Rhinotocinclus, new genus

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Type-species. Parotocinclus longirostris Garavello, 1988.

Diagnosis. Rhinotocinclus is diagnosed from all other genera in Hypoptopomatinae by having the canal cheek plate on the ventral surface of the head posteriorly elongated and contacting the cleithrum (Fig. 1; vs. canal cheek plate rounded or mesially elongated and not expanded backwards to contact the pectoral girdle). The new genus is also distinguished from other genera except Curculionichthys, Otocinclus, and Parotocinclus by having the dorsal-fin locking mechanism functional with the dorsal-fin spinelet V-shaped (vs. dorsal-fin locking mechanism non-functional and dorsal-fin spinelet roundish or absent). It is further distinguished from Curculionichthys by having a single rostral plate (vs. paired rostral plate) and less numerous lateral abdominal plates (2–5, but up to 8 in the R. collinsae Group; vs. 5–8). It is further distinguished from Parotocinclus by the shape of the coracoid, which is expanded anteriorly as a lamina partially covering the cleithrum ventrally (Fig. 1; vs. coracoid less expanded anteriorly), and from Otocinclus by having the preopercle exposed and bearing part of the mandibular branch of the laterosensory canal (vs. preopercle not exposed in the surface and not bearing laterosensory canal). Rhinotocinclus is further distinguished from the genera of Neoplecostomini (Euryochus Pereira & Reis, 2017, Hirtella Pereira, Zanata, Cetra & Reis, 2014, Isbrueckerichthys Derijst, 1996, Kronichthys Miranda Ribeiro, 1908, Neoplecostomus Eigenmann & Eigenmann, 1888, Pareiorhaphis Miranda Ribeiro, 1918, Pareiorhina Gosline, 1947) by having the pectoral girdle widely exposed and bearing odontodes on the ventral surface of both cleithrum and coracoid (vs. pectoral girdle covered by thick skin) and by the small body size (maximum standard length of its species 19.9–33.0 mm, vs. 49.2 mm in Hirtella, usually above 100 mm in remaining genera).

Sexual dimorphism. Species of Rhinotocinclus exhibit conspicuous secondary sexual dimorphism. As all hypoptopomatines, males of Rhinotocinclus possess a urogenital papilla immediately behind the anus (Figs. 2B,C), and a variably deep skin fold along the first, unbranched pelvic-fin ray (Fig. 2D), both characteristics being absent in females. In addition, males possess a much larger nostril than females (Fig. 3), causing the internarial distance to be smaller in males. The larger size of the olfactory organ of males also causes an elevation in the snout profile immediately in front of the eyes, which can be easily seen in lateral view (Fig. 3). In most species, males also possess longer pelvic fins, which reach or almost reach to the anal-fin origin (Figs. 2B,C), which does not happen in females. On the other hand, females usually attain larger size than males.

Characters and phenotypic species groups. The genus Rhinotocinclus as herein rearranged includes 23 species previously attributed to Parotocinclus, Hisonotus, and Curculionichthys. These species can be grouped in four phenotypic species groups that share characters that may or may not have phylogenetic significance, but which are easily verified and useful to identify the species.

The Rhinotocinclus britskii Group. Species in this group were all described in Parotocinclus and are characterized by possessing (1) a normally developed adipose fin (Figs. 4A,B), (2) dark brown oral teeth (Fig. 5A), (3) a Y-shaped light mark from the snout tip to the nostrils (Figs. 6A,B), and (4) a dominant color pattern formed by five dark bars on body (first at the anterior portion of the dorsal fin, usually continuous with a triangular spot at the anterior portion of the dorsal fin and anteriorly inclined, second at posterior portion of dorsal fin, usually not reaching dorsal midline and posteriorly inclined, third from adipose to anal fin, fourth before end of the caudal peduncle, usually connected to the fifth, which is contiguous with a dark blotch at the base of the caudal fin (Figs. 7A,B). Species in this group include R. bristskii, R. eppleyi, R. kwarup, R. longirostris, R. polyochrus, R. variola, R. yaka, and three new species, which are widely distributed in the Amazon, Orinoco, and coastal rivers of the Guianas.

The Rhinotocinclus collinsae Group. Species in this group were also originally described in Parotocinclus and are characterized by possessing (1) an adipose fin, which may be coalesced to the dorsal plates and lack a membrane (Fig. 4C), (2) brown oral teeth (Fig. 5A), (3) lack of clearly defined light marks from the snout tip to the nostrils (Figs. 6G,H), (4) absence of a triangular dark spot in the dorsal fin (Fig. 8A) and dark bars 2 and 3 of body fused (Fig. 7C), (5) unicuspid accessory oral teeth on both the premaxilla and dentary (Fig. 5A, arrows), and (5) odontodes on the ventral surface of the pelvic-fin ray aligned with main ray axis and not bent mesially (Fig. 9B). Species in this group include R. collinsae, R. halbothi, and R. hardmani, which are distributed on the Guiana Shield in the coastal rivers of the Guianas and some northern tributaries of the lower Amazon in Brazil.

