Pygidianops amphioxus, de Pinna & Kirovsky, 2011

de Pinna, Mário C. C. & Kirovsky, Alexandre L., 2011, A new species of sand-dwelling catfish, with a phylogenetic diagnosis of Pygidianops Myers (Siluriformes: Trichomycteridae: Glanapteryginae), Neotropical Ichthyology 9 (3), pp. 493-504 : 494-499

Pygidianops amphioxus , new species

Figs. 1-4

Pygidianops n. sp.; Mendonça et al., 2005: 759 [community ecology].

Pygidianops sp. ; Schaefer et al., 2005: 5 [comparative material examined]; Soares-Carvalho, 2010 [natural history].

Holotype. INPA 34661 , 25.7 mm SL, female, Brazil, Amazonas , Manaus , Reserva Adolfo Ducke , igarapé do Acará , tributary to rio Tarumã, itself tributary to rio Negro, 03°08’S 60°02’W, 10 Nov 1996, A. Kirovsky & M. de Pinna.

Paratypes. All from Brazil . INPA 12427 , 4 ex (1 male, 3 females), 23.3-24.8 mm SL (3, 23.3-24.8 mm SL), collected with holotype ; INPA 16556 , 2 ex (1 female, 1 sex indeterminate), 20.1-27.8 mm SL, same locality as holotype , 25 Nov 2000, F. Mendonça & M. de Pinna; MZUSP 86951 , 14 ex (8 males, 6 females), 20.2-28.7 mm SL, Amazonas , rio Preto da Eva , igarapé Sucuriju (trib. to rio Preto da Eva), 14 Aug 2004, M. de Pinna, L. H. Rapp Py-Daniel & L. Sousa ; MZUSP 87676 , 21 ex (2 c&s, 10 males, 11 females), 23.3- 30.4 mm SL (8, 24.0- 30.4 mm SL), Amazonas , rio Preto da Eva , igarapé Sucuriju , next to Bom Jesus farm, at km 13 of Ramal Francisca Mendes, 2º45’15.8”S 59º37’29.6”W, 04 Jul 2003, O. Oyakawa et al .; MZUSP 105878 , 6 ex (all males), 24.2-25.8 mm SL, Amazonas , Manaus , igarapé at Recanto do Chupisco, at side road leaving at km 36 of road Manaus-rio Preto da Eva, 2º49’48.7”S 59º58’11.2”W, 09 Jul 2003 , MZUSP team; MZUSP 105879 , 3 ex (1 male, 1 female, 1 sex indeterminate), 17.7-25.9 mm SL, same data as INPA 16556; MZUSP 105880 , 6 ex (2 c&s, 2 males, 3 females, 1 mutilated), 22.0- 26.5 mm SL (3, 22.0- 26.5 mm SL), collected with holotype .

Non-type specimens. INPA 31552 , 7 ex (1 c&s), 23-25 mm SL, Brazil, Amazonas, Santa Isabel do rio Negro , rio Daraá , rio Negro drainage, cachoeira Panã-Panã , 0°22’15.2”S 64º47’37.6”W, M. S. Rocha & V. L. Masson ; MZUSP 104675 , 7 ex (3 males, 4 females), 23.7-31.8 mm SL, Pará, Terra Santa, igarapé Jamari (trib. to rio Nhamundá ), near Terra Santa , 2º00’03.5”S 56º31’05.1”W, 23 Sep 2009. H. A. Lazzarotto , R. P. Leitão & R. Frederico .

Diagnosis. Distinguished from the three other known species in Pygidianops by the caudal fin continuous with the remainder of the body profile (vs. caudal fin well differentiated from the body); the elongate body (HL 10-12% SL, vs. 16-18%); the relatively even depth of body (vs. body markedly less deep close to caudal fin); the short barbels, approximately half as long as HL (vs. approximately as long as, or longer than, HL); the presence 58 or 59 vertebrae (vs. 35-41 in P. magoi , 41 in P. cuao , 37-41 in P. eigenmanni ); the lack of a lacrimal bone (infraorbital one); the presence of a single continuous ligament connecting the dorsal processes of the premaxillae; and the deep shape of the opercle, resulting from a hypertrophied ascending process. The latter two characteristics are putative autapomorphies for the species. The lack of an anal fin; the presence of a single pair of pleural ribs, the strongly dorsallydeflected tip of the mesethmoid and the lack of mesethmoid cornua further distinguishes P. amphioxus from all congeners except P. magoi . The presence of eyes separates P. amphioxus from all congeners except P. cuao .

