Paracanthopoma saci Dagosta & de Pinna, 2021

Pinna, Mário de & Dagosta, Fernando Cesar Paiva, 2022, A taxonomic review of the vampire catfish genus Paracanthopoma Giltay, 1935 (Siluriformes, Trichomycteridae), with descriptions of nine new species and a revised diagnosis of the genus, Papéis Avulsos de Zoologia 62, pp. 1-90 : 55-59

Paracanthopoma saci Dagosta & de Pinna, 2021 ( Fig. 33 )

Paracanthopoma saci Dagosta & de Pinna, 2021 [ Brazil, Mato Grosso do Sul, Alcinópolis, rio Taquarizinho (tributary to rio Taquari, rio Paraguay drainage) (18°12′14.8″S, 53°34′11.3″W)].

Holotype: MZUSP 125624 , 19.6 mm SL, Brazil, Mato Grosso do Sul, Alcinópolis, rio Taquarizinho (tributary to rio Taquari , rio Paraguai drainage) (18°12′14.8″S, 53°34′11.3″W), alt. 363 m, col., F. Dagosta, A. Ferreira, R. Zanon, 18 Sep 2019.

Paratypes: MZUSP 125626,13ex(3c&s), 14.5-21.8mm SL, collected with holotype. MZUSP 125622, 2 ex, 19.5-19.9 mm SL, same locality and collectors as holotype, 17 Sep 2019. MZUSP 115585, 1 ex, 19.3 mm SL, Brazil, Mato Grosso do Sul, Alcinópolis, stream tributary to rio Taquari (rio Paraguai drainage) at dirt road between Alcinópolis and road MS-217 (18°12′16.5″S, 53°34′13.5″W), col., O. T. Oyakawa et al., 26 Aug 2013.

Diagnosis: Distinguished from all other species of Paracanthopoma by the short and anteriorly-displaced opercular odontodophore,which leaves a large posterior free area of periodontodal fold surface continuous with the rest of the integument around it. As a consequence of that morphology, in dorsal view the posterior tips of the opercular odontodes do not reach the vertical through the base of the pectoral fin. Also unique in the genus is the three-rayed pelvic fin (vs. five) and the absence of an ascending process on the opercle. The presence of three teeth on the median premaxilla (vs five or more) also distinguishes Pc. saci from congeners ( Pc. ahriman , Pc. cangussu , Pc. capeta , and Pc.irritans , all consistently with five median premaxillary teeth, often have one or two teeth in replacement, which may yield erroneous counts of three or four under superficial examination). Also distinguished from all congeners (except for extreme upper value in Pc. cangussu ) by the long caudal peduncle (24.0-26.6% SL; vs. 14.7-24.0) and from all congeners except Pc.satanica by the anteriorly-located pelvic fin (prepelvic length 56.0-61.1% SL; vs. 61.6-74.4).

Description: Morphometric data for the holotype and paratypes are provided in Table 10 . Body elongate (HL 15.1-16.1% SL). Cross-section of body as broad as deep at pectoral-fin insertion and increasingly compressed posterior to that point, tapering to caudal fin. Dorsal profile of body in broad gentle arc, nearly straight, from head to origin of dorsal fin ( Fig. 33 ). Dorsal midline with transparent fin fold anterior to dorsal fin. Dorsal and ventral profiles of caudal peduncle straight immediately posterior to ends of dorsal and anal fins, then expanded by dorsal and ventral procurrent caudal-fin rays resulting in symmetrically spatulate caudal peduncle ( Fig. 33 ). Ventral profile of body nearly straight until pelvic-fin origin, but greatly distented in some specimens due to gut contents. Myotomes and longitudinal skeletogenous septum clearly visible through thin integument along whole body. Axillary gland large, elongate in shape, positioned dorsal to pectoral-fin base and extending posteriorly approximately to midlength of adpressed pectoral fin, its large round or oval pore located at its posterior terminus. Some specimens preserved with large amount secretion still attached to pore.

