Psilorhynchus platydorsalis

Conway, Kevin W., 2018, ‘ On Psilorhynchus sucatio and P. nudithoracicus’, the sequel: Unnecessary and unscientific names lead to rapid synonymization and taxonomic time wasting-A response to Arunachalam et al. (2018, Zootaxa 4418 (6), pp. 594-600: 594-595

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Psilorhynchus platydorsalis


Problems with the diagnosis of Psilorhynchus platydorsalis 

Arunachalam et al. (2018) described Psilorhynchus platydorsalis  based on three specimens collected from a single location (Son River, Koilwar Village, Bhojpur District, Bihar, India). Based on the characters listed in the description of P. platydorsalis  (Arunachalam et al., 2018: 217–218) and based on the gestalt of the holotype (Arunachalam et al., 2018; Fig. 1) it is apparent that this newly described species is very similar in external appearance to P. nudithoracicus  (compare the holotype specimen of P. platydorsalis  shown in figure 1 of Arunachalam et al., 2018 with any specimen of P. nudithoracicus  illustrated in Conway et al., 2013 [figures 15–17, 20]). In the diagnosis of P. platydorsalis, Arunachalam et al. (2018: 214)  list 20 characters, including counts of fin rays and scales and measurements of the fins, body and head, which serve to differentiate the allegedly new species from P. nudithoracicus  . Critical appraisal ( Table 1) of these 20 characters reveals that the differences reported between P. platydorsalis  and P. nudithoracicus  are either: (1) negligible (e.g., “distance between dorsal-fin origin and pectoral-fin insertion (28.7–29.3 [in P. platydorsalis  ] vs. 30.6–32.2 %SL [in P. nudithoracicus  ])” a difference of 1.3 % which equates to ~ 0.5 mm when one considers the size of the specimens or “caudal peduncle [width] (3.1–3.2 [in P. platydorsalis  ] vs. 1.9–2.9 %SL [in P. nudithoracicus  ])” a difference of 0.2 % which equates to less than 0.1 mm when one considers the size of the specimens); (2) ephemeral, and likely to disappear when the actual variation in P. nudithoracicus  is considered (e.g., pre-anal length, pre-pelvic length, pectoral-fin length, pelvic-fin length, post-dorsal length etc; Table 1 in Conway et al., 2013 and Table 1 herein); or (3) entirely suspect because the values reported in the diagnosis are either in conflict with information provided elsewhere in the text (e.g., number of “pre-anal scales” listed as 7 for P. platydorsalis  in diagnosis but “anal scale rows [=pre-anal scales]” listed as “8.5–9” in Table 3 in Arunachalam et al., 2018) or unlikely to be obtainable from material in the way it was examined by Arunachalam et al. (2018) (e.g., the values reported for the length of the upper or lower jaws, which cannot be observed in preserved specimens due to the thick skin that contributes to the rostral cap, upper lip and lower jaw cushion in Psilorhynchus  ).

This demonstrates that Arunachalam et al. (2018) have not provided convincing character evidence that P. platydorsalis  can be distinguished from P. nudithoracicus  and therefore P. platydorsalis  is a junior synonym of P. nudithoracicus  .