Rivulus uatuman, Wilson J. E. M. Costa, 2004

Wilson J. E. M. Costa, 2004, Rivulus uatuman sp. n. (Teleostei: Cyprinodontiformes: Rivulidae): a new miniature killifish from the central Brazilian Amazon., Zootaxa 696, pp. 1-8 : 2-7

Rivulus uatuman new species

(Figs. 1-2)

Holotype. UFRJ 6022 , male, 16.6 mm SL; Brazil: Estado do Amazonas: Balbina, temporary pool near stream close to Rio Uatuma , rio Amazonas basin (1°56’19.6”S 59°59’27.30.6”W; altitude 43 m); W. J. E. M. Costa, R. Paiva and D. Ramos, 12 June 2004.

Paratypes. UFRJ 6023 , 3 females, 15.3-18.7 mm SL; UFRJ 6024 , 1 male, 22.2 mm SL, 1 female, 18.6 mm SL (c&s); collected with holotype.

Diagnosis: Similar to R. amanapira Costa , R. atratus Garman , R. ornatus Garman , R. kirovskyi Costa , R. rectocaudatus Fels & de Rham , R. romeri Costa , R. tecminae Thomerson, Nico and Taphorn , and R. uakti Costa , and distinguished from all other species of the genus by possessing a frontal squamation pattern consisting of one scale with all margins exposed just posterior to snout (vs. scale with all margins exposed near the center of median portion of frontal region) and an oblique infraorbital dark gray bar through chin (vs. never a similar color pattern). Similar to R. amanapira , R. kirovskyi , R. rectocaudatus , R. romeri , R. tecminae , and R. uakti Costa , and distinguished from all other congeners by the absence of dermosphenotic (vs. presence), absence of preopercular canal (vs. conspicuous short canal), and frontal scales arranged transversely (vs. circularly). Similar to R. kirovskyi and R. romeri , and distinguished from all other congeners by having the third neuromast of the anterior supraorbital series posteriorly placed (vs. close to first and second neuromasts), and by the absence of vomerine teeth (vs. presence). Distinguished from R. kirovskyi and R. romeri in having dorsal-fin origin in a vertical through the base of the last anal-fin ray or posterior to it, between neural spines of 20th and 21st vertebrae (vs. in a vertical through posterior third of the anal-fin base, between neural spines of 17th and 19th vertebrae in R. romeri and neural spines of 19th and 20th vertebrae in R. kirovskyi ), 12 pectoral-fin rays (vs. 14 in R. romeri and 15 in R. kirovskyi ), 31-32 vertebrae (vs. 29-30), and a distinctive white bar on the distal margin of the caudal fin in male (vs. white bar absent). Further distinguished from R. romeri by having a short anal fin, its tip reaching vertical through caudal peduncle (vs. long anal fin in males, tip reaching vertical through caudal-fin base), 31-33 scales on the longitudinal series (vs. 29-30), and 16 scale rows around caudal peduncle (vs. 12).

Description: Morphometric data given in Table 1. Male larger than male, largest male 22.2 mm SL. Dorsal profile slightly convex from snout to end of dorsal-fin base, approximately straight on caudal peduncle. Ventral profile convex on head, almost straight to gently convex from anterior portion of venter to end of anal-fin base, nearly straight on caudal peduncle. Body slender, subcylindrical anteriorly, slightly wider than deep, to compressed posteriorly. Greatest body depth at level of pelvic-fin base.

Tip of dorsal and anal fins rounded. Caudal fin rounded. Pectoral fin rounded, posterior margin on vertical anterior to pelvic-fin base. Tip of pelvic fin reaching between urogenital papilla and base of 1st anal-fin ray in male, and reaching anus in female. Pelvic-fin bases in close proximity. Dorsal-fin origin on vertical through base of last anal-fin ray or slightly posterior to anal-fin base, between neural spines of 20th and 21st vertebrae. Analfin origin between pleural ribs of 15th and 16th vertebra. Dorsal-fin rays 6-7; anal-fin rays 9-10; caudal-fin rays 23-24; pectoral-fin rays 12; pelvic-fin rays 6.

Scales large, cycloid. Body and head entirely scaled, except anterior ventral surface of head. Few scales on caudal-fin base; no scales on dorsal and anal fins. Frontal squamation S-patterned; E-scales not overlapping medially; scales arranged in transverse pattern, all scales with exposed posterior margin. Longitudinal series of scales 31-33; transverse series of scales 7; scale rows around caudal peduncle 16. Ctenii-like contact organ on each scale of anteroventral portion of male flank. Supraorbital neuromasts 3 + 3; last neuromast of anterior supraorbital series separated by interspace occupied by scale. Preopercular canal absent.