Dorsal profile of head continuous with that of dorsum. Head longer than broad (head width 83.6-100.0% HL), snout very broad, semicircular with a continuous round anterior margin ( Fig. 33 ). Head muscles not entering skull roof. Head depressed (head depth 41.8-54.6% HL) with dorsal profile mostly continuous from nape to tip of snout. Ventral profile of head straight, flattened, though externally irregular due to integument folds. Eye medium-sized (11.3-13.6% HL), without free orbital rim, located dorsally on head and directed dorsolaterally. Integument over eye thin and transparent. Center of eye located slightly anterior to middle of HL, interorbital width equal to, or slightly larger than, longitudinal diameter of eye. Eyelens unconstricted by iris, entirely exposed on external aspect of eye. Anterior nostril small, surrounded by short tubule of integument produced posteriorly into small pointed process. Anterior internarial width slightly smaller than interorbital. Posterior naris slightly larger than anterior ones, oval (longer than broad) in shape, located close to anteromesial margin of eye. Center of posterior naris slightly anterior to transverse line through anterior margin of eyes. Posterior internarial width narrower than interorbital and approximately twice as wide as anteroposterior length of one nostril.

Opercular odontodophore tiny, dorsolaterally located on head, on dorsal half of head depth in lateral view, anterodorsally to pectoral-fin base.Opercular odontodes 6, disposed in two irregular rows. Odontodes with distal portions curved medially. Opercular periodontodal fold vestigial or absent, instead fused with large well-delimited roundish area of thickened integument resembling a vastly hypertrophied periodontodal fold but being in fact a novel structure. Interopercular odontodophore miniscule, nearly invisible on surface of head, located ventrolaterally on head, horizontally aligned with origin of pectoral fin. Interopercular odontodes 4. Interopercular odontodophore closer to opercular one than to eye. Interopercular periodontodal fold absent or vestigial. Epiodontodeal velum absent.

Mouth inferior (ventral) and small, strongly flattened ventrally. Each premaxilla with one scalpelloid tooth attached to its distal tip, and one additional tooth socket with partly-formed tooth in parallel ( Figs. 4I , 34 ). Scalpelloid teeth deeply hidden in labial tissue and difficult to expose in preserved specimens without damaging soft tissue.Conical teeth absent on premaxilla. Upper lip very broad but poorly-differentiated, continuous with ventral surface of snout. Median premaxilla miniscule, broader than long, with 3 closely-set tiny teeth ( Figs. 4I , 34 ). Median premaxillary velum poorly-differentiated. Hypodontal pad of median premaxilla semicircular. Lower jaw narrow, composed mostly of produced dentary lobes, continuous with mental region posteriorly. Jaw cleft short, strongly directed posteriorly, but curved laterally at posterior end. Dentary diastema as deep median concavity between dentary lobes. Dentary teeth 4, large but difficult to visualize in alcoholic specimens, concentrated at mesial end of dentary and directed anteromesially, arranged in two ventral and two dorsal ones, not aligned ( Figs. 4I , 34 ). Dentary teeth long, their axis anteriorly-directed at base, but curved dorsally or dorsolaterally at distal half. Median tooth of ventral row longer than others.

Branchiostegal velum forming large, continuous, round and posteriorly concave, free fold across whole of mental region. Dorsal portion of branchial membrane reaching and slightly overlapping anterior margin of pectoral-fin base. Branchial opening small, located anteriorly to pectoral-fin base, approximately equal to space between opercular and interopercular odontodophores. Maxillary barbel ranging from extremely short to vestigial, extending maximally for one-third distance between its base and base of interopercular odontodophore. Posterior point of its base slightly anterior to vertical through anterior margin of eye in lateral view. Rictal vestigial, reduced to small knob mesially to base of maxillary one, absent in some specimens. Nasal barbel vestigially represented by posterior elongated portion of fold around anterior naris described above.

