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Accepted by M.R. de Carvalho: 18 May 2015; published: 17 Jun. 2015
ZOOTAXA
ISSN 1175-5326 (print edition)
ISSN
1175-5334
(online edition)
Copyright © 2015 Magnolia Press
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http://dx.doi.org/10.11646/zootaxa.3973.2.10
http://zoobank.org/urn:lsid:zoobank.org:pub:4E6E002C-309C-47EB-964C-BF5A06FEC62E
Retroculus acherontos, a new species of cichlid fish (Teleostei)
from the Rio Tocantins basin
MARIA ISABEL LANDIM
1
, CRISTIANO R. MOREIRA
2
& CARLOS A. FIGUEIREDO
3
1
Museu de Zoologia da Universidade de São Paulo, Caixa Postal 42494, 04218-970, São Paulo, SP, Brazil. E-mail: milandim@usp.br
2
Museu Nacional/Universidade Federal do Rio de Janeiro, Setor de Ictiologia, Departamento de Vertebrados, Quinta da Boa Vista s/
n, 20940-040, Rio de Janeiro, RJ, Brazil. E-mail: moreira.c.r@gmail.com
3
Universidade Federal do Estado do Rio de Janeiro, Instituto de Biociências, Departamento de Ciências do Ambiente. Av. Pasteur,
458/sala 512F, 22290-240, Rio de Janeiro, RJ, Brazil. E-mail: carlos.figueiredo@gmail.com
Abstract
Retroculus acherontos is described from the Rio Tocantins basin, Brazil. It is distinguished from all congeners by the pres-
ence of a nuchal hump in large specimens, an irregular black blotch located on the middle of the anterior soft dorsal-fin
rays, fewer pectoral-fin rays (15–16), a deeper body, a convex ventral profile, a more transversely convex ventral surface
of the body, and by chest scales similar in size to the scales on the side. An expanded diagnosis for the genus Retroculus
is provided.
Key words: taxonomy, Cichlinae, rheophily, Neotropical
Resumo
Retroculus acherontos é descrita da bacia do rio Tocantins, Brasil. É distinta de todos seus congêneres pela presença de
uma protuberância nucal em indivíduos grandes, uma mancha negra irregular mediana nos raios moles da nadadeira dor-
sal, por possuir menos raios na nadadeira peitoral (15–16), corpo mais alto, perfil ventral do corpo mais convexo, super-
fície ventral do corpo comprimida e pela região peitoral ventral com escamas de tamanho similar às escamas do flanco. É
apresentada uma diagnose expandida do gênero Retroculus.
Introduction
The cichlid genus Retroculus currently comprises three rheophilic species occurring in north-central South
America, in the Araguaia, Capim, Tocantins, Tapajós, Xingu, Oyapock and Araguari river basins (Kullander,
2003). Retroculus was proposed by Eigenmann & Bray (1894) for R. boulengeri Eigemann & Bray from Brazil.
These authors considered it to be related to Acaropsis (known at the time by a single species, A. nassa, currently
Acaronia nassa) despite of the remark noting the “high development of the snout and the preorbitals” (Eigenmann
& Bray, 1894: 614). Subsequently, Regan (1906) considered R. boulengeri to be a junior synonym of Chromys
lapidifera Castelnau, 1864 described from “la grande cascade de l’Araguay (Caxoeira grande)” (Castelnau,
1864:459–463), which corresponds to the Cachoeira de Santa Isabel at Santa Isabel do Araguaia, Goiás state,
Brazil. The present known distribution of R. lapidifer includes the Araguaia, the Tocantins, and the Capim river
basins (Kullander, 2003).
Gosse (1971) revised the genus, and redescribed Retroculus lapidifer based on additional material. Since the
holotype was already lost (Gosse, 1971; M. Desoutter, pers. comm., 2003), Gosse designated a neotype from
Xavantina Island, Rio das Mortes, Mato Grosso state, Brazil. Gosse (1971) also described R. xinguensis from the
Cachoeira von Martius, Xingu River basin, Brazil, and R. septentrionalis from the Oyapock River, French Guiana.
