D. assimilis was formerly included in the genus Puntius and Puntius filamentosus 'group' of related species which also contained P. arulius, P. assimilis, P. exclamatio, P. filamentosus, P. rohani, P. rubrotinctus, P. singhala, P. srilankensis and P. tambraparniei, but all of these were moved to the new genus Dawkinsia by Pethiyagoda et al. (2012).
Dawkinsia species are defined by the following combination of characters: adult size normally 80 – 120 mm3.1 – 5 in SL; rostral barbels absent; maxillary barbels present or absent; last unbranched dorsal-fin ray smooth; 4 unbranched and 8 branched dorsal-fin rays; 3 unbranched and 5 branched anal-fin rays; lateral line complete, with 18-22 scales on body; gill rakers simple, acuminate (not branched or laminate); no antrorse predorsal spinous ray; free uroneural present; 4-5 supraneurals; 15 precaudal and 14-17 caual vertebrae; post-epiphysial fontanelle absent; infraorbital 3 slender, not overlapping preoperculum; juvenile (< 50 mm2.0 in SL) colour pattern consisting of three black bars on body, retained in adults of some species; a black, horizontally elongate blotch on the caudal peduncle in adults.
D. assimilis is told apart from all other Dawkinsia spp. by the following combination of characters: tip of caudal-fin lobes with a transverse black band approximately as wide as the eye (expressed faintly or absent in the Kallada River population); lower lip continuous; presence of a dark blotch on the caudal peduncle approximately 2-5 body scales in length, and not reaching anal-fin origin; no prominent markings on body anterior to anal-fin origin; mouth inferior; maxillary barbel length 23.5 - 33.3 % HL.
The branched dorsal rays are also extended into filament-like threads in some adult individuals of the Chalakudy and Kallada River populations.
Within the group the most similar-looking species are D. filamentosa and D. rohani, but the former is readily identifiable by possession of a subterminal mouth (vs. inferior in D. assimilis) and shorter maxillary barbels (0.5 - 2.2% SL, vs. 5.5 - 9.3 %), while in the latter the caudal-fin lobes lack transverse black bands.
D. assimilis has been misidentified as Puntius mahecola (Valenciennes, 1844) in the past but the identity of both species was resolved by Pethioyagoda and Kottelat (2005a, 2005b).
They inspected the syntypes of P. mahecola and concluded that though valid it isn't closely related to any member of Dawkinsia (then the P. filamentosus group) but is rather a smaller, silvery fish with a single dark blotch on the caudal peduncle, located entirely posterior to the anal-fin.
It's widely-distributed in Kerala state, southern India and has been pictured in some older literature as Puntius amphibius (Valenciennes, 1842).
P. mahecola had previously been considered a synonym of D. filamentosa, with a fish resembling the latter collected from the Chalakudy River and exported for the aquarium hobby as 'P. mahecola' in 1996.
Several Dawkinsia species, including D. assimilis, continue to appear on trade lists as 'mahecola barb' while a beautiful colour morph with additional red markings is traded as D. assimilis 'red', 'super red' or 'neon red'.
Kottelat and Pethiyagoda (2005a) suggested that some populations of D. assimilis might represent distinct species but were unable to form any definitive conclusions due to a lack of adult material from Karnataka, in particular.
The precise relationships within the genus are still open to question in some respects with Knight et al. (2011) suggesting that members may hybridise naturally at some localities.
For example, a D. arulius-like fish (possibly D. rubrotincta) co-occurs with D. assimilis in the Kallada River at Thenmalai, which also happens to be the type locality of D. exclamatio.
The latter is somewhat anomalous since it's the only Dawkinsia species other than D. rubrotincta to feature a (roughly) W-shaped mid-lateral blotch, but also has a laterally-elongated blotch on the caudal peduncle as in other genus members.
D. exclamatio should also have a sub-terminal mouth and lack dorsal-fin filaments but some specimens possess a terminal mouth and/or dorsal filaments, and one specimen also had black caudal-fin tips as typically seen in D. filamentosa, whereas the description states that the fin tips are only dusky and lack distinctive markings.
Forms apparently intermediate between D. filamentosa and D. tambraparniei have also been collected where the two occur together in the Thamirabarani River, Tamil Nadu.
While no conclusions were reached because DNA testing was not performed, hybridisation between D. filamentosa and D. arulius in that river had previously been speculated and is a phenomenon known to be more common in the family Cyprinidae than in any other group of fishes, meaning future research may yield interesting results.
The genus Puntius was for a number of years viewed as a polyphyletic catch-all containing over 100 species of small to mid-sized cyprinid until Pethiyagoda et al. (2012) published a partial review covering South Asian members.
The majority of sub-Himalayan Puntius species were reclassified and new genera Dawkinsia, Dravidia, and Pethia erected to accomodate some of them, with the remainder either retained in Puntius or moved to the existing Systomus assemblage, though the definition of the latter was altered meaning some Southeast Asian species formerly placed there are no longer members.
It subsequently became clear that the name Dravidia was preoccupied by a genus of flesh fly, therefore the replacement name Haludaria was made available by Pethiyagoda (2013).
No species from Indochina, China, or Indonesia were included in the study meaning a significant number of former Puntius are currently classed as incertae sedis, i.e., of uncertain taxonomic placement, and this also applies to a number of South Asian species of unresolved status.
They’re perhaps best referred to as ‘Puntius‘ for the time being whereby the genus name is surrounded by quotation marks to denote its questionable usage, and that is the convention used here on SF at the moment.
Thanks to J. D. Marcus Knight.
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