Lateral line short, approximately half of pectoral-fin length, and straight, extending alongside dorsal margin of anterior portion of axillary gland. Terminal lateral-line pore immediately dorsal to axillary gland opening. Short secondary branch splitting off ventrally from proximal portion of main canal, with corresponding pore opening anteriorly to midlength of main canal. Single lateral-line tubule poorly calcified, extending over part of main canal immediately posterior to bifurcation.

Pectoral fin small (61.5-78.8% HL), with i + 4 rays in all specimens (except i + 3 in one side of one specimen). Pectoral-fin morphology with pronounced variation, with most specimens (n = 9, including holotype) having triangular pointed shape with first ray slightly longer than others. Some specimens (n = 5) with all rays equally long, resulting in a truncate fin profile. Two exceptional specimens in MZUSP 125626 (14.5 and 15.0 mm SL) with hypertrophied first pectoral-fin ray forming long filament. Filamentous ray corresponding to 36% SL in smaller specimen and 26% SL in larger specimen (see Remarks below). Pelvic fins minute, closely set together at base, with i + 2 rays (with all three rays unbranched in small specimens; one specimen with vestigial additional ray posteriorly). Pelvic splint present in single of three c& s specimens. Origin of pelvics close to origin of anal fin, well anterior to vertical through origin of dorsal-fin, extending posteriorly well beyond anus and urogenital papilla and slightly beyond origin of anal fin. Posterior margin of pelvic fin gently convex. Dorsal fin elongate, roughly triangular with roundish edge and gently convex, sinusoidal or straight distal margin. Dorsal-fin rays i + 5, ii + 5 or i + 6, plus prominent series of 5 to 8* procurrent ones. Anal fin similar in shape to dorsal fin, with i + 5 rays, plus 5 to 9 (8*) procurrent ones. Origin of anal fin at or slightly posterior to vertical through origin of dorsal-fin. Caudal fin roughly rectangular in normal preserved position but into more fan-like shape when expanded. Margin of caudal fin truncate with round edges and gently convex margin. Principal caudal-fin rays 5 + 5* or 4 + 5. Procurrent caudal-fin rays 22-29 dorsally and 20-29 ventrally.

Vertebrae 40 to 44 (42*, n = 10), with single specimen with 37. First dorsal-fin pterygiophore positioned subsequent to neural spine of vertebra 22 (n = 3). First anal-fin pterygiophore positioned subsequent to neural spine of vertebra 22 (n = 1) or 23 (n = 2). Dorsal- and anal-fin pterygiophores 6, poorly calcified or entirely cartilaginous, clearly visible only in c&s preparations. Branchiostegal rays 3.

Pigmentation in preservative: General aspect of fish almost entirely white. Post-orbital part of skull roof with extensive dark field formed by brain pigment seen by transparency, its anterior margin strongly concave, in continuous arc or angulate, immediately posterior to eyes. Well-defined elongate dark field extending anterior to eye, along lateral margin of olfactory capsule. In some specimens, mesial margin of olfactory capsule also with some dark pigment, but much weaker than lateral one. Dark pigment on body restricted to uniform weblike or dotted covering on dorsal part of abdominal wall and few isolated small spots along base of dorsal fin and, rarely, those of anal and caudal fins and muscular margins of caudal peduncle. Posterior portion of vertebral column with internal dark pigment on each individual vertebrae, forming series of spots visible externally by transparency along caudal peduncle (presumably more evident in life). All fins hyaline.

Geographical distribution: The species is so far known from a single locality in the rio Taquari system (rio Paraguay drainage) ( Fig. 45 ). It is the only species of Paracanthopoma from the Paraná-Paraguay and in fact the only one from outside of the broad Amazonian drainages (Amazon, Orinoco, Essequibo and coastal Guyanan drainages). It also marks the southernmost limit of the genus.

