Coregonus supersum, Selz & Seehausen, 2023

Selz, Oliver M. & Seehausen, Ole, 2023, A taxonomic revision of ten whitefish species from the lakes Lucerne, Sarnen, Sempach and Zug, Switzerland, with descriptions of seven new species (Teleostei, Coregonidae), ZooKeys 1144, pp. 95-169 : 95

Coregonus supersum sp. nov.

Figs 9 , 14

Coregonus ‘Balchen’: Douglas and Brunner 2002.

Coregonus crassirostris compactus : Fatio 1885 (see also synonymy of C. zugensis ).

Coregonus lavaretus nat. riusensis, oekot. primigenius: Steinmann 1950 (see also synonymy of C. sarnensis , C. suidteri , and C. litoralis ).

Coregonus schinzii helveticus var. zugensis : Fatio 1890 (see also synonymy of C. litoralis )

Coregonus suidteri : Kottelat 1997; Kottelat and Freyhof 2007; Vonlanthen et al. 2012, 2015 (see also synonymy of C. litoralis and C. suidteri ).

Coregonus sp. ‘Zugerbalchen’: Vonlanthen et al. 2012.

Coregonus sp. ‘Zugerseebalchen’: Steinmann 1950.

Material examined.

Holotype. Historical specimen (year 1939): NMBE- 1076275, 288 mm SL, male; Switzerland: Lake Zug . Paratypes. All from Switzerland, Lake Zug: Historical specimens (years 1907, 1937, 1939, 1941): NMBE-1076265, NMBE-1076267, NMBE-1076268, NMBE-1076277, MHNG 2786.063, N = 5, 263-411 mm SL .

Diagnosis.

Coregonus supersum is a large whitefish species with strong pigmentation of all fins and the body; greenish blue colour on the flanks above the lateral line; deep bodied (23.2-26.6% SL, mean = 24.4); blunt snout; short head (19.8-21.8% SL, mean = 20.9); sub-terminal mouth; small eye (eye diameter: 18.2-21.7% HL, mean = 19.9) with a thick and triangular-shaped eye socket; short and stout caudal peduncle (caudal peduncle depth: 6.7-8.1% SL, mean = 7.2; caudal peduncle length: 12.2-14.4% SL, mean = 13.1); few and short gill rakers (longest gill raker: 8.9-12.4% HL, mean = 10.1; total gill raker number: 21-27).

Description.

Shape: Generally, deep bodied with greatest body depth anterior of dorsal fin. Dorsal profile from tip of snout to anterior origin of dorsal fin is strongly convex and ventral profile moderately convex or almost straight from interorbital area to the pelvic fin origin. Head short. Mouth short, wide and sub-terminal. Rostral plate pronounced and a bit wider than deep resulting in a slightly rectangular shape. Tip of snout often blunt. Small eye with a thick and mostly sickle cell-shaped eye-socket. Pectoral fin moderately tapered and moderately short. Dorsal fin long. Caudal peduncle stout and short with caudal fin moderately forked. Unbranched rays of anal fin slightly bent posteriorly. Anal fin longest anteriorly and progressively shortening posteriorly with outer margin of anal fin mostly straight and only rarely slightly concave. Meristics: Few and short gill rakers. Colour: Pigmentation of fins and body overall strong. Pectoral fin moderately pigmented at the median to distal parts of the fin. All other fins are strongly pigmented. Silvery appearance along the flanks. The preserved specimens have moderate to many pigmented small dots (aggregation of melanophores) on the scales along the flank and the dorsum. Dorsally above the lateral line the silvery appearance changes to a greenish blue colour. The dorsal part of the head and the snout around the nostrils is strongly pigmented. The pre-operculum and operculum are silvery with one black spot on the lower margin of the pre-operculum. Preserved specimens are pale in colouration with similar pigmentation as described for live specimens. Preserved fish are brownish in colouration.

Differential diagnosis.

