Sorubim Cuvier, 1829

Michael W. Littmann, 2007, Systematic review of the neotropical shovelnose catfish genus Sorubim Cuvier (Siluriformes: Pimelodidae)., Zootaxa 1422, pp. 1-29 : 2-8

Genus Sorubim Cuvier, 1829

Platystoma Spix and Agassiz, 1829 : 23. Type species: Silurus lima Bloch and Schneider, 1801 . Type by subsequent designation by Jordan (1917: 131). Gender: neuter. Preoccupied by Platystoma Meigen, 1803 in Diptera.

Sorubim Cuvier, 1829 : 293. Type Species: Silurus lima Bloch and Schneider, 1801 . Type by subsequent designation. Gender: masculine.

Sorubim Spix and Agassiz, 1829 : 24. Type Species: Silurus lima Bloch and Schneider, 1801 . Type by subsequent designation by Bleeker (1862). Gender: masculine. First appeared in synonomy in Spix and Agassiz as above, but made available back to that authorship and date because of use as a valid name by Bleeker (1862: 10).

Abron Gistel, 1848 : X. Type species: Silurus lima Bloch and Schneider, 1801 . Type by being a replacement name. Gender: neuter. Replacement for Platystoma Valenciennes [Spix and Agassiz, 1829].

Diagnosis: A genus of catfishes distinguished from other genera of Pimelodidae by a combination of the following characters: (1) very depressed head; (2) long upper jaw greatly projecting over lower jaw; (3) premaxillary tooth patch covered by minute, villiform teeth and exposed ventrally; (4) eyes positioned laterally, usually visible from below; (5) black lateral stripe of highly variable width from snout to distal tip of median rays on caudal fin.

Description: Dorsal fin formula II6, the first spine a spinelet, the second pungent and serrated posteriorly, followed by six branched rays; anal fin with 18-25 total rays (17-20 pterygiophores), usually 14-18 branched, the two most posterior rays often joined at base represented by one anal pterygiophore; adipose fin present, triangular in shape, posterior edge usually rounded, position of origin even with or slightly posterior to anal fin origin; pectoral fin with single, pungent spine, serrated on the distal end anteriorly, throughout posteriorly, supported by 7-10 branched rays; pelvic fin supported by one unbranched and five branched rays, its origin at mid-body, well behind insertion of dorsal fin; caudal fin supported by 16-21 principal branched rays, upper lobe always with eight, lower lobe variable with 8-13 usually larger than upper-lobe; caudal fin forked, lower lobe with slightly rounded edges or deeply forked and pointed. Thread-like filaments often present on second dorsal spine or first branched fin-ray, pectoral spine, first pelvic ray, and upper- and lower-most principal caudal rays. Filaments are longest on middle caudal rays of both inferior and superior lobes. Juveniles often exhibit extremely long filaments, often longer than length of body; this trait is common among several pimelodid species (i.e., Brachyplatystoma spp., Platysilurus , Platystomatichthys ). Vertebrae 50-58, gill rakers 13-37, and brachiostegal rays 12-15.

Members of the genus Sorubim are elongate, medium to large-sized catfishes (to ~800 mm SL). Body shape ranges from thin, elongate and round in cross-section, to stout and slightly compressed laterally, body tapering from its widest point near cleithral process to caudal peduncle. Dorsal fin situated about mid-way between tip of snout and adipose fin origin. Lateral-line complete and stitched in appearance, slightly arched above pectoral fin, anterior section ossified and covered by ossicles; ossicles large and thin, small and ovate, or joined to form a single, continuous and smooth plate. Ossicles located at distal points of lateral-line pores, often enlarged in many adults forming plates of variable size and shape. Head densely ossified, length greater than three times its width; bones of skull roof ornamented with small pit-like striations radiating from center, covered by thin layer of skin (preservation may leave skull bones visible through skin); skull dorsoventrally flattened; anterior fontanelle long and wide, between frontal bones; posterior fontanelle small and tear-drop shaped, sometimes extending posteriorly as thin groove on supraoccipital bone. Neurocranium and nape scattered with dendritic arrangement of ampullary organs (see Gelinek 1978).

One pair of maxillary barbels, two pairs of mental barbels; fleshy, oval or flat in cross-section. Maxillary barbel length variable; in some species reaching only to pectoral insertion, in others to pelvic fin origin and beyond. Outer mental barbels extend just beyond pectoral origin; inner mental barbels do not extend posterior to bony operculum.

Arrangement of teeth on palate forming four patches, two on vomer, one elongated patch on each metapterygoid(fig. 4, Littmann et al. 2000). All tooth patches have numerous minute and villiform teeth.