Retroculus was long considered to be related to the Geophaginae because of its overall body shape. This
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commenced with Steindachner (1875), who suggested that Retroculus was part of Geophagus Heckel and, closely
related to G. acuticeps Heckel in the subgenus Satanoperca Günther. Later, the presumed shared presence of a lobe
on the upper part of the first branchial arch (Regan, 1906) was used as additional evidence for the relationships of
Retroculus with Geophagus. Steindachner (1911), in a pronounced shift from his earlier hypothesis, suggested that
Retroculus was related to African species with a pharyngeal pad then included in the genus Pelmatochromis
Steindachner. Gosse (1971) alternatively maintained that Retroculus was closely related to the geophagine genera
Geophagus and Acarichthys Eigenmann. This was followed by Cichocki (1976) who considered Retroculus as a
subgenus of Geophagus, and Stiassny (1991) who included the genus within the geophagine assemblage.
Kullander (1998) was the first author to approach the relationships of Retroculus with a parsimony analysis,
and proposed the genus as the sister-group of all remaining South American cichlids plus the African
Heterochromis Regan. Subsequent molecular studies partially corroborate Kullander's hypothesis, recovering
Retroculus as the basal-most genus of a monophyletic Neotropical clade, excluding the African genus
Heterochromis (Farias et al., 1999; Farias et al., 2000; Farias et al., 2001; and; Sparks & Smith, 2004). Recently,
Smith et al. (2008) proposed Cichla as sister-taxon of Retroculus and this clade as the sister-group to the clade
including all other Neotropical cichlids.
The present study has as its primary purpose to describe a new species of Retroculus from the upper Rio
Tocantins. Because of the morphological differences between the new species and the ones previously described
within the genus, we provide an expanded diagnosis for the genus.
Material and methods
Measurements and counts follow Kullander (1980) and Kullander & Nijssen (1989) and were taken on the left side
of the specimens. Count frequencies in the text are given in parentheses, with the values from the holotype
indicated by asterisks. Counts of fin-rays, pterygiophores and vertebrae were taken from specimens cleared and
stained (CS) for bone and cartilage following Taylor & Van Dyke (1985) and from radiographed specimens.
Osteological nomenclature follows Barel et al. (1976), ligament nomenclature Anker (1989), and scale
nomenclature Roberts (1993). Conservation status was assessed using the categories and criteria of IUCN Red List
(IUCN, 2012). Extension of occurrence (EEO), and area of occupancy (OOC) were calculated with the aid of the
GeoCAT tool (Bachman et al., 2011). Institutional abbreviations are: AMNH, American Museum of Natural
History, New York; FMNH, Field Museum of Natural History, Chicago; MCP, Museu de Ciências e Tecnologia,
Pontifícia Universidade Católica do Rio Grande do Sul, Porto Alegre; MNRJ, Museu Nacional, Universidade
Federal do Rio de Janeiro, Rio de Janeiro; MZUSP, Museu de Zoologia da Universidade de São Paulo, São Paulo;
NRM, Swedish Museum of Natural History, Stockholm; UMMZ, University of Michigan Museum of Zoology,
Michigan.
Retroculus Eigenmann & Bray, 1894
Diagnosis. Retroculus is distinguished from all other cichlid genera by the presence of two exclusive characters:
epibranchial lobe supported by a thickening of the first epibranchial (vs. epibranchial lobe supported by the second
epibranchial, or by a laminar expansion of the first epibranchial; Fig. 1); and the presence of a vomero-palatine
apparatus on roof of the mouth (vs. roof of mouth not distinctively specialized; Fig. 2). Retroculus can also be
distinguished by a combination of the following diagnostic characters: the presence of two lachrymal plates plus
five tubular infraorbitals (Fig. 3); a first tubular infraorbital only posteriorly associated with the orbit; the lip folds
meeting without overlap at the corner of the mouth (African lips of Kullander, 1986; or type-I lip of Stiassny,
1987); presence of two supraneurals; the seven lateralis canal foramina in the preopercle; the five lateralis canal
foramina on the dentary; a black blotch on the anterior soft dorsal-fin rays; the orobranchial mucosa laterally
covered by papillae; by the presence of skin flaps and a lack of microbranchiospines along the external margin of
the first ceratobranchial; attachment of the ligament hypohyal-hypobranchial 1 along the antero-medial spiny
process of the first hypobranchial (Fig. 4); supracleithrum laterosensory canal posteroventrally angled (Fig. 5);
posterior portion of the dentigerous arm of the dentary slender; and rakers at base of epibranchial lobe (Fig. 6B).
