Hypoptopoma Günther, 1868a (Hypostomatin.)

Aquino, Adriana E., 2010, Systematics Of The Genus Hypoptopoma Günther, 1868 (Siluriformes, Loricariidae), Bulletin of the American Museum of Natural History 2010 (336), pp. 1-110 : 7-14

Hypoptopoma Günther, 1868a Figure 2

Hypoptopoma (Hypostomatin.) Günther, 1868a: 477 (type species: Hypoptopoma thoracatum Günther, 1868a , by monotypy).

Hypoptopoma (Hypostomatinum.) Günther, 1868b: 234 (description of H. thoracatum ).

Aristommata Holmberg, 1893: 96 . Aristommata inexspectata Holmberg, 1893 type species by monotypy.— Isbrücker, 1980: 87.—Aquino and Miquelarena, 2001: 2.— Isbrücker, 2002: 26.— Ferraris, 2007: 249.

Aristomata Eigenmann, 1910: 412 (misspelling).

Hypoptoma Miranda Ribeiro, 1911: 97–99 (misspelling; list of fishes of Brazil).

Otocinclus not of Cope, 1871.—Eigennman, 1914: 229 (description of O. spectabilis ).

Diapeltoplites Fowler, 1915: 237 (proposed as subgenus of Hypoptopoma Günther, 1868a ).— Isbrücker, 1980: 87 (placed as synonym of Hypoptopoma Günther, 1868a ).

Diapaletoplites Jordan, 1920: 556 (misspelling; list of fish genera).

Nannoptopoma Schaefer, 1996a: 915 (type species: Nannoptopoma spectabilis ( Eigenmann, 1914)) .

DIAGNOSIS: Distinguished from all other loricariids by having the nuchal plate laterally expanded to such an extent that the distance between distal tips is at least twice the width of the base of the dorsal spine at its widest point (fig. 3B–C). In members of the Loricariinae and Hypoptopomatinae except Niobichthys and Acestridium , the nuchal plate is round to polyhedral, its width equal or slightly surpassing that of the base of the pectoral spine (fig. 3A). In Niobichthys , and members of the Ancistrinae and Hypostomatinae, there is a pair of nuchal plates in contact in the midline. Species of Acestridium possess 2–4 median unpaired predorsal plates (Reis and Lehman, 2009).

Hypoptopoma can also be distinguished from all other loricariids, except the hypoptopomatin genus Oxyropsis , by the shape of the head, which is depressed, and by the ventrolateral placement of the eyes. This typical ‘‘ Hypoptopoma ’’ head shape is further reflected anatomically by a series of uniquely derived osteological characters (shared with Oxyropsis ): the lateral ethmoid laterally elongated; the T-shaped third infraorbital, with its lateral platelike side positioned ventrally on the head (fig. 4); the canalbearing plate presenting more than threefourths of its surface area positioned ventrally on the head, and the fourth infraorbital reduced, smaller than the fifth infraorbital (fig. 5) (except in H. spectabile ). In other taxa, the position of the eyes is lateral to dorsolateral and not visible from below. Accordingly, the lateral ethmoid is not laterally elongated, the third infraorbital is platelike and laterally positioned on the head, the canal-bearing plate presents less than three-fourths of its surface area positioned ventrally on the head, and the fourth infraorbital is larger than the fifth infraorbital.

Hypoptopoma is distinguished from all other Hypoptopomatini , including Oxyropsis , by the shape of the caudal peduncle. The cross section of the trunk posterior to the base of the anal fin is distinctly ovoid, with the dorsoventral axis as the longest. The ovoid shape is enhanced by the uniform size of the odontodes in longitudinal rows across the trunk and, in adults of the largest species in average size, by the smoothness of the plates. Oxyropsis , Acestridium , and Niobichthys possess a narrow and depressed caudal peduncle. The trunk cross section posterior to the base of the anal fin is roughly rectangular to oblate spheroidal in shape, with the horizontal axis as the longest. The lateral edges are enhanced by rows of variably enlarged odontodes, which in Oxyropsis form a keellike ridge in cross section ( Aquino and Schaefer, 2002). In Otocinclus , which possess a robust caudal peduncle and the dorsoventral axis the longest, the cross section of the caudal peduncle is progressively rectangular in shape toward the caudal fin ( Schaefer, 1997). The rectangular shape is further enhanced by rows of variably enlarged odontodes along the dorsolateral edges of the peduncle.

Hypoptopoma , except for H. spectabile , can be further distinguished among loricariids by the presence of a column of odontodes positioned along the posterior margin of the trunk plates (hereafter termed ‘‘marginal odontodes’’; fig. 6). The marginal odontodes are not visible in the juvenile stages, becoming progressively conspicuous in ontogeny. Their development takes place with a progressive smoothening of the plate surfaces (in the largest specimens, the plates are smooth, the odontodes reduced in number, and their distribution is restricted to the posterior portion of the plate surface). The marginal odontodes are slightly larger and flattened relative to the typical conical odontodes.

