Partial Success Breeding the Dwarf Rasbora

Author: Rob Torrens

The dwarf rasbora (Boraras maculatus), also known as the pygmy or spotted rasbora, is a small (adult size of 2.5 cm) member of the cyprinid family. It is reputed to be the smallest known member of this family, although there are other species, such as the Mosquito Rasbora (Boraras brigittae), Somphong's Rasbora (Boraras somphongsi) or Axelrod's Rasbora (Boraras axelrodi) that could equally claim the title. In addition to being the smallest cyprinid, they are about the 10th smallest vertebrate.

Boraras maculatus was long considered to be the juvenile form of the Clown Rasbora (Rasbora kalochroma, adult size of 10cm), but is now recognised as a true species.

Originating from slow moving waters, such as ponds, bogs and ditches in South East Asia, dwarf rasboras are a peaceful, agile schooling fish. They are best kept with other relatively small and peaceful fish. Energetic swimmers, such as danios may intimidate this quite rasbora, causing it to seek refuge in available plant cover.

	Boraras maculatas - Dwarf Rasbora

The colouring of these fish is a general pink/red blush with a black spot in the middle of their side and also at the base of the anal and caudal fins. The first rays of the dorsal and anal fins may have a black colouration and the exterior rays of the caudal fin may have slight pink/red tinges. When well feed and in ideal water conditions the colouring becomes more intense, with the males developing slight violet overtones. The males, in addition to a more intense colouration, are also more slender than the females.

Dwarf rasboras are an irregular import into New Zealand and, due to their small size, often overlooked. They are easy to keep and feed when housed with suitable tankmates. They readily accept flake food (providing it is small enough) but also relish (as most fish do) feedings of live food such as small daphnia, grindal worms and newly hatched brineshrimp (which enhances their colouration). They do best when kept at 24-26°C in water that is softish (<12 dGH) and acidic (pH 6-6.8). They appear most relaxed when housed in a tank that has dark gravel and is well planted. They also appreciate subdued lighting provided by floating plants or peat stained water. They should be kept in a school of at least 5 or 6.

Breeding the Dwarf Rasbora

Dwarf rasboras don't appear to be too difficult to breed providing the adults are well conditioned and care is taken to provide the required water conditions. The water conditions considered the optimum for spawning these fish are 26-28°C, soft (2-3 dGH) and acidic (pH 5.8-6.3). The spawning tank can be small and should be reasonably shallow (in my case the tank was 12" x 6" x 6", L.B.H) and should have some densely planted, fine leaved plants. The fish place their eggs on the plants (rather than some sort of top-dresser style), however the eggs are not the most adhesive so a spawning grate (for me this was a double layer of marbles) can also be beneficial as the parents may prey on their eggs.

The species is not overly prolific with a maximum of about 50 eggs laid. Breeder pairs should be well fed during their stay in the breeding tank to minimise egg eating and removes once spawning is complete. The eggs hatch within 24-36 hours and need an extremely small food (such as green water or one of the commercial Liquifry type foods). Once the fry are able to accept freshly hatched brine shrimp they grow rapidly and without further problems.

So why is the title of this article only "partial success"? The reason is that, despite having two pairs in the breeding tank, I only succeeded in raising 15 fry to adulthood. There are a couple of reasons for this: I didn't condition the females separately from the males and I left the adults in the breeding tank too long (so they probably ate a number of the fry). The reason for leaving the adults in so long is that I'm always crap at spotting eggs (unless fish have nicely piled them in one place) - this has meant that I've develop a 'put the female in, condition the female, add the male, leave them a week, take them out, cross fingers' approach. This has worked well with rainbows and blue-eyes, but these are continuous spawners (a relatively small number of eggs laid per day for a number of days, with the eggs taking about a week to develop prior to hatching). However it's not as appropriate for rasboras where an 'isolate and condition female, add male one evening then remove both the following day and cross fingers' approach would be better.

So next time I try I'll still pay attention to the water conditions, I'll separate and condition the female on live foods then add the male and remove both of them in one or two days. There will still be a lot of finger crossing just to be on the safe side.

References:

Aquarium Atlas vol 1, Dr Rudiger Riehl & Hans Baensch, Microcosm Ltd.
Breeding Behavior of Aquarium Fishes, Dr Wolfgang Wickler, T.F.H. Publications Inc.
Rasboras, Dr Martin R Brittan, T.F.H. Publications Inc.