The Rhinotocinclus bockmanni Group. Species in this group were originally described either in Parotocinclus, Hisonotus, or Curculionichthys, and are characterized by having (1) small azygous platelets at the adipose-fin position (except for R. hera), (2) light yellow oral teeth (except R. hera, which has light ochre teeth; Figs. 5B,C), (3) two separate light lines from the snout tip diverging toward each nostril (Figs. 6E,F), (4) a dominant color pattern formed by four or five dark bars on body, usually connected by a lateral dark stripe of variable intensity (Fig. 7D). Species in this group include R. bockmanni, R. dani, R. hera, R. pentakelis, and two new species, which are distributed in Amazon tributaries draining the western portion of the Brazilian Shield.

The Rhinotocinclus chromodontus Group. Species in this group were all described in Hisonotus and are characterized by (1) lacking an adipose fin and small azygous platelets at the adipose-fin position, (2) having brown or light ochre teeth (Figs. 5A,B), (3) a V-shaped light mark from the snout tip to the nostrils (Figs. 6C,D), and (4) a dominant color pattern formed by a dark stripe from the snout tip, passing through the eye and extending to the end of caudal peduncle, the transverse dark bars being inconspicuous laterally (Fig. 7E). Species in this group include R. acuen, R. chromodontus, R. dinizae, R. jumaorum, which are distributed in the rivers Araguaia, Xingu, Tapajós, and Madeira, all Amazon tributaries draining the Brazilian Shield.

Remarks. Among the Hypoptopomatini, some species of Hypoptopoma (H. brevirostratum Aquino & Schaefer, 2010, H. elongatum Aquino & Schaefer, 2010, H. guianense Boeseman, 1974, H. incognitum Aquino & Schaefer, 2010, H. inexspectatum (Holmberg 1893), and H. steindachneri Boulenger, 1895) may variably possess a small adipose fin, but their relationships were already demonstrated to be with their congeners and other hypoptopomatin genera (Delapieve et al., 2017), and this character bears no phylogenetic signal among the two groups. The adipose fin is also present in most Neoplecostomini, but these are also distantly related to the new genus (Pereira, Reis 2017; Reis et al., 2017). The main diagnostic feature of Rhinotocinclus is the canal cheek plate posteriorly elongated and contacting the cleithrum. A very similar configuration of the cheek plate, however, occurs in a few species of Hisonotus from southern Brazil and Uruguay (H. megaloplax Carvalho & Reis, 2009, H. montanus Carvalho & Reis, 2009, H. ringueleti Aquino, Schaefer & Miquelarena, 2001, H. thayeri Martins & Langeani, 2016 and H. vireo Carvalho & Reis, 2011). Again, this similarity is better interpreted as a convergence as Hisonotus have been demonstrated not to be directly related to Rhinotocinclus (Cramer et al., 2011; Reis et al., 2017; Roxo et al., 2019).

Parotocinclus longirostris was chosen as type-species for the new genus following Recommendation 69A of the International Code of Zoological Nomenclature (ICZN, 1999). It is adequately described and illustrated, type-material exists and is readily available at MZUSP, and it is a common and well distributed species in central Amazon.

Etymology. Rhinotocinclus masc., from the Greek Pniv q c (Rhinos), beak, snout and Otocinclus, a genus of Hypoptopomatinae, in allusion to the conspicuous and elegant snout of most of its species.

Key to the species of Rhinotocinclus

1a. Adipose fin absent, sometimes small platelets at adipose-fin position ..................14

1b. Adipose fin present, even if spine adnate to dorsal plates, without membrane......2

2a. Triangular dark spot on anterior portion of dorsal-fin membrane (Figs. 8B,C); with Y-shaped light mark from snout tip to nostrils (Figs. 6A,B); odontodes on ventral surface of first pelvic-fin ray bent and pointing mesially (Fig. 9A); eye large (25.5–40.5% snout length).......................................... 5 (R. britskii group)

2b. No triangular dark spot on anterior portion of dorsal-fin membrane (Fig. 8A); no light mark on snout tip (Figs. 6G,H); odontodes on ventral surface of pelvic-fin ray aligned with main ray axis (Fig. 9B); eye small (18.9–24.6% snout length) .......................................3 (R. collinsae group)

