Taxonomy & naming
Mesonoemacheilus herrei was described by the loach specialists Theodor Nalbant and Petru Bănărescu in 1982, in the genus Mesonoemacheilus, part of the family Nemacheilidae — the stone loaches, the largest and most species-rich family within the loach superfamily Cobitoidea. The Catalog of Fishes, the authority for valid fish names, retains the species in Mesonoemacheilus, and that combination is used here without alteration.
The species epithet honours Albert W. Herre (1868–1962), the American ichthyologist who collected the type specimen in 1941 in the Anaimalai Hills. Nemacheilid taxonomy in the Western Ghats has been reshuffled repeatedly over the decades as older catch-all genera such as Nemacheilus and Noemacheilus were split apart; Mesonoemacheilus is one of the resulting genera recognised for a cluster of small, southern-Indian hill-stream loaches.
Morphology
Like other members of its family, Mesonoemacheilus herrei is a small, elongate, cylindrical loach built for holding position on the bottom of fast-flowing water rather than for open swimming. Seriously Fish records an adult standard length of only about 2–2.5 in, making it one of the smaller stone loaches in the region. As in related Mesonoemacheilus species, sexually mature males develop a fleshy suborbital flap and tuberculate (roughened) pectoral-fin rays — secondary characters typical of breeding-condition male nemacheilids rather than a true erectile spine of the kind seen in the spined and botiid loaches.
As is typical of the family, the skin carries small, deeply embedded scales and no heavy body armour, leaving it — like most loaches — comparatively sensitive to harsh water conditions and medications. Beyond this, detailed morphometric data for the species are sparse; FishBase itself notes the record is thin on measurements, and Seriously Fish's care profile remains the most complete published account of its appearance and habits.
Habitat
This species is endemic to the Anaimalai Hills near Valparai, in the Western Ghats of Tamil Nadu, southern India — a single, narrowly defined upland catchment and nowhere else on Earth. It inhabits clear, fast-flowing hill streams that run cool and highly oxygenated over stone and gravel substrates, the classic environment of a stone loach: cascading, well-aerated water rather than the slow, still, warm water many aquarium fish are adapted to.
Seriously Fish records a working temperature range of roughly 64–78 °F, distinctly cooler than most popular tropical aquarium fish, with a near-neutral to slightly alkaline pH of about 6.0–7.5. As a single-stream endemic restricted to one hill range, its entire habitat is small in area and correspondingly vulnerable to any disturbance within that catchment.
Feeding
Mesonoemacheilus herrei is recorded as omnivorous but with a clear preference for small aquatic invertebrates — the diet of a benthic micro-predator picking its way over stream-bed stone and gravel for insect larvae and other small prey rather than a herbivore or detritivore.
In the aquarium context reported by Seriously Fish, it takes bloodworm, brine shrimp (Artemia) and Daphnia readily, and a keeper should expect to offer small live, frozen or sinking foods rather than rely on flake, which a bottom-feeding stream loach like this is unlikely to prioritise.
Mating
Specific courtship and spawning behaviour for Mesonoemacheilus herrei has not been documented. What is known is limited to secondary sexual characters: mature males develop a fleshy flap below the eye and roughened tubercles on the pectoral-fin rays, features associated with breeding condition in this genus, which presumably play some role in courtship or male competition as in other nemacheilids.
A close relative, M. triangularis, has spawned in captivity, giving some indication of what the genus may do — eggs hatching in roughly 24 hours with fry reared on brine shrimp nauplii — but this has not been reported for M. herrei itself, and the data for this species specifically remain sparse.
Breeding
Breeding of Mesonoemacheilus herrei has not been recorded in the aquarium hobby. Like other stone and brook loaches, the expected mode — based on the family and the related M. triangularis spawning — is egg-scattering over gravel or stones in fast-flowing water, with no parental care once the eggs are released.
This species is essentially unknown in the hobby and is rarely, if ever, exported, so there is no established captive-breeding record to draw on. Anyone encountering it should treat it as an unbred, poorly documented species rather than assume any particular spawning routine will work; the honest picture is that home breeding of this fish has not been demonstrated.
In the aquarium
Mesonoemacheilus herrei is not an established aquarium fish — Seriously Fish notes it is unrecorded in the hobby and rarely exported, so any care guidance is necessarily drawn from its known wild ecology rather than from a body of keeper experience. Anyone who does obtain it should replicate a cool, clear, fast-flowing, highly oxygenated hill-stream setup: efficient filtration producing strong current, a stone-and-gravel substrate, and stable water in the 64–78 °F, pH 6.0–7.5 range reported for its native streams — decidedly cooler than a typical tropical community tank.
As with other small, near-scaleless loaches, caution with medications and copper-based treatments is prudent even though specific sensitivity data for this species have not been published. Given how restricted and poorly known this fish is, and its Critically Endangered status in the wild, it is not a species to source casually from the wild-collected trade; any interest in keeping it should be weighed against the acute conservation pressure on its single native stream system.
Conservation
Mesonoemacheilus herrei is listed as Critically Endangered on the IUCN Red List, assessed 30 March 2010 under criteria B1ab(iii)+2ab(iii) — criteria tied to an extremely restricted range and continuing decline in the extent or quality of habitat. As a single-catchment endemic confined to streams in the Anaimalai Hills near Valparai, its entire population occupies a small, well-defined area, which is precisely what makes B-criteria assessments like this one so severe: any degradation of that one stream system threatens the whole species.
The Western Ghats, including the Anaimalai Hills, face ongoing pressure from plantation agriculture (notably tea and coffee), hydropower development and associated water diversion, and general land-use change — the kinds of catchment-level disturbance that most directly threaten a narrow-range hill-stream fish like this one. There is no indication the aquarium trade is a meaningful driver of decline for this species, since it is rarely if ever exported; the conservation story here is almost entirely about habitat integrity in one small corner of southern India.