The tiny rasboras in the Boraras genus have been shown to represent a monophyletic group among rasborine cyprinids.
New research by Kevin Conway from the Department of Biological Sciences at Imperial College of Science, Technology and Medicine in London, has provided new evidence to support the monophyly of the Boraras genus.
In a paper in the journal Ichthyological Exploration of Freshwaters, Conway identifies four synapomorphic features which he says show that Boraras evolved from a single common ancestor and help to define a distinct group among rasborine cyprinids.
Conway says that the urohyal (a bony or cartilaginous part of the hyoidean arch) has a unique shape in Boraras which isn't seen in other rasborines. The members of the genus also lack a supraorbital canal, have an elongated fourth pleural rib and have a postcleithrum attached high on the cleithrum.
However, although Conway managed to provide evidence to suggest that the genus is monophyletic, he wasn't able to resolve the phylogeny sufficiently to show how the members of the group were interrelated, further emphasising the tricky taxonomy of the group.
The fishes, which reach only a few centimetres in length and have a very delicate appearance, are sometimes imported from the Far East. They fair well in small aquaria, but need to be kept away from larger boisterous species, and often benefit from soft water.
The Boraras genus was originally erected by Kottelat and Vidthayonon in 1993 to hold the just described Boraras micros, B. brigittae, B. maculatus, B. merah and B. urophthalmoides. The group may also include some undescribed species, including: Boraras sp. "South Thailand".
Boraras were previously considered members of the Rasbora genus, but Kottelat and Vidthayonon believed them to be sufficiently different enough to merit erecting a new genus for them. Their decision was based on the fishes' distinctive colour pattern, a distinctive reproduction system, different vertebral counts and the presence of an elongated fourth pleural rib.
For more details on the genus see the paper: Conway, KW (2005) - Monophyly of the genus Boraras (Teleostei: Cyprinidae). Ichthyological Exploration of Freshwaters, Volume 16, No. 3, pp 249-264.