Ecology: The rio Taquarizinho is ca. 15 m wide at the collection locality. Water is clear, slightly milky and with medium current. Specimens were collected by seining on sand banks in the middle of the river, especially in sectors shaded by riparian vegetation. There was no aquatic vegetation and depths of collection ranged from 30 to150 cm. Paracanthopoma saci is sympatric with Paravandellia oxyptera (MZUSP 125623, 125625) with both relatively abundant at the type locality. The two species are psammophilic, but with different microhabitat preferences. Paracanthopoma saci favors fine sand, while Pv. oxytera prefers sectors with coarser granulation. Once the subtelties of their preferences are understood, it is possible to target one or the other species for collection with reasonable accuracy. Segregation is not complete however, and occasionally they were captured together in the same net (information above is based on pers. comm. by F. Dagosta). The abdomen is distended with blood in several specimens. Some female specimens have large eggs visible by transparency.

Remarks: Paracanthopoma saci is the only species of Paracanthopoma outside of the core Amazonian drainages (see Geographical Distribution above) and only the second species of Vandelliinae from the entire ParanáParaguai basin (the other being Paravandellia oxyptera ). Other fish collected with Pc. saci were all typical members of the Paraná-Paraguai fish fauna, including some endemics (e.g., Cyphocharax gillii, Steindachnerina brevipinna, Creagrutus meridionalis, Brachychalcinus retrospina, Bryconamericus exodon). Paracanthopoma saci is obviously an outlier Amazonian component in the rio Taquari and an exception to the general composition of the Paraguai basin. Though exceptional, this situation is not unique. It has been known for a long time that the headwaters of the rio Paraguay include some odd Amazonian components, usually very restricted in distribution. Ribeiro et al. (2013) explains such cases as a result of events related to the formation of the Pantanal wetlands. A subsidence of the Upper Paraguai, ca. 2.5 mya resulted in the capture of a component of the upper rio Tapajós into the upper rio Paraguai. The ecological barrier represented by the Pantanal prevented such upper-water course, high-energy, components from spreading to the rest of the basin, thus explaining their restricted distributions. Paracanthopoma saci is clearly an additional element in that pattern. If such biogeographical association is correct, Pc.saci will be a useful marker to calibrate molecular divergence in Paracanthopoma .

Two paratypes of Pc. saci have extraordinarily elongated first pectoral-fin rays (part of MZUSP 125626, see Description above) ( Fig. 35 ). This is the most extreme case of filamentous pectoral-fin ray known in Trichomycteridae . No other specimens of the species show any sign of fin elongation. The significance of this trait is difficult to interpret with the information available. The two filamentous specimens are identical in all other relevant characteristics to other putative conspecifics, there being thus no reason to suspect that they might represent a different taxon. At 14.5 and 15.0 mm SL, those two specimens are among the smallest known of Pc.saci . Sexual dimorphism in pectoral-fin size and structure exists in all species of Paravandellia and at least one of some of Paracanthopoma ( Pc. irritans ; see above) with mature males having a stouter and larger pectoral fin (see section on Sexual Dimorphism). No such association is evident in Pc.saci . An alternative hypothesis of juvenile especialization is possible. The longest filament, corresponding to 36% SL, is seen in the smallest specimen. In the second specimen, only slightly larger, the filament is reduced to 26% SL. Other specimens similar in size to the latter show no sign of filament. So, if the filament is indeed a juvenile especialization,it is reduced very abruptly in the course of ontogeny (or perhaps broken off). Resolution of this question will have to await additional material and data.

The extremely reduced opercular and interopercular armatures of Pc. saci , a reduction reflected in their vestigial (opercular) or absent (interopercular) periodontodal folds, raises questions about the functionality of that complex in the species. One specimen in MZUSP 125626 is preserved with opercular odontodes erect, a sign that the biomechanical links usually associated with movement of the opercular odontodophore are still functional in the species. No such evidence exists for the still more reduced interopercular armature. Given that the latter is not only tiny in size in Pc. saci but also quite buried in integument, it is possible that it is reduced to a vestigial condition.Reductions of opercular and interopercular armature are recurrent in different lineages of sand-dwelling trichomycterids, such as psammophilic taxa in Glanapteryginae and Sarcoglanidinae . Despite such reductions, it is certain that Pc. saci is hematophagous like all other vandelliines, because several specimens in MZUSP 125626 have coagulated blood in their guts.

