Synodontis irsacae Matthes, 1959

Englmaier, Gernot K., Blažek, Radim, Zimmermann, Holger, Bartáková, Veronika, Polačik, Matej, Žák, Jakub, Mulokozi, Deogratias P., Katongo, Cyprian, Büscher, Heinz H., Mabo, Lwabanya, Koblmüller, Stephan, Palandačić, Anja & Reichard, Martin, 2024, Revised taxonomy of Synodontis catfishes (Siluriformes: Mochokidae) from the Lake Tanganyika basin reveals lower species diversity than expected, Zoological Journal of the Linnean Society 202 (3) : -

Synodontis irsacae Matthes, 1959

( Fig. 7; Supporting Information, Video S2) Synodontis irsacae Matthes, 1959: 78 (type locality: Kalundu , Lake Tanganyika, Democratic Republic of the Congo).

Material examined: Type material: MRAC 130315 , holotype, S. irsacae , 113.5 mm SL, Kalundu , North Lake Tanganika, Democratic Republic of the Congo, coll. Matthes , 1960. MRAC 130316–130321 , six, paratypes, S. irsacae , 45.1–122.8 mm SL, coll. Matthes , 1960.

Non-type material: MRAC 2003.033 .P.0002–0004, three, 70.7–85.6 mm SL, Magara , route Bujumbura-Nyanza Lac, Lake Tanganyika, Burundi, coll. De Vos , 1994. MRAC 90279 , one, 198.0 mm SL, à 2 km au sud immédiat d’Albertville , stat. 103, depth 7–10 m, trawl with panels, Lake Tanganyika, Democratic Republic of the Congo, coll. Poll , 1947. MRAC 130987–131006 , four, 66.5–96.6 mm SL, Kalungwe , Kivu region, Lake Tanganyika, Democratic Republic of the Congo, coll. Leleup , 1961. MRAC 2003.033 .P.0041–0044, two, 108.9–130.3 mm SL, Pemba, S. d’Uvira , Lake Tanganyika, Democratic Republic of the Congo, coll. De Vos , 1995. NMW 100762 , six, 55.3–87.8mm SL, Chimba Rocks (8°25 ʹ 13.4″S, 30°27 ʹ 41.8″E), Lake Tanganyika, Zambia, coll. Reichard, Zimmermann, Bartáková, Polačik , 2021. NMW 100763 , one, 73.4 mm SL, Cape Kachese (8°29 ʹ 18.6″S, 30°28 ʹ 32.3″E), Lake Tanganyika, Zambia, coll. Reichard, Zimmermann, Bartáková, Polačik , 2021. NMW 100764 , one, 65.9 mm SL, Mpende Fisheries (8°28 ʹ 56.9″S, 30°28 ʹ 00.1″E), Lake Tanganyika, Zambia, coll. Reichard, Zimmermann, Bartáková, Polačik , 2021. NMW 100765 , six, 66.6–96.4 mm SL, Kalambo (8°37 ʹ 24.9″S, 31°11 ʹ 59.5″E), Lake Tanganyika, Zambia, coll. Reichard, Blažek, Zimmermann, Bartáková, Polačik , 2021. NMW 100766 , three, 69.1–76.1 mm SL, Mbita Island (8°45 ʹ 14.8″S, 31°05 ʹ 04.7″E), Lake Tanganyika, Zambia, coll. Reichard, Blažek, Zimmermann, Bartáková, Polačik , 2021. NMW 100767 , five, 67.0– 97.2 mm SL, Katukula (8°43 ʹ 20.3″S, 30°56 ʹ 35.1″E), Lake Tanganyika, Zambia, coll. Blažek, Zimmermann, Bartáková, Polačik, Koch , 2022. NMW 100768 , one, 80.9 mm SL, Kalambo (8°37 ʹ 24.9″S, 31°11 ʹ 59.5″E), Lake Tanganyika, Zambia, coll. Blažek, Zimmermann, Bartáková, Polačik, Koch , 2022. NMW 100769 , three, 83.9–99.0 mm SL, Mbita Island (8°45 ʹ 14.8″S, 31°05 ʹ 04.7″E), Lake Tanganyika, Zambia, coll. Blažek, Zimmermann, Bartáková, Polačik, Koch , 2022. NMW 100770 , four, 71.0– 114.4 mm SL (and one C&S specimen in a separate jar, 75.5 mm SL, NMW 100781 ), Crocodile Island (8°42 ʹ 28.45″S, 31° 7 ʹ 17.82″E), Lake Tanganyika, Zambia, coll. Blažek, Zimmermann, Bartáková, Polačik, Koch , 2022. NMW 100771 , two, 87.8–105.2 mm SL, Nondwa Point (4°51 ʹ 50.1″S, 29°36 ʹ 28.6″E), Kigoma, Lake Tanganyika, Tanzania, coll. Reichard, Zimmermann , 2022. NMW 100772 , one, 68.4 mm SL, Nondwa Point (4°51 ʹ 50.1″S, 29°36 ʹ 28.6″E), Kigoma, Lake Tanganyika, Tanzania, coll. Reichard, Blažek, Zimmermann , Englmaier, 2023. NMW 100773 , seven, 76.1–113.9 mm SL, Jacobson’s Beach (4°55 ʹ 3.00″S, 29°35 ʹ 44.01″E), Kigoma, Lake Tanganyika, Tanzania, coll. Reichard, Blažek, Zimmermann , Englmaier, 2023. NMW 100774 , two, 104.6–108.1 mm SL, Cape Kabogo , site 1 (5°27 ʹ 42.45″S, 29°44 ʹ 50.19″E), Lake Tanganyika, Tanzania, coll. Reichard, Blažek, Zimmermann , Englmaier, 2023. NMW 100775 , two, 90.0–115.0 mm SL, Cape Kabogo , site 2 (5°28 ʹ 38.62″S, 29°45 ʹ 35.82″E), Lake Tanganyika, Tanzania, coll. Reichard, Blažek, Zimmermann , Englmaier, 2023. NMW 100776 , two, 103.3–120.3 mm SL, Cape Kabogo , site 3 (5°30 ʹ 9.56″S, 29°47 ʹ 14.94″E), Lake Tanganyika, Tanzania, coll. Reichard, Blažek, Zimmermann , Englmaier, 2023. NMW 100777 , six, 74.9–105.1 mm SL, Kalilani (6°0 ʹ 56.77″S, 29°44 ʹ 46.86″E), Lake Tanganyika, Tanzania, coll. Reichard, Blažek, Zimmermann , Englmaier, 2023. NMW 100778 , five, 70.1–99.8 mm SL, Mabilibili (6°27 ʹ 26.51″S, 29°55 ʹ 2.41″E), Tanzania, coll. Reichard, Blažek, Zimmermann , Englmaier, 2023. NMW 100779 , five, 73.8–103.3 mm SL, Karema (6°54 ʹ 57.18″S, 30°30 ʹ 1.66″E), Lake Tanganyika, Tanzania, coll. Reichard, Blažek, Zimmermann , Englmaier, 2023. NMW 100780 , 10 , 75.1–101.2 mm SL, Kibige (5°43 ʹ 8.06″S, 29°24 ʹ 38.73″E), Lake Tanganyika, Democratic Republic of the Congo, coll. Jiten Vaitha, Whiteboard Enterprises Ltd , 2022.