Coregonus supersum - Coregonus zugensis

Coregonus supersum can be differentiated from C. zugensis by having a shorter head (dorsal head length: 13.6-15.4% SL, mean = 14.4 vs. 14.9-16.8% SL, mean = 15.7; head length: 19.8-21.8% SL, mean = 20.9 vs. 21.3-23.5% SL, mean = 22), a smaller eye (eye diameter: 18.2-21.7% HL, mean = 19.9 vs. 19.7-25.3% HL, mean = 22.9; eye height: 19.4-22.2% HL, mean = 20.9 vs. 21.7-26.7% HL, mean = 23.9), a longer upper and lower jaw and maxilla (upper jaw: 24.3-28% HL, mean = 26.5 vs. 26.7-33.3% HL, mean = 30.4; lower jaw: 36.3-41.6% HL, 37.9 vs. 38.4-44.6% HL, mean = 42; maxilla: 17.8-21.3% HL, mean = 18.8 vs. 20.2-25.4% HL, 22.9), a deeper snout (9.2-12% HL, mean = 10.6 vs. 6.6-9.2% HL, mean = 8), shorter gill rakers (middle gill raker: 6.8-11.3% HL, mean = 8.4 vs. 10.2-15% HL, mean = 12.6; longest gill raker: 8.9-12.4% Hl, mean = 10.1 vs. 11.2-17.7% HL, mean = 14.3) and less gill rakers (upper arch gill raker number: 7-9, mode = 9 vs. 11-15, mode =12; lower arch gill raker number: 14-18, mode = 15 vs. 21-26, mode = 22; total number of gill rakers: 21-27 vs. 33-40) (Tables 8 , 13 ). Based on ratios C. supersum can be differentiated from C. zugensis by having a larger 'caudal peduncle depth / dorsal head length’ ratio (CD/DHL: 0.46-0.58 vs. 0.37-0.44) (Table 11 ).

Coregonus supersum - Coregonus obliterus

Coregonus supersum can be differentiated from C. obliterus by having a shorter caudal fin (15.8-21.7% SL, mean = 19.4 vs. 21.2-24.1% SL, mean = 22.2), a smaller eye (eye diameter: 18.2-21.7% HL, mean = 19.9 vs. 21.5-24.7% HL, mean = 23.2; eye cavity: 23.1-27.2, mean = 25.2 vs. 27.9-31% HL, mean = 28.9; eye height: 19.4-22.2% HL, mean = 20.9 vs. 23-25.2% HL, mean = 23.9) and longer longest gill raker (8.9-12.4% HL, mean = 10.1 vs. 7.6-10.6% HL, mean = 8.8) (Table 8 ). Based on ratios C. supersum can be differentiated from C. obliterus by having a larger 'predorsal length / eye height’ ratio (PreD/EH: 10.52-12.07 vs. 8.46-9.73) and a larger 'adipose fin base / eye depth’ ratio (AdFB/ED: 1.15-1.42 vs. 0.64-1) (Table 11 ).

Coregonus supersum - Coregonus litoralis

Coregonus supersum can be differentiated from C. litoralis by having a shorter dorsal head length (13.6-15.4% SL, mean = 14.4 vs. 14.4-17.9% SL, mean = 15.2), a shorter middle gill raker (6.8-11.3% HL, mean = 8.4 vs. 8.9-13.4% HL, mean = 11.1) and by having a larger 'mouth width / snout depth’ ratio (MW/SD: 1-1.21 vs. 0.75-1) (Tables 2 , 8 , 11 ).

