Sewellia Hora, 1932

Sewellia Hora, 1932 View in CoL

The genus Sewellia is diagnosed by a series of features apparently unique: the pelvic valve, the fan-shaped rostral barbels, and the duct between the fossa behind anterior rostral barbel and the oral cavity.

In Sewellia , the two pairs of rostral barbels appear as a kind of plate or flap, compressed, with a posterior fringe of 4–7 digitations, each ending in a conical papilla; additional papillae may be present on the barbel and small tubercles at the base ( Fig. 1 View Fig ). The anterior barbel posteriorly touches the upper lip, or is fused at the base of the upper lip. The posterior barbel has the lateral part of the flap free, and the medial part is fused with the upper lip. There is one fossa behind each barbel. The posterior extremity of the anterior fossa is continued by a duct, whose entrance is apparently closed by a sphincter; when the duct is open, a needle inserted in the duct reaches without obstacle until about the level of the base of the anterior ray of the pectoral fin ( Fig. 1 View Fig ). In some species (e.g., S. lineolata ) the anterior barbel apparently covers the whole anterior fossa and possibly functions as a valve. All these structures are already present in the smallest examined specimens of S. lineolata (CMK 16676, 27.9 mm SL), S. diardi (CMK 22308, 19.6 mm SL), S. pudens (CMK 22380, 20.9 mm SL) and S. speciosa (CMK 22279, 17.2 mm SL). But in S. elongata , the largest of the available species, in the smallest specimens (e.g., CMK 15529, 15.1 mm SL; Fig. 2a View Fig ), the rostral barbels are still cylindrical, but they already connect the rostral cap and the upper lip. The outer rostral barbel starts to have the diagnostic flat and digitate shape around 20 mm SL ( Fig. 2b View Fig ). Noteworthy is also that in small juveniles (up to about 23 mm SL), the gill opening extends downwards until shortly above the anterior pectoral-fin rays ( Fig. 2c View Fig ).

The upper lip is thin, smooth or covered by tiny papillae. The upper jaw is covered by a sheath, itself covered by unculi (K. W. Conway, pers. comm.). The lower lip is missing. Behind the middle of the lower jaw, is a thick ‘mental pad’ with or without papillae; this has sometimes been called lower lip in Balitoridae and Gastromyzontidae ; I retain ‘mental pad’, following Roberts (1982a: 515, fig. 15). I had called ‘mental lobe’ a possibly homologous structure in Barbuccidae ( Kottelat, 2025) .

In two species ( S. pterolineata , S. marmorata ) there is a post-oral pouch behind the mouth (Fig. 3). The mouth is located in a depression, margined posteriorly by a postoral pouch. This is a skin fold extending across the ventral surface of the head, behind the mouth, covering the posterior part of the cavity or even the mandibular barbel and part of the posterior fossa. This leaves a pouch opening forward and extending backwards until about the level of the basis of pectoral-fin rays 1 to 3. The duct at the posterior extremity of the anterior fossa extends at least to the posterior margin of the pouch. The ventral surface of the rostral cap is covered by large tubercles.

Hora (1932: 315, 327) already hypothesised that the fossa could be connected with the mouth cavity. When adpressed on a flat structure, the pectoral and pelvic fins, the oro-pectoral fold, the posterior rostral barbel, and the upper lip form a tight sucker, opening posteriorly through the pelvic valve ( Roberts, 1998: 273) and enclosing the mouth. It might be speculated that the anterior fossa and the duct allow the passage of waters to the gills when the sucker is closed. A video ( Conway, 2022) shows the high frequency of mouth, gill opening, and pectoral fin movements.

Roberts (1998: 272) mentions an ‘oro-pectoral membrane’ and an ‘oro-pectoral fold’ but he gives no explicit definition. He explains that “the lateral oral fold extends from the rostral cap to the pectoral fin origin”. I understand this to mean the hard, thickened structure ( Fig. 1 View Fig ) that extends from the basis of the outer rostral barbel to the origin of the pectoral fin, which may appear as folded (with a groove for part or all its length), not visible in smallest specimens, becoming gradually more distinct with increasing size. I have more difficulties to understand what is the oropectoral membrane, which Roberts described as: “The head may appear either oblong (almost rectangular) or broadly ovate. This depends on whether the oropectoral membrane is folded in or folded out” and “[... the oropectoral fold extends ...] from the corner of the mouth (or rictus of the jaws) to the pectoral fin origin”. The oropectoral membrane is mentioned in none of the species’ descriptions. I do not understand what membrane this could be.