3a. Belly covered with middle abdominal plates between lateral abdominal plates (Figs. 10A,B); males with dorsal skin fold on first pelvic-fin ray (Fig. 2D); urogenital papilla normally developed (approximately same size of anal tube; Fig. 2B) ......................................................4

3b. Belly naked or almost naked between lateral abdominal plates (Fig. 10C); males without dorsal skin fold on first pelvic-fin ray; urogenital papilla 3–4 times bigger than normal (2–3 times longer than anal tube; Fig. 2C)............................. R. hardmani (Essequibo River basin, Guyana)

4a. One irregular series of middle abdominal plates between the lateral abdominal plates (Fig. 10B); adipose fin normally developed (Figs. 4A,B).......................................... R. collinsae (Essequibo River basin, Guyana)

4b. Four to seven irregular series of middle abdominal plates between the lateral abdominal plates (Fig. 10A); adipose-fin spine coalesced to dorsal plates (Fig. 4C) ............................................................................ R. halbothi (Rio Trombetas basin, Brazil and upper Marowijne basin, Suriname)

5a. Snout more acutely pointed (Fig. 11B); dark bars on body wider and closer (Fig. 7B); 2–4 plates between posterior border of rostral plate and nostril (Fig. 12A) ................................................................................................ 7

5b. Snout more broadly rounded (Fig. 11A); dark bars on body narrower and spaced (Fig. 7A); one plate between posterior border of rostral plate and nostril (Fig. 12B) ....................................................................................... 6

6a. More numerous premaxillary (28–34, mode 32; Tab. 1) and dentary (27–33, mode 29; Tab. 2) teeth; color pattern with more broken marks, mottled................................................... R. kwarup (Upper rio Xingu basin, Brazil)

6b. Fewer premaxillary teeth (15–29, mode 23; Tab. 1) and dentary (15–29, mode 20 and 25; Tab. 2) teeth; color pattern with less broken marks................................................................................................ R. britskii (Coastal rivers of Guyana and Suriname; upper rio Branco basin in Brazil and Guyana; Cuyuni and Caroni rivers, Venezuela; lower and middle portions of tributaries to the eastern Amazon basin, Brazil)

7a. Zero to 2 (rarely 3) irregular series of middle abdominal plates (Figs. 10B,C); four dark bars on body (bars 1+2 or 2+3 fused) ..............................................................................................9

7b. Four or 5 irregular series of middle abdominal plates (Fig. 10A); five dark bars on body ..............................................................................................8

8a. Fewer premaxillary (22–30, mode 26; Tab. 1 and dentary (21–27, mode 25; Tab. 2) teeth; .............................. R. eppleyi (Upper Río Orinoco basin, Venezuela)

8b. More numerous premaxillary (28–36, mode 30; Tab. 1) and dentary (27–31, mode 30; Tab. 2) teeth;........................................ R. longirostris (Tributaries to central Amazon basin, Brazil)

9a. One or 2 lateral abdominal plates between pectoral-fin axilla and pelvic-fin.................................................................................................10

9b. Three to 5 lateral abdominal plates between pectoral-fin axilla and pelvic-fin.................................................................................................11

10a. Belly naked or almost naked between lateral abdominal plates (Fig. 10C); caudal peduncle shallower (6.3–7.5% SL) ................ R. pilosus n. sp. (Rio Madeira basin near Humaitá, Brazil)

10b. Belly covered with 1–2 series of middle abdominal plates between lateral abdominal plates (Fig. 10B); caudal peduncle deeper (8.1–8.8% SL) .............. R. isabelae n. sp. (Río Tigre and Río Nanay, Loreto, Peru)

11a. Fewer premaxillary (23–32) and dentary (19–31) teeth........................................13

11b. More numerous premaxillary (33–51) and dentary (33–43) teeth........................12

12a. Dark dots smaller than pupil diameter broadly distributed dorsally and ventrally; triangular dark spot on anterior portion of pectoral-fin membrane (Fig. 13B); triangular dark spot occupying more than half of dorsal fin (Fig. 8C) ........................ R. variola (Western Amazon basin, Colombia and Brazil)

12b. No dark dots smaller than pupil diameter (sometimes darkened sensory pores on head); no triangular dark spot on pectoral-fin (Fig. 13A); triangular dark spot occupying less than half of dorsal fin (Fig. 8B)................................. R. yaka (Rio Tiquié, Upper rio Negro basin, Brazil)

13a. Conspicuous light bar in front of dorsal fin, extended on head as Y-shaped mark towards each eye; triangular dark spot of dorsal fin well developed; body dark bars 1+2 fused; caudal peduncle shallower (6.8–7.1% SL); pectoral-fin spine longer (29.2–32.3% SL); orbit smaller (38.9–40.6% interorbital distance) .............................................................................. R. polyochrus (Río Mawarinuma, Río Orinoco basin at Neblina mountains, Venezuela)

