Rasbora microcephalus (Jerdon, 1849)

Rasbora microcephalus (Jerdon, 1849) View in CoL ( Fig. 7B View Figure 7 )

The sample comprises five specimens. With a size of 26.6 mm SL to 35.3 mm SL, all these specimens are juveniles.

The closest recent records of a Rasbora species to the hot springs are R. microcephalus ( Silva et al., 2010) and R. cf. adisi and R. dandia ( Sudasinghe et al., 2020) and were documented from tributaries of the Mahaweli Ganga with the localities approximately 35 km ( R. microcephalus ) and 50 km ( R. cf. adisi and R. dandia ) distant to Kanniya. These three species are the only species of Rasbora , which occur in the lowlands of the northern dry zone of Sri Lanka ( Sudasinghe et al., 2020). The coloration, the absence/presence of a ventral projection at the jaw articulation, the absence/ presence of a lachrymal groove and the prominence of the symphysal knob are the main morphological traits to separate adults of these three species ( Sudasinghe et al., 2020). With 35.3 mm SL, the largest specimen from Hacker’s sample is not fully grown. It is distinctly smaller than adult specimens of the three species potentially occurring in the area ( R. cf. adisi 50.8-73.8 mm SL, R. dandia 45.0- 89.6 mm SL, R. microcephalus 47.8.8- 99.9 mm SL) ( Sudasinghe et al., 2020). The absence of a lachrymal groove, the presence of a slight ventral projection at the jaw articulation, and the colour pattern ( Fig. 7B View Figure 7 ) indicate that both specimens are R. microcephalus . Unfortunately, genetic analysis of the five specimens is not possible because Hacker preserved all the samples in formalin.

Duncker (1912) mentioned Rasbora daniconius from close to the hot springs of Kanniya. But R. daniconius is confined to India ( Silva et al., 2010; Sudasinghe et al., 2020). Duncker brought two specimens to the Museum der Natur Hamburg (ZMH–ICH) ( Fig. 8 View Figure 8 ). With a SL of 39.3 mm and 40.1 mm these specimens are somewhat larger than those from the hot springs collected by Hacker. Also, these two specimens have no lachrymal groove but an indistinct projection of the jaw articulation ( Fig. 8 View Figure 8 ) and are in fact R. microcephalus .

The five specimens collected in the most downstream spring (well 7) ( Fig. 3B View Figure 3 ) in 1982 by R. Hacker are the first record of this species from the hot springs of Kanniya .