Pigmentation: Pigmentation intraspecifically variable. In alcohol, dorsal surface dark to light brown or gray; ventral surface completely white or cream-colored; black lateral stripe extends from tip of snout to inferior edge of lower caudal lobe. Lateral stripe variable in width, occasionally running through entire width of eye, other times barely touching its dorsal edge; not extending below lateral line. Dark dorsal surface and lateral stripe sometimes separated by light lateral band, usually white or gray, sometimes light brown. Dorsal pigment may shift color shade quickly in conjunction with background change (personal observation).

Fins hyaline with pigment confined to rays on live and freshly preserved specimens; posterior rays of anal fin often speckled with chromatophores. Preserved specimens often with lemon-yellow pigment at bases of pelvic, anal, and caudal fins; sometimes chromatophores present on all fin rays. Juvenile fins generally exhibit more pigment than adults. Color pattern distinct from other genera of Pimelodidae (no other pimelodid species possesses horizontal stripe stretching entire length of head and body). Bloch and Schneider (1801) described paired and anal fins as being red or rose-colored (this may describe hemorrhaging of blood vessels caused by handling stress). I have not observed red pigment on any specimens in the field or laboratory. No sexual dimorphism in any species of Sorubim has been identified or reported. However, median lengths observed at first sexual maturity for S. cuspicaudus in the Magdalena River basin were 533 mm and 425 mm for females and males, respectively, indicating the possibility of sexual size differences (Escobar et al. 1983).

Etymology: The name Sorubim probably originates from a native language. In a brief discussion of subfamilial relationships among the Pimelodidae, Silfvergrip (1996) mentioned that Swainson (1838) considered Spix and Agassiz's (1829) use of Sorubim to be a misspelling of Sorubium . Swainson gives no explanation for this comment, but Silfvergrip and others note that the common vernacular name for many large pimelodids in South America is Sorubí, a common usage for related shovelnose catfishes (i.e., Pseudoplatystoma spp.). Likewise, Robins et al. (1991) used the name Sorubim as a common name applied to several large pimelodids in the genus Pseudoplatystoma ( Pseudoplatystoma tigrinum = Tiger Sorubim, P. fasciatum = Barred Sorubim, P. corruscans = Spotted Sorubim).

Distribution: Known to occur in the Amazon, Cauca, Essequibo, Lake Maracaibo, Magdalena, Orinoco, Paraná, Sinu basins and in at least one Atlantic Ocean drainage in northeastern Brazil, the rio Parnaíba.

Remarks: The tremendously rich ichthyofaunas in river systems of tropical South America present formidable opportunities to ecologists (Ibarra & Stewart 1989). Little is known about the reproductive biology and natural history of species of Sorubim . The turbid white-water rivers inhabited by Sorubim make direct observation of these fishes nearly impossible. Most natural history information on Sorubim stems from observations of captive fish in aquaria.

Burgess (1989) reported one instance of two adult Sorubim excavating a small pit for a nest. Both fish were guarding freshly hatched young, although none survived. The curious behavior of vertical posturing amidst the elongate stems of submerged grass and reeds has been observed in aquaria by Reid (1986) and Burgess(1989) who interpreted this as a cryptic behavior used to hide from predators and/or to stalk prey utilizing a lie-and-wait strategy. Sorubim catfishes eat crustaceans and other invertebrates as well as fish (Ringuelet et al. 1967; Reid 1986; Burgess 1989; pers. obs.). Adult Sorubim species maintain a protein-rich, piscivorous diet as do many of their pimelodid relatives.

Near Iquitos, Peru, I observed juvenile Sorubim among branches, sticks, root masses and reed-like grasses in small caños, tributary streams, and backwaters of large river channels. The darker fin pigmentation and longer fin-ray filaments of juveniles may provide camouflage while hiding in grasses and leaf detritus. Larger adults living in more open-water habitats lose these cryptic features (Reid 1986). I have collected Sorubim mainly in turbid small streams and large rivers with aquatic vegetation and little or no flow, but only occasionally in clearer black water or over a strictly sand or mud-bottomed substrate. Bag and straight seines are best for collecting juveniles or sub-adults less than 150 mm SL, whereas gill nets and cast-nets work well for larger adults.

Taxonomic history:

Sorubim is at once recognized by its distinctive shovel-like projecting upper jaw, depressed snout, laterally placed eyes and black horizontal stripe extending along body from snout tip to posterior margin of caudal fin (Bloch & Schneider 1801; Cuvier 1829; Spix & Agassiz 1829; Bleeker 1862-1863; Günther 1864; Weyenbergh 1877; Eigenmann & Eigenmann 1890; Miranda-Ribeiro 1920; Ringuelet et al. 1967; Burgess 1989). Several osteological characters demonstrate the monophyly of the genus. A detailed analysis of these characters will be presented elsewhere.

