| Sulphur molly | |
|---|---|
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Actinopterygii |
| Order: | Cyprinodontiformes |
| Family: | Poeciliidae |
| Genus: | Poecilia |
| Species: | P. sulphuraria |
| Binomial name | |
| Poecilia sulphuraria (Álvarez, 1948) | |
| Synonyms[2] | |
| |
Thesulphur molly (Poecilia sulphuraria), locally known asmolly del Teapa, is anendangered species ofpoeciliid fish. It isendemic to thesulfur springs of theMexican states ofChiapas andTabasco and one of few fish able to withstand the toxicity ofhydrogen sulfide. The high concentration of hydrogen sulfide in the water forces the fish to spend most of their time breathing on the surface, which exposes them to predatory birds; the species has evolved a wave-making behavior to avoid predation.
Poecilia sulphuraria belongs to the shortfin mollyclade within thesubgenusMollienesia. It is thesister species of the widespread Atlantic molly,P. mexicana.[3] Thetype locality isBaños del Azufre. Closely relatedPoecilia populations have been found in the nearbysulfide springs ofLa Gloria in thePichucalco andIxtapangajoya river drainages and designatedP. sulphuraria.Morphological,phylogenetic, andpopulation genetics analyses have shown the Baños de Azufre population to be more closely related toP. thermalis than to La Gloria population, meaning that they should either be treated as a distinctspecies orP. sulphuraria should be treated assynonymous withP. thermalis.[4]
Poecilia sulphuraria has a deep, compact body with a noticeably slendercaudal peduncle (tail base).[5] The maximum recordedstandard length is 50 mm.[6] The head is broad, and the snout-to-dorsal-fin distance is long, giving the fish a somewhat front-heavy appearance. The lower lip projects beyond the upper one and bears a distinctive flat, plate-like extension that points forward and downward. This unusual lip structure is one of the most striking features of the species. Theteeth are arranged in rows, with the outer teeth flattened, slightly curved backward, and only weakly attached. The eyes are large, and the space between them is broad.[5]
Thecaudal fin is truncate, rather than rounded or forked. Thedorsal fin is short and set far back on the body, especially in females. Theanal fin differs between sexes, with males having agonopodium that is shorter than the head. Thepectoral fins are rounded, and thepelvic fins show the typical features of relatedlivebearing fishes. The dorsal and caudal fins carry scattered dark spots, while the other fins are unmarked.[5]
The body is covered with moderately largescales, with a consistent pattern along the sides and back. The fish are dark on the back and upper sides and silvery below. The darker scales along the upper body often have black edging, which becomes more pronounced toward the back. Thegill cover is dark, and a shadowy patch on the belly reflects the dark internal lining of the body cavity. Some individuals show faint vertical bars toward the rear of the body.[5]
Poecilia sulphuraria is restricted to theAtlantic slope. The type locality isBaños del Azufre and their outlet stream, Arroyo Azufre, within theGrijalva River basin, about 12 km west ofTeapa, on the border between theMexican states ofChiapas andTabasco.[6] A molly population from La Gloria springs (Ojos del Azufre) has also been designated asP. sulphuraria.[7]
Baños de Azufre aresulfur springs, now altered;[6] thespecific epithet refers tosulfur.[5]Water clarity varies from transparent to cloudy, often taking on a bluish-gray hue, while temperatures reach approximately 32 °C. The outlet stream averages is about 6.0 m wide and characterized by weakly to moderately flowing, sometimes nearlystagnant water around 26 °C. Bottom sediments consist ofmarl andclayey mud.Aquatic vegetation is almost entirely lacking; small patches ofgreen algae occur sporadically. Water depth may reach 1.4 m but is more commonly between 0.3 and 0.9 m.[6]
Hydrogen sulfide (H₂S), which is abundant in the sulfur molly's habitat, is a potentrespiratory toxin; the few animals that tolerate it tend to beinvertebrates. The sulfur molly is one of few fish that permanently inhabit sulfidic habitats, and tolerates higher concentrations of hydrogen sulfide than the other speciesendemic to Baños de Azufre,Gambusia eurystoma. Despite occurring only a few metres apart,Poecilia sulphuraria andP. mexicana occupied sharply different chemical environments, indicating a clear division of habitat use. The presence of hydrogen sulfide likely restrictsP. mexicana from entering sulfidic waters, whileP. sulphuraria, although capable of surviving in non-sulfidic conditions, may be kept out of those habitats bycompetition with its sister species.[8]
The physiological or biochemical mechanisms that allowPoecilia sulphuraria to tolerate these toxic conditions have not yet been identified. In spring habitats where hydrogen sulfide levels are particularly high,P. sulphuraria is frequently seen breathing at the water surface. It has two prominent lip extensions, a feature unique among poeciliids, which may enhance surface-based oxygen uptake and represent an adaptation to the severely oxygen-poor conditions of these environments;[8] alternatively, these may be taste organs.[9] Tolerance to hydrogen sulfide varies within the species: fish originating from highly sulfide-richmicrohabitats survive higher H₂S levels than those from areas with lower concentrations, a pattern consistent with either physiological adjustment through exposure or population-level adaptation to sulfide-rich conditions.P. sulphuraria is the sole fish species occupying the sulfurous zones immediately downstream of major sulfide springs.[8]
Poecilia sulphuraria is listed by theInternational Union for Conservation of Nature as anendangered species. Although it isprotected byMexican law, no specific conservation measures are in place.[1]Human pressure differs sharply between the two known populations. The isolated Ojo del Azufre site is only lightly affectedby nearby agriculture, whereas the Baños del Azufre population faces severe disturbance. There, the springs are intensively used for recreation, and the original forested streamside zone has been completelycleared and converted into pastureland, resulting in majorhabitat degradation.[10]
_with_Sulphur_Molly_(Poecilia_sulphuraria)_in_beak%2c_Teapa%2c_Tabasco%2c_Mexico.png)
The diet ofPoecilia sulphuraria is based ondetritus,sulfide bacteria, andchironomids.[11] Sulphur mollies feed on the bottom, but have to spend most of their time breathing at the water surface to survive. This exposes them topredation from birds.[12]P. sulphuraria displays a striking collective defense against bird predators such asgreen kingfishers,great kiskadees,herons andegrets. When attacked from above, thousands of fish dive downward at the same time from the water surface, creating large, repeated surface waves that can continue for up to two minutes. These coordinated wave displays are a highly visible and rhythmic anti-predator behavior, likely confusing birds or by acting as a conspicuous signal that alerts them to detection.[13]
Juveniles measuring 11 mm standard length were collected in mid-February, while an immature female of 19 mm was recorded in mid-December, indicating reproduction likely occurs over an extended portion of the year.[6] Newly born young are about 8.0 mm long.[14]Sulfur mollies produce fewer but larger young compared to related species.[15][16]