A single specimen in MBUCV-V 32270 (15.6 mm SL), Brasil, Mato Grosso, rio do Peixe, rio Negro basin, ParanaParaguay drainage, at road to Perdigão (19°23,25′S, 54°58,79′W), col., R. Barriga et al., 26 Aug 1998, probably represents Pc. saci , but this could not be directly verified for the present study.

Figures

Figure 4. Comparative chart of jaws and anterior portion of head of Paracanthopoma species, cleared and stained specimens, ventral views: (A) Pc. ahriman FMNH 105525;(B)Pc.alleynei MZUSP 103052;(C) Pc.cangussu MZUSP 86250;(D)Pc.capeta MZUSP 29154;(E) Pc.daemon MZUSP 95597;(F)Pc.irritans INPA 20529; (G) Pc.malevola MCP 36217;(H) Pc.parva MZUSP 30400;(I) Pc.saci MZUSP 125626; (J) Pc.satanica MZUSP 100149;(K) Pc.truculenta MZUSP 30404;(L) Pc.vampyra MZUSP 100138. Scale bars = 500 μm. Pc.carrapata not shown.

Figure 33. Paracanthopomasaci,holotype,MZUSP125624,19.6 mmSL.Brazil,Mato GrossodoSul,Alcinópolis,RioTaquarizinho.(A) Lateralview of body;(B)Dorsal view of head;(C) ventral view of head.

Figure 34. Paracanthopoma saci, holotype, MZUSP 125624, CT scan images of head skeleton, (A) Lateral; (B) Dorsal; (C) Ventral. Specimen poorly calcified, some structures not properly shown.

Figure 35. Paracanthopoma saci,paratypes,MZUSP 125626,specimens with filamentous pectoral fins,14.5 and 15.0 mm SL (seetext,Remarks on the species).

Figure 45. Map of northern South America showing geographical distribution of Paracanthopoma saci (square); Pc. satanica (5-tip star); Pc.truculenta (dot); and Pc. vampyra (10-tip star). Open symbols represent type localities. Some symbols may represent more than a single locality or lot of specimens.

Tables

Table10.Morphometric data of Paracanthopoma saci.Ranges,mean and SD include holotype. Head subunits were obtained with an ocular micrometer and therefore as projections.Abbreviations:min = minimum value;max = maximum value;n = number of specimens;SD = standard deviation.

	n	holotype	min	max	mean	SD
Standard length (mm)	8	19.1	15.4	21.1	18.1
Percentages of SL
Total length	8	1.1	1.1	1.1	1.1	0.0
Body depth	8	13.1	11.7	15.7	13.9	1.5
Caudal peduncle length	8	25.1	24.0	26.6	25.0	1.0
Caudal peduncle depth	8	9.9	8.9	11.3	9.8	0.8
Predorsal length	8	68.6	66.5	69.2	68.1	0.9
Preanal length	8	67.5	65.5	68.8	67.2	1.1
Prepelvic length	8	60.2	56.0	61.1	59.4	1.5
Dorsal-fin base length	8	9.4	6.8	10.3	8.6	1.2
Anal-fin base length	8	11.0	7.6	11.0	9.0	1.1
Pectoral-fin length	8	11.5	9.7	12.3	10.8	0.8
Head length	8	15.7	15.1	16.1	15.6	0.4
Percentages of HL
Head width	8	83.6	83.6	100.0	89.1	5.1
Head depth	8	41.8	41.8	54.6	50.3	4.0
Pectoral-fin length	8	73.3	61.5	78.8	69.4	5.3
Interorbital	8	13.6	12.6	15.5	13.8	1.0
Eye diameter	8	12.7	11.3	13.6	12.8	0.7
Snout length	8	36.4	35.8	39.8	37.2	1.4
Mouth width	8	23.6	14.6	28.6	24.5	4.3
Anterior internarial width	8	15.5	14.6	21.8	17.2	2.5
Posterior internarial width	8	10.9	9.7	13.6	11.0	1.2