Silurichthys exortivus, Ng & Kottelat, 2022

Ng, Heok Hee & Kottelat, Maurice, 2022, Silurichthys exortivus, a new catfish (Teleostei: Siluridae) from eastern Borneo Indonesia, Zootaxa 5138 (2), pp. 185-190 : 186-188

Silurichthys exortivus , new species

( Fig. 1 )

urn:lsid:zoobank.org:act:2E86DB97-C617-41B7-847C-3793F6C46C3A

Type material. Holotype: MZB 17240, 60.6 mm SL; Indonesia: Borneo: Kalimantan Timur: Mahakam River drainage, Belayan River system, REA Plantations , Long Buluh Damai Estate , 0°14’29”N 116°19’14”E; M. Kottelat & R. K. Hadiaty, 22 November 2009.

Diagnosis. Silurichthys exortivus can be distinguished from all congeners in having 4 (vs. 6–7) principal rays on the upper caudal-fin lobe, lacking (vs. having) a dorsal fin, and, except for S. sanguineus , in lacking (vs. having) pelvic fins. It further differs from S. ligneolus and S. sanguineus in having a caudal fin with distinct, asymmetrical lobes (vs. obliquely truncate and without distinct lobes), and in the number of anal-fin rays (54 vs. 44–45 in S. ligneolus and 60 in S. sanguineus ). The following combination of characters additionally distinguishes it from congeners: body depth at anus 14.0% SL, caudal peduncle depth 5.1% SL, upper lobe of caudal fin 1.1 times longer than lower lobe, 1 raker on first gill arch, 48 vertebrae and brown mottled body.

Description. Biometric data are given in Table 1 . Body laterally compressed. Head somewhat depressed. Dorsal profile straight, descending gradually from dorsal-fin origin to snout tip. Anterior profile of snout rounded. Anterior pair of nostrils tubular and anteromedial to maxillary-barbel base. Posterior pair of nostrils bordered by fleshy dorsal and ventral membranes, situated posteromedial to maxillary-barbel base. Eyes small, subcutaneous; located in anterior half of head; visible dorsally, not visible ventrally.

Mouth subterminal; gape horizontal or very slightly oblique. Well-developed rictal fold present, consisting of large and fleshy upper lobe joined at corner of mouth with lower lobe; lower lobe subtended by short submandibular groove.

Teeth villiform. Dentary teeth in slightly curved, elongate bands narrowing posteriorly, reaching from symphysis almost to mouth corners; premaxillary teeth in broader, slightly curved rectangular bands; vomerine teeth in a single crescent-shaped band.

Maxillary barbels slightly flattened, reaching to anterior quarter of anal fin. One pair of mandibular barbels present; located slightly anterolateral to gular fold; barbels flattened for most of proximal length, reaching to middle of pectoral-fin base.

Gill membranes separate, overlapping, free from isthmus. Branchiostegal rays 9. Gill rakers short, small; 0+1.

Dorsal fin absent. Distal margin of pectoral fin broadly convex, with 8 rays. Segments of the proximal twothirds of first pectoral-fin element co-ossified, forming spine. Pelvic fin absent. Distal margin of anal fin straight, with 54 rays; joined to caudal fin for length of last anal-fin ray. Integument over anal fin thickened proximally for slightly more than half of ray lengths; fin-ray erector muscles extending along anterior edges of anal-fin rays, ventralmost extent of muscles that of thickened integument. Caudal fin forked, with upper lobe 1.1 times longer than lower lobe; principal rays i,3,4,i. Vertebrae 12+36=48.

Coloration. In 70% ethanol: flanks and thickened integument over anal fin brown, with numerous pale and dark patches forming mottled pattern. Tympanic region dark. Dorsal surface and sides of head brown with numerous scattered dark spots, fading to dark yellow on ventral surfaces of head, breast and belly. Maxillary and mandibular barbels mottled brown, fading to yellow distally. Pectoral, anal and caudal fins hyaline, with scattered brown patches along fin rays imparting diffuse mottled pattern.

Etymology. The species name comes from the Latin adjective exortivus , meaning “towards the sunrise”. This name is used in allusion to distribution of this species (eastern Borneo), which represents the easternmost limit of its distribution.

Figures

FIGURE 1. Silurichthys exortivus, holotype, MZB 17240, 60.6 mm SL; Borneo: Mahakam River drainage.

Tables

TABLE 1. Morphometric data for the holotype of Silurichthys exortivus

Standard length (mm)	60.6
% SL
Preanal length	31.2
Prepectoral length	17.5
Anal-fin length	71.9
Pectoral-fin length	14.9
Pectoral-spine length	5.6
Caudal-fin length	24.1
Body depth at anus	14.0
Caudal peduncle depth	5.1
Head length	17.3
Head width	12.9
Head depth	10.9
% HL
Snout length	37
Interorbital distance	42
Eye diameter	12
Maxillary barbel length	300
Mandibular barbel length	166