Freyhof & Serov (2000: 218) also mention the oro-pectoral membrane and refer to the definition of Roberts (1998).

However, they mention “no skin-fold between oropectoral membrane and ventral surface of head” (p. 221, for S.

lineolata; p. 225, for S. marmorata ; p. 229, for S. pterolineata ;

p. 230, for S. speciosa ), and “skin fold between oropectoral membrane and ventral surface of head running from pectoral-fin origin to second rostral barbel” (p. 229, for S.

diardi; p. 230, for S. elongata ; p. 231, for S.

breviventralis; p. 235, for S. patella ). I retain the wording

‘oro-pectoral fold’ for the hard structure continuing the rostral cap from the base of the posterior rostral barbel to the basis of the anterior pectoral-fin ray. The ‘oropectoral fold’ is also present in S. hypsicrateae and S. Fig. 3. Sewellia pterolineata, CMK 16331, males; mouth, ventral pudens . view. a, 32.8 mm SL; b, 26.7 mm SL. Dotted line shows posterior extent of post-oral pouch. fa, anterior fossa; fp, posterior fossa; Roberts (1998: 273) called the structure made of the posterior mb, mandibular barbel.

rays of the pelvic fin the ‘pelvic valve’ ( Fig. 4 View Fig ). The posterior row of irregular pointed projections (often confused with tubercles). In some species, the extensions appear as a single longitudinal blade (e.g., S. speciosa , S. lineolata ), or a blade with several interruptions (e.g., S. pterolineata ), or a triangular plate at midlength of ray ( S. patella ). These extensions maybe missing (e.g., S. pudens ). See Discussion for the details of the different species.

In all species, pads of unculi are present on the ventral surface of the unbranched pectoral- and pelvic-fin rays, and on the two branches of the about 9 to 12 anterior branched pectoral-fin rays and about 9 to 12 anterior branched pelvic-fin rays. A row of small, blunt tubercles may be present along anterior edge of unculiferous pads of anteriormost rays of both fins. two or three rays of the pelvic fin are adjacent, without or with only narrow membranes between them; they are folded over the preceding rays, between them and the body. These three rays are encased in thick tissue, forming a slender vertical crest along its dorsal margin. The three rays are adnate to the body and partly covered by the pelvic axillary lobe. I have also observed the pelvic valve in Pseudogastromyzon myersi . Beaufortia has fused pelvic fins, but the fins are not fused between the posteriormost ray of each fin. They are fused, but along the penultimate or antepenultimate ray, the rays are turned upward, with several slender branches, and not encased in thick tissues (observed in B. daon , B. kweichowensis , B. leveretti , B. zebroidus ; not in B. cyclica ).

Tubercles of a variety of sizes are present on the head, inferior part of the body, and on all fins (except the dorsal fin). Their development seems to be sexually dimorphic. Among the species I examined, large tubercles are present on the cheek, snout and rostral cap in S. elongata , S. pudens , some S. diardi and S. pterolineata ( Figs. 7 View Fig , 10 View Fig , 11 View Fig , 12 View Fig ).

Tubercles and unculi on pectoral and pelvic fins show a variety of development in the different species examined. Small tubercles are usually present on the 3 to 5 anterior pectoral-fin rays. In most species, tubercles are present on some median pectoral-fin rays (usually between rays 15 to 23). In some species, anterior pectoral-fin rays may have a swelling with the posterior side covered with densely set unculi (e.g., S. speciosa , S. lineolata ). These rays are arched and rigid, forming a kind of concavity under the anterior part of the fin. In several species, on pelvic-fin rays between about 8 to 15, segments have each a lamellar extension of the bone from the dorsal surface of the ray. These extensions may appear as a crenated ridge (e.g., S. diardi , S. elongata ). With increasing size, these extensions may develop as a