Boraras - marvels in miniature

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They may be small in size but they're big in character, and that's why Boraras remain so popular. Kevin Conway sings their praises.

Members of the genus Boraras are tiny cyprinid (carp-like) fishes that range from just 13-22mm/0.5-0.9”. As tinies they are not terrors, being peaceful and, even better for you and me, cheap to buy.  A decent shoal will only cost £20!

All species, except for the smallest, Boraras micros, were previously within the genus Rasbora. These were removed and placed together with B. micros, the type species of the genus Boraras, in 1993 — based on several differences between them and the larger species of Rasbora by Kottelat and Vidthayanon.

Aquarium care

Because of their small size and nature, Boraras are best kept on their own in a dedicated species or community tank with other small, peaceful fishes such as other small cyprinids or characins. When kept with larger fishes, or even small boisterous ones, they will spend most of the time avoiding more aggressive tank mates.

In the wild B. micros and B. urophthalmoides inhabit swamps or marshes that have abundant aquatic vegetation. B. brigittae, B. maculatus and B. merah, meanwhile, live in small, slow-flowing rivers more associated with peat-swamp forests.

I have had most success keeping Boraras from both of those habitats in groups of six to ten in 41 x 25 x 25cm/16 x 10 x 10” or 46 x 25 x 25cm/18 x 10 x 10” tanks with warm (25°C/77°F) soft water with a pH of 6.5 and very little hardness.

I made sure that both were always choked with Java moss (Vesicularia sp.), Indian fern (Ceratopteris sp.) and duckweed (Lemna sp.)  and added a thin layer of gravel for substrate and occasionally a piece of bogwood to help stain the aquarium water a light tea colour.

I added a small sponge filter and lighting was via a desktop lamp. Water changes at 30% were undertaken twice weekly with either aged tapwater treated with peat extract or rainwater.

These fish accept a variety of foods but, being micro-predators, do best on a variety of small live foods such as newly hatched brineshrimp, small Daphnia and microworms. However, they will happily accept dry foods such as crushed flakes or micro-pellets and take frozen foods.

When grouped in a dedicated species tank and fed on small live foods they will quickly settle and start to display their characteristic stunning colours.

Sexing and breeding

Sexing them is fairly easy. All except B. micros are sexually dichromatic. Males of B. brigittae, B. maculatus and B. merah, when in condition, exhibit a more intense red body colour than the females, which are plumper than males when in condition. Males also exhibit intense black and red markings on dorsal and anal fins.

Males of B. urophthalmoides are not as colourful as males of B. brigittae, B. maculatus or B. merah, but can be easily distinguished from females by a more intense coloration, particularly an orange/yellow stripe above the lateral stripe, the amount of iridescent blue in the lateral stripe and a slimmer overall appearance.

When in good condition males will become more aggressive towards each other,  their colours will intensify and they will begin fighting for and defending a small territory. Once they have established one they will begin to display and, if the female shows herself to be receptive, spawning should be imminent.

Different conditions

Species have spawned in aquariums under several water conditions and in 1998 Brittan wrote that a well-conditioned pair of B. maculatus adults should be introduced during the evening to a small tank of water with a pH of 6.5, DH 1-4, temperature of 24-28°C/75-82°F and comprising a mixture of broad and fine-leafed aquatic plants.

They should spawn the following morning, with the females laying a small number of eggs.

Other enthusiasts, including myself, have had more success breeding Boraras in small groups without removing any pairs to a separate aquarium.

Hellweg said only dominant males of B. brigittae spawned with females in a continuous fashion in his aquariums. A few eggs were laid just after first light over a number of days. Surprisingly, he has had great success keeping and breeding several species in water with a pH of 7.2-7.4 and a DH of around 6.

Females lay small numbers of very small eggs and the pair provide no parental care  whatsoever. They will eat their own eggs, given the chance.

In a mature, well-planted aquarium some fry may survive to maturity if they avoid any adults. However, a larger number may be raised by removing plants along with eggs from the adult’s aquarium to a separate one filled with water from the original tank.

Newly hatched fry are tiny and once they have absorbed the yolk sac need tiny foods to survive. I was never able to provide my B. maculatus fry with those foods and they never survived long.

Hellweg said he successfully initially fed B. brigittae fry with Paramecium before moving on to frozen Cyclops and baby brineshrimp. He also reported that placing slimy driftwood in the fry tank likely provided an additional source of microorganisms for them.

With a small, heavily planted tank and small live foods, all species of Boraras available in the trade are easy to keep.

What species are available?

Five described species of Boraras and a small number of undescribed species are found in aquarium shops.

