Oreichthys cosuatis

Cyprinidae

Probably restricted to the Ganges and Brahmaputra River basins in Nepal, India and Bangladesh with apparent occurences in Thailand, Myanmar and the Western Ghats mountains in southern India likely to represent undescribed species. The fish pictured above were collected from Ghoti Ganga Creek, a northern tributary of the Ganges.

Appears to favour slow-moving tributaries and minor rivers with plenty of marginal vegetation and relatively clear water; it is found in hill streams in some areas but never in the fast-flowing sections. Some sources state it has also been recorded from still waters including ponds and ditches but as yet we have been unable to confirm these details.

Fully grown at around 1.57"/4cm.

A tank measuring 24" x 15" x 12"/60cm x 37.5cm x 30cm/70.8 litres is big enough to house a small group of these.

This species will thrive in a heavily-planted set-up where the multitude of hiding places will provide it with the sense of security it needs in order to display natural behaviour. It will also appreciate the dimly-lit conditions towards the bottom of the tank where it will spend most of its time. Use as soft a substrate as is feasible for the plant species being grown because Oreichthys species have miniscule sensory bristles on the lower jaw which are used when foraging the substrate for food. Driftwood branches, floating plants and leaf litter can all be added to lend a more naturalistic feel if you wish. It will not do well at all under sparsely-furnished or starkly-lit conditions.

It can be a little tricky to acclimatise to aquarium life as it often arrives in poor condition and seems to be rather sensitive to swings in water chemistry. As a result it should not be introduced to biologically immature aquaria and small, regular water changes of around 10% tank volume are the ideal in terms of maintenance.

75 - 82°F/24 - 28°C

Will not do well in very acidic or alkaline conditions so a value between 6.5 - 7.5 is best.

5 - 15°H

It will accept dried foods of a suitable size but should not be fed these exclusively. Daily meals of small live and frozen fare such as Daphnia, Artemia and suchlike will result in the best colouration and encourage the fish to come into breeding condition. A degree of vegetable matter should also be included on the menu; a decent quality dried product containing Spirulina or similar will suffice for this. It is noted as something of a shy, reluctant feeder (see below).

Not really suitable for most community tanks as it has a retiring nature and may be intimidated or outcompeted for food by larger/more boisterous tankmates. Small, peaceful cyprinids such as Trigonostigma or Boraras species make good choices and we suspect it will also do ok with many South American characins, Otocinclus or pygmy Corydoras catfish. Accommodating it in a biotope-style community of Indian fish is more tricky although suitable species from the Ganges basin include Badis badis , Pangio pangia and Colisa chuna .

Although it is gregarious by nature it is a shoaling rather than schooling fish which develops a distinct pecking order. Indeed the behaviour between rival males of this species is really something to behold with some wonderful displays involving the impressive dorsal fin. It therefore should always be maintained in a group of 8 or more BUT the tank must be of a sufficient size which will allow weaker individuals some respite from dominant conspecifics and decorated in such a way that many broken lines of sight are provided. Conversely if kept singly, in a very small group or in cramped conditions it can become withdrawn and subdominant fish may be bullied incessantly.

Males grow a little larger than females, exhibit more colour on the body and fins and develop an extended dorsal fin. Sexually mature females are rounder in the belly than males.

Like many small cyprinids this species is an egg-scattering, continuous spawner that exhibits zero parental care. That is to say when in good condition and in the presence of both males and females relatively small numbers of eggs will be laid daily. In a well-furnished, mature aquarium it is therefore possible that small numbers of fry may start to appear without human intervention.

However if you want to increase the yield of fry a slightly more controlled approach is required. The adult group can still be conditioned together but one or more small, say 18" x 12" x 12"/45cm x 30cm x 30cm/42.5 litre, containers should also be set up. Water chemistry parameters ought to be within the values suggested above, the most important factor being that they remain stable. Decor can be very simple; a couple of sinking spawning mops or good-sized clumps of Java moss will give the fish somewhere to deposit their eggs. Filtration is not absolutely necessary although you may wish to include a small air-powered sponge filter bubbling away gently.

One or two pairs of well-conditioned adult fish should then be introduced to each container. It is wise to make the transfer slowly in order to avoid excessive levels of stress but if conditions are to their liking they should begin to spawn the next afternoon or evening. Whether this species eats its eggs or not is unclear although it certainly appears not to actively hunt for them as is the case with many small Puntius and other related species. Once spawning has commenced it should continue sporadically on a daily basis without risk of any undue aggression towards the female(s).

The pair(s) can be left in situ for two or three days then removed as the first eggs should hatch by the third day after the initial spawning. The tiny young will survive on their yolk sacs for around 5 more days after which they will require Paramecium or other microscopic food. After 8 days they should be large enough to accept Artemia nauplii/microworm etc. As the days pass additional fry should start to appear from later spawning events. It's best to wait a week or two before starting to perform small water changes in order to avoid unduly shocking the young fish.

An assortment of undescribed Oreichthys from India, Myanmar and Thailand continue to be exported under the name Oreichthys cosuatis which has resulted in confusion as a few of them are now pictured as such by some resources. Dr. Paul Loiselle of the New York Aquarium told us that the defining characteristic for the 'real' cosuatis is the presence of red ventral fins as noted by Hamilton in his original description of the taxon. One of the most notable cases of misidentification has occured with the recent introduction of O. sp. 'sail fin' to the hobby which has been widely sold as cosuatis but is clearly a different species. Many believe that this fish will turn out not to be an Oreichthys at all although the results of a 2009 US study in which the DNA of both described and undescribed members of the genus was sequenced suggest otherwise. At any rate Oreichthys cosuatis is easily distinguished from sp. 'sail fin' by its more tapered head shape, slimmer body and less extended dorsal fin in males among numerous other factors.

Currently the only other recognised member of the genus is O. parvus but it seems certain that new species will be described in the future as there exist at least four undescribed forms from Myanmar, four from India (including one species that is said to originate in the Western Ghats mountain range; most intriguing as the genus was previously thought confined to northeastern parts of the country) and possibly others from Thailand. The described species are quite easy to distinguish from one another as Oreichthys cosuatis exhibits a yellow blotch in the dorsal fin whereas in O. parvus the same fin is coloured red. Telling some of the undescribed ones apart from cosuatis is trickier; one form from the Dibru River in Tinsukia District, Assam state, India is very similar indeed but has a less rounded dorsal fin for example. Other variants from Myanmar differ in fin colour or patterning such as the extent of the dark blotch at the caudal peduncle (this being absent altogether in some forms).

Members of this genus possess rows of sensory papillae on the snout, cheeks, throat, opercle and interorbital area that are lacking in most other cyprinids but present in a few other genera including Eirmotus, Cyclocheilichthys, Neobarynotus and a handful of Puntius species.

Literature cited

1. Tan, H.H. and M. Kottelat. 2008 - Raffles Bull. Zool. 56(2):423-433.
   Revision of the cyprinid fish genus Eirmotus, with description of three new species from Sumatra and Borneo.
2. V. Vilasri. 2002 - The Natural History Journal of Chulalongkorn University 2(1):64-65
   Oreichthys parvus Smith, 1933 (Teleostei: Cyprinidae), an Addition to the Fish Record from the Peat Swamp, Southern Thailand.
3. www.fishbase.org
4. www.petfrd.com