Description. General aspect of new species shown in Figs. 1 and 2 and morphometric data presented in Table 1. Body elongate, round in cross section close to head and progressively more compressed towards caudal fin. Body in lateral view constant in depth along its whole length. Conspicuous fin-fold extending along nearly whole of dorsum, from occiput anteriorly to caudal fin posteriorly. Dorsal fin-fold deepest along caudal peduncle, fusing with caudal fin. Posterior half or third of dorsal fin-fold with small fatty bodies alongside base. Gravid females with tumescent posterior part of abdomen, with large eggs seen by transparency. Mature males often with testes seen by transparency as white opaque cloud inside posterior portion of abdomen. Ventral part of body with paired metapleural keels extending longitudinally along ventrolateral edge of trunk, from vertical through origin of pectoral fins to anterior third or fourth of caudal peduncle, posterior to anal and urogenital openings. Ventral margin of caudal peduncle with ventral finfold along its posterior two-thirds or three-fourths, with fatty bodies along base similar to those in dorsal fin-fold. Large conical urogenital papilla present in males ( Fig. 5) and absent or very small in females. Head elongate, widest at snout ( Figs. 2 and 3). Maxillary, rictal and nasal barbels short and thick, with blunt round tips and similar lengths. Internal core transparent but clearly visible in all three pairs of barbels. Maxillary barbels deflected strongly anteriorly in preserved and live specimens. Nasal barbels located closer to eyes than to anterior margin of snout. Proximal portion of rictal barbels directed straight posteriorly, but bending ventrolaterally in many preserved specimens. Nostrils minute, nearly invisible in alcohol-preserved specimens but evident in cleared and stained and SEM preparations ( Figs. 3a and 6). Both nostrils in form of simple orifices, lacking occlusion valves. Anterior nostril located slightly anterior to base of internal core of nasal barbel. Posterior nostrils slightly closer to anterior margin of eye than to anterior ones. Posterior nostrils slightly closer together than anterior nostrils. Anterior and posterior nostrils united by sinusoidal ridge of integument, forming small lobe anterior to posterior nostril. Branchiostegal membranes attached to isthmus only anteriorly, their margin forming nearly continuous semicircular arc when expanded (v-shaped when collapsed), with small semicircular flap (often not visible due to shrinkage or folding) anteromesially to base of pectoral fin in ventral view ( Fig. 3b), covering corresponding recess of isthmus. Branchial openings wide, each one extending from base of pectoral fin nearly to ventral midline. Profile of branchial membrane semicircular when completely expanded ( Fig. 3b), but often collapsed into inverted v-shape (as in holotype; see Fig. 2a) according to position of branchial basket at preservation. Eyes located at middle of HL, with well-formed lens directed antero-laterally. One paratype with eye on one side partly degenerate, or malformed. Opercular and interopercular odontodes absent. Five latero-sensory pores present on side of head ( Fig. 4, top), corresponding to (anterior to posterior) infraorbital pore, preopercular canal pore, pore 1 of lateral line, pore 2 of lateral line, and terminal pore of lateral line. Short lateral line following path between two posterior pores, oriented at approximately 45 degree angle relative to body axis. Pore of axillary gland absent.

Pectoral fin short and narrow (more so towards tip), slightly curved posteriorly, resembling short barbel in external aspect. Single unsegmented and unbranched pectoral-fin ray. Anal, dorsal, pelvic and adipose fins absent. Caudal fin very small and round, continuous with caudal peduncle, with 7+6, [6+6], 6+7 or 5+6 poorly calcified rays, all unsegmented and unbranched. Only two or three procurrent caudal-fin rays dorsally and ventrally. Fold-like expansions of caudal peduncle without procurrent rays for most of their lengths. Post-Weberian vertebrae 58 (n=1) or 59 (n=4), all with welldeveloped neural spines and all but two anterior ones also with hemal spines (in one specimen, last free vertebra has neither spine). Single pair of pleural ribs, directed posteriorly, on first free centrum. Five branchiostegal rays.

Coloration. Dark pigmentation very reduced in preserved fish. Scattered dark chromatophores along whole of dorsum, forming two faint dorsal longitudinal stripes alongside dorsal first such case in Trichomycteridae .A genital papilla has been recorded in other species of Pygidianops and Typhlobelus (cf. Schaefer et al., 2005) and it is possible that the papilla, and its putatively associated insemination, is synapomorphic for the two genera. Hypertrophied genital papillae are not present in species of Glanapteryx and Listrura , which have either small partly concealed papillae or no papilla at all. Females of P. amphioxus reach larger sizes than males. In all samples available, largest specimens are always females, which also tend to be more heavily built when mature.

head of Pygidianops amphioxus, MZUSP 87676. Scale bars = 100  m.

fin-fold. Small faint field of melanophores dorsal to pectoral fin. Some mature females with few scattered dark fields deep inside abdominal cavity, around eggs. Head with dark brain pigment seen by transparency, forming web-like pattern. Eyes very dark, sharply visible in external aspect. Remainder of fish white, all other color differences resulting from muscle limits and different underlying tissues seen by transparency. In life the fish is light pink with a faint iridescence under direct light. Although not transparent, its general color makes it almost invisible against the sand of its natural habitat. At close range the dark eyes are evident.

