Retroculus acherontos, Landim, Maria Isabel, Moreira, Cristiano R. & Figueiredo, Carlos A., 2015

Landim, Maria Isabel, Moreira, Cristiano R. & Figueiredo, Carlos A., 2015, Retroculus acherontos, a new species of cichlid fish (Teleostei) from the Rio Tocantins basin, Zootaxa 3973 (2), pp. 369-380 : 371-378

Retroculus acherontos , new species.

( Figure 7 ; Table 1)

Retroculus sp. 1 Bartolette et al., 2012: 66 [listing—DEPRJ 2332].

Holotype., MZUSP 85824, 1, 124.9 mm SL, Brazil, Goiás, Rio Tocantins basin, Rio das Almas, tributary of left margin of Rio Maranhão, 14˚37′51.2″S, 49˚1′56.6″W, 12–13 August 1997, E. P. Caramaschi et al.

Paratypes. All localities in Brazil, Estado de Goiás, Rio Tocantins basin. MZUSP 85823, 112.53 mm SL, same as holotype; AMNH 233644, 4, 113.7–158.6 mm SL; MZUSP 85825, 4 (2 CS) 107.3–168.7 mm SL, Rio Maranhão, below rapids of Cachoeira do Machadinho, 14˚38′18″S, 48˚59′03.3″W, 5–19 August 1997, E. P.

Caramaschi, D. F. Moraes and J. K. Brondi; MNRJ 26352 5, 118.7–151.3 mm SL; MZUSP 85826, 4, 66.3– 194 mm; NRM 50260, 4, 131.6– 136 mm SL, Rio Maranhão, rapids below Cachoeira do Machadinho, 14˚38′18.3″S, 48˚59′03.3″W, 0 9 October 1995, E. P. Caramaschi, D. F. Moraes and D. A. Halboth; MZUSP 85827, 1, 130 mm SL, female, Rio Maranhão, rapids below Cachoeira do Machadinho, 14˚38′18.3″S, 48˚59′03.3″W, 15–16 February 1997, E. P. Caramaschi, D. F. Moraes and J. K. Brondi; MZUSP 85828, 1, 183.5 mm SL, male, Rio Traíras, between 14˚06′15.7″S, 48˚29′27.8″W and 14˚12′00.2″S, 48˚32′09.8″W, 4– 5 October, E. P. Caramaschi, D. F. Moraes and A. Suppa; MZUSP 85830, 1, 136.8 mm SL, Rio Traíras, 06–07 December 1997, E. P. Caramaschi et al.; MZUSP 85829, 2, 110.2 mm SL, male, and 121.1 mm SL, female, Rio das Almas, tributary of left margin of Rio Maranhão, 14˚37′51.2″S, 49˚1′56.6″W, 07–08 October 1997, E. P. Caramaschi et al.; MZUSP 85831, 1, 167.6 mm SL, female, Brazil, Goiás, Rio Tocantins basin, Rio Traíras, between 14˚06′15.7″S, 48˚29′27.8″W and 14˚12′00.2″S, 48˚32′09.8″W, 0 4 October 1997, E. P. Caramaschi, D. F. Moraes and C. A. Figueiredo; MZUSP 85832, 2, 154.2 mm SL and 187.2 mm SL, male, Rio Maranhão, near old bridge between the Uruaçú and Niquelândia towns, between 14˚32′08.4″S, 49˚01′46.8″W and 14˚31′20.1″S, 49˚02′07.3″W, E. P. Caramaschi, D. F. Moraes and D. A. Halboth; MZUSP 85833, 1, 116 mm SL, female, Rio Traíras, 20 July 1996 by, E. P Caramaschi, D. F. Moraes, D. A. Halboth; MZUSP 85834, 1, 137.3 mm SL, left margin of Serra da Mesa reservoir, Rio Palmerinha arm, Porto Serrinha station, 14˚02′57.9″S, 048˚29′57.1″W and 14˚02′22.1″S, 048˚31′28.6″W, 0 8 October 1996, D. F. Moraes and D. A. Halboth; MZUSP 85835, 3, 100.7 –141.3 mm SL, Rio Traíras, 10 December 1997, E. P. Caramaschi et al.; MZUSP 85836, 5, 63.3–185.7 mm SL, Rio das Almas, tributary of the left margin of Rio Maranhão, 14˚37′51.2″S, 49˚1′56.6″W, 0 8 October 1996, E. C. Caramaschi et al.; MZUSP 85837, 1, CS, 151 mm SL, male, Rio Maranhão, near bridge between Uruaçú and Niquelândia, 20 June 1996, E. P. Caramaschi, D. F. Moraes, D. A. Halboth; MZUSP 85838, 2 (1 CS). 135mm SL, male and 162.8 mm SL, Rio Maranhão, between 14˚40′22.8″S, 48˚54′40.2″W and 14˚40′30.5″S, 48˚54′59.7″W; 13–13 October, 1998, R. Iglesias et al.; MZUSP 85839, 130 mm SL, Rio Passa Três, 14 October 1998, R. Iglesias et al.

