Most of the 150 or so types of cave fish pale in comparison to a newly discovered species from an isolated karst sinkhole in south-western Madagascar. Fish living in perpetual darkness typically share a syndrome of convergent features, including the loss of eyes and pigmentation accompanied by enhanced non-visual sensory structures on the head. The new species, Typhleotris mararybe, is small, about 38mm, and combines dark pigmentation with an absence of eyes and well-developed sensory canals and pores on its head. Its body is uniformly brown in colour, as are the basal one-third of its caudal, pelvic, pectoral and anal fins, whose extremities are without pigment and white.

Cave fishes have been known to exist in Madagascar for a century, but the extent of the fauna and details of their biology, relationships and distributions remain largely unstudied. While the four cave-inhabiting fishes from the island nation are all gobioids, they represent two separate evolutionary lineages.

First is a single species of the Gobiidae genus, Glossogobius, whose other species are found in Africa and Indonesia. Second is the genus Typhleotris of the family Milyeringidae, all three species of which are endemic to Madagascar.

John S Sparks, of the American Museum of Natural History, and Prosanta Chakrabarty, of Louisiana State University, described the new species, giving it a name with a unique derivation. The specific epithet is from the Malagasy words "marary", meaning ill or sick, and "be", meaning big.

The resulting name, mararybe (or "big sickness"), commemorates a strange and debilitating viral "sinkhole fever" that members of the field team suffered after diving in Grotte de Vitane. The Grotte de Vitane sinkhole is on the arid coastal plain below and west of the Mahafaly plateau in south-western Madagascar, near the town of Itampolo, and is a challenging place to collect. The two specimens of the new species collected by the senior author are a tribute to his resolve. After descending to the water by way of a chain ladder, it took four hours swimming and snorkelling to capture them with a handheld net. Both were caught at a depth of no more than 1.5 metres.

The sinkhole is a sacred site visited by locals who come to offer prayers. Interestingly, while locals were aware of other Malagasy cave-inhabiting fish before their discovery by scientists, T mararybe was unknown to them, even though they sometimes descend to the water climbing down tree roots.

The authors make a compelling case that the new species evolved from an ancestral species that was both blind and unpigmented and that pigmentation has been secondarily regained. The sinkhole is part of a vast and unmapped system of subterranean waterways, explaining the arrival of an ancestor. The dark pigmentation of the new species is correlated with the direct sunlight that portions of the sinkhole receive and perhaps make the fish more difficult to see. T mararybe is a rather slow swimmer, like the other two species in the genus, although it was observed to move away from approaching objects more energetically than the related species T madagascariensis and to escape by diving.

There is evidence that T madagascariensis and T mararybe are sister species, sharing a number of fin and scale characters that aren't shared by a third species, T pauliani. Both morphology and molecular data point to a relationship between Typhleotris and an Australian genus, Milyeringa, that occurs in very similar habitats. The available geographic and geologic evidence associated with the fish are intriguing. While much of the coastal plain is younger Quaternary sandstone, the genus is so far restricted to older limestone karst of the Eocene age.

The two previously known species of the genus are both red listed by the IUCN as endangered. T pauliani is found in caves and sinkholes to the north of the Onilahy river drainage, while T madagascariensis is found to its south in the Mahafaly plateau and in isolated caves, sinkholes and wells on the coastal plain to the west. Given the single locality of the new species, it is almost certainly endangered as well. It would not be surprising to find additional species of the genus, but many sinkholes are accessible only to very experienced cavers and are located in remote and hostile environments, often a day or more's travel by ox cart from the nearest village.