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FIGURE 1. First epibranchial of (A) Retroculus acherontos, MZUSP 85838, (B) R. septentrionalis, MZUSP 38947, paratype,
and (C) Satanoperca lilith, MZUSP 8503. Ventral view, anterior to top. Cartilage black. Scale bar = 1 mm.
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.
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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, 09 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, 04 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, 08
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, 08 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.
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.
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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 = 1mm.
FIGURE 5. Supracleithrum of Retroculus acherontos, MZUSP 85838. Lateral view, anterior to left. Scale bar = 1 mm.
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FIGURE 6. Epibranchial lobe on the first branchial arch of (A) Geophagus altifrons, MZUSP 8547, 148 mm SL, and (B)
Retroculus acherontos, MZUSP 85826, 161.3 mm SL. Lateral view, anterior to left.
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).
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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. Anal-
fin 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. M
ORPHOMETRIC
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
Holotype Range Mean N
Standard length (mm) 112.5 65.8–194.0 135.3 44
Head length (mm) 37.9 22.5–69.3 46.9 44
Percentage of standard length
Head length 33.7 32.4–37.8 34.6 44
Caudal peduncle length 14.2 8.0–15.0 11.4 44
Body depth 39.7 35.6–45.8 39.4 44
Head depth 32.1 24.3–36.6 29.7 44
Caudal peduncle depth 15.2 13.8–17.0 15.2 44
Dorsal fin base length 54.6 50.4–58.9 54.6 44
Anal fin base length 14.2 12.6–16.8 14.4 44
Last dorsal-fin spine 11.5 9.1–15.6 12.2 42
Last anal-fin spine 11.0 9.9–13.8 11.7 43
Pectoral fin length 24.1 20.4–26.2 23.8 44
Anal fin length 24.9 16.8–27.5 22.9 31
Preanal length 73.3 69.5–78.0 73.7 41
Predorsal length 42.5 40.4–47.9 43.8 44
Percentage of head length
Head width 49.1 35.8–54.2 45.9 44
Orbit diameter 23.9 18.9–29.0 23.2 44
Interorbital width 28.1 25.5–34.1 29.0 44
Postorbital length 36.8 30.3–39.9 35.2 44
Preorbital length 34.2 27.3–37.6 33.6 44
Snout length 50.6 42.7–57.2 52.1 44
Upper jaw length 38.7 34.0–44.0 39.2 44
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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).
FIGURE 7. Retroculus acherontos: (A) holotype, MZUSP 85824, 124.9 mm SL, and (B) paratype MZUSP 85823, 112.5 mm
SL.
Orobranchial cavity, gill arches and gill rakers: Vomero-Palatine apparatus on mouth roof as for genus (Fig.
2A). 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. 4A). First epibranchial deep, with ventral margin concave and uncinate
process longer than first arm (Fig. 1A). 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.
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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.
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
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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 km
2
) and
area of occupancy (OOC 36.0 km
2
), 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.
