Redescription of the Tuvirão, Gymnotus inaequilabiatus Valenciennes, 1839, Using High-Resolution X-ray Computed Tomography
The tuvirão (Gymnotus inaequilabiatus) attains the largest body size (c. 1 m) of any species within Gymnotus, and the type specimen is the largest known specimen of the genus. Gymnotus inaequilabiatus is one of several species in Gymnotus that often have been misidentified or mistaken for one another, in part because of their similar external appearance and their overlapping geographic ranges in southern Brazil and northern Argentina. Here we redescribe G. inaequilabiatus using conventional osteological and morphological tools, and also High Resolution X-Ray Computed Tomography of the 184-year-old type specimen of the species. We report characters showing that this species is a member of the G. tigre species group, rather than the G. carapo species complex, as had previously been believed. We also provide a detailed description of the cranial osteology of the large-bodied type specimen, which represents the greatest extent of hypermorphosis known in the Gymnotus. O tuvirão (Gymnotus inaequilabiatus) é a maior espécie de Gymnotus (c. 1 m) e seu espécime tipo é um dos maiores espécimens conhecidos deste grupo. Gymnotus inaequilabiatus é uma das espécies deste gênero erroneamente identificada e confundida com outro congênere que possue uma distribuição similar no sul do Brasil e nordeste da Argentina. Nesse trabalho nos redescrevemos G. inaequilabiatus baseando-se em osteologia convencional e a tomografia computadorizada de alta resolução do espécime tipo. Demonstra-se que esta espécie faz parte do grupo de espécies G. tigre e não parte do complexo de espécies do grupo G. carapo como previamente proposto. Uma detalhada descrição osteológica do crânio do espécime tipo é fornecida, e este representa o extremo máximo de hipermorfose no gênero.
Gymnotus inaequilabiatus, neotype, MNHN 4615, 920 mm TL. (A) Body (modified from Claude Ferrara, MNHN); (B) head.
Original illustration of Gymnotus inaequilabiatus (from Valenciennes, 1839:plate 14).
Geographical distribution of Gymnotus inaequilabiatus. Circle, location of nontype material; Diamond, possible approximate location of type locality inferred from geographical information available on type material; Triangle, approximate location of type locality provided by MNHN ichthyology collection database.
Variation in size and coloration in Gymnotus inaequilabiatus. (A) MZUSP 46001, Argentina, Rio Paraná, 998 mm TL; (B) MLP 3353, Argentina, Misiones, ca. 250 mm TL (photo by T. Carvalho); (C) MACN 5979, Argentina, Rosario, ca. 250 mm TL (photo by T. Carvalho); (D) LBP 12616, Brazil, Rio Paraguay, ca. 650 mm TL; (E) MCP 39375, Brazil, Rio Grande do Sul, ca. 550 mm TL.
Right lateral view of head skeleton of Gymnotus inaequilabiatus. (A) Exploded view. (B) Intact skull view. Abbreviations: ang-ar, anguloarticular; bh, basihyal (glossohyal); bo, basioccipital; br, branchiostegal; c, centrum; cb, ceratobranchials; cha, anterior ceratohyal; chp, posterior ceratohyal; cl, cleithrum; co, coracoid; d, dentary; eb, epibranchial; enp, endopterygoid; epo, epioccipital; exo, exoccipital; fr, frontal; h, hyomandibula; hhd, dorsal hypohyal; hhv, ventral hypohyal; iop, interopercle; met, mesethmoid; mpt, metapterygoid; na, neural arch; obs, orbitosphenoid; op, opercle; pa, parietal; pas, parasphenoid; pfr, pectoral-fin rays; pmx, premaxilla; pop, preopercle; pp, parapophysis; pro, prootic; pt, posttemporal; pto, pterotic; pts, pterosphenoid; q, quadrate; r, rib; ra, radials; rar, retroarticular; sc, scapula; scl, supracleithrum; sn, supraneurals; soc, supraoccipital; sop, subopercle; spo, sphenotic; sym, symplectic; tr, tripus; uhy, urohyal; vet, ventral ethmoid.
Lateral view of a subset of head bones of the neotype specimen of Gymnotus inaequilabiatus. (A) Right lateral view of the head skeleton and associated bony elements of the anterior vertebrae and pectoral girdle, with lateral dermal bones removed (i.e., premaxilla, maxilla, opercular series, branchiostegal rays) to allow clear view of neurocranium. (B) Dorsal view of parietals, frontals, and mesethmoid (all fused). (C) Ventral view of parasphenoid bone. (D) Right lateral view of dentary bone. (E) Right lateral view of urohyal. (F) Right lateral view of gill arch bones, from left to right: (dorsal) pharyngobranchials and epibranchials, (ventral) posterior and anterior ceratohyal, hypohyal, and basihyal.
Overview of cranial external features and proportions of the neotype specimen (MNHN 4615) of Gymnotus inaequilabiatus.
Contributor Notes
Associate Editor: R. E. Reis.