Boraras urophthalmoides

Boraras urophthalmoides
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Actinopterygii
Order:	Cypriniformes
Family:	Cyprinidae
Genus:	Boraras
Species:	B. urophthalmoides
Binomial name
Boraras urophthalmoides (Kottelat, 1991)[1]

The exclamation point rasbora or least rasbora (Boraras urophthalmoides) is a species of ray-finned fish in the genus Boraras. It was first described from a freshwater swamp associated with the Sai Buri River in Pattani province, southern (peninsular) Thailand, but currently considered to range throughout much of southern and central parts of the country, with the province of Narathiwat on the border, with Peninsular Malaysia appearing to represent the southern limit of its range.

Additional records are not extensive, but it seemingly occurs throughout the Mae Klong and Chao Phraya river basins and has been recorded in both Vietnam and Cambodia, where one notable population inhabits the moats and ponds of the Angkor Wat temple complex near Siem Reap.

Habitat

Boraras uriphthalmoides inhabits shallow bodies of water such as swamps, marshes, floodplains and rice paddies, where it is typically associated with submerged vegetation in predominantly clear water. It is also known to move into temporarily inundated areas during the wet season.

Size

This species is very small, ranging from 12 to 16 mm.

Mature females are noticeably rounder-bellied and often a little larger than the slightly more colourful males.

Notes

This species appears to vary in colouration depending on collection locality, which has led to some confusion with B. brigittae as some populations can develop a strong orange/red stripe above the dark lateral marking when in good condition.

B. urophthalmoides can be easily identified by its smaller adult size, distinct round, dark blotch on the caudal peduncle, comparatively long, unbroken lateral stripe and less uniformly red patterning. Its sometimes seen on sale under the trade names of ‘sparrow rasbora’ or ‘exclamation-point rasbora’, the latter in reference to the distinctive pattern formed by the dark body markings.

Boraras was erected in 1993 in order to separate a small group of species from the larger Rasbora assemblage on the basis of differences in morphology and reproductive strategy.

In older literature they are therefore referred to as members of Rasbora and following Liao et al. (2010) the genus is a member of the rasborin sub-group within the subfamily Danioninae (the other sub-group contains the danionins).

This group is further subdivided into six clades (clusters of closely related species), of which Boraras (or at least B. brigittae, the only species involved in the study) is included in ‘clade three’ alongside Horadandia atukorali, Rasboroides vaterifloris, Trigonostigma heteromorpha and three species previously included in Rasbora but moved into new genera by the authors: Trigonopoma gracile, T. pauciperforatum and Rasbosoma spilocerca.

Of these, it was found to form a monophyletic group with Rasboroides, meaning these two derived from the same common ancestor. The results for B. brigittae and T. heteromorpha were found to be inconclusive in some respects, and further work regarding their phylogenetic position was recommended.

Shortly afterwards a paper investigating systematics of the subfamily Danioninae was published by Tang et al. (2010). Their results differed from those of Liao et al., and their conclusion was to synonymise the four new genera plus Boraras and Trigonostigma with Rasbora, based on an incomplete knowledge of relationships within the group, an approach they describe as "more conservative". This has not been followed by the majority of authors, however, and the separate groupings are maintained in the extensive work by Kottelat (2013), which is followed here.

The small adult size in Boraras species evolved via a process known as miniaturisation, characterised by sexually mature adults with a significantly reduced size of less than 20 mm SL.

Among bony fishes, cyprinids are one of the few groups in which this phenomenon occurs repeatedly, with all Barboides, Danionella, Microdevario, Microrasbora, Horadandia, Boraras, Paedocypris, Sawbwa and Sundadanio species representing miniaturised taxa, along with a few members of Danio, Laubuca and Rasbora. All show a preference for still or slow-moving waters, often in nutrient-poor habitats such as forest peat swamps.

The anatomical structure of miniaturised cyprinids can vary greatly, and there are two principle "groupings", with some species possessing intermediate features to some degree. The first contains those fishes which, though small, are essentially proportionally dwarfed versions of their larger relatives, e.g., Barboides, Microdevario, Microrasbora, etc.

The other includes those in which anatomical development stops at a point where adults still resemble a larval form of their larger ancestor, i.e., Danionella and Paedocypris.

The latter are usually referred to as "developmentally truncated" or "paedomorphic" and are thought to have evolved via a process known as "progenetic paedomorphosis", i.e. paedomorphosis brought about by accelerated maturation. They typically exhibit a simplified skeletal structure along with species-specific morphological peculiarities.

Britz et al. (2009) consider that developmental truncation may have facilitated the development of such novelties "by freeing large parts of the skeleton from developmental constraints, dissociating developmentally linked pathways and creating a greater potential for more dramatic changes".

Boraras species from the Malay Archipelago (B. brigittae, B. maculatus and B. merah) tend to have a slimmer, more elongate body profile than those from Indochina (B. micros, B. urophthalmoides, B. naevus), which may assist when attempting to identify them.

References

Kottelat, M., 1991 - Ichthyological Exploration of Freshwaters 2(2): 177-191 Notes on the taxonomy of some Sundaic and Indochinese species of Rasbora, with description of four new species (Pisces: Cyprinidae). Conway, K. W., 2005 - Ichthyological Exploration of Freshwaters 16(3): 249-264

Monophyly of the genus Boraras (Teleostei: Cyprinidae). Conway, K. W. and M. Kottelat, 2011 - Zootaxa 3002: 45-51

Boraras naevus, a new species of miniature and sexually dichromatic freshwater fish from peninsular Thailand (Ostariophysi: Cyprinidae). Kottelat, M., 2013 - The Raffles Bulletin of Zoology Supplement 27: 1-663

The fishes of the inland waters of southeast Asia: a catalogue and core bibliography of the fishes known to occur in freshwaters, mangroves and estuaries. Liao, T. Y., Kullander, S. O. and F. Fang, 2009 - Zoologica Scripta 39(2): 155-176

Phylogenetic analysis of the genus Rasbora (Teleostei: Cyprinidae). Mayden, R. L., K. L. Tang, K. W. Conway, J. Freyhof, S. Chamberlain, M. Haskins, L. Schneider, M. Sudkamp, R. M. Wood, M. Agnew, A. Bufalino, Z. Sulaiman, M. Miya, K. Saitoh, and S. He, 2007 - Journal of Experimental Zoology, Molecular Development and Evolution 308B: 1-13

Phylogenetic relationships of Danio within the order Cypriniformes: a framework for comparative and evolutionary studies of a model species. Tang, K. L., M. K. Agnew, W. J. Chen., M. V. Hirt, T. Sado, L. M. Schneider, J. Freyhof, Z. Sulaiman, E. Swartz, C. Vidthayanon, M. Miya, K. Saitoh, A. M. Simons, R. M. Wood and R. L. Mayden, 2010 - Molecular Phylogenetics and Evolution 57(1): 189-214 Systematics of the subfamily Danioninae (Teleostei: Cypriniformes: Cyprinidae).