Distichodus antonii Schilthuis 1891

Moelants, Tuur, Zebe, Victorine Mbadu, Snoeks, Jos & Vreven, Emmanuel, 2014, A review of the Distichodus antonii assemblage (Characiformes: Distichodontidae) from the Congo basin, Journal of Natural History 48 (27 - 28), pp. 1707-1735 : 1716-1722

Distichodus antonii Schilthuis 1891

( Tables 4 and 5; Figures 3A and 4 )

Distichodus antonii Schilthuis, 1891: 89 .

Distichodus fasciolatus Boulenger, 1898 (partim): 27, pl. XIV.

Distichodus langi Nichols and Griscom, 1917 (partim): 687–688.

Type material

Holotype. BMNH 1899.9 .6.10, Kinshasa , Stanley Pool ( DRC) (± 4°15′02″ S, 15°25′ 00″ E), coll. Greshoff, 1899 (103.3 mm SL).

Note on the holotype. Distichodus antonii was described by Schilthuis in 1891 based on a single specimen from “Bayari Sea”, collected by Greshoff ( Schilthuis 1891). From the introduction of Schilthuis’ s article, it is clear that “Bayari Sea” is either situated near Kinshasa, or near Boma [ Lower Congo ( DRC)]. Curiously, two specimens are labelled as type in the BMNH: a “ holotype ” BMNH 1898.11 .17.10 from “cataracts of Manyanga” with a size of 513.0 mm SL and a “type” BMNH 1899.9 .6.10 from “Kinshasa, Stanley Pool” with a size of 103.3 mm SL (125.5 mm TL). The latter specimen and not the former clearly has to be considered as the holotype as it corresponds to the size (12.8 cm TL) given in the original description by Schilthuis (1891). We were unable to trace back the exact location of “Bayari Sea” and to the best of our knowledge this locality has nowhere else been used in the ichthyological literature of the Congo basin. Possibly, the widening of the Congo River channel at Pool Malebo (Stanley Pool) has been called a “Sea” by the collector .

Other material examined (all lengths are SL)