DESCRIPTION: Adult body size moderately small relative to most other loricariids; maximum size of Hypoptopoma species ranging from 18 to 105 mm SL, attaining greatest body length among genera of Hypoptopomatinae . Head and body almost entirely covered by bony dermal plates, except for area surrounding lips and base of pectoral, pelvic, dorsal, and anal fins. Dermal bones exposed on head and pectoral girdle, and fin rays covered by dermal teeth or odontodes ( Bhatti, 1938). Odontodes rather evenly distributed on head, slightly enlarged and variably aligned in dorsal and ventral series on rostral margin of snout, and typically arranged in well-defined longitudinal series on trunk plates. Distinct column of slightly enlarged and flattened marginal odontodes arranged along posterior margin of trunk plates. Anterior surface of pectoral-fin spine and ventral surface of pelvic-fin spine with enlarged odontodes. Series of smaller, irregularly shaped pararostral plates present anterior to each nasal plate between mesethmoid and lateral rostral plates (first to third canal-bearing infraorbitals) and adjacent to naris between nasal plate and lateral rostral plates (fig. 7A). In some species, one or more paranasal plates can be present (one in H. gulare and H. machadoi ; two or more in H. steindachneri ) (fig. 7B–C).

Dorsal-fin rays I,7; anal-fin rays i,5; pectoral-fin rays I,6; pelvic-fin rays i,5; caudal-fin rays i,7-7,i. Adipose fin variably present. Premaxillary teeth 10–36; dentary teeth 10–37. Oral disk rounded, surface papillose and with scattered unculi.

Dermal plates on trunk arranged in five longitudinal, serially homologous rows (fig. 1). Lateral line complete and continuous; total lateral plates (medial series) 20–26; dorsal series 17–22; middorsal series 3–5; midventral series 11–15; ventral series 17–22. Number of midventral series plates between posterior process of cleithrum and first plate of ventral series (roughly triangular plate at base of pelvic-fin spine) three ( H. bianale , H. elongatum , H. gulare , H. incognitum , H. inexspectatum , H. machadoi , H. spectabile , H. sternoptychum ) (figs. 1B, 8B) or four ( H. baileyi , H. brevirostratum H. guianense , H. muzuspi , H. psilogaster , and H. thoracatum ) (figs. 1A, 8A), rarely five. Second plate of midventral series contacting a single plate ( H. bianale , H. elongatum , H. gulare , H. incognitum , H. inexspectatum , H. machadoi , H. spectabile , H. sternoptychum ) (figs. 1B, 8B) or two plates of medial series ( H. baileyi , H. brevirostratum H. guianense , H. muzuspi , H. psilogaster , and H. thoracatum ) (figs. 1A, 8A). Abdominal plates arranged according to three main patterns: (1) plates arranged in paired series of lateral sickle-shaped plates and medial series of roughly squared plates, each series composed of more than three plates ( H. baileyi , H. brevirostratum , H. guianense , H. muzuspi , H. psilogaster , and H. thoracatum ) (fig. 9A); (2) plates arranged in paired series of lateral sickle-shaped plates and only one roughly triangular medial plate between anteriormost pair of lateral plates (all other species, except H. spectabilis and H. sternoptychum ) (fig. 9B–C), and (3) presence of a pair of slender plates posterior to the coracoids, followed by a series of 1–3 unpaired abdominal plates anterior to the anal plate ( H. spectabilis and H. sternoptychum ) ( Schaefer, 1996a: fig. 1). Thoracic plates present or absent ( H. baileyi , H. brevirostratum , H. guianense , H. muzuspi , H. psilogaster , H. spectabilis , and H. thoracatum ); when present, forming shield between left and right ventral canal-bearing plates (fig. 9B–C). Anal plate composed of single plate (fig. 9A–B), except in H. bianale , whose shield is two plates (fig. 9C). Fully developed anal plate shaped as blunt, posteriorly oriented arrowhead.

Complete development of lateral series of trunk plates achieved ontogenetically before full development of abdominal shield and plates anterior to naris (fig. 10). Development of trunk-plate series progresses in rostral direction, with individual plates near- est caudal fin developing first. Plates of medial series and bony canals of lateral line develop asynchronously, with lateralis canal ossifying prior to ossification of surrounding plate lamina. Platelike connecting bone (sensu Schaefer, 1987), counted as second middorsal plate, develops prior to plates lying immediately anterior and posterior to middorsal series. Development of abdominal plates initiates with lateral series in an approximately lateral to medial direction. Fully developed individuals can be recognized by having ventral region between pectoral fins entirely covered by abdominal plates.

