Taxonomy & naming
Pethia didi was described by Sven O. Kullander and Fang Fang in 2005. The Catalog of Fishes (Eschmeyer, California Academy of Sciences) records the valid combination as Pethia didi (Kullander & Fang, 2005), the parenthetical authority indicating that the species was not originally placed in Pethia.
The genus Pethia was established by Pethiyagoda, Meegaskumbura and Maduwage in their 2012 revision of South Asian small barbs, splitting numerous species out of the long-standing but paraphyletic genus Puntius. Pethia now comprises a group of small, spotted or plain Indian and Sri Lankan barbs characterised by particular combinations of lateral-line scales, barbel absence or reduction, and fin structure. If Pethia didi was originally described under a different genus combination, primary literature records that earlier placement; the current valid name follows the Catalog of Fishes.
Morphology
Pethia didi is a small barb, estimated at under 2 in standard length based on the typical size range of its congeners. The genus Pethia generally comprises slender to moderately deep-bodied fish with a moderately large eye, a slightly terminal mouth without barbels (or with vestigial ones), and a relatively high dorsal fin. Coloration among Pethia species varies from plain silvery-gold to forms with one or more dark lateral blotches or spots.
Detailed morphometric and meristic data for Pethia didi are not widely reported outside the original description. Specific information on coloration, lateral-line scale counts, and fin-ray formulae should be taken from the 2005 Kullander and Fang paper.
Habitat
The species is associated with hill-stream habitats in the Western Ghats region of peninsular India, a landscape of steep, fast-flowing rivers draining the Ghats escarpment before reaching the Arabian Sea or the Deccan plateau interior. These streams are characterised by clean, well-oxygenated water over rocky or sandy substrates, with dense riparian vegetation providing shade and leaf litter.
Water conditions in Western Ghats hill streams are typically cool relative to lowland Indian rivers, with temperatures in the range of approximately 64–75 °F, near-neutral to mildly acidic pH, and soft to moderately hard water reflecting the largely silicate geology of the Ghats. These are naturally low-nutrient, high-clarity systems with minimal tolerance for sedimentation or pollution.
Feeding
Pethia species are generalised omnivores in the wild, feeding on small aquatic invertebrates, algal films, biofilm scraped from rock surfaces, and organic detritus carried in the current. In fast-flowing hill streams, much of the invertebrate food comprises larval insects — particularly chironomids, mayflies, and caddisflies — drifting in the water column or sheltering among gravel.
In an aquarium setting, congeners accept a wide variety of foods including quality dried flakes and micro-pellets, live and frozen bloodworm, brine shrimp, and daphnia, with some vegetable matter or spirulina supplementation reflecting the algal component of the natural diet.
Mating
Like all members of the genus Pethia and the broader barb clade, Pethia didi is an egg-scattering, non-guarding spawner. There is no nest construction, no territory defended beyond the brief moment of courtship, and no pair bond. A driving male pursues a gravid female through vegetation or over substrate, the two pressing close together as eggs and milt are released simultaneously.
Spawning readiness is typically signalled by the swelling of the female's abdomen with eggs and by heightened colour and activity in males. The adhesive eggs fall among plant stems, fine roots, or into gaps in gravel and are abandoned immediately after fertilisation.
Breeding
Pethia didi has not been documented in the aquarium hobby literature to a sufficient degree to describe captive breeding in detail. By analogy with well-studied congeners such as Pethia conchonius and Pethia nigrofasciata, the species would be expected to spawn readily under conditions that replicate the cool, clear hill-stream environment: a separate breeding tank with soft, slightly acidic water around 68–72 °F, fine-leaved plants or a spawning mop, and a mesh bottom layer to protect eggs from the adults.
Egg-scattering barbs give no parental care; adults will consume eggs and fry given the opportunity and should be removed after spawning. Eggs of congeners typically hatch within one to two days at appropriate temperatures, with free-swimming fry emerging a few days later and accepting infusoria-grade and then newly hatched brine shrimp as first foods.
In the aquarium
Pethia didi is rarely encountered in the aquarium hobby and is not established in the ornamental trade to the extent of popular congeners such as the rosy barb or black ruby barb. The species' apparent restriction to Western Ghats hill streams suggests it would require cooler, well-oxygenated, high-clarity water — conditions more demanding than those needed for tropical lowland barbs, but manageable in a dedicated setup with a chiller or in a temperate climate.
Should the species become available, the approach for comparable hill-stream Pethia would apply: a spacious tank with good filtration and flow, soft to moderately hard near-neutral water, a mixed group to allow natural social behaviour, and a varied diet. Long-finned or slow tankmates are best avoided, as barbs can be active and may nip in inadequate group sizes.
Conservation
Pethia didi has not been evaluated by the IUCN Red List and carries the designation Not Evaluated (NE). This is a common situation for recently described Indian endemic barbs, many of which were formally named only in the past two decades as systematic revisions of South Asian cyprinids proceeded.
The Western Ghats is recognised as one of the world's biodiversity hotspots and as a global Freshwater Ecoregion of conservation concern. Hill-stream endemics in this system face mounting pressure from dam construction, water extraction for agriculture, sand mining, and agricultural runoff. Species with restricted ranges — potentially a single river system or a small cluster of streams — are inherently vulnerable to local habitat loss even in the absence of formal threat assessments. A formal IUCN assessment for Pethia didi would be a valuable contribution to regional conservation planning.