Taxonomy & naming
Pethia arunachalensis was described in 2020 by Shangningam, Kosygin and Chowdhury on the basis of specimens collected from the Nao-dhing River, a tributary of the Brahmaputra Basin in Arunachal Pradesh, north-east India. The description was published in Records of the Zoological Survey of India (volume 119, issue 4, pages 328–333), a peer-reviewed journal of the Zoological Survey of India that has published species novelties from the subcontinent for well over a century.
The genus Pethia was erected as part of the broad revision of the old catch-all genus Puntius by Pethiyagoda and colleagues in 2012, which broke that heterogeneous assemblage into several smaller, better-defined genera on morphological and phylogenetic grounds. Pethia now contains a growing number of small, barbel-less, spotted or blotched barbs from the Indian subcontinent and Sri Lanka. Pethia arunachalensis fits within this group by its lack of rostral and maxillary barbels, and its incomplete lateral line — characters shared with other Pethia from peninsular and north-east India. No subspecies or synonyms are recorded.
Morphology
Pethia arunachalensis is a small, laterally compressed barb with a maximum recorded standard length of 1.5 in, placing it among the smaller members of the genus. The species is distinguished by an incomplete lateral line with only six to seven pored scales, well short of the full series that runs from shoulder to caudal base in most cyprinids. Rostral and maxillary barbels are absent, a character shared with most Pethia and a key feature separating the genus from related barb groups that retain at least one barbel pair.
A distinct black spot marks the caudal peduncle, and the original description lists this spot as one of the diagnostic features separating P. arunachalensis from its congeners. Detailed meristic and body-proportion data appear in the original description in Records of the Zoological Survey of India; additional colour data in life are not extensively documented in the published literature to date, given the species' recent discovery.
Habitat
The type locality is the Nao-dhing River in Arunachal Pradesh, a tributary feeding the Brahmaputra drainage system in the eastern Himalayan foothills. The rivers and streams of this region are characterised by swift, well-oxygenated water draining through forested hill terrain; they tend toward neutral to slightly alkaline pH and are generally cool and clear, carrying sediment loads that vary with monsoon flow.
Arunachal Pradesh lies in a zone of high rainfall and is climatically distinct from the warmer lowland plains — streams here can be considerably cooler than the great Brahmaputra itself. No detailed physico-chemical survey of the collection site is available in the published record, but the regional environment of hill-stream tributaries in this part of north-east India suggests cool, well-aerated, near-neutral water with moderate current.
Feeding
No dedicated feeding study exists for Pethia arunachalensis. Like other small Pethia, the species is presumed to be an opportunistic omnivore, taking small invertebrates, algae, fine plant material, and organic detritus from the substrate and water column — a foraging strategy shared across hill-stream barbs of comparable size on the Indian subcontinent.
In aquarium conditions, small Pethia generally accept finely crushed flake and micro-pellet foods alongside live and frozen items such as micro-worm, daphnia and baby brine shrimp. A varied diet would be appropriate for any individual that found its way into specialist care.
Mating
Pethia arunachalensis has not been studied in spawning behaviour, but all members of the genus are egg-scattering, non-guarding spawners consistent with the broader barb pattern. In courtship, a ready male pursues and drives a gravid female through vegetation or over the substrate, the two pressing briefly together as eggs and milt are released simultaneously.
No pair bond is formed, and no territory is held beyond transient courtship chasing. Eggs are adhesive and fall among plants, roots or fine gravel — left to the adults, they are typically eaten. There is no nest construction, no bubble-nest, and no mouthbrooding in any Pethia species.
Breeding
No captive breeding records for Pethia arunachalensis are available in the literature. By analogy with other small, barbel-less Pethia from north-east India and the Western Ghats that have been spawned in aquaria, a likely approach would involve a small, well-filtered tank with fine-leaved plants or a spawning mop, soft to moderately hard neutral water, cool temperatures reflecting the hill-stream origin of the type locality, and removal of adults after the spawning act to protect eggs from predation.
Females scatter a relatively small clutch of adhesive eggs that would be expected to hatch within one to two days at cool temperatures, with free-swimming fry appearing a few days later. The species is not established in the ornamental trade and has not been produced commercially.
In the aquarium
Pethia arunachalensis is effectively unknown in the aquarium hobby. It was described as recently as 2020 from a remote river system in Arunachal Pradesh, and there is no record of it reaching specialist fishkeepers or being bred in captivity. Its small size — under 1.5 in standard length — and hill-stream origin would make it a candidate for a cool, well-oxygenated, lightly planted nano or species tank if it were ever collected and exported, but no husbandry data exist.
Any hobbyist fortunate enough to acquire specimens would be advised to maintain them in a group in cool, clean, well-aerated water and to document their behaviour and breeding attempts, as virtually nothing beyond the taxonomic description is on record.
Conservation
Pethia arunachalensis has not been assessed by the IUCN Red List; it was described only in 2020 and the assessment process for recently described species from the Brahmaputra region has not yet caught up with the newest additions. FishBase records its status as Not Evaluated (NE).
The Brahmaputra drainage and its Arunachal Pradesh tributaries face pressures from dam construction, deforestation, and monsoon-season sedimentation, and the broader north-east Indian hill-stream fauna contains many localised species of uncertain status. As a species apparently known only from the Nao-dhing River, P. arunachalensis may have a restricted range that warrants attention once survey data are available. No collection pressure from the aquarium trade is recorded, as the species does not appear to have entered commerce.