Abstract
The miniature seasonal killifishSpectrolebiascostae, first described for the middle Araguaia River basin, has been also recorded from two areas in the middle Tocantins River basin, from where male specimens exhibit some differences in their colour pattern. Analyses directed to species delineation (GMYC and bPTP), using a fragment of the mitochondrial gene COI, strongly support two species,S.costaefrom the Araguaia River basin and a new species from the Tocantins River basin.Spectrolebiasgracilissp. n.is described on the basis of specimens collected from two localities separated by about 530 km, Canabrava River floodplains near Alvorada do Tocantins and Tocantins River floodplains near Palmeirante. Field inventories were unsuccessful in finding additional populations in the region, which is attributed to the high environmental degradation, including several large dams that have permanently inundated typical killifish habitats.Spectrolebiasgracilisis member of a clade also includingS.costae,S.inaequipinnatus, andS.semiocellatus, diagnosed by having the dorsal and anal fins in males with iridescent dots restricted to their basal portion, caudal fin in males hyaline, and caudal-fin base with two pairs of neuromasts. Within this clade, a single miniaturisation event is supported for the most recent common ancestor of the subclade comprisingS.costaeandS.gracilis, which differ from other congeners by reaching only about 20 mm standard length as maximum adult size.
Highlights
The great species diversity, striking colour patterns and the broad array of unique biological specializations make aplocheiloid killifishes important members of the tropical biota of Americas, Africa, and southern Asia (Costa 2008)
Both the generalized mixed Yule coalescent (GMYC) and Bayesian implementation of the Poisson tree processes (bPTP) models delimited the Tocantins River basin populations as two distinct specific entities, whereas haplotypes of the Canabrava and Palmeirante populations were clustered in a single species (Fig. 2)
Morphometric and meristic data obtained from specimens representing the populations of the Tocantins River basin were similar to data recorded for specimens collected along the Araguaia River basin (Costa 2007a)
Summary
The great species diversity, striking colour patterns and the broad array of unique biological specializations make aplocheiloid killifishes important members of the tropical biota of Americas, Africa, and southern Asia (Costa 2008). The subequatorial South American area comprising the largest southern tributaries of the Amazonas River (Tocantins, Araguaia, Xingu and Tapajós River drainages), is remarkable by concentrating numerous endemic aplocheiloid fishes (Costa 1990, 2007a, b, 2011, 2016), many of them consisting of miniatures not surpassing 30 mm of standard length (SL) when adults (Costa 1998) Some endemic groups, such as the genera Maratecoara Costa, 1995, Pituna Costa, 1989, Plesiolebias Costa, 1989, and part of the genus Spectrolebias Costa & Nielsen, 1997 are members of the aplocheiloid clades known as seasonal or annual killifishes (Myers 1942; Costa 2002a), that comprise species completing their whole life cycle in seasonal pools formed during the rainy seasons.
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