Interhyal vestigial, not ossified. Five branchiostegal rays. Second pharyngobranchial teeth absent. Gill-rakers of first branchial arch 1 + 6. Vomerine teeth absent. Desmosphenotic absent. Ventral process of posttemporal absent. Total vertebrae 31-32.

Coloration: Male: Side of body light brown, with longitudinal rows of red, dark brown and light blue dots. Dorsum light brown, venter light gray. Side of head dark reddish brown with light purple dots. Infraorbital region and ventral surface of head light gray; transverse black bar on chin. Iris green, dorsal portion of orbit bright silver. Dorsal fin light yellow with broad black median stripe. Anal fin bright yellow, base light blue. Caudal fin dark gray with distal white bar; basal portion with light gray and black dots. Pelvic fin yellow. Pectoral fin hyaline.

Female: Side of body light brown with six narrow dark brown stripes. Dorsum light brown, venter pink. Side of head light brown with dark brown dots. Infraorbital region and ventral surface of head light pink; transverse dark brown bar on chin. Iris yellow, dorsal portion of orbit bright silver. Dorsal fin hyaline with gray median stripe. Anal fin yellow to pale orange. Caudal fin hyaline with small dark gray spots on basal portion. Pelvic fin pale orange. Pectoral fin hyaline.

Distribution: Known only from the type locality, temporary pools near stream close to Rio Uatumã, Rio Amazonas basin, central Amazon, northern Brazil. Rio Uatumã is a northern tributary of the Rio Amazonas, placed between Rio Negro and Rio Trombetas.

Habitat notes: All the specimens of the type series were collected in isolated temporary pools within the Amazonian forest. The pools were located near the forest border (Fig. 3) and were shallow (about 3-15 cm deep). The bottom was litter and the water dark tea colored, becoming turbid after collection (Fig. 4).

Etymology: The name uatuman refers to the river basin in which the new species was collected. The name is probably derived from the local language of the Waimiri-Atroari, linguistic group Karib, an indigenous tribe of people inhabiting Rio Uatumã and adjacent river basins.

Discussion

Characters of the cephalic latero-sensory system and frontal squamation have been consistently employed in the systematics of cyprinodontiform fishes. Gosline (1949) described the latero-sensory system of the head in the fundulid genus Fundulus Lacepede , providing interesting comparisons with other cyprinodontiform taxa. Subsequently, Hoedeman (1958) described the frontal scale patterns in some cyprinodontiforms, establishing a new terminology based on the frontal squamation in the genus Rivulus Poey . Since then, both characters of the cephalic laterosensory system and the frontal squamation have been used in systematic studies of different cyprinodontiform groups, including poeciliids (e.g., Rosen & Mendelson, 1960), fundulids (e.g., Wiley, 1977), and rivulids (e. g., Costa, 1998, 2001). In rivulids, however, these traits are highly modified. The frontal scales are reduced and primarily arranged in a circular pattern, and all cephalic canals are opened or absent and consequently represented by exposed neuromasts (e.g., Parenti, 1981; Costa, 1998, 2004a).

Rivulus uatuman possesses an uncommon pattern of frontal squamation, already reported and illustrated for a few species of Rivulus by some authors (e.g., Fels & de Rham, 1982; Thomerson et al, 1992; Costa, 2003b, 2004b) (Fig. 5A). This pattern consists of one scale with all the margins exposed and placed just posterior to the snout, instead of the frontal scale with all margins exposed being positioned near the center of the frontal region (Fig. 5B) as in other rivulids. This unique pattern is present in R. amanapira , R. atratus , R. ornatus , R. kirovskyi , R. rectocaudatus , R. romeri , R. tecminae , R. uakti , and R. uatuman , all of these species also sharing a derived color pattern in which there is a distinctive infraorbital dark gray bar through the chin (Costa, 2003b, 2004b). In addition, the frontal scales are arranged transversely (Fig. 5A), instead of circularly as in other species of Rivulus (Fig. 5B), only in R. amanapira , R. kirovskyi , R. rectocaudatus , R. romeri , R. tecminae , R. uakti , and R. uatuman . These species also exhibit some other unique apomorphic conditions such as the absence of dermosphenotic and absence of preopercular canal (Costa, 2003b, 2004b).

A unique arrangement of supraorbital neuromasts is present in R. kirovskyi , R. romeri , and R. uatuman . In these species, the third neuromast of the anterior supraorbital series is separated from the two anterior neuromasts by an interspace occupied by one scale (Fig. 5A), in contrast to all three supraorbital neuromasts forming a continuous row (Fig. 5B). The putative monophyly of the group comprising Rivulus kirovskyi , R. romeri , and R. uatuman is also corroborated by other characters. Besides constituting the smallest species of the genus, reaching 22.7 mm SL as maximum adult size, these three species also share the absence of vomerine teeth, a condition not occurring in basal rivulid genera ( Kryptolebias Costa , Prorivulus Costa , and Rivulus ).