SEXUAL DIMORPHISM: Male urogenital papilla pointed, short, variably slender, more or less covered by anterior flaplike anus. Males of some species with patch of tightly arranged small odontodes positioned lateral to urogenital papilla and anus, variably developed on plates 1–4 of ventral trunk series. In some species, males with variably developed fringe of soft tissue along posterior margin of pelvic-fin spine, proximal to first branched ray when developed, typically restricted to basal one-third to one-half of spine length. Anus of females tubular, without separate urogenital papilla. In fe- males of all species, size and arrangement of odontodes on plates lateral to anus similar to adjacent plates, without distinct patch of differentially arranged odontodes.

DISTRIBUTION: Widely distributed in lowland cis-Andean South America, with representatives in the major tributaries of the Amazon, Orinoco, Essequibo, Nickerie, Tocantins, and Paraguay /Paraná rivers (fig. 11).

COMPARISONS: The pattern of odontode distribution on the trunk plates changes during ontogeny. In the smallest specimens of each species, odontodes on the plate lamina have a relatively dense and regular distribution in multiple longitudinal rows, a condition observed widely among species of Loricariidae . In larger specimens (actual size of specimen depends on the species), the distribution of odontodes is more uneven and irregular over the plate surface, resulting in one or more relatively open areas lacking odontodes and giving the plate surface a smoother appearance. A pattern of increased irregularity in odontode distribution and greater proportion of the plate surface without odontodes is correlated with increase in the size of specimens. It has not been determined whether trunk-plate smoothing is due to actual loss of odontodes or plate growth in surface area without corresponding increase in number of odontodes, thus expanding the space among odontodes. Observation of open ‘‘sockets,’’ or alveoli, would suggest both odontode loss and surface ossification as responsible for plate smoothing during ontogeny. Although odontode loss would not be uncommon among loricariids, progressive plate smoothing has not been reported. Among Hypoptopoma species , the greatest degree of plate smoothing is observed mostly among species of the clade at node 7 (see fig. 45) and the largest specimens of H. thoracatum and H. psilogaster . Among non-loricariid loricarioids, some callichthyids have odontodes arranged along the posterior margin of the trunk plates. However notable differences in morphology of individual odontodes and in arrangement of odontodes suggest nonhomology with the condition observed among Hypoptopoma species.

Within the Hypoptopomatinae , Hypoptopoma is most similar to Oxyropsis in external appearance. Representatives of these two genera share the depressed head and the ventrolateral position of eyes ( Schaefer, 1997). In addition to the unique presence of the laterally expanded nuchal plate and marginal trunk-plate odontodes, Hypoptopoma can be further distinguished from Oxyropsis by having a trunk ovoid in cross section, the ratio of the caudal-peduncle depth in the caudal-peduncle width.1 (vs. cross section of trunk depressed, rectangular to oblate spheroid shaped in cross section, ratio of caudal-peduncle depth in caudalpeduncle width,1), and by the absence of the single row of enlarged odontodes along the trunk midline lying adjacent and immediately dorsal to the lateral-line canal, a derived character state unique for Oxyropsis ( Aquino and Schaefer, 2002) .

TAXONOMIC REMARKS: Günther (1868a) distinguished Hypoptopoma by characteristics related to the head: head depressed and spatulate, with the eyes located on the lateral margins of the head. Following this diagnosis, Steindachner (1879) assigned his new species carinatum to the genus Hypoptopoma . Aquino and Schaefer (2002) provided evidence in support of Oxyropsis monophyly and demonstrated that the characteristics used by Günther to distinguish Hypoptopoma are shared with Oxyropsis . A phylogenetic diagnosis of Hypoptopoma ( Schaefer, 1991) distinguished the genus on the basis of two osteological characters: (1) canal in the preopercle forming a semicircle, and (2) preopercular canal passing through the fifth infraorbital before entering the preopercle. The present study shows that those two characters are not shared by all species of Hypoptopoma . In a revised analysis, Schaefer (1998) recognized four derived unreversed characters supporting the Hypoptopoma clade (Schaefer, 1998: fig. 3): (1) sphenotic with expanded anterior lamina, (2) preopercle canal semicircular, (3) presence of notch on canal-bearing ventral plate, and (4) trunk plates with enlarged odontodes concentrated along posterior margin (in this paper, marginal odontodes). Likewise, the present study shows that character (1) is shared with other genera of the tribe, and characters (2), (3), and (4) are not shared by all species of Hypoptopoma .

Figures

Fig. 2. Holotype and jar label of the type species of the genus Hypoptopoma, H. thoracatum, BMNH 1867.6.13:38.