Friday, July 20, 2018

[Ichthyology • 2018] Spectrolebias gracilis • A New Miniature Cryptic Species of the Seasonal Killifish Genus Spectrolebias (Cyprinodontiformes, Aplocheilidae) from the Tocantins River basin, central Brazil


Spectrolebias gracilis  Costa & Amorim, 2018 


Abstract
The miniature seasonal killifish Spectrolebias costae, 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. costae from the Araguaia River basin and a new species from the Tocantins River basin. Spectrolebias gracilis sp. 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. Spectrolebias gracilis is member of a clade also including S. costae, S. inaequipinnatus, and S. 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 comprising S. costae and S. gracilis, which differ from other congeners by reaching only about 20 mm standard length as maximum adult size.

Key Words: Amazon, Biodiversity conservation, Integrative taxonomy, Miniaturization, Molecular taxonomy, Species delimitation


Taxonomic accounts
Spectrolebias gracilis sp. n.

Diagnosis: Spectrolebias gracilis is member of a clade endemic to the Araguaia-Tocantins River System, also including S. costae, S. semiocellatus Costa & Nielsen, 1997 and S. inaequipinnatus Costa & Brasil, 2008, and morphologically diagnosed by: dorsal and anal fins in males with iridescent dots restricted to the basal portion of fins (vs. scattered over the whole fin), caudal fin in males hyaline (vs. variably coloured, usually dark red or grey), caudal-fin base with two pairs of neuromasts (vs. one). Spectrolebias gracilis is similar to S. costae and distinguished from S. semiocellatus and S. inaequipinnatus by having dorsal fin rounded in males (vs. pointed), dark brown to black pigmentation on the flank in males (vs. light brownish grey), and a subdistal bright blue stripe on the dorsal and anal fins in males (vs. subdistal bright blue absent). Spectrolebias gracilis differs from S. costae by the iridescent light blue colour pattern in males, comprising the presence of 10–12 small blue spots irregularly arranged on opercle, surrounded by diffuse blue iridescence (Fig. 4; vs. 6–8 small blue spots, usually arranged in three vertical series, contrasting with dark brown colour ground, Fig. 3) and one or two series of dots irregularly arranged on the basal portion of the dorsal fin (Fig. 4; vs. blue dots arranged in single longitudinal row close to fin base, Fig. 3).
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Figure 4. Spectrolebias gracilis sp. n., UFRJ 6440, holotype, male, 19.2 mm SL; Canabrava floodplains. 

Figure 5. Spectrolebias gracilis sp. n., UFRJ 6441, paratype, female, 17.8 mm SL; Canabrava floodplains.

Etymology: From the Latin gracilis, meaning thin, referring to the thin body of the small-sized new species.

Distribution and habitat: Spectrolebias gracilis is known from temporary pools of two localities of the middle Tocantins River basin, central Brazil (Fig. 6). In both localities pools were shallow, about 80 cm in deeper places, and densely occupied by aquatic vegetation.


Figure 3. Specrolebias costae, UFRJ 3549, male, 18.8 mm SL; das Mortes River floodplains.


Wilson J. E. M. Costa and Pedro F. Amorim. 2018. A New Miniature Cryptic Species of the Seasonal Killifish Genus Spectrolebias from the Tocantins River basin, central Brazil (Cyprinodontiformes, Aplocheilidae).  Zoosystematics and Evolution. 94(2): 359-368.  DOI: 10.3897/zse.94.28085

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