Diagnosis: Synodontis irsacae is distinguished from congeners in LT by a unique combination of characters: axillary pore absent or macroscopically inconspicuous; no granular papillae on head, body, and fins; dorsal spine pigmented and dark, with only its tip and the upper flexible part unpigmented and white; anal fin dark, posterior margin unpigmented and white; 18–35 (median 24) mandibular teeth, commonly arranged in six symmetric rows of replacement teeth; 32–88 (median 57) primary premaxillary teeth; 2–11 (median 6) secondary branches on inner mandibular barbels; and 38–41 (median 40) total vertebrae, with 21–24 (median 22) caudal vertebrae.

Description: The general appearance of S. irsacae is shown in Figure 7 and Supporting Information, Video S2; axial skeletons in Supporting Information, Figure S8C, D; and relative measurements, meristic counts, and coded characters are given in Supporting Information, Table S12.

Longest examined specimen 198.0 mm SL (female, MRAC 90279). Males and females of similar size (males 69.1–130.3 mm SL, mean 92.7 mm, N = 39; females 66.6–198.0 mm SL, mean 92.3 mm, N = 39). Body moderately compressed laterally. Dorsal head profile and predorsal back rising straight or slightly convex to dorsal-fin origin at an angle of 20°–25° (against midline of body). Postdorsal profile falling straight to adipose-fin origin and slightly convex to caudal-fin origin. Body depth at anal-fin insertion greater than half of body depth at dorsal-fin origin. Head dorsoventrally compressed; depth at posterior eye margin approximately equal to maximal cranium width. Snout blunt and longer than half of head length. Mouth subterminal, with abundant papillae on lips; width 70.3%–108.6% of snout length. Eye diameter approximately one-third of snout length.