The taxonomic history of Sorubim is somewhat convoluted (see Littmann et al. 2000, Table 1). Silurus lima was described by Bloch and Schneider (1801) based on a specimen from Brazil, and later became the type species of Sorubim by monotypy. In 1829, Agassiz and Cuvier each used the generic name Sorubim almost simultaneously. Authorship of Sorubim was long attributed to Agassiz, in Spix and Agassiz (1829), but Kottelat (1988) and Whitehead and Myers (1971) identified Cuvier (1829) as the first author of the name. According to Kottelat (1988) and Whitehead and Myers (1971), Cuvier's manuscript was in print by March 1829; the print date for Spix and Agassiz's "Brazilian fishes" could not have been earlier than May 1829. Thus, Cuvier's description has two months priority over Agassiz's (Boeseman 1962). For a detailed discussion on the dates, authorships, and status of Spix and Agassiz's Brazilian fishes including Sorubim and other South American genera, see Boeseman (1962), Whitehead and Myers (1971), and Kottelat (1988).

In Spix and Agassiz's (1829) well-known “Brazilian fishes,” Agassiz used the name Platystoma to describe several species now placed in three different genera ( Pseudoplatystoma , Sorubim , and Sorubimichthys ). Eigenmann and Eigenmann (1890) found that Platystoma Agassiz 1829 was preoccupied by Platystoma Meigen 1803 , a genus of insects. Most of the species listed in combination with Sorubim were published as junior synonyms (i.e., S. carapary as a synonym of Platystoma corruscans ; S. infraoculare as a synonym of Platystoma lima ; S. jandia as a synonym of Platystoma spatula ; and S. pirauaca as a synonym of P. planiceps ). Interestingly, the names originally read in combination with Sorubim were provided by Spix, who was responsible for drawing the plates; thus, Sorubim appears with several specific epithets on Spix's figure plates thought to represent the same fishes described in Agassiz's text. The only Agassiz name appearing with a plate and not published as a synonym is S. truncatum , now recognized as a synonym of Pseudoplatystoma tigrinum (see Lundberg & Littmann 2003).

The genus Sorubim , as currently delimited, contains five valid species; S. lima (Bloch & Schneider 1801) , S. trigonocephalus Miranda-Ribeiro 1920 , S. cuspicaudus Littmann, Burr and Nass 2000 , S. elongatus Littmann, Burr, Schmidt and Isern 2001 , and S. maniradii Littmann, Buitrago and Burr 2001 . The current status of each specific epithet published in combination with Sorubim is listed in Lundberg and Littmann (2003).

Key to species of the pimelodid genus Sorubim

1 Gill rakers 31-37 (Fig. 1B)......................................................................................................... S. maniradii

- Gill rakers 13-23 (Fig.1A, 1C)....................................................................................................................2

2 Pectoral fin-rays 8; gill rakers on first arch modally 21; tooth patches on vomer not connected (Littmann et al. 2001b, fig. 3b); insertion point of inner mental barbels even with or posterior to gular apex; head and body elongate, body ovate in cross-section; eye diameter into interorbital distance 2-3 times; gape width into head length more than 3.6 times.......................................................................................... S. elongatus

- Pectoral fin-rays 9, rarely 8 or 10; gill rakers on first arch 18 or fewer; tooth patches on vomer fused (Littmann et al. 2000, figs. 4a and 4c,); insertion point of inner mental barbels anterior to gular apex; head and body stout, body moderately compressed laterally; eye diameter into interorbital distance 3 times or more; gape width into head length approximately 3..............................................................................................3

3 Caudal fin with rounded lobes, distal caudal rays on upper and lower lobes curved (Fig. 2A); distributed east of the Andes Cordillera........................................................................................................................4

- Caudal fin deeply forked and elongate, distal caudal rays on upper and lower lobes nearly straight and pointed (Fig. 2B); restricted to the Lake Maracaibo, Sinu and Magdalena River basins....... S. cuspicaudus

4 Premaxillary tooth patch greatly exposed, exposed tooth patch length about equal to width (Fig. 3C); head triangular and spear-shaped (Fig. 3B).............................................................................. S. trigonocephalus

- Premaxillary tooth patch moderately exposed, exposed tooth patch length contained 1.5 to 2.5 times in width (Fig. 4); head oblong, not triangular......................................................................................... S. lima