Discussion
The species of the genera Teleocichla Kullander and Retroculus are among the only South-American rheophilic
cichlids. Not only do the species of the genera occur in rapids, but also their morphology is that commonly
associated with rheophilic fishes, including depressed body, the pointed snout with eyes situated posteriorly on the
head, the more compressed ventral region of the body, and the scales of the anteroventral scales smaller and more
embedded in skin (Hora, 1922; Kullander, 1988). However, Retroculus acherontos has these features less
specialized. Its body is less depressed (Fig. 7), the snout less acute, the eyes more anterior, the ventral portion of the
body is rounded, and the ventral scales not conspicuously smaller than flank scales. Because of these differences,
we examined several species representing most of the monophyletic groups in Cichlidae, in order to confirm the
generic assignment of the new species. As a consequence, we provide an expanded diagnosis to include characters
not related to rheophilic habitat, including two characters not found in other cichlids. These two characters are
probably synapomorphies for Retroculus, pending a comprehensive phylogenetic analysis of the family, including
the species of the genus.
Among these characters, one previously unreported morphological specialization found in all Retroculus
species is the presence of a vomero-palatine apparatus (Fig. 2). The vomero-palatine apparatus is composed by a
ventral projection of the vomer bordered laterally by a depression on each side of this projection. Along the medial
margin of each of these depressions occurs a posteriorly divergent ventrally projecting skin flap, running
posteriorly from the projection of the vomer. Each flap is bordered by a series of papillae, and in all members of the
genus, except for R. septentrionalis, an additional anterior series of papillae is present along the ventral projection
of the vomer in an arrangement approximately perpendicular to the other series. In R. xinguensis this anterior series
of papillae is irregular, forming a second series in some specimens, and is confluent with the posterior papillae
series (Fig. 2B). Retroculus acherontos and R. lapidifer have a small gap between the anterior series and the
posterolateral series. Within this gap, we find a few (one-three) irregularly distributed papillae. These two species
also have multi-lobed papillae, mainly on the anterior series, contrasting to the simple papillae of R. xinguensis.
Retroculus septentrionalis has papillae only on the distal portion of the divergent skin flap (Fig. 2C). Papillae and
flaps may also be present in other regions of the orobranchial cavity.
A similar structure is found in some species of two unrelated group of fishes, the Curimatidae of the order
Characiformes (Vari, 1989) and the Cypriniformes (Reid, 1982). Reid (1982) proposed that this complex structure
probably functions in Cypriniformes as a bolus-formation, mixing mucous with ingested particles. A similar
hypothesis was advanced for the Curimatidae by Vari (1989). Both groups feed on aufwuchs, and the mucous
would trap and filter the ingested food. This hypothesis cannot be readily applied to Retroculus, since R. lapidifer
and R. xinguensis, and probably all Retroculus species, feed mainly on immature autochthonous insects (Moreira &
Zuanon, 2002). However, as observed by Zuanon (1999), Retroculus take in the mouth some substrate along with
food items, selects the food items in the orobranchial cavity and then expels the substrate items through its mouth
or gill opening. It is common to observe large sand grain in the cavities that border laterally the vomer ventral
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projection (pers. obs.; Fig. 2B). This indicates that the vomero-palatine apparatus probably plays an important role
in the selection of food items.
Comparative material examined. Indo-Malagasy species: Etroplus suratensis AMNH 217756, 1;
Paratilapia polleni MZUSP 43788, 1; Paretroplus kieneri AMNH 97365, 6; AMNH 229560, 2; Paretroplus
polyactis UMMZ 23016, 1 CS; Paretroplus tsimoly AMNH 229556, 2; Ptychochromis oligacanthus UMMZ
226591, 1 CS; and Ptychochromoides vondrozo UMMZ 235934, 1 CS. African species: Heterochromis multidens
AMNH 5963sw 1 CS, AMNH 5966, 1; Tylochromis labrodon AMNH 57163 2, T. lateralis AMNH 58041sw 1 CS,