However, described and undescribed are often confused in the trade — likely stemming from the fact that differences are only slight, relating only to the shape of dark markings on the body and differences in numbers of fin rays, which are difficult to count without a microscope.

However, it is possible to tell all of these described species apart, based solely on coloration.

Scientific name: Boraras brigittae (Vogt, 1978).

Common name: Mosquito or Chilli rasbora.  

Maximum size: 18mm/0.7”.

Distribution and habitat: Indonesian (South-eastern Borneo, Kalimantan Tengah and Kalimantan Selatan). Black waters in peat swamp forests.

Colour pattern: Overall reddish with dark stripe along body and smaller dark spots at base of anal and caudal fin. Dorsal edge of dark stripe highlighted by intense red band. Dorsal and anal fin with dark markings along anterior edge highlighted with intense red in males. Breeding condition males exhibit intense red markings on pelvic and caudal fins.

Notes: B. brigittae is the most attractive of the genus. Similar to B. urophthalmoides, it is distinguished by larger adult body size and intense red coloration, particularly above dark stripe along body and on fins.

B. urophthalmoides is orange/yellow instead of red above the dark stripe along body.

B. brigittae is also similar to B. merah, which exhibits a similar red body. B. merah differs from B. brigittae in having a dark ovoid spot and short interrupted dark stripe along body as against a single dark stripe in B. brigittae.

Both species are reportedly found together in areas of southern Kalimantan where distributions overlap, explaining why they are sometimes mixed in dealers’ tanks.

Scientific name: Boraras maculatus (Duncker, 1904).

Common name: Pygmy or dwarf rasbora.

Maximum size: 22mm/0.9”.

Distribution and habitat: Malaysia, Indonesia (Sumatra) and South-east Thailand. Black waters in peat swamp forests.

Colour: Overall body colour orange/red with large dark spot on side of body and smaller dark spots at base of anal and caudal fin. Dorsal and anal fin with dark markings along anterior edge highlighted with intense red on males. Size and shape of dark marking at base of anal fin is highly variable and may be represented by two distinct markings.

Notes: B. maculatus was the first species of Boraras to be described and has been popular since the 1950s.

The first Boraras successfully bred and raised in the aquarium, this is the largest member of the genus, distinguished by a larger adult size and colour pattern of a single large spot on the side of the body.

There’s a currently undescribed species, called B. ‘micros red’, superficially similar to B. maculatus but lacking pronounced dark markings along the anterior edge of anal fins.

Scientific name: Boraras merah, Kottelat 1991

Common name: None

Maximum size: 17mm/ 0.7”

Distribution and habitat: Indonesia (South-western Borneo, Kalimantan Barat and Kalimantan Tengah). Inhabits black waters in peat swamp forests.  

Colour: Overall reddish with dark longitudinal, ovoid spot on side of body and interrupted stripe along posterior half of body. Smaller dark spots at base of anal and caudal fin.

Dorsal and anal fin also have dark markings along anterior edge, highlighted with intense red markings on males.

Notes: B. merah is common in the trade, but usually offered as B. brigittae. Both exhibit an overall red body, but B. merah has a dark ovoid spot followed by interrupted dark stripe as against a complete dark stripe along side of body.

Scientific name: Boraras urophthalmoides, Kottelat, 1991.

Common name: Exclamation point or Sparrow rasbora.

Maximum size: 16mm/ 0.6”.

Distribution and habitat: Widely throughout Thailand and reported from Cambodia and Vietnam. Swamps and marshes with abundant aquatic vegetation.

Colour: Overall brownish orange body with dark stripe along side of body and smaller dark spots at base of anal and caudal fin.

Dorsal edge of dark stripe is highlighted by intense orange/yellow band.  Dorsal and anal fin marked with dark markings along anterior edge. Dorsal and anal fins of males also exhibit a light red to orange colour.

Notes: B. urophthalmoides is the smallest species commonly available in the aquarium trade. According to Kottelat, wild specimens do not reach much more than 13mm/0.5” in standard length, but may grow larger under aquarium conditions.

B. urophthalmoides is most similar to B. brigittae but easily distinguished by smaller size and more orange coloration.    

Although older aquatic literature referred to B. urophthalmoides as Rasbora urophthalma. In 1991 Kottelat showed that the type specimens of the latter represented a Puntius-like cyprinid and not the species to which that name had been given.

He corrected this nomenclatural problem by providing a new name for the species. This was urophthalmoides, meaning the appearance of urophthalma.

Scientific name: Boraras micros, Kottelat and Vidthayanon, 1993.

Common name: None.