Diagnosis. Distinguished from all congeners by the presence of the following exclusive characters: a nuchal hump in specimens of both sexes longer than 121.4 mm of SL (vs. hump absent); a black blotch in the median portion of the first to seventh dorsal-fin soft rays (vs. blotch situated near their base); 33–37 longitudinal series scales (vs. more than 38); 15–16 pectoral-fin rays (vs. 17–18); chest scales not distinctively smaller than flank scales and with posterior margin free from skin (vs. scales much smaller and embedded); fourth vertebra hypapophysis elongated (vs. hypapophysis not elongated; Fig. 8 ), and by a deeper first epibranchial (vs. first epibranchial shallower; Fig. 1 ). Retroculus acherontos is also further distinguished from R. lapidifer and R. septentrionalis by the absence of gill rakers on the fifth ceratobranchial (vs. present); from R. septentrionalis and R. xinguensis by the presence of faint, slender, dark bars on the caudal fin (vs. larger conspicuous bars) and by the presence of anterior regular series of papillae on the vomero-palatine apparatus (vs. anterior series irregular and occasionally forming a second series in R. xinguensis , or anterior series absent in R. septentrionalis ; Fig. 2 ); from R. septentrionalis the presence of a less pronounced spiny process on the first hypobranchial (vs. pronounced spiny process; Fig. 4 ), and from R. lapidifer and R. xinguensis by a lower vertebral count 31–32 (vs. 33–34).

Description. Morphometric data for holotype and paratypes are given in Table 1. Body laterally compressed. Dorsal profile straight, posterodorsally inclined, from tip of upper lip to vertical through middle of eye. Specimens smaller than 121.4 mm SL with profile convex from above eyes to anterior terminus of dorsal fin. Nuchal hump present in larger individuals. Profile in all individuals slightly convex from first dorsal-fin spine to last dorsal-fin ray. Caudal peduncle gently concave dorsally. Ventral profile gently convex from mouth to first anal-fin ray. Analfin base straight, posterodorsally inclined. Ventral profile of caudal peduncle almost straight or slightly concave. Branchiostegal membrane united to isthmus, reaching beyond posterior margin of opercle.

TABLE 1. MORPHOMETRIC DATA OF Retroculus acherontos , new species. Data based on holotype and 43 paratypes.

Jaws and dentition: Mouth terminal, jaws isognathous. Lip folds meeting without overlap at corner of mouth. Teeth conical, arranged in four or five irregular rows in upper jaw, and in two or three rows in lower jaw, larger in outer rows. Dentary with teeth restricted to anterior half. Edentulous posterior half forming ascending slender curve parallel with articular web of anguloarticular. Small gap present between coronoid process of dentary and primordial process of anguloarticular where dorsal dental-articular ligament attaches. Dentary with five foramina along laterosensory mandibular canal.