Dense, non-granular papillae on head; smaller and less dense (or absent) on lateral and ventral side of body and on fins. Occipitonuchal shield and humeral process rugose and of granular texture. Humeral process narrow and elongated; maximal depth 23.5%–32.9% (N = 4) of its length. Axillary pore absent or macroscopically inconspicuous, never large and prominent.

Adipose fin usually elongated; basal length 26.2%–40.9% of SL, extending over 51.5%–80.9% of distance between dorsal-fin insertion and end of caudal peduncle. Adipose-fin insertion near to end of caudal peduncle. Dorsal fin with two unbranched rays and commonly seven branched rays; dorsal spine moderately serrated on lower anterior edge, poorly marked serrations on upper anterior edge, posterior edge with 5–12 serrae; length of dorsal spine, excluding upper flexible part, 57.9%–86.1% of head length. Anal fin with four or five unbranched rays and commonly eight branched rays; longest rays commonly reaching beyond level of posterior edge of adipose fin. Pelvic fin with a single unbranched ray and six branched rays. Pectoral fin with a single unbranched ray and commonly eight branched rays; pectoral spine densely serrated on anterior (8–26 serrae) and posterior (9–20 serrae) edges. Pectoral fin shorter than pectoral–pelvic distance, not reaching pelvic-fin origin in folded position. Caudal fin forked with 2 + 15 principal rays; number of upper procurrent rays (10–14, median 12, N = 15) less than number of lower procurrent rays (13–16, median 15, N = 22).

Mandibular teeth 18–35 (median 24, N = 90) (holotype 23), unicuspid, arranged in four or six symmetric rows of replacement teeth. Primary premaxillary teeth 32–88 (median 57, N = 90) (holotype 51), arranged in 2–4 irregular rows. Secondary and tertiary premaxillary teeth small, arranged in two rows and one row, respectively.

Maxillary barbels short, usually reaching beyond pectoral-fin origin, only in 24% of specimens (N = 90) reaching beyond pectoral-fin insertion. Outer mandibular barbels 33.1%–63.8% of head length, with 2–8 (median 4, N = 90) primary branches; secondary branches absent (present in only seven specimens). Inner mandibular barbels with 6–14 (median 9, N = 90) primary branches and 2–11 (median 6, N = 90) secondary branches.

Lateral-line canals ossified, extending beyond posterior margin of last complex centrum. One specimen (NMW 100781) with 73 ossified canals. Frontal fontanel (examined in one specimen, 75.5 mm SL, NMW 100781) originating posterior to centre between eyes and extending beyond posterior margin of mesethmoid; shape oval, anterior and posterior margins slightly acute; length shorter than interorbital distance between frontal margins; resembling an intermediate state between B and C in the paper by Pinton (2008: fig. 15) where the bony bridge is not closed.

Total vertebrae 38–41 (median 40, N = 50) (holotype 41), abdominal vertebrae 16–19 (median 18), preanal caudal vertebrae 1–3 (median 2), and postanal caudal vertebrae 18–21 (median 20).

Length of digestive tract (not stretched) ~81%–213% (mean 140%) of SL (N = 20, 67.7–113.1 mm SL).

Coloration: In life (Supporting Information, Video S2), body coloration light brown or cream coloured, occasionally yellowish. Dorsal side darker than belly. Head, body, and adipose fin usually covered with abundant black spots of variable size/shape (smallest on head). Rayed fins pigmented and black, broad posterior margins unpigmented and white. Some specimens with light-brown or cream-coloured blotch at base of fins. Dorsal spine black, only its tip and upper flexible part unpigmented and white. Lobes of caudal fin with black bars, outer margins white. Maxillary and mandibular barbels light brown or cream coloured. Iris copper coloured.

In formalin (initial fixation) and later transferred to 75% ethanol ( Fig. 7), coloration similar to that in live specimens, commonly slightly darker. In 19% of specimens (N = 90), spots absent on ventral side. First branched dorsal-fin ray: 56%–76% (mean 66%, N = 29) pigmented and dark (from base) vs. 24%– 44% (mean 34%) unpigmented and cream coloured (from tip).

After long-term preservation in 75% ethanol, historical specimens brown; dark-brown spots visible on head, dorsal, lateral, and ventral side of body and on adipose fin.

Distribution and habitat: Endemic to LT ( Fig. 1). Synodontis irsacae was observed mainly below 12 m in both the southern and northern lake basins. The species usually appears to be less abundant in comparison to S. multipunctatus , which might be attributable to the more cryptic lifestyle between and under medium- to large-sized rocks. It is found almost exclusively in rocky habitats or the transition zones between rocky and sandy habitats where the rocks provide sufficient shelter.