Taxonomy & naming
Barbodes rhombeus was described by Maurice Kottelat in 2000 in his account of the freshwater fishes of Laos, where it was placed in the then-broad genus Puntius as Puntius rhombeus Kottelat, 2000. The specific epithet rhombeus is Latin for rhombus-shaped or diamond-shaped, referring to the body outline or possibly the pattern of the diagnostic spots.
Following the comprehensive revision of Puntius by Pethiyagoda, Meegaskumbura and Maduwage (2012), which dismembered the old polyphyletic genus and resurrected or erected several natural groupings, rhombeus was transferred to Barbodes — the genus that covers many of the larger, two-pair-of-barbels barbs of the Asian mainland and archipelago. The Catalog of Fishes (Eschmeyer, CAS) records the valid combination as Barbodes rhombeus (Kottelat, 2000). FishBase synonymy confirms the original combination Puntius rhombeus and the subsequent reclassification.
Morphology
Barbodes rhombeus is a small barb reaching a maximum standard length of about 2.5 in. The body is moderately elongate and laterally compressed, typical of the hill-stream barb body plan. Two pairs of barbels are present — a feature shared by other members of the genus Barbodes and useful in distinguishing them from one-pair genera such as Puntigrus or Puntius sensu stricto.
The species is characterised by two bold black spots that give it the common name Indochinese spotted barb: one positioned below the dorsal fin base and a second at the base of the caudal peduncle. The body ground colour is pale silvery to yellowish. It is a benthopelagic schooling fish, spending time in mid-water and near the substrate in flowing, clear water.
Habitat
The Indochinese spotted barb inhabits shallow hill streams with clear water and open, sunlit conditions across the Mekong, Chao Phraya and Meklong river basins, and also occurs in peninsular Thailand north of the Isthmus of Kra. Its range encompasses Thailand, Laos, Cambodia and Vietnam. The species has also been introduced to Singapore and parts of Peninsular Malaysia, where established populations exist outside its native range.
FishBase characterises the species as tropical and benthopelagic — at home in both the mid-water column and near the bottom of hill-stream habitats. The clear, well-oxygenated water of its native riffles and shallow runs contrasts with the peat-swamp blackwaters of more southerly Sundaland barbs; B. rhombeus is a fish of moving, sunlit freshwater rather than stained, still pools.
Feeding
Like most small Asian barbs, Barbodes rhombeus is an opportunistic omnivore. In the clear hill streams it inhabits, it picks small invertebrates — insect larvae, microcrustaceans and worms — from the substrate and water column, supplementing these with algae, biofilm and organic detritus. The open, sunlit nature of its preferred habitat suggests significant grazing on periphyton growing on rocks and submerged surfaces.
In the aquarium, it will accept quality flake and small pellet foods, with live and frozen supplements such as bloodworm, daphnia and brine shrimp to maintain condition. Feeding with a variety of items keeps the fish active and their colours at their best.
Mating
Barbodes rhombeus is an egg-scattering, non-guarding spawner in the manner typical of the great majority of barbs. No nest is constructed and no parental care is provided at any stage. When a gravid female is in condition and water parameters are favourable, one or more males will drive her through vegetation or over the substrate, pressing alongside her briefly as eggs and milt are simultaneously released.
Adhesive eggs settle among plants, on gravel or in crevices in the substrate. Adults pay no further attention to the spawn and will consume eggs and larvae given the opportunity. In the wild, the flowing, clear-water habitat means eggs are partly dispersed by current, reducing the risk of mass predation.
Breeding
Breeding Barbodes rhombeus in captivity follows the standard approach for small Asian barbs. A shallow spawning tank with soft, neutral to slightly acidic water, gentle filtration and a layer of fine-leaved plants or a spawning mop over a mesh or marble substrate allows eggs to fall beyond the reach of the adults. Well-conditioned fish are introduced and spawning typically follows within a day or two, often in the morning hours.
Once spawning is complete, adults should be removed immediately to prevent egg predation. Eggs hatch in approximately one to two days at tropical temperatures, and the free-swimming fry are fed on infusoria and then newly hatched brine shrimp as they grow. The species is seldom deliberately bred in the hobby owing to its rarity in the trade, but its biology suggests it would prove straightforward given appropriate conditions.
In the aquarium
Barbodes rhombeus is rarely encountered in the aquarium hobby outside specialist collections focused on Indochinese stream fishes. Its modest size of around 2.5 in SL and preference for clear, well-oxygenated water make it a suitable candidate for a hillstream or general Asian stream biotope aquarium maintained with good current, clean water and a temperature range consistent with its native Indochina range.
As a schooling species it should be kept in groups of six or more individuals to express natural behaviour and reduce stress. It is compatible with other small, non-aggressive stream fishes of similar size. Its two-spotted pattern and active mid-water swimming make it a distinctive addition to a well-planted stream tank, though its availability in the trade is limited.
Conservation
Barbodes rhombeus is assessed as Least Concern (LC) on the IUCN Red List, with the most recent assessment dated 31 January 2019. The species has a broad distribution across the major river systems of mainland Southeast Asia — the Mekong, Chao Phraya and Meklong basins — and FishBase assigns it a high resilience to fishing pressure and low vulnerability score, consistent with a wide-ranging, reproductively active small barb.
Despite the LC status, the freshwater ecosystems of Indochina face growing pressure from hydroelectric dam construction, water extraction for agriculture, sedimentation from deforestation, and agricultural runoff. The clear hill streams that B. rhombeus favours are particularly susceptible to increased turbidity and reduced flow. Monitoring of small-barb assemblages across the Mekong basin is warranted as dam projects continue to alter hydrology in the region.