Scales: Transforming ctenoid scales. Horizontal scale series, 33*(5), 34 (11), 35(19), 36(5), 37(1). Upper lateral line series, 22(1), 24(3), 25*(10), 26(9), 27(9), 28(2), 23(6), 29(1). Lower lateral line series, 13(2), 14(10), 15(14), 16*(13), 17(1), 19(1), three scales in single caudal-fin lateral line. Transverse scale rows, 16*(1), 17(7), 18(23), 19(8), 20(3). Circumpeduncular scale rows, 19(1), 20(10), 21*(20), 22(8), 23(1). Dorsal fin scaled from base of last spine to middle of base of soft rays. Caudal fin scaled except for distal tip of rays. Scales larger on proximal one fourth of fin length. Base of anal fin scaled. Base of pectoral fin scaled. Opercle with three irregular vertical series of scales. Scales on cheek interrupted anteriorly at vertical through margin of eye, and ventrally above posterior tip of premaxillary dentigerous arm. Preopercle without scales. Chest scales only slightly smaller than flank scales and not embedded in skin.

Fins: Dorsal fin XVI, 12(23), XVI, 13*(11), XV, 12(2), XV, 13(5), XV, 14(2), XIII, 13(1). Anal fin III, 7*(28), III, 8(16). Caudal fin truncate, principal rays 16. Pectoral-fin rays 15(7), 16(36). Dorsal-fin pterygiophores 27–28, and anal-fin pterygiophores eight (4).

Orobranchial cavity, gill arches and gill rakers: Vomero-Palatine apparatus on mouth roof as for genus ( Fig. 2 A). Orobranchial mucosa irregularly covered by papillae. First epibranchial gill rakers 10–11, elongate, without teeth. Gill rakers on first ceratobranchial 13, expanded laterally as flaps with several small projections and with conical and spatulate tooth on its most medial projection. Skin flaps on first ceratobranchial on the base of branchial filaments. Microbranchiospines absent. Slender basibranchials and hypobranchials. First hypobranchial with median, anterior, spinous process ( Fig. 4 A). First epibranchial deep, with ventral margin concave and uncinate process longer than first arm ( Fig. 1 A). Tooth plates present on fourth ceratobranchial. No gill rakers associated with fifth ceratobranchials. Teeth on the tooth-plates of fifth ceratobranchial more robust near symphysis and slender posterolaterally. Teeth molariform anteriorly, recurved posteriorly. Urohyal with dorsal ridge groove.

Axial skeleton: Total vertebrae count 32–33, with 16 (16*) abdominal and 16 (16*) caudal. Well-developed hypapophyses on fourth vertebra entirely separated, not forming single spine-like structure. Two supraneurals.

Miscellaneous osteology: Two lachrymal plates ( Fig. 3 ). First plate strongly elongated with four pores, three anterior and one dorsal slightly posterior to nostril. Second plate slender, expanded anteriorly, connecting with first tubular infraorbital rostrally below first lachrymal plate. Five separate tubular infraorbitals. First tubular infraorbital situated almost entirely ventral of first lachrymal plate, and only distally associated with orbital margin ( Fig. 3 ). Nasal slender and elongate and of same length as first lachrymal plate. Highly interdigitated suture between parasphenoid and vomer. Preopercle with seven lateral-line foramina. Parasphenoid apophysis well developed. Canal in supracleithrum descending along its posterior margin ( Fig. 5 ).

Color in alcohol. Base body coloration pale yellow, darker dorsally and paler ventrally. Two dark preorbital stripes, between eye and upper lip, darker to middle of snout. Upper preorbital stripe running from anteroventral portion of eye, lower stripe along ventral margin of eye. Stripe present on cheek from corner of the mouth to preopercle. Upper lip darker than lower. Dark blotch covering opercle darker on its center. Seven or eight irregular vertical dark bars along body with last one on median rays of caudal fin. Dorsal-fin rays pale yellow, darker distally. Interradial membrane gray. Black irregular blotch extending from middle to distal margin of first to seventh soft dorsal-fin rays. Last soft dorsal-fin rays dark gray with small white spots. Caudal-fin with irregular median dark bar immediately posterior to its base, followed by yellowish coloration for about two thirds of its length and dark pigment in its less scaled posterior most portion. About five irregular faint dark narrow bars on posteromedian portion of fin. Anal-fin rays yellowish on base, becoming darker distally, with hyaline small spots mainly on posterior rays, forming longitudinal stripes in some specimens. Pelvic fin darker distally.

