part of the aquifer. Based on few sightings, it has been observed under the stones in muddy bottoms at the shallow parts of cave cenotes, although Navarro y Valdés (1990) reported a sighting at a depth of 24 m. The principle source of energy in this environment is feces from bats and swallows. Ophisternon feeds on the stygobitic shrimp Creaseria morleyi. It tolerates low oxygen concentration levels and is able to breathe atmospheric oxygen (Schmitter-Soto, 1998a).

Life History: This species has been reported as oviparous (Schmitter-Soto, 1998a).

Evolutionary Origins: O. infernale appears to be clustered in caves located along the old Pliocene shoreline (Wilkens, 1982). Although other cavernicolous species have secondarily spread over northern Yucatan through channels in the subterranean aquifer, both O. infernale and Typhliasina pearsei seem to be primarily restricted to caves and cenotes near their original sites of cave colonization (Wilkens, 1982).

Conservation Status: Endangered (Williams et al., 1989), Endangered (IUCN 1990, 1994), Endangered (criteria A1ac+2c, B1+2c) (IUCN 1996). The IUCN (1996) criteria describe the threats which are: An observed reduction of at least 50% over the past 10 years, or 3 generations, because of a decline in extent of occurrence (EO), area of occupancy (AO), and/or habitat quality; together with a similar reduction over the next 10 years for the same reason. EO less than 5000 km² or AO less than 500 km² plus population known from less than 5 sites with a continuing decline in EO, AO and/or habitat quality (Medina-Gonzáles et al., 2001).
The main threats to this species, and to the syntopic Typhliasina pearsei (Bythitidae), are from human-induced water pollution, particularly bacterial in the form of fecal coliforms and from excess nitrate. If their occurrence is restricted to the shallow parts of caves, they are more exposed to human disturbance, either directly by modification and use of the systems, or indirectly by actions on the surface of the ground, like waste and waste water disposal. The human population obtains its water from the aquifer underlying the plateau and, until recently, disposed of its waste water almost directly back into the same aquifer, often only a few meters from where it was obtained. This situation is particularly acute under the largest city, Mérida although most of the people here, as in other urban areas use piped water, but it seems likely that the same situation will exist elsewhere on the peninsula. In rural areas there is little or no sewage treatment and that industrial and domestic waste is sometimes discarded into caves. The population of the peninsula is growing (censused at 2.9 million in 1995; Mérida estimated at 649,770 in 1995) and this will place greater and greater strain on the water resources of the area.
Wastewater in Mérida is now collected and pumped to various treatment plants. The resulting treated product is now injected into the saline groundwater 200 m below the surface of the aquifer. In theory, the injected water should become saline and remain below the less dense freshwater lens. However in practice, this is not working properly due to leakage at shallower depths causing damage to the freshwater aquifer.
Mérida is only one of many growing towns and cities on the Yucatán Peninsula which has a growing population as well as increasing numbers of tourists. Thus, there is a steadily increasing demand for clean water for domestic purposes and concurrently, the necessity that resulting wastewater be completely and rapidly removed. The continued existence of all of the freshwater organisms of the plateau depends wholly on their water supply. Human water supply should be augmented by the collection (and subsequent treatment) of rainfall to supplement that obtained from the aquifer. All wastewater should be collected and properly treated before injection into the aquifer. Detailed studies of the aquifer need to be made to monitor its health. It is necessary to intensively survey the occurrence of cave and groundwater species such as Ophisternon in order to have an up to date distribution records.

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Contributor: Graham Proudlove, University of Manchester, UK; Roger Medina, Universidad Autónoma de Yucatán, Merida, Mexico