Sexual dimorphism. A very large conical genital papilla is present in several specimens of P. amphioxus ( Fig. 5) and examination of gonads has shown that all such specimens are male. The presence or absence of a hypertrophied genital papilla can be determined even in the smallest specimens examined (17.7 mm SL), as long as the urogenital region is well preserved. It is a reliable non-invasive method for sexing of specimens. Presence of a sexually-dimorphic papilla is often associated with insemination in fishes. In characiforms, where the phenomenon has been investigated in recent years, all known taxa bearing a urogenital papilla are inseminating (N. Menezes, pers. comm.; see examples in Burns & Weitzman, 2006 and Menezes et al., 2009). This is indication that P. amphioxus is inseminating and, if confirmed, this will be the Distribution. Pygidianops amphioxus has the broadest reported distribution of any glanapterygine species yet known, spanning over 900 km in straight line ( Fig. 7). Its westernmost record is the rio Daraá, tributary of the upper rio Negro, and its eastern limit is an affluent to the rio Nhamundá, a northern Amazonian tributary just west of the rio Trombetas. Within the Ducke Preserve, the species occurs in igarapé do Acará and igarapé Tinga ( Soares-Carvalho, 2010), which belong to two distinct systems. The igarapé do Acará opens on the rio Tarumã, which is a tributary to the lower rio Negro; while the igarapé Tinga is part of the rio Puraquequara which opens directly into the rio Amazonas. In all cases, P. amphioxus is restricted to lowland terra firme rivers on the northern Amazonian versant. This suggests some association with the Guiana Shield, a link further highlighted by the close relationship of P. amphioxus to P. magoi , from the río Orinoco (see Discussion below). However, at this time it is premature to speculate further on the significance of that association. It is likely that current distributional data are still incomplete and that the species will be found in many other localities as suitable microhabitats are more thoroughly sampled.

Ecological notes. The type locality of P. amphioxus is the place where the species has been most closely observed by the authors. The igarapé do Acará, is a stream located inside the forest preserve Adolfo Ducke, a 10,000 ha square of protected primary forest north of the city of Manaus (between 02º55’ 03º01’S and 59º53’ 59º59’W). The preserve is covered by mostly undisturbed “terra firme” tropical rainforest, growing over sediments from the Barreiras group ( Chauvel et al., 1987). Until ca. 10 years ago, the preserve was largely continuous with surrounding forest, but today it is almost entirely isolated by urban growth of Manaus. The igarapé do Acará is a tributary of the rio Tarumã, itself tributary to the rio Negro. The waters of the igarapé do Acará are clear to slightly tea-stained, quite transparent (4 m visibility), highly acidic (pH 3.4-3.8) and poorly conductive (12.7 Us 20 /cm). The course of the igarapé is densely shadowed by overlying forest (12.17% vertical solar light penetration), its water is moderately cool (24 o C) and highly oxigenated (6.4 mg /l). The stream-bed is mostly sandy (average granulation 0.8 mm), interrupted by rocky outcrops which form small waterfalls and rapids at many sites. Leaf litter and debris accumulate only at few spots. The average width of the stream is 4.5 m, and the average depth is 70 cm. A few pools may be up to 2.5 m deep. During rains, the specimen of Pygidianops amphioxus, MZUSP 87676. Anterior to left. Scale bar = 20  m.

water may raise up to 1 m beyond its normal level, but only for a short period of time (less than 24 hours). The current speed is modally 1 m /4 sec, but varies markedly according to sector, from very slow pools to fast running rapids. Pygidianops amphioxus is the only trichomycterid yet found in the igarapé do Acará, although at least 36 other fish species occur therein, three of which ( Gymnorhamphichthys rondoni , Imparfinis pristos and Characidium pteroides ) in the same microhabitat as the new species.