Color in life. (Based on photograph of recently preserved specimen). Dorsal region of head greenish, ventral bluish. Upper lip greenish but bluish distally. Lower lip bluish. Branchiostegal membrane ventral to opercle red. Black spot on opercle. Preorbital stripes bluish. Body background color greenish dorsally and paler on ventral region with small irregular red spots distributed along body and followed posteriorly by blue spot. Greater concentration of red pigmentation on scapular area. Dorsal fin greenish over scaled portion, with small faint bluish or white irregular blotches over red background over membrane. Blotches on posterior soft rays became lenticulate spots and with black blotch on anterior six dorsal-fin soft rays. Distal margin of posterior rays with grayish background. Caudal fin greenish over scaled portion with distal margin grayish. Central portion of fin with some slender dark bars. Anal fin with 11 irregular white stripes over distinctly red background and grayish towards distal margin. Pectoral fin hyaline. Pelvic fin grayish, becoming darker towards margin.

Distribution and habitat. Retroculus acherontos is known from the region of the Serra da Mesa Hydroeletric Power Dam, in the upper Rio Tocantins basin. It has been collected, in several small to medium tributaries of the main Rio Tocantins, where it is sympatric with R. lapidifer . The waters where R. acherontos were collected range from clear, with some humic acid, to white-waters.

The habitat of Retroculus acherontos are unknown but based on morphological differences (see Discussion) from remaining Retroculus species, we expect that the species occurs in less rapidly flowing environment, possibly being restricted to more marginal portions of the rivers.

Conservation note. Retroculus acherontos have restricted extension of occurrence (EEO 1,281.3 km2) and area of occupancy (OOC 36.0 km2), sensu IUCN (2012). This portion of the Rio Tocantins has undergone drastic changes in habitat after the impounding of the Serra da Mesa dam. Although the species was found in the area after the dams’ reservoir reached its full capacity, the restricted EEO and OOC, alongside with the environmental changes classify Retroculus acherontos in the threatened species category of “Endangered” according to criteria B1B2ab(iii) (IUCN, 2014).

Etymology. From the Greek Acherontos , meaning 'of Acheron', in reference to the river which in the greek mythology flows to Hades, land of the souls. This is in reference to the Rio das Almas, which means river of the souls in Portuguese. An adjective.

Figures

FIGURE 1. First epibranchial of (A) Retroculus acherontos, MZUSP 85838, (B) R. s eptentrionalis, MZUSP 38947, paratype, and (C) Satanoperca lilith, MZUSP 8503. Ventral view, anterior to top. Cartilage black. Scale bar = 1 mm.

FIGURE 2. Vomero-Palatine apparatus of (A) Retroculus acherontos, MZUSP 85826, 161.3 mm SL, (B) R. xinguensis, MZUSP 40682, 78.2 mm SL, and (C) R. septentrionalis, paratype, MZUSP 38947, 106.2 mm SL. Ventral view, anterior to top. Arrows indicate the lateral depressions. V indicate the ventral projection of vomer.

FIGURE 3. Lachrymals and tubular infraorbitals of Retroculus acherontos, MZUSP 85838. Lateral view, anterior to right. Scale bar = 3 mm.

FIGURE 4. First hypobranchial of (A) Retroculus acherontos, MZUSP 85838, and (B) R. septentrionalis, MZUSP 38947, paratype. Dorsal view, anterior to top. Cartilage black. Scale bar = 1 mm.

FIGURE 5. Supracleithrum of Retroculus acherontos, MZUSP 85838. Lateral view, anterior to left. Scale bar = 1 mm.

FIGURE 7. Retroculus acherontos: (A) holotype, MZUSP 85824, 124.9 mm SL, and (B) paratype MZUSP 85823, 112.5 mm SL.

FIGURE 8. Radiograph of (A) Retroculus acherontos holotype, MZUSP 85823, 112.5 mm SL, and (B) Retroculus lapidifer, MZUSP 40682, 138.5 mm SL. Arrows indicate the hypapophyses. Lateral view, anterior to left.