Taxonomy & naming
Monopterus rongsaw was described in 2018 by Ralf Britz, David Sykes, David Gower, and Rachunliu Kamei from a single holotype specimen, BNHS FWF 611, measuring 7 in total length, collected at Nongriat Village near Sohra (Cherrapunji) in the Khasi Hills, Meghalaya, India. The original description placed the species in the genus Monopterus, the most species-rich genus in the family Synbranchidae. The Catalog of Fishes (Eschmeyer, CAS) subsequently records it under the combination Rakthamichthys rongsaw, reflecting a 2020 generic reclassification that erected Rakthamichthys for hypogean members of the Monopterus lineage with these specific morphological traits; both combinations appear in the literature and the underlying identity of the taxon is not in dispute.
Synbranchidae belongs to the order Synbranchiformes — the same order that contains the spiny eels of the family Mastacembelidae — and has no close relationship with the true eels of the order Anguilliformes. The eel-like body of swamp eels is a product of convergent evolution on an elongate, burrowing way of life, and the two groups differ in virtually every anatomical detail that is not directly related to body shape. Within Synbranchidae, Monopterus (sensu lato) is distributed across South and Southeast Asia, with some species introduced and invasive elsewhere; the Khasi Hills population belongs to a distinctly subterranean lineage adapted to life in soil and rock crevices, paralleled only by Monopterus luicolus among known swamp eels.
The epithet rongsaw was taken from the local Khasi word for the species or its habitat; the etymology is documented in the original description.
Morphology
The single known specimen measures 7 in total length. Like all swamp eels, the body is extremely elongate, cylindrical toward the head and laterally compressed posteriorly, with no pectoral or pelvic fins; the dorsal, anal, and caudal fins are reduced to a low continuous fold around the tail tip. The gill apparatus is reduced and the fish relies primarily on aerial respiration through a modified branchial chamber.
The defining diagnostic features that distinguish M. rongsaw from other Synbranchidae are its complete absence of skin pigmentation and its very small eyes, which are covered by skin and non-functional. The loss of both pigment and eyes is characteristic of obligate hypogean fishes and has arisen independently in several lineages of cave and soil vertebrates. The absence of pigmentation exposes the underlying blood vessels, producing the striking hot-pink coloration that attracted popular attention at the time of the species' description. The vertebral formula of 92 precaudal plus 69 caudal vertebrae (161 total), recorded from the holotype, is consistent with other swamp eels of comparable size.
Habitat
The type locality is a patch of secondary rainforest at roughly 440 metres elevation near Nongriat Village, Sohra district, Khasi Hills, Meghalaya, India. The single specimen was found while researchers were excavating moist, rocky soil in search of caecilians (limbless amphibians), at a depth of approximately 40 centimetres, about 50 metres from an unnamed stream. This places the species in a terrestrial, soil-dwelling microhabitat unlike the aquatic and semi-aquatic settings that most Synbranchidae occupy.
The Khasi Hills receive some of the highest annual rainfall on Earth — Cherrapunji (Sohra) holds long-standing world records — and the soils of the lower forest here remain saturated much of the year. This permanently moist substrate appears to be the habitat that permits a fish to live terrestrially: the soil provides the continuous film of water over the body surface, and the modified air-breathing apparatus of swamp eels tolerates the low dissolved-oxygen conditions of waterlogged soil. Beyond this single observation, nothing is documented about the species' microhabitat preferences, population density, or depth range.
Feeding
No feeding observations exist for Monopterus rongsaw. With only one specimen ever collected — the holotype used for the formal description — the diet is entirely unknown.
By analogy with other Monopterus species, the fish is likely a predator of soil invertebrates: earthworms, insect larvae, small arthropods, and similar prey encountered in the subterranean environment. Other hypogean fishes worldwide tend to be opportunistic predators of whatever invertebrate fauna shares their habitat, relying on chemosensory and mechanosensory input rather than vision. Whether M. rongsaw retains functional chemosensory systems, and how these compare with its surface-dwelling relatives, has not been studied.
Mating
Nothing is known of the mating behaviour of Monopterus rongsaw. No second specimen has been found, no females have been identified, and no reproductive structures were reported from the holotype.
Within Synbranchidae, and particularly within Monopterus, the dominant reproductive strategy is protogynous hermaphroditism: individuals are born and first mature as females, and a proportion change sex to male as they grow larger. In the better-studied species such as Monopterus albus, the male constructs a floating nest of mucus bubbles at the entrance to a bank burrow, and both courtship and egg-deposition occur in association with this nest. Whether the hypogean lifestyle of M. rongsaw has modified or abolished this system — and whether a soil-dwelling fish can build a surface bubble-nest at all — is an open question that cannot be answered from one specimen.
Breeding
Breeding in Monopterus rongsaw is entirely undocumented. In the wider genus, males typically build and guard a floating bubble-nest of mucus at a burrow opening, and attend both eggs and larvae through early development — a degree of parental care unusual among swamp eels. Females deposit eggs into the nest and parental duties are primarily male. The species is not in the aquarium trade and has never been maintained in captivity.
Given the completely subterranean habitat and the single-specimen data, it is not possible to say whether this species follows the standard Monopterus reproductive pattern, has evolved a modified strategy adapted to soil life, or whether protogynous sex change occurs on the same schedule as in surface relatives. The breeding biology must remain marked as unknown pending the collection of additional material.
In the aquarium
Monopterus rongsaw is not available in the aquarium trade and should not be considered a potential aquarium fish. The species is known from a single specimen collected from its type locality; its rarity, its narrow endemic range, and the complete absence of any captive-husbandry data mean it has no place in home aquaria.
For completeness: like other swamp eels, any specimen would require a sealed, escape-proof enclosure — swamp eels are air-breathers that can travel short distances over moist ground and are adept at squeezing through gaps — and would be sensitive to the copper-based medications that cannot be used safely with scaleless fish. The species' soil-dwelling habits would presumably require a deep, saturated substrate rather than an open water column. These points are noted for biological context only; collection of this fish from its only known locality would be ecologically irresponsible and is likely contrary to Indian wildlife protection frameworks.
Conservation
Monopterus rongsaw has not been formally assessed by the IUCN Red List. At the time of writing it carries a status of Not Evaluated (NE), almost certainly because the species was described only in 2018 and the data available — a single specimen, a single locality — are insufficient for a standard IUCN assessment under current criteria.
The conservation picture is nonetheless stark. The entire documented range of the species consists of one patch of secondary forest beside one unnamed stream in the Khasi Hills. The Khasi Hills are part of the broader Northeast Indian biodiversity hotspot, a region under persistent pressure from agriculture, plantation expansion, deforestation, and road construction. The forests around Nongriat and Sohra are in somewhat better condition than many parts of the region — the area is a well-known ecotourism destination — but the microhabitat of moist forest soil is inherently vulnerable to any disturbance that compacts the ground, alters drainage, or removes canopy cover. A species known from one site and one individual has, by definition, no buffer against local extinction events. A formal IUCN assessment, were one conducted, would likely yield a Data Deficient or Critically Endangered categorisation on geographic-range grounds alone.