Distichodus antonii Schilthuis, 1891 . Democratic Republic of the Congo ( DRC). MRAC 88 Manyanga (± 4°54′ S, 14°23′E), coll. Wilverth 1896 (279.9 mm) . MRAC 96 , Léopoldville (± 4°18′ S, 15°18′ E), coll. Wilverth 1896 (106.4 mm) . MRAC 2148 , Léopoldville (± 4°18′ S, 15°18′ E), coll. Christy 30/06/1912 (363.0 mm) . MRAC 2263 , Léopoldville (± 4°18′ S, 15°18′ E), coll. Christy 7/05/ 1912 (416.0 mm) . MRAC 2649 , Stanley Pool (± 4°18′ S, 15°18′ E), coll. Dubois 1912 (148.7 mm) . MRAC 2650 . Léopoldville (± 4°18′ S, 15°18′ E), coll. Dubois 1912 (362.0 mm) . MRAC 40938–40941 , Léopoldville (± 4°18′ S, 15°18′ E), coll. Tinant 1/ 01/1934 – 24/04/1934 (59.5–73.8 mm) . MRAC 67448–449 , Léopoldville (± 4°18′ S, 15° 18′ E), coll. van Moorsel 07/1944 (98.8–102.2 mm) . MRAC 48072 , Kalamu river , near Boma (± 5°49′ S, 13°03′ E), coll. Dartevelle 01/01/937– 4/08/1937 (181.2 mm) . MRAC 96207 , Near Léopoldville (± 4°18′ S, 15°18′ E), coll. Van de Weyer 1954 (58.4 mm) . MRAC 117317 , Stanley Pool , Nduka indigenous dam (± 4°20′ S, 15°24′ E), coll. Mission Brien-Poll-Bouillon, 23/09/1957 (211.3 mm) . MRAC 177320 , Stanley Pool , meadows near Kingabwa (± 4°19′ S, 15°21′ E), coll. Mission Brien- Poll-Bouillon, 25/9/1957 (103.8 mm) . MRAC 177381 , Stanley Pool (± 4°20′ S, 15°23′ E), coll. Van Orshoven 21/05/1964 (96.3 mm) . MRAC 177604–607 , Stanley Pool (± 4°18′ S, 15°18′ E), coll. Brichard 1967 (46.8–85.9 mm) . MRAC 73-22 -P-1333, Stanley Pool (± 4°06′ S, 15°15′ E and 4°20′ S, 15°23′ E), coll. Mandeville 22/09/1954 (24.8 mm) . MRAC 73-22 -P-1335, Stanley Pool ( DRC) (± 4°06′ S, 15°15′ E and 4°20′ S, 15°23′ E), coll. Mandeville 24/11/1954 (27.0 mm) . MRAC 73-22 -P-1336- 337, Stanley Pool (± 4°06′ S, 15°15′ E and 4°20′ S, 15°23′ E), coll. Mandeville 10/01/ 1955 (47.2–55.8 mm) . MRAC 73-22 -P-1352, Stanley Pool (± 4°06′ S, 15°15′ E and 4° 20′ S, 15°23′ E), coll. Mandeville 1/02/1958 (250.3 mm) . MRAC 86-21 -P-30, Inga (± 5°39′ S, 13°39′ E), coll. Mutambwe Shango 1967 (43.7 mm) . MRAC A7-014 -P- 0002-0008, Pool Malebo at Kinkole, Molondo Island (± 4°16′47.8″ S, 15°27′59.7″ E), coll. Mbadu Zebe 29/12/2004 (85.4–139.7 mm) . MRAC B0-021-P-0873-0874, Loboya River , right bank, Bambondji II, site 115 (± 0°11′33.1″ N, 25°31′50.1″ E), coll. Danadu and Moelants 31/08/2010 (85.5–89.4 mm) . MRAC B0-021-P-0875, Lobaye River , right bank, Djabir, Site 6B (± 0°29′28.1″ N, 24°10′42.8″ E), coll. Danadu and Moelants 9/08/ 2010 (153.2 mm) . MRAC B0-021-P-0876-0877, Lomami River , left bank, Djabir surroundings, site Lom 2G2 (± 0°30′40.7″ N, 24°10′30.9″ E), coll. Danadu and Moelants 13/08/2010 (99.6–119.9 mm) . MRAC B0-021-P-0878, Congo River , right bank, village Batikamondji I, site 101 (± 0°08′36.1″ N, 25°31′42.0″ E), coll. Danadu and Moelants 30/08/2010 (134.7 mm) . MRAC B0-021-P-0879-0882, Loboya River , right bank, site 119B (± 0°11′27.0″ N, 25°31′39.7″ E), coll. Danadu and Moelants 31/ 08/2010 (115.3–130.3 mm). Paralectotypes D. fasciolatus Boulenger, 1898 : MRAC 28 , Boma ( DRC) (± 5°50′ S, 13°03′ E), coll. Wilverth 1896 (86.3 mm) . MRAC 63 , Matadi ( DRC) (± 5°49′ S, 13°27′ E), coll. Wilverth 1896 (62.1 mm) . MRAC 102 , Léopoldville ( DRC) (± 4°18′ S, 15°18′ E), coll. Wilverth 1896 (76.7 mm) .

Republic of the Congo (Brazzaville). MRAC A4-046 -P-1399-1407 (1/8), Kintelé, Congo River , (± 4°9.12′ S, 15°21.44′ E), coll. Mamonekene, Mady-Goma, Opoya et al. 19/6/2003, (95.7 mm) . MRAC A4-046 -P-1438, Djoué River, tributary of Congo River , downstream from hydroelectric dam (± 4°19′ S, 15°14′ E), coll. Mamonekene, Bakabana, Ibala Zamba 24/7/2003 (180.5 mm) .

Differential diagnosis

Within the Congo basin, D. antonii can be distinguished from D. affinis , D. altus , D. decemmaculatus , D. noboli , D. notospilus and D. teugelsi by its higher total number of LL scales, i.e. 52–64 (versus <46 in the six other species) and from D. maculatus by the absence of large, dark spots all over the body (versus 9–14 vertical dark bars instead) and a higher number of dorsal fin rays, i.e. 21–25 (versus 19–21). Distichodus antonii can be separated from all remaining Congo species by its terminal (versus inferior) mouth, with the exception of D. lusosso (which has a distinctive elongate snout and only six to eight vertical, dark bars) and by its low number of scales between the LL and the dorsal fin, i.e. 10–12 (versus 13–17) ( Table 4).

