Wednesday, December 25, 2019

[Ichthyology • 2019] Tyttobrycon shibattai • On A Remarkable Sexual Dimorphic Trait in the Characiformes related to the Olfactory Organ and Description of A New Miniature Species of Tyttobrycon Géry (Characiformes: Characidae)

Tyttobrycon shibattai
Abrahão, Pastana & Marinho, 2019

Among the order Characiformes, secondary sexual dimorphism is commonly associated to the occurrence of bony hooks on fins, shape and length of the dorsal and anal fins, and sexual dichromatism. The analysis of a new miniature Characidae species of the genus Tyttobrycon, described herein, yielded to the discovery of a sexually dimorphic trait related to nostril aperture and number of olfactory lamellae. In this type of dimorphism, mature males present larger nostril aperture and higher number of olfactory lamella than females. A dimorphic olfactory organ is for the first time recorded and described for a member of the Characiformes. Gross morphology and development of brain and peripheral olfactory organ of Tyttobrycon sp. n. are described and compared to other species of Characidae. It is hypothesized that such dimorphic trait is related to male-male detection during cohort competition in small characids. The new species of Tyttobrycon is diagnosed from its congeners by the number of branched anal-fin rays (19–21) and the absence of a caudal-peduncle blotch. It occurs in a small tributary of Rio Madeira basin, near to the limit between Brazil and Bolivia, Acre State, Brazil.

Fig 1. Tyttobrycon shibattai. (A) MZUSP 125268, holotype, male, 17.1 mm SL, Brazil, Acre, Xapuri, Seringal Cachoeira road, Rio Iná, tributary of Rio Abunã, Rio Madeira drainage, Rio Amazonas basin; (B) MZUSP 125269, paratype, female, 17.7 mm SL, collected with the holotype.

Tyttobrycon shibattai, sp. nov.   

Diagnosis: Tyttobrycon shibattai can be distinguished from all congeners, except T. spinosus, by having 19–21 branched anal-fin rays (vs. up to 17). It can be diagnosed from T. spinosus by lacking a caudal-peduncle blotch (vs. blotch present). Additionally, it is further distinguished from all species, except T. dorsimaculatus, by the presence of both tricuspid and conical teeth on premaxilla and dentary (vs. teeth exclusively conical). It is distinguished from T. dorsimaculatus by having hyaline dorsal-fin, with few scattered melanophores (vs. dorsal fin with a conspicuous dark blotch) and by the presence of a parallel line of subjacent melanophores dorsal to anal-fin base (vs. melanophore line absent).

Fig 8. Type locality of Tyttobrycon shibattai. Rio Iná, tributary of Rio Abunã, Rio Madeira drainage. Photograph taken in October 2010.

Geographical distribution: Tyttobrycon shibattai is only known from its type locality at Rio Iná, a tributary of Rio Abunã, near to the limit between Brazil and Bolivia, Rio Madeira drainage, Rio Amazonas basin, Acre State, Brazil (Fig 7).

Ecological notes: 
Specimens of Tyttobrycon shibattai were collected in lentic habitats in the Rio Iná (Fig 8). The type locality was relatively shallow, with its deepest portion reaching 60 cm, and exhibiting a muddy-brown water. The river bottom was composed of clay and sand. Water physicochemical parameters during samples was 30.5°C, 6.3 mg/L of O2 dissolved, pH 6.31, and 62.1 conductivity microSiemens (uS)/cm. The only syntopic species collected with Tyttobrycon shibattai was Tridentopsis sp., which was abundant in the area.

Stomach contents of two adults containing mostly sediments, plant and algae debris, and some autochthonous invertebrates with predominance of ostracods. The stomach content of one juvenile specimen presented mostly plant and algae debris.

Etymology: The specific name honors O. A. Shibatta, a renowned icththyologist from Universidade Estadual de Londrina-MZUEL, for his great contributions to the systematic of fishes and support to new generations of ichthyologists. This honor is also dedicated to the vast influence and importance on the early ichthyological formation of the leading author of this paper (VPA). Shibatta was responsible for encouraging his studies with nervous system of Neotropical fishes.

Vitor Pimenta Abrahão, Murilo Pastana and Manoela Marinho. 2019. On A Remarkable Sexual Dimorphic Trait in the Characiformes related to the Olfactory Organ and Description of A New Miniature Species of Tyttobrycon Géry (Characiformes: Characidae). PLoS ONE. 14(12): e0226130. DOI: 10.1371/journal.pone.0226130