Fewer ecological data are available for the rio Preto da Eva locality, which is a site heavily impacted by human activity. The stream in the sector visited is less rocky and the water murkier than in the Ducke Preserve, while the riparian vegetation is mostly shrubby, rather than dense forest. The two latter factors seem to be due to deforestation and the presence of a dirt road crossing the stream. However, the microhabitat favored by P. amphioxus seems to be similar to that in the Ducke preserve, with obvious preference for the few shaded sectors of the stream. At rio Preto da Eva, Stauroglanis gouldingi , an undescribed species of Paracanthopoma and Ituglanis sp. are other trichomycterids co-occurring with P. amphioxus .

In every locality, P. amphioxus is an interstitial inhabitant of the sandy sectors of streams. It was collected in the superficial 20 cm of sand, in fast flowing portions, 30-60 cm in depth. The distribution of individuals seems to be spotty, with a preference for submerged sand banks immediately below small waterfalls or at steep turns of the stream course. Collecting was done by scooping large amounts of sand in a fine-mesh seine, and then looking for trapped fish. The choice of sites to be seined was done either randomly in suitable microhabitats, or by aiming at spots where fish were seen (by underwater observation) to bury after being dislodged by disturbing the sand. The fish moves extremely fast amidst submerged sand, but becomes immobilized as soon as the water drains through the mesh and the sand grains amalgamate. When hand-freed from the sand, small specimens move slowly in undulating of nasal barbel of Pygidianops amphioxus, MZUSP 87676. Anterior to left. Scale bar = 30  m.

movements over the drained substrate, resembling large aquatic nematodes. Larger specimens tend to wriggle frantically in more normal catfish manner. Despite their delicate aspect, the fish is actually quite tough to the touch.

Underwater observation of live P. amphioxus in its natural environment occurred only during careful disturbance of its natural microhabitats, by slowly running fingers through the superficial layer of sand. And even so, only for a fraction of a second before it darted again into the sand. Despite six hours of underwater observation by both authors, the fish has never been observed otherwise, either during the day or night. It seems to us that P. amphioxus never leaves the sand in normal conditions.

Pygidianops amphioxus is different from most sanddwelling fishes, which bury in the sand by making jerky body movements and then stay close to the surface or with part of their head emerging from the sand. The new species, contrastingly, seems to hide deep in the sand, and to do so in a remarkably effortless manner. To an underwater observer, the fish looks like it dives, rather than buries, into the sand.

Before preservation, the captured fish were placed in a water-filled plastic bag, where they went straight to the bottom to hide amongst the few available sand grains. In the absence of sand, the fish seems to be unable to maintain proper resting balance, and lies on its side. If disturbed in those circumstances, it darts in convulsive eel-like movements for a few moments before dropping again to the bottom.

Pygidianops amphioxus probably feeds upon the tiny interstitial invertebrate fauna of its microhabitat. Gut contents contain many small sand grains, along with unidentified arthropod remains and other finely-macerated organic debris. Sand grains in the gut (in this case considerably larger) are also visible in the cleared and stained specimen examined of P. eigenmanni . It seems like in the process of capturing its tiny prey, species of Pygidianops also swallow some of the surrounding sand. They possibly also take nutrition from the organic layer surrounding ingested sand grains (biofilm). In either case, it seems that P. amphioxus feeds while buried in the sand, which further supports the view that those fishes never leave the sand as part of their normal activities.

The material available includes several mature females with large eggs seen by transparency, indicating that the specimens studied are adult individuals. The eggs in each ovary are disposed roughly in single file, but the two rows of eggs are arranged in a partly imbricate manner along the abdominal cavity. This arrangement seems to be related to the minute size of the fish and its elongate shape, which result in eggs that are large relative to the internal space in the abdominal cavity. Similar space-saving strategies are common in ovaries of various unrelated paedomorphic taxa, such as some phengodid beetles ( Costa et al., 1999). No small juveniles were caught, possibly because they are so small as to escape the collecting gear employed. Soares-Carvalho (2010) concluded that P. amphioxus reproduces throughout the seasonal cycle, with an increased rate in the dry season. The sex ratio in the species is approximately 50/50.

Etymology. The specific name, a noun in apposition, refers to the cephalochordate amphioxus (a common name that applies to Recent cephalochordates in general, now mostly included in the genus Branchiostoma ), in allusion to obvious similarities in body shape and superficial aspect.

Remarks. Little morphological divergence exists among the four populations known of P. amphioxus . Specimens from the rio Daraá, on average, are slightly darker along the dorsum than those of other populations. Individuals from the rio Preto da Eva have the articular process of the palatine somewhat more produced than those from the Ducke Preserve (specimens of other populations have not been examined for osteology). Such differences are small and fit expected variation normally observed within single species of glanapterygines. Specimens from the igarapé Jamari and rio Daraá have not been designated as types because of their outlying localities and relatively limited study material at this time.