Polymorphism in Conservative Structures? The Scapulocoracoids in Skates Genus Psammobatis (Chondrichthyes, Arhynchobatidae) and the Validity of P. parvacauda

E Mabragaña,V Gabbanelli,J M Díaz De Astarloa,M González-Castro,D M Vazquez

doi:10.3389/fmars.2020.00291

Abstract

Skeletal structures, especially the clasper, neurocranium and scapulocoracoids have been widely used to characterize and describe new species of elasmobranchs. Intra and interspecific variation of scapulocoracoids in 3 species of Psammobatis (n=67) from the Southwest Atlantic were analyzed to assess its utility as a diagnostic character in skates. Moreover, based on scapulocoracoids and body morphometric analysis, the validity of Psammobatis parvacauda McEachran, 1983 was evaluated. A remarkable sexual dimorphism in scapulocoracoids was observed in all species in accordance with original descriptions, which was also evident in the Principal Component (PCA) and Discriminant Analyses. The number and arrangement of post-dorsal and post-ventral fenestra was highly variable at intra-specific level, in contrast with original descriptions. Particularly, one of the morphotypes observed in females of P. normani was identical to the one registered in P. parvacauda. Results showed intraspecific variation in this structure, as was previously observed in Sympterygia species. Therefore, this structure should not be used as a diagnostic character in skates, at least for these genera. Regarding body morphometry the PCA showed that P. parvacauda grouped with P. normani females and the cross-validated analysis showed that P. parvacauda was classified in the group of P. normani females. P. parvacauda is only known for its original description (around Malvinas/Falkland Islands) and a likely second specimen, but our analyses revealed lack of diagnostic characteristics. Based on observation and measurement in the holotype of P. parvacauda, and comparisons with congeneric the species is here synonymized with P. normani.

Highlights

Skates (Chondrichthyes, Rajiformes), constitute a monophyletic and cosmopolitan group of cartilaginous fishes, with representatives in all oceans
Skeletal material was obtained from specimens of P. rudis and P. normani, collected by trawl fishing during research cruises conducted by the National Institute for Fisheries Research and Development (INIDEP) in southern Patagonian waters (49◦28 S, 67◦03 W to 52◦00 S, 53◦49 W, and from 84 to 182 m) in April 2000, and from specimens of P. lentiginosa collected off Buenos Aires Province (36◦23 S, 63◦59 W to 39◦05 S, 58◦04 W, from 66 to 99 m depth) in October 2006
A sexual dimorphism has been observed in the three Psammobatis species analyzed, according to the original descriptions (McEachran, 1983), being the scapulocoracoids of females more rectangular than those of males, which were more triangular shaped

Summary

Introduction

Skates (Chondrichthyes, Rajiformes), constitute a monophyletic and cosmopolitan group of cartilaginous fishes, with representatives in all oceans. The endoskeleton of skates -like all chondrichthyan fishes- is comprised predominantly of a hyaline-like cartilage that persists throughout the entire life and is not replaced by bone (Compagno, 1999; Seidel et al, 2017). The lateral face of the scapulocoracoid has an articular surface for the pectoral basal cartilages and foramina for blood vessels and nerves (Compagno, 1999). Fenestrae on the lateral face of the scapulocoracoid of primitive batoids include anterodorsal and anteroventral fenestrae between the pro and mesocondyles, and postdorsal and posventral fenestrae between the meso and metacondyles (Compagno, 1999)

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