Lamprologus lethops Roberts and Stewart, 1976

SCHELLY, ROBERT C. & STIASSNY, MELANIE L. J., 2004, Revision of the Congo River Lamprologus Schilthuis, 1891 (Teleostei: Cichlidae), with Descriptions of Two New Species, American Museum Novitates 3451, pp. 1-40 : 26-28

Lamprologus lethops Roberts and Stewart, 1976

Figures 21–23 , Table 6, Plate 2b

Lamprologus lethops Roberts and Stewart, 1976: 284 , pl. 9, figs. a–c (Type locality: Congo River mainstream near Bulu, Zaire, 5 ° 01 ̍ S, 14 ° 01 ̍ E).

HOLOTYPE: MCZ 50248, Congo River mainstream near Bulu, West of Luozi, 5 ° 1 ̍ S, 14 ° 1 ̍ E, 7/15/1973, T.R. Roberts and D.J. Stewart.

PARATYPE: MCZ 50249, collected with holotype .

MATERIAL EXAMINED: Holotype and single paratype.

DIFFERENTIAL DIAGNOSIS: Lamprologus lethops , the only known blind cichlid, is further distinguished from all other Congo River species of Lamprologus save L. symoensi by its irregularly sized flank scales, among which clear parallel rows are not apparent, and a scaleless patch on the head extending posteriorly well beyond the dorsal fin origin. In addition, L. lethops has by far the highest lateral line scale count (43–49) of the Congo River Lamprologus and a low value for BD as a percentage of SL (17.9%) that overlaps only with L. werneri and L. tigripictilis , n.sp.

DESCRIPTION: Counts and measurements for the holotype and single paratype are given in table 6. Remarkably autapomorphic; lacking pigmentation, cryptophthalmic and cylindrically shaped. Raised, fleshy nostril tubes particularly well­developed. In holotype, two small, dark spheres visible beneath covering of thick skin in place of fullyformed eyes (corresponding tissues missing in damaged, eviscerated paratype). In overall appearance this unusual species superficially resembles members of the genus Teleogramma . Lamprologus lethops , along with L. werneri , with which its body depth range overlaps, has one of the shallowest body depths among fluviatile lamprologines (body depth measures 17.9% SL in both specimens). Head length 31.9–32.1%, mean 32.0% SL. Ascending process of premaxilla rises straight at angle of about 40 °, blending into head profile without sharp angle; dorsal body profile gently convex, with greatest incline immediately ahead of caudal peduncle. Ventral body profile approximately straight. To date this species known only from two specimens, one eviscerated and dehydrated, and material is insufficient to comment on sexual dimorphism.

Fins: Dorsal fin XX 7. Anal fin VI–VII 5– 6. Spines in both fins gradually increase in length posteriorly. Dorsal and anal fins with filamentous extensions to end of caudal peduncle. Caudal fin large, rounded, and paddle­shaped, with 14 branched rays; appears lance­shaped, subacuminate in preserved specimens. Pectoral and pelvic fins short, not reaching vertical through anus.

Teeth: Jaws isognathous. Both outer and inner row teeth unicuspid and sharply pointed. Six greatly enlarged, recurved, procumbent canines situated anteriorly in both jaws, increasing in size laterally. Inner teeth in about 7 poorly defined rows of tightly packed, small, recurved, caniniform teeth anteriorly, gradually thinning to single row posteriorly. Inner premaxillary teeth in lateral row are slightly enlarged.

Gill Rakers: Relatively slender, elongate, non­denticulate. Holotype with 12 gill rakers along hypobranchial and ceratobranchial of first gill arch, single raker in angle of arch and 4 rakers along epibranchial. Gill arches in paratype are damaged and partially missing.

Lower Pharyngeal Jaw: Wider than long, strongly interdigitating along ventral suture. Paratype with 38 teeth in posterior row. Median teeth, especially posteriorly, more robust. Teeth are beveled or simply coneshaped and pointed.

Scales: Flank scales ctenoid and variable in size, presenting jumbled pattern with parallel oblique scale rows mostly obscured, particularly so posteriorly on flanks. Lateral line 43–49. Lateral line branches variably fragmented by occasional scales lacking canals. Upper and lower branches of lateral line overlap. Cheek naked, subopercle and opercle with few large scales. Gradual transition to uniformly small scales above lateral line anteriorly, with uniformly sized small scales on belly. Nape scaleless, with scaleless patch between lateral line and dorsal fin extending below dorsal fin to about third spine. Dorsal and anal fins scaleless. Small scales occur over most of caudal fin.

Vertebrae: 32; 14 + 18 (2).

Additional Osteology: Infraorbital series comprised of broad, platelike lachrymal with 4 sensory canal openings and 1–2 tubular infraorbitals adjacent to lachrymal (fig. 22). Dermosphenotic absent. Supraneurals absent. Supraoccipital crest low and poorly developed. No frontal ridge extending to median coronal pore (NLF0).

Coloration: Depigmented. Scaleless but depigmented opercular spot present. Preserved coloration yellowish brown.

DIET: Unknown.

DISTRIBUTION (fig. 23): Known only from the lower Congo River rapids near Bulu.

REMARKS: Lamprologus lethops is remarkable as the only blind cichlid. Roberts and Stewart (1976) described L. lethops as cryptophthalmic, indicating that the eyes are reduced and covered over by thick skin. Interestingly, this condition is found independently in at least five other lower Congo River rapids endemics, and one species (a clariid catfish, Gymnallabes nops ) appears to lack eyes entirely, while in a further 26 lower rapids species the eyes are reduced although not covered by skin. The highly derived features of L. lethops alone make it attractive for further study, and the similarities it shares with Teleogramma , such as small scales and a gracile, dorsoventrally compressed body, suggest that it may be important to the determination of close outgroups to lamprologines. Unfortunately, the window of opportunity to collect more specimens of L. lethops may be closing. A new dam, planned to span the entire channel near Inga ( SNEL, 2002), would, if constructed, make the survival of L. lethops and other rapids adapted endemics doubtful.

Figures

Plate 2. (a) Lamprologus symoensi, paratype, MRAC 79001.6284–85, female, 46 mm SL; (b) Lamprologus lethops, holotype, MCZ 50248, male, 26 mm SL; (c) Lamprologus teugelsi, n.sp., holotype, AMNH 233611, female, 60 mm SL; (d) Lamprologus tigripictilis, n.sp., holotype, AMNH 233609, male, 62 mm SL.

Fig. 21. Lamprologus lethops holotype, MCZ 50248, male, 26 mm SL, drawn by Patricia Wynne.

Fig. 22. Lamprologus lethops paratype, MCZ

Fig. 23. Collection localities of Lamprologus lethops.