Geometric morphometrics as a tool to identify species in multispecific flatfish landings in the Tropical Southwestern Atlantic

Fisheries statistics of many countries are still limited by the use of common names in multispecific captures, where little is known about the correct taxonomic identification of captured species. Homonymy is a known obstacle for commercially exploited Pleuronectiformes, traditionally grouped as flatfishes in Brazilian fisheries statistics. The present study was based on visits to 14 artisanal fish landing points and four industrial landing sites along the coast of Rio de Janeiro, in the southwestern Atlantic, between 2013 and 2016. A total of 176 Pleuronectiformes specimens were collected under the flatfish category. Standardized and scaled images were used for a geometric morphometric analysis based on anatomical landmarks in order to identify how species were distributed in the morphospace and to point out diagnostic character not used by the known literature for the group. A total of four different groups were identified as the main target for the commercial flatfish category, being associated with four species of Paralichthyidae: Paralichthys isosceles Jordan, 1891, P. patagonicus Jordan, 1889, P. orbignyanus (Valenciennes, 1839) and Xystreurys rasile (Jordan, 1891). A canonical analysis independently isolated P. isosceles and X. rasile, while strongly associating P. patagonicus and P. orbignyanus. These results were corroborated by an UPGMA analysis of Procrustes distances based on the consensus form. The analysis identified that tail shape and relative eye and jaw positions are of significant importance for species separation. The combination of the results of the analysis and specific ocelli positioning allowed for the selection of diagnostic characters for all species without the need for laboratory examinations. Conflicts between the literature and the specific composition of flatfish landings are also discussed. Geometric morphometrics were demonstrated herein as useful not only in quantifying differences in scarcely studied body parts, but also as a tool able to identify the consistency of a multispecies commercial category used by the fishery statistics, as well as useful in pointing out characteristics capable of dismembering flatfishes into monospecific categories.