Two new cis-Andean species of the South American catfish genus Megalonema allied to trans-Andean Megalonema xanthum, with description of a new subgenus (Siluriformes: Pimelodidae)

John G Lundberg,Wasila M Dahdul

doi:10.1590/s1679-62252008000300018

Abstract

A revised diagnosis of the pimelodid catfish genus Megalonema is given based on synapomorphic features of the Weberian complex and gas bladder. Megalonema xanthum from the Magdalena River is redescribed. Two new cis-Andean species of Megalonema are described, M. amaxanthum n. sp. from the Amazon River basin, and M. orixanthum n. sp. from the Orinoco River basin. These three species are differentially diagnosed by shape and size of the supraoccipital posterior process, adipose-fin shape, vertebral counts, eye size, premaxillary bone shape and dentition, length of the anal-fin base, width between the posterior nostrils and presence/absence of dentations on the pectoral spine. Eretmomegalonema new subgenus is established for M. xanthum, M. amaxanthum and M. orixanthum and supported by the uniquely synapomorphic paddle-like structure of its pelvic fin and hypertrophied basipterygium. Unambiguous synapomorphies indicate a sister-group relationship between M. amaxanthum and M. orixanthum, with M. xanthum basal to this pair. This topology is congruent with the Neogene origins of separate Magdalena, Amazon and Orinoco basins suggesting vicariant control of diversification of Eretmomegalonema.

Highlights

The pimelodid catfish genus Megalonema has an ample distribution in South America throughout the Paraná, Amazon, Essequibo, Orinoco, Maracaibo and Magdalena basins
The purpose of this paper is to describe two new species that are related to M. xanthum, and to recognize these three as a monophyletic subgroup of Megalonema characterized by oddly hypertrophied pelvic fins
Stewart found that Megalonema exhibits unique features of the gas bladder and Weberian vertebral structure that set it apart from the Calophysus group, he pointed out other possible synapomorphies in the reduced structure of the pectoral spine, elevated pectoral-fin ray count and loss of the posterior cleithral process

Summary

Introduction

The pimelodid catfish genus Megalonema has an ample distribution in South America throughout the Paraná, Amazon, Essequibo, Orinoco, Maracaibo and Magdalena basins. The species of Megalonema inhabit medium to large rivers mostly in flowing marginal reaches and over fine substrates of mud or sand. These are streamlined catfishes of modest size with adult lengths ranging among species from ca. In color Megalonema are mostly plain yellowish to tan and generally without prominent marks except for the common presence of an embedded dark spot in the base of the upper caudal-fin lobe, and sometimes the lower lobe. Description of a new subgenus and two new species of Megalonema

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