Taxonomy & naming
Danio annulosus was formally described by Kullander, Rahman, Norén and Mollah in 2015 in Zootaxa (vol. 3994), based on specimens collected from Shuvolong Falls in Rangamati district, Chittagong division, Bangladesh. The Catalog of Fishes (Eschmeyer, the authority for valid names) records the valid combination as Danio annulosus Kullander, Rahman, Norén & Mollah, 2015.
The species belongs to the informally recognised 'chain danio' group within Danio — a cluster of South and Southeast Asian cyprinids sharing the characteristic series of dark rings or chainlike markings along the flank. The chain danio pattern is found in several described species, and Danio annulosus is distinguished from its relatives primarily by fin proportions and the shape of its humeral spot, which is wider than deep rather than round or deeper than wide as in other chain danios. No synonyms or previous combinations are recorded: it was new to science at description.
Morphology
Danio annulosus is a moderately deep-bodied, laterally compressed cyprinid with a maximum recorded standard length of 3 in, placing it at the larger end of the danio size range. Its most striking feature is the series of dark, oval to ring-shaped markings along each flank, separated by pale golden or silvery interspaces — the chain pattern that defines its species group. The humeral spot, located just behind the gill cover, is distinctively wider than it is deep, a character that separates annulosus from otherwise similar chain danios.
The species is further distinguished by its relatively short pectoral and pelvic fins compared to related chain danios, a difference noted in the original description as a key diagnostic feature. The overall body form is that of a typical benthopelagic danio, adapted for life in and around the rocky substrate and turbulent water at the base of a waterfall. Detailed meristic and morphometric data are in the original Zootaxa description.
Habitat
The entire known range of Danio annulosus is a shallow pool at the base of Shuvolong Falls, a waterfall that drains into Kaptai Lake in the Rangamati district of the Chittagong Hill Tracts, Bangladesh. Kaptai Lake itself is a reservoir created in 1962 by damming the Karnafuli River; the falls and the small stream connecting the pool to the lake predate this impoundment and represent a remnant of the original fluvial system.
At the type locality, the pool was described as approximately 15.5 in deep, with rocky substrate and no significant aquatic vegetation. A very shallow connecting channel (roughly 0.5 in deep at collection time) links it to the lake margin. Associated fauna collected alongside the species included Esomus danrica, Rasbora daniconius, Dermogenys burmanica, Aplocheilus panchax and Trichogaster lalius, along with shrimp, snails and small Garra species — suggesting warm, tropical freshwater with moderate flow. No temperature or pH data from the site are currently published.
Feeding
No dietary study of Danio annulosus in the wild has been published. By analogy with closely related chain danios and the guild of small cyprinids that shared its type locality, it is likely an opportunistic omnivore and micropredator, taking small invertebrates, insect larvae, microcrustaceans and organic detritus from the water column and rocky substrate at the waterfall pool.
In the aquarium, chain danios in general accept all standard small foods — flake, micro pellets, frozen or live daphnia, brine shrimp and bloodworm. Given its benthopelagic habits and rocky-bottom microhabitat, annulosus may feed more at mid-water and near the substrate than at the surface, but data are lacking. A varied diet of quality prepared and live foods is appropriate for any chain danio kept in captivity.
Mating
No published spawning observations exist for Danio annulosus in the wild or in captivity. As a member of Danio, it is expected to follow the typical danio pattern of egg-scattering and non-guarding reproduction. Courtship in danios involves an active male driving a gravid female through vegetation or over substrate, the pair coming together briefly as eggs and milt are released simultaneously.
There is no pair bond, no nest, and no bubble-nest construction. After spawning, adults show no interest in the eggs. In a single-locality species with a turbulent, rocky-bottom waterfall habitat, it is plausible that eggs are scattered into rock crevices or carried by current into quieter zones, but the precise microhabitat of natural spawning is undocumented.
Breeding
Breeding in captivity has not been documented in the literature for Danio annulosus, and the species is rarely if ever encountered in the aquarium trade given its highly restricted range and recent description. Based on the biology of congeners, it would be expected to spawn in the manner of other chain danios: adhesive or weakly adhesive eggs scattered over plants, moss or rocky substrate, with no parental care whatsoever — adults would be removed after spawning to protect the eggs.
Breeding mode is classified as egg-scatterer with no parental care. Should captive specimens become available, a shallow tank with fine-leaved plants or java moss, clean well-oxygenated water, and conditioning on live foods would form the basis of any breeding attempt. Eggs and fry, if produced, would follow the general danio pattern of hatching within two to three days and becoming free-swimming shortly thereafter.
In the aquarium
Danio annulosus is essentially unknown in the aquarium hobby. As a species described in 2015 from a single waterfall site in rural Bangladesh with no established collection or trade route, it has not entered the ornamental fish market in any meaningful numbers. Its existence is known principally from ichthyological literature and database records.
Should it become available, the requirements of the chain danio group provide a reasonable guide: a spacious tank with good oxygenation, moderate current, clean water with a tropical temperature range, and a social group of at least six individuals. The relatively large maximum size of 3 in SL suggests a tank of 31.5 in or more would be appropriate. Its association at the type locality with other cyprinids and small anabantoids hints at a tolerant community species, but detailed husbandry data remain entirely absent from the literature.
Conservation
Danio annulosus has not been evaluated by the IUCN Red List. Its entire documented range consists of a single waterfall pool connected to Kaptai Lake, a reservoir constructed in 1962 on the Karnafuli River in the Chittagong Hill Tracts of Bangladesh. The formation of the reservoir fundamentally altered the lower Karnafuli system, and the persistence of Shuvolong Falls as a functioning waterfall habitat above the lake level represents a narrow refuge.
Pressures on this microhabitat include sedimentation, water-level fluctuations tied to hydroelectric operations at the Kaptai Dam, agricultural runoff and development pressure in the Rangamati district. A species known from one pool at one waterfall is intrinsically vulnerable to any single disturbance event. Given the hydrological control exerted by the dam and the rapid pace of landscape change in the Chittagong Hill Tracts, a formal conservation assessment and targeted surveys to establish whether any additional populations exist elsewhere in the Karnafuli watershed are urgently warranted.