Atlantirivulus, a new subgenus of the killifi sh genus Rivulus from the eastern Brazilian coastal plains (Teleostei: Cyprinodontiformes: Rivulidae)

Rivulus Poey is a widespread, speciose and poorly diagnosed assemblage of American aplocheiloid killifi shes (COSTA, 2006). Over 100 nominal species are placed in Rivulus, ranging from Mexico to southern Brazil, but no unique apomorphic condition is known to occur in all species (COSTA, 2006). Rivulus has been diagnosed by a combination of characters not occurring in other rivulid genera (e. g., HUBER, 1992), and monophyly is poorly supported by morphological characters (COSTA, 2006), whereas molecular analyses suggest that Rivulus is not monophyletic (HRBEK & LARSON, 1999; MURPHY et al., 1999). However, monophyly of some species groups, sometimes formally recognized as subgenera, are well corroborated both by morphology and molecular studies (COSTA, 1998, 2006; HRBEK & LARSON, 1999; MURPHY et al., 1999). COSTA & BRASIL (1991) fi rst recognized monophyly of a geographically isolated, species assemblage from eastern Brazil based on the apomorphic morphology of the angulo-articular bone. Six species were assigned to this unnamed assemblage: Rivulus santensis KÖHLER, R. luelingi SEEGERS, R. haraldsiolii BERKENKAMP, R. nudiventris COSTA & BRASIL, R. janeiroensis COSTA, and R. depressus COSTA (COSTA, 1991). This assemblage was named as “the santensis superspecies” by HUBER (1992), “the R. janeiroensis group” by COSTA (1998), and “the R. santensis species group” by COSTA (2004). Two new species were described in recent years (COSTA, 2004, 2007a), and a new diagnostic feature for the group (a unique pattern of infra-orbital neuromasts) was described (COSTA, 2007a, Atlantirivulus, a new subgenus of the killifi sh genus Rivulus from the eastern Brazilian coastal plains (Teleostei: Cyprinodontiformes: Rivulidae)


Introduction
Rivulus Poey is a widespread, speciose and poorly diagnosed assemblage of American aplocheiloid killifi shes (COSTA, 2006).Over 100 nominal species are placed in Rivulus, ranging from Mexico to southern Brazil, but no unique apomorphic condition is known to occur in all species (COSTA, 2006).Rivulus has been diagnosed by a combination of characters not occurring in other rivulid genera (e. g., HUBER, 1992), and monophyly is poorly supported by morphological characters (COSTA, 2006), whereas molecular analyses suggest that Rivulus is not monophyletic (HRBEK & LARSON, 1999;MURPHY et al., 1999).However, monophyly of some species groups, sometimes formally recognized as subgenera, are well corroborated both by morphology and molecular studies (COSTA, 1998(COSTA, , 2006;;HRBEK & LARSON, 1999;MURPHY et al., 1999).

Materials and methods
Fin-ray counts include all elements.Number of vertebrae, gill-rakers, and caudal-fi n rays were recorded only from cleared and stained specimens.The compound caudal centrum was counted as a single element.Osteological preparations were made according to TAYLOR & VAN DYKE (1985).Terminology for frontal squamation follows HOEDEMAN (1958)
Description.Small, maximum adult length 31.4-42.7 mm SL.Dorsal profi le gently convex from snout to end of dorsal-fi n base, nearly straight on caudal peduncle.Ventral profi le about straight to slightly convex from lower jaw to end of anal-fi n base, approximately straight on caudal peduncle.Body slender, cylindrical anteriorly, about so deep as wide, to compressed posteriorly.Greatest body depth at vertical just in front to pelvic-fi n base.Jaws short, lateral profi le of snout slightly pointed.
Extremity of dorsal and anal fi ns rounded.Caudal fi n rounded to subtruncate.Pectoral fi n rounded, posterior margin anterior to vertical through pelvic-fi n base.Pelvic fi n short and elliptical, tip reaching between anus and base of fi rst anal-fi n rays; pelvic fi n and pel-Vertebrate Zoology ■ 58 (1) 2008 vic girdle absent in one species (R. nudiventris).Pelvic-fi n bases medially in close proximity.Dorsal-fi n origin on vertical through posterior portion of anal-fi n base.Dorsal-fi n rays 7-10; anal-fi n rays 13-17; cau-dal-fi n rays 28-34; pectoral-fi n rays 13-15; pelvic-fi n rays 4-6.
Scales small, cycloid.Trunk and head entirely scaled, except anterior ventral surface of head.Body squamation extending over anterior 25 % of caudalfi n base; no scales on dorsal and anal-fi n bases.Frontal squamation E-patterned; E-scales not overlapping medially; scales arranged in regular circular pattern around A-scale without exposed margins; transverse row of scales anterior to H-scale.Longitudinal series of scales 31-36; transverse series of scales 8; scale rows around caudal peduncle 14-16.Contact organs on scales of fl ank in males.
Color pattern variable among species.Females often with black spot on upper portion of caudal-fi n base.
Atlantirivulus is diagnosed by a curved shape of the ventral process of the angulo-articular (vs.straight) (Fig. 2), and a unique pattern of infraorbital neuromasts, which are numerous (1+19-28+1, vs. 1+9-16+1) and arranged in a zigzag pattern (vs.aligned around orbit).These conditions are not found in other aplocheiloid fi shes.
species.Rivulus depressus COSTA, R. haraldsiolii BERKENKAMP, R. janeiroensis COSTA, R. lazzarotoi COSTA, R. luelingi SEEGERS, R. nudiventris COSTA & BRASIL, R. santensis KÖHLER, and R. simplicis COSTA.Distribution.Coastal river basins of eastern Brazil.Habitat.Small, shallow streams and surrounding pools, usually within forest, or sometimes in open places adjacent to the forest.Water often light yellow to orange and acid (pH 3.5-6.0).Etymology.The name Atlantirivulus is derived from the Latin Atlanticus (Atlantic) and rivulus (stream), referring to the occurrence in the streams of the Atlantic forest of eastern Brazil.Gender masculine.