Coregonus supersum - Coregonus suidteri

Coregonus supersum can be differentiated from C. suidteri by having a shallower pelvic fin base (3.6-4.3% SL, mean = 3.9 vs. 4.6-4.9% SL, mean = 4.7), a shallower pectoral fin base (2.8-3.7% SL, mean = 3.2 vs. 3.5-3.9% SL, mean = 3.7), a shorter erected anterior anal fin (11.3-12.8% SL, mean = 11.8 vs. 12.4-14.2% SL, mean = 13.3), a longer distance from the posterior part of the adipose fin to the caudal fin base (17.2-19.7% SL, mean = 18.4 vs. 13.7-15.6% SL, mean = 14.6), a shorter pre-dorsal distance (46.4-48.6% SL, mean = 47.3 vs. 49.5-50.2% SL, mean = 49.8), a smaller eye (eye diameter: 18.2-21.7% HL, mean = 19.9 vs. 21.9-23.9% HL, mean = 22.9; eye cavity: 23.1-27.2% HL, mean = 25.2 vs. 29.6-30% HL, mean = 29.8; 19.4-22.2% HL, mean = 20.9 vs. 23.6-23.7% HL, mean = 23.7), a less wide and shorter mouth (mouth width: 8.4-10.5% HL, mean = 9.3 vs. 10.3-11.1% HL, mean = 10.7; upper jaw length: 24.3-28% HL, mean = 26.5 vs. 27.8-30.1% HL, mean = 28.9; lower jaw length: 36.3-41.6% HL, mean = 37.9 vs. 41.5-42.2% HL, mean = 41.9; maxilla: 17.8-21.3% HL, mean = 18.8 vs. 21.3-23% HL, mean = 22.1), a wider interorbital width (25.8-29.7% HL, mean = 27.5 vs. 19.7-25.4% HL, mean = 22.6), a less wide internarial width (10.7-13.9% HL, mean = 12 vs. 13.8-15.4% HL, mean = 14.6) and lower jaw width (7.2-9.2% HL, mean = 8.1 vs. 11.7-12.3% HL, mean = 12) (Table 8 ).

Distribution and notes on biology.

Coregonus supersum occurs in Lake Zug and it has been shown with genetic analysis ( Hudson et al. 2011) that translocated populations of whitefish in Lake Maggiore (local name of the population is ‘Lavarello’) and Lake Lugano group in a neighbour-joining tree with the extant population of whitefish from Lake Zug, suggesting that C. supersum also occurs in these two lakes south of the alps. Coregonus supersum is most likely the only species out of three species that did not go extinct in Lake Zug (this is discussed in more detail in the Suppl. material 1: paragraph 4). Lake Zug used to harbour three whitefish species, of which two are extinct, C. zugensis and C. obliterus . The only extant whitefish species of Lake Zug is C. supersum . The extinction of C. obliterus and C. zugensis can be attributed to strong anthropogenic-induced eutrophication that was present in many Swiss lakes, which was accompanied by population collapse, speciation reversal, and extinction of Swiss whitefish ( Vonlanthen et al. 2012; Frei et al. 2022a, b). Steinmann (1950) notes that C. supersum feeds on zooplankton and benthic prey items. A habitat-stratified fishing campaign in Lake Zug during a short time window in summer showed that the whitefish population of Lake Zug only occupies the first 20 metres of the pelagic and benthic water column, despite Lake Zug being almost 200 metres deep; below 81 metres no fish were caught, which is attributed to anoxic conditions in the hypolimnion ( Vonlanthen et al. 2015). The spawning season of C. supersum used to take place in the last few weeks of December ( Steinmann 1950), but in recent years this has shifted to the first weeks of January (P. Reichlin, pers. comm.). Coregonus supersum spawns in depths of ~ 10-40 m ( Steinmann 1950; P. Reichlin, pers. comm.).

Etymology.

The specific name supersum means in Latin to be 'left over’ or to have ‘survived’. The name refers to the fact that this is the only species to exist to date in Lake Zug. A verb.

Common name.

We suggest the German name Zugerbalchen.

Figures

Figure 9. Coregonus supersum, Lake Zug, Switzerland A holotype, NMBE- 1076275, 288 mm SL, male, preserved specimen B paratype, MHNG- 2786.063, 286 mm SL, sex unknown, preserved specimen. Scale bars: 1 cm

Figure 14. Illustrations of specimens of each species of Coregonus from Lake Lucerne and Sarnen. Illustrations of Lake Lucerne species are based on several individuals. Illustration of C. sarnensis is based on NMBE- 1078159, 230 mm, male. Scale bar for C. sarnensis: 1 cm.