T. leonensis AMNH 59400 3, T. variabilis AMNH 5901 4; Callochromis macrops AMNH 11720, 1; C.
pleurospilus AMNH 217381 57; Chromidotilapia kingsleyae AMNH 231282, 2; C. mamonekenei AMNH 232357,
3; Ectodus descampsii AMNH 58464 4; Grammatotria lemairii AMNH 11725, 1; G. lemairii AMNH 217345, 1;
Haplochromis gigliolii AMNH 215932sw, 3; H. pectoralis AMNH 221753sw, 1 CS, H. squamipinnis AMNH
224667sw, 2 CS; Hemichromis elongatus AMNH 216052, 1 CS; Hemichromis cf. bimaculatus AMNH 12318, 1
CS; Pelvicachromis cf. humilis AMNH 97493, 8; H. pulcher AMNH 097564 20; Lethrinops micrentodon AMNH
225562sw, 1 CS; L. polli AMNH 223185sw, 2 CS; Parananochromis caudifasciatus AMNH 11749, 3; Xenotilapia
sima AMNH 217359, 1. South American species: Retroculinae: Retroculus lapidifer MZUSP 40574, 7, 1 CS; R.
lapidifer MZSUP 40682, 174, 1 CS; R. septentrionalis paratypes MZUSP 38947, 19 ., 1 CS; R. xinguensis MZUSP
38972, 7.; R. xinguensis MZUSP 33058, 108.; 2 CS; R. xinguensis MZUSP 36873, 84., 1 CS. Cichlinae: Cichla sp.
MZUSP 50246, 81., 2 CS; Crenicichla haroldoi MZUSP 16038, 3., 2 CS; C. labrina MZUSP 41202, 3., 2 CS.
Astronotinae: Astronotus ocellatus MZUSP, 26320 3., 2 CS. Cichlasomatinae: Acaronia nassa MZUSP, 7489 5., 1
CS; A. nassa MZUSP, 46775, 4., 1 CS; A. nassa MZUSP 57484, 5. Geophaginae: Acarichthys heckelii MZUSP
6239, 5., 1 CS; A. heckelii MZUSP 46464, 2., 1 CS; Apistogramma taeniata MZUSP 9219, 9., 2 CS; Biotodoma
cupido MZUSP 34797, 5., 1 CS; Biotoecus opercularis MZUSP 5754, 13., 2 CS; Crenicara punctulatum MZUSP
45810, 3., 1 CS; C. punctulatum FMNH 101912, 23. , 2 CS; Dicrossus filamentosus MZUSP 29719, 5., 2 CS;
Geophagus altifrons MZUSP 44956, 8., 1 CS; G. altifrons MZUSP 8547, 9., 2 CS; Guianacara owroewefi UMMZ
216831, 6., 1 CS; Gymnogeophagus meridionalis MCP 10582, 5., 2 CS; Microgeophagus ramirezi MZUSP 27976,
4., 2 CS; Satanoperca acuticeps MZUSP 44263, 6., 2 CS Satanoperca jurupari MZUSP 44203, 3., 2 CS;
Satanoperca lilith MZUSP 8503, 8., 2 CS; Taeniacara candidi MZUSP 7807, 2., 1 CS.
Acknowledgments
We are grateful to Érica P. Caramaschi (LEP) for providing the material of Retroculus from the Rio Tocantins basin
collected in the course of the project: “Estudos Básicos sobre a Ictiofauna do AHE Serra da Mesa, GO”. Thanks to
Osvaldo Oyakawa (MZUSP), Scott Schaefer, Barbara Brown and R. Arrindell (AMNH), Mary Anne Rogers
(FMNH), and Roberto Reis (MCP); for the loan of the comparative material. Thanks to Martine Desoutter
(MNHN) for confirming the loss of the Cromys lapidifera type. Thanks to Sandra Raredon (NMNH) for providing
radiographs and photographs of the Retroculus boulengeri holotype. Special thanks are due to Melanie Stiassny for
kindly receiving MIL at the AMNH. This study benefitted from comments from M. Stiassny, Sven Kullander,
Richard Vari, Mário de Pinna, Flávio Lima and Fábio Di Dario. Thanks are due to the Department of Grants and
Fellowships, AMNH for providing MIL a Study Collection grant and the Axelrold Fund, through Melanie Stiassny,
for additional support to visit AMNH. MIL was sponsored by a FAPESP fellowship (03/01059-7) and CRM by a
FAPESP fellowship (03/01056-8).
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