Description

Morphometric and meristic data are given in Table 5. Body relatively deep (within D. antonii assemblage). Dorsal head profile straight, dorsal body outline concave from posterior to head to end of dorsal fin, straight from end of dorsal to adipose fin, and convex from adipose to caudal fin. Ventral head profile straight, concave from posterior to head to end of anal fin, and convex from end of anal to caudal fin. Head broad with nasal openings widely set apart (10.1–26.5 %HL), and head relatively shallow (35.2–67.2 %HL), although these characteristics are positively allometric. Mouth terminal. Posterior edge of maxillary not passing nostrils. Two rows of bicuspid teeth in each jaw. Origin of dorsal fin slightly in front of pelvic fin origin along vertical axis. Distal margin dorsal fin straight or slightly concave, distal margin of anal fin straight to slightly convex. Base and distal end of pelvic fin relatively far from vent; these distances are positive allometric, i.e. 21.0–33.6 %SL and 3.9–18.0 % SL, respectively. Pectoral and pelvic fin rays decreasing in length from outer to inner fin margin. Adipose fin situated approximately half-way between dorsal and caudal fins. Caudal peduncle deeper than long. Caudal fin forked with two rounded lobes covered with numerous small scales except for translucent distal area.

Maximum reported total length 550 mm ( Boulenger 1909). Duren (1943) stated it can reach a size of more than 800 mm.

Colouration

Body in vivo dark brown-olive, belly whitish. Dorsal fin scattered with numerous small, dark spots. Caudal fin dark brown or blackish. Adipose fin with a well-marked black distal margin ( Figures 3A and 4 ). Pectoral fins uniform pale. Pelvic fins uniform pale or with slightly darker fin ray borders. Between 9 and 14 more or less distinct vertical dark bars on each flank that disappear with increasing size, starting to fade at c. 200 mm SL, disappearing at about 250 mm SL. Preserved specimens brownish with a lighter belly. The black margin of the adipose fin and vertical bars can be absent because of poor and/or long-term conservation.

Ecology

According to Gosse (1963), the fry and juveniles of D. antonii live in aquatic prairies and in plant fringes. Adults feed in Echinochloa patches and live at the edge of streams along islands and even on the bottom in the middle of the stream. The species is a phytophagous species; stomachs almost always contain plant fragments, with easily recognizable twigs and leaves of Echinochloa , which comprises the largest part of their diet. The same author stated that this species has two annual periods of reproduction, and that like Citharinus and all other Distichodus species , spawning and egg deposition have to take place at the border of the stream, just before the flood. Recent results from stomach content analyses in Pool Malebo confirmed the all year round herbivorous diet of D. antonii , mainly feeding on leaves, complemented with some detritus ( Mbadu Zebe et al. 2010a). Matthes (1964) reported the species as occurring in lakes and large rivers. Mbadu Zebe et al. (2010b) reported that low water levels during the dry season in Pool Malebo leads to the formation of scale annuli due to the scarcity of macrophytes (its main food source).

Distribution

This species is a widespread Congo basin endemic ( Figure 5 ). Poll (1976) reported the species to be widespread in the Congo basin, including the Lualaba River, but absent from Luapula-Mweru. However, we found one specimen (MRAC 121238) from “Kasenga, Luapula River”, identified as D. antonii by Poll in 1959. We checked the identification and found it to be correct. As this specimen was originally registered as originating from “Kasenga, Lualaba”, a labelling error probably occurred. Therefore, we agree with Poll (1976) that D. antonii is absent from the Luapula system. Distichodus antonii seems to be sympatric with the D. atroventralis complex, D. fasciolatus and D. langi . In recent collections (2010–2011) from the region of Kisangani, the first three taxa were even found syntopically in the same gillnet in all three possible combinations of two species.

Etymology

Distichodus antonii was most probably named by Schilthuis (1891) in memory of Anton Greshoff 1855–1905 ( Nieuw Letterkundig Magazijn 2010) who collected the holotype.

Figures

Figure 3. Illustrations of type specimens of the Distichodus antonii assemblage. (A) Holotype of D. antonii (BMNH 1899.9.6.10), 103.3 mm SL; (B) Lectotype of Distichodus fasciolatus (MRAC 23), 278.2 mm SL; (C) Holotype of Distichodus langi (AMNH 5915), 364 mm SL. Picture taken by M.L.J. Stiassny (AMNH); (D) Syntype of Distichodus atroventralis (BMNH 1891.12.29.13), 63.5 mm SL.

Figure 4. Distichodus antonii (B0-021-P-0873-0874) from the Loboya River, affluent of the Maiko River, Upper Congo (DRC), 89.4 mm SL. (A) Lateral view; (B) Detail of the colour pattern on the adipose fin.

Figure 5. Distribution map of Distichodus antonii and Distichodus langi based on the specimens identified at the AMNH and MRAC. Note: Distichodus antonii: () holotype and (●) other specimens; D. langi: (*) lectotype and (*) specimen.