13b. No light bar in front of dorsal fin and Y-shaped mark on head; triangular dark spot of dorsal fin inconspicuous; body dark bars 2+3 fused; caudal peduncle deeper (7.7–8.6% SL); pectoral-fin spine shorter (24.8–27.3% SL); orbit larger (52.1–63.7% interorbital distance) .................... R. discolor n. sp. (Río Orinoco basin in southern Venezuela)

14a. Dominant color pattern formed by dark stripe from snout tip, through eye and extending to end of caudal peduncle (Fig. 7E);V-shaped light mark from snout tip diverging to each nostril (Figs. 6C,D) .................................................................... 20 (R. chromodontus group)

14b. Dominant color patter formed by four or five dark bars on body (Fig. 7D); two separate light lines from snout tip diverging to each nostril (Figs. 6E,F); ........................................................................ 15 (R. bockmanni group)

15a. No triangular dark spot on anterior portion of dorsal-fin membrane (Fig. 8A); five dark bars on body, variably united by irregular dark stripe ..........................16

15b. Triangular dark spot on anterior portion of dorsal-fin membrane (Fig. 8B); dark bar 2 absent or inconspicuous, bars 1, 3–5 united by thin, regular dark stripe ......................... R. bockmanni (Middle rio Tapajós basin, Brazil)

16a. One to 3 azygous platelets at adipose-fin position; tooth cusps light yellow ......17

16b. Azygous platelets absent; tooth cusps ochre................................. R. hera (Rio Curuá-Una basin, Pará, Brazil)

17a. Dark bars on body regularly arranged, usually connected by irregular midline dark stripe; nasal bone projected laterally and contacting infraorbital 2 .................................................................................18

17b. Dark bars on body somewhat fragmented and inclined, such that they connect to form a zig-zag pattern; nasal bone not contacting infraorbital 2 .................................................................................. R. loxochelis n. sp. (Jamanxim National Forest, rio Tapajós basin, Brazil)

18a. Caudal fin with one slanted dark band at base of rays and irregularly dispersed dark dots; caudal peduncle shallower (8.8–10.1% SL or 22.9–26.3% HL) .......................................................................19

18b. Caudal fin with three slanted dark bands; caudal peduncle deeper (10.1–11.7% SL or 26.6–29.7% HL) ...................................................... R. pentakelis (Upper rio Tocantins basin, Brazil)

19a. Body dark bars 2 and usually 3 reaching to the ventral midline; more numerous premaxillary teeth (19–28, mode 21; Tab. 1) and dentary (16–22, mode 19; Tab. 2) ........... R. dani (Rio Teles Pires and rio Jamanxim basins, Brazil)

19b. Body dark bars barely passing lateral dark stripe; fewer premaxillary teeth (12–18, mode 16; Tab. 1) and dentary (11–16, mode 13; Tab. 2)........................... R. marginalis n. sp. (Lower rio Xingu and rio Iriri, Brazil)

20a. Tooth cusp chestnut brown or reddish brown (Fig. 5A); 20–40 premaxillary (Tab. 1) and 18–34 dentary (Tab. 2) teeth............................21

20b. Tooth cusp light ochre (Fig. 5B); 17–22 premaxillary (Tab. 1) and 14–19 dentary (Tab. 2) teeth ...........................................................................22

21a. Caudal fin mostly hyaline, with 2–3 irregular dark bands; dorsal- and pectoral-fin spines with 2–3 dark dots; body narrower (cleithral width 22.4–24.9% SL or 59.4–67.2% HL); pectoral-fin spine shorter (23.3–26.7% SL or 60.8–68.9% HL) ............................. R. jumaorum (Lower rio Madeira basin, Brazil)

21b. Caudal fin mostly brown, with hyaline spot on upper and lower lobes; dorsal- and pectoral-fin spines homogeneously dusky; body wider (cleithral width 24.9–27.9% SL or 67.7–76.2% HL); pectoral-fin spine longer (26.7–29.5% SL or 71.4–78.4% HL) ......................... R. chromodontus (Upper rio Tapajós basin, Brazil)

22a. Pectoral-fin spine short (23.1–26.9% SL); head short (37.0–40.0% SL), 4–6 lateral abdominal plates between pectoral-fin axilla and pelvic–fin ................................................... R. acuen (Upper rio Xingu basin, Brazil)

22b. Pectoral-fin spine long (28.1–30.0% SL); head long (40.0–41.8% SL); 3–4 lateral abdominal plates between pectoral-fin axilla and pelvic-fin............................................. R. dinizae (Upper rio Araguaia basin, Brazil)