Descriptive osteology of fishes referred to the genus Rasboroides Brittan ( Teleostei : Cyprinidae )

Owing to the spectacular colouration, fish species of the genus Rasboroides have a substantial economic value in the aquarium trade of the world. All the species of Rasboroides categorized as threatened fishes. The studies revealed that some species of the genus Rasboroides are having a stable population in the South-western wet zone of Sri Lanka. Osteology of these important species was studied and the findings further support the recently described or re-described species within the genus. One of the important characters of the genus is danionin notch which was only observed in R. pallidus. This character is always observed in related Cyprinid genera Devario. The presence of danionin notch in R. pallidus is mystifying since it is independently found in two different tribes of subfamily Danioninae. Further studies ought to reveal the status of this character within the Danioninae.


INTRODUCTION
Sri Lanka possesses a highly diverse endemic fauna and flora (Myers et al., 2000) and is a reservoir of unique evolutionary history (Bossuyt et al., 2004).During the British rule of the island, several large scale-taxonomic studies on the fishes of Sri Lanka have been carried out.However, the aspects of osteology, ethology and ecology of fishes are handful in Sri Lanka (Costa and Fernando, 1967;De Silva et al., 1977;Kortmulder, 1986;Moyle and Senanayake, 1984;Wickramanayake, 1990).Recently, De Silva et al. (2015) made a comprehensive compilation on freshwater fishes, where almost all reported fishes to date have been included.But that compilation too lacks the aspects of biology and osteology of fish.
Rasboroides vaterifloris (Rasbora vaterifloris) was originally reported in Gilimale, Sabaragamuwa Province of Sri Lanka (Deraniyagala, 1930).Brittan (1972) was the first reviser, who recognized the uniqueness of Rasbora vaterifloris and placed in subgenus, Rasbora (Rasboroides).Kottelat and Vidthayanon (1993) recognized the validity of the subgenus Rasboroides and resurrected to generic rank.Batuwita et al. (2013) resolved the identity of subdivision of R. vaterifloris by recognizing four different species (Deraniyagala, 1958).Previous study of Batuwita et al. (2013) on the genera Horadandia and Rasboroides discusses about a few characters of cranium, suspensorium, and 5th gill arch.Hence, this study discusses descriptive osteology of all congeners of genera Rasboroides.
Description.Infraorbitals with sensory canals; outer margin of 3rd infraorbital with downward extension; 4th infraorbital greatly reduced; 5th infraorbital absent; anterior dorsal extension of the maxilla slightly overlaps the premaxilla dorsally; outline shape of tip of the ascending process of the premaxilla straight; dorsal boarder of operculum with a deep indentation; dorsal outline of lower jaw with a concavity next to symphyseal knob; no concavity before the coronoid process of dentary; superior border of anguloarticular not elevated, truncate; danionin notch absent; narrow coronoid process of dentary; frantoparietal fontanelle present.Abdominal vertebrae12; caudal vertebrae16; total vertebrae28.Ventral view of the shape of supracleithrum: L-shaped; anterior outline of horizontal limb of cleithrum in ventral view convex; absence of a foramen on anterior wall of horizontal limb of cleithrum; coracoid foramen well-developed.Basipterygium broad, not deeply notched; basihyal with two lateral processes, apophysis wide; absence of Rasborin process on 4th epibranchial; pharyngeal teeth in 3 rows (3,4,5-5,4,3); pharyngeal teeth with terminal grooves; no notch on posterior lateral border of 5th ceratobranchial; uroneural is absent from pleurostyle; parahypural and first hypural fused posteriorly; six hypurals present.
Description.Infraorbitals with sensory canals; 1st and 3rd infraorbitals greatly enlarged; outer margin of 3rd infraorbital with downward extension; 4th infraorbital greatly reduced; 5th infraorbital absent; anterior dorsal extension of the maxilla slightly overlaps the premaxilla dorsally; outline shape of tip of the ascending process of the premaxilla straight; dorsal boarder of operculum slightly convex; dorsal outline of lower jaw with a concavity next to symphyseal knob; a shallow depression before the coronoid process of dentary; a broad coronoid process of dentary; superior border of anguloarticular convex, elevated; danionin notch present; frantoparietal fontanelle present.Abdominal vertebrae 11-12; caudal vertebrae 16-17; total vertebrae 28-29.Ventral view of the shape of supracleithrum Lshaped; anterior outline of horizontal limb of cleithrum in ventral view straight; absence of a foramen on anterior wall of horizontal limb of cleithrum; coracoid foramen well-developed.Basipterygium narrow, deeply notched; absence of hypohyal process on the basihyal, apophysis wide; absence of Rasborin process on 4th epibranchial; pharyngeal teeth in 3 rows (3,4,4,3); pharyngeal teeth with terminal grooves; no notch on posterior lateral border of 5th ceratobranchial; uroneural is absent from pleurostyle; parahypural and first hypural fused posteriorly; six hypurals present.
Description.Infraorbitals with sensory canals; outer margin of 3rd infraorbital without downward extension; 4th infraorbital greatly reduced; 5th infraorbital absent; anterior dorsal extension of the maxilla slightly overlaps the premaxilla dorsally; outline shape of tip of the ascending process of the premaxilla concave; dorsal boarder of operculum without a deep indentation; dorsal outline of lower jaw with a shallow concavity next to symphyseal knob; shallow concavity before the coronoid process of dentary; superior border of anguloarticular straight; danionin notch absent; broad coronoid process of dentary; frontoparietal fontanelle present.Abdominal vertebrae 12-13; caudal vertebrae 17; total vertebrae 29-30.Ventral view of supracleithrum L-shaped; anterior outline of horizontal limb of cleithrum in ventral view convex; absence of a foramen on anterior wall of horizontal limb of cleithrum; coracoid foramen absent; basipterygium narrow, deeply notched; basihyal with two lateral processes, apophysis wide; absence of Rasborin process on 4th epibranchial; pharyngeal teeth in 3 rows (2,4,5-5,4,2); pharyngeal teeth with terminal grooves; no notch on posterior lateral border of 5th ceratobranchial; uroneural is absent from pleurostyle; parahypural and first hypural fused posteriorly; six hypurals present.1).Pectoral and pelvic girdles have distinct differences among species.Morphology of dentary showed distinctly separable characters within the congeners and the consistency of these characters further supports the identity of four discrete species within the Rasboroides.Kottelat and Witte (1993) placed Rasbora axelrodi Brittan, 1976 in the monotypic genus Sundadanio and considered it to be a member of the danionine lineage because it had a danionin notch in the dentary.However, Fang (2003) stated, that she did not observe a danionin notch in S. axelrodi (in 10 specimens).This describes intraspecific variations and/ or presence of additional species in the genus (Conway et al., 2011).
We examined about eight specimens (c&s) from the genus Rasboroides from Sri Lanka; only all R. pallidus (3 specimens) have the danionin notch (Figures 1C, 2C).However, the presence of shallow depression in R. vaterifloris (Figure 2A) has to be verified by further studies.Based on an on-going study of Devario, danionin notch appears to be a well-defined in all the Sri Lankan species (Batuwita et al., pers. comm.).Fang (2003) also concluded that danionin notch is not a Danionin (s.l.) synapomorphy.

CONCLUSIONS
The findings of this study confirm the species recognition based on morphology and morphometry by the previous study of Batuwita et al. (2013).The results further revealed that one of the important characters of subfamily Danioninae, the danionin notch occurs also within the Rasboroides, suggesting that danionin notch is not a Danionin (s.l.) synapomorphy as mentioned by Fang (2003).