Maximum size: 13mm/0.5”

Distribution and habitat: Known only from two separate marshes in Thailand in provinces of Udon Thani and Nongkhai.

Colour: Overall olive brown. Pattern of spots similar to B. maculatus but spot on side of body much smaller. Species also lacks dark pigmentation on anterior edge of dorsal and anal fins in all other described species.  

Notes: Smallest and most recently described species, having a very small natural distribution and known only from two swamps in Thailand. B. micros appears to be rarely available in the trade and mostly when offered for sale under the name B. micros is undescribed as B. ‘micros red’.

B. micros can be distinguished by a complete lack of red and dark pigmentation on dorsal and anal fins, its olive brown body and by its tiny adult size.

Did you know?

The generic name of Boraras is an anagram of Rasbora, alluding to the fact that species of Boraras have more caudal vertebrae than abdominal vertebrae — the opposite in species of Rasbora. Despite this, species are still often referred to under the common names of Pygmy, Dwarf, Midget or Mosquito rasbora in some shops.

Ask the expert

Kevin Conway is regarded as a leading expert on Boraras, specialising in the species’ evolutionary development.

What was the main reason for placing these tiny fishes in their own genus?

Back in 1993 ichthyologists felt that these small species were so distinct from the larger Rasbora species that the erection of a new genus was warranted.

The biggest differences between Boraras and Rasbora (other than size) are mostly related to aspects of their skeleton and sensory canal system, including differences in number of vertebrae, differences in number of fin rays, and differences in the extent of the sensory canal system of the head and body.

You studied Boraras at the Natural History Museum in London. Can you tell us more about that?

I studied the skeleton and evolutionary relationships of Boraras as part of my Master’s degree at the Natural History Museum in 2004/2005.

I was able to look at all of the described species over the course of six months in the museum’s zoology department. Initially the museum actually had very few specimens of Boraras in their collections. At the start of the project they only had specimens of B. maculatus from Malaysia, but we managed to get specimens of all the other described species from the Raffles Museum in Singapore, which has collections from all over South-East Asia).

I spent a lot of time looking through a microscope at specimens preserved in alcohol or looking at skeletal preparations. It was not easy to dissect them, particularly specimens of B. micros, but I persevered.

Did you discover anything new about them?

Nothing really ground breaking. I only really confirmed what others had suggested previously — that Boraras species are more closely related to each other than to species of Rasbora. I was, however, the first person to look at the entire skeleton of all five species and was also first to propose a hypothesis on how they are related to each other. I suggested that B. maculatus, B. brigittae and B. merah were more closely related to each other that they were to B. micros or B. urophthalmoides.

Have you ever collected them in the wild?

Unfortunately not. That’s one of the good things about working in a natural history museum as nine times out of ten someone else has done the hard part for you and all I ever needed to do was walk to the collection and pick a jar from the shelf — or get in touch with another museum and have them delivered.

To date all of my fieldwork has been in African countries or in the Himalayas and so I have never had the chance to collect them. Most of the live Boraras I have encountered were purchased on a Saturday afternoon from Wholesale Tropicals in the East End of London!

Are there any differences between those species that live in marshes and those that live in peat swamp forest streams?

Not really. The only difference that I can think of is that those species that live in the marshes and swamps of Thailand (B. micros and B. urophthalmoides) seem to be a little smaller on average than the other species that inhabit the peat swamp forests of peninsular Malaysia, Singapore and Indonesia (B. brigittae, B. maculatus, B. merah). However, I can’t think of any biological explanation for this.

We believe Boraras  to be the smallest commonly available aquarium fish. Is this true?

Definitely. Though much smaller fishes are known to science (such as Paedocypris), species of Boraras, B. urophthalmoides in particular, are without doubt the smallest freshwater tropical fish regularly available in the UK.

Miniature fishes seem to be getting a lot of press lately. Do Boraras share anything in common with other miniatures like Paedocypris?

As far as Boraras go they are relatively boring anatomically. Other than being sexually dichromatic, which is pretty common among cyprinids, they do not exhibit any interesting sexual dimorphisms such as those present in some other tiny species like Paedocypris or Sundadanio.

In fact some scientists refer to Boraras as ‘proportioned dwarfs’, meaning they are almost identical copies of larger close relatives but shrunk to a tiny size — or have been miniaturised.

This is quite the opposite of the situation regarding Paedocypris, the adults of which resemble an earlier developmental stage of their close relatives and are now referred to as being ‘developmentally truncated’.

This article was first published in the November 2009 issue of Practical Fishkeeping magazine. It may not be reproduced without written permission.