New species of Harlequin rasbora described

A new species of Trigonostigma has been described, called Trigonostigma truncata. Coming from the coastal swamp forests of eastern Malaysia, the new fish looks superficially similar to the well-known Harlequin rasbora, but is more slender-bodied, and with a different shaped and placed black wedge marking on its body.

Trigonostigma heteromorpha, the Harlequin rasbora, is available both wild-caught and domestically bred, and some slender-bodied, different shaped rasboras were obtained through the aquarium trade and examined next to existing material from the Lee Kong Chian Natural History Museum in Singapore.

Several features including body shape, anal fin colour, the place the black wedge terminates and mouth position of the slender fish versus the common Harlequin have resulted in the new species description by Dr Tan Heok Hui and the new species name Trigonostigma truncata.

It’s all about the Axine

The scientific paper names that black wedge that all Trigonostigma have as an Axine, and its where the axine ends on the body of the new truncata, which helps to make it different from T.heteromorpha. In all of the four other species, the axine goes into the caudal peduncle, but in truncata it doesn’t, and the word “truncata” means cut-off. The new fish is also visibly more shallow-bodied than the common Harlequin and is said to have a bluish-lilac sheen in life versus the reddish or purplish sheen of T.heteromorpha, although we guess this in reference to sexually mature male colouration.

That takes the Trigonostigma species number in the genus to five – T.heteromorpha, espei, hengeli, somphongsi and now truncata. T.somphongsi is the least Harlequin-looking of the five species, with its axine being little more than a black line than a wedge. Its also found only in Thailand, not Malaysia like the other four, and until recently was non-existent in the hobby before being made available by a handful of specialist aquatic stores with the right connections to Thai exporters.

But baring in mind that T.truncata was discovered in wild-caught stock destined for aquariums, however, its likely that the new species may have already been distributed throughout the aquarium fish trade before being discovered as something different to the norm. Keep your eyes peeled the next time you spot Harlequins and other Trigonostigma for sale.

Fishkeeping News contacted Dr Tan Heok Hui and asked for permission to use the photos.

You can read the full paper and species description here.

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