Lectotype. MHNG-816.022, Switzerland, Lake Thun (46°40'N, 7°46'E), 165 mm SL, sex unknown.

Non-types. NMBE-1077186–1077202, NMBE-1077221–1077237, Switzerland, Lake Thun (46°40'N, 7°46'E), N = 34, 177–271 mm SL; NMBE-1059754; 1059768; 1059791; 1059801; 1059814, NMBE-1077129–1077131, NMBE-1077318–1077341, Switzerland, Lake Brienz (46°43'N, 7°57'E), N = 32, 101–164 mm SL.

Coregonus albellus is a very small whitefish species in Lake Brienz and a small whitefish species in Lake Thun with weak pigmentation of all fins and body; the colouration on the flanks above the lateral line of specimens from Lake Thun are pale rose to brown and from Lake Brienz pale brown to light green; no or few small pigmented dots on the edge of the scales along the flank for specimens from Lake Thun and specimens from Lake Brienz sometimes have rather large pigmented dots more or less in a row on the upper dorsum; elongate slender body; large eye with a thin and roundish eye socket; tip of snout fleshy and roundish; many and long gill rakers.

No single character was sufficient to distinguish C. albellus against all the five other species from Lake Thun and the species is diagnosed by a combination of characters. Based on ratios for the subset of whitefish from Lake Brienz smaller than 163.5 mm, C. albellus can be distinguished from the other three species from Lake Brienz by a smaller "postdorsal length / eye height" ratio (PostD/EH: 5.47–6.93 vs. 7.5–8.9). Also, when taking the full-size range (100–290 mm) of all species from Lake Brienz C. albellus can be distinguished from the three other species by a smaller "predorsal length / eye height" ratio (PreD/EH: 6.1–7.58 vs. 8.12–10.5) (Table 11).

Coregonus albellus-Coregonus alpinus

The specimens from lakes Thun and Brienz of C. albellus differ from those of C. alpinus of both lakes in having a higher number of gill rakers (UGR#: 9–17, mode = 13 vs. 8–11, mode = 10; LGR#: 20–29, mode = 25 vs. 15–23, mode = 19; total GR: 32–44, mode = 38 vs. 25–34, mode = 29), a longer longest gill raker (14.1–21.8% HL, mean = 17.7 vs. 10–15.2% HL, mean = 11.9), a deeper adipose fin (4.5–9.2% SL, mean = 6.5 vs. 3.4–5.5% HL, mean = 4.4), a longer lower jaw (38.4–49.2% HL, mean = 43.6 vs. 33.8–41.4% HL, mean = 38.4) and a thinner eye socket (2– 4.9% HL, mean = 3.4 vs. 3.3–6.3% HL, mean = 5).

In Lake Brienz C. albellus further differs from C. alpinus by having translucent pelvic and anal fins compared to the moderately to strongly pigmented pelvic and anal fins of C. alpinus, a longer pectoral fin (Pectoral fin 1 length: 15.7–22.6% SL, mean = 18 vs. 13.9– 17.9% SL, mean = 16.3; Pectoral fin 2 length: 16.9–23.8% SL, mean = 19.4 vs. 14.4–17.7% SL, mean = 16.9), a longer distance from the anal fin to the hypural plate of the caudal peduncle (17.7– 24.2% SL, mean = 20.7 vs. 15.3–19.5% SL, mean = 17.6), a longer head (16.1– 23.1% SL, mean = 17.9 vs. 14–16.3% SL, mean = 15.4), a larger eye and eye cavity (eye diameter: 26.1–32% HL, mean = 29.3 vs. 21.8–27.2% HL, mean = 24.3; eye height: 26.5– 30.6% HL, mean = 28.7 vs. 22.4– 27.1% HL, mean = 23.9; eye cavity: 30.4–36.8% HL, mean = 33.3 vs. 26.4–31.5% HL, mean = 29), and a longer upper jaw (28.6–34.9% HL, mean = 32.1 vs. 25.4–29.1% HL, mean = 26.8). Finally, C. albellus smaller than 163.5mm SL can be distinguished from C. alpinus by a smaller "preanal length / lower jaw" ratio (PreA/LJ: 6.33–7.44 vs. 9.24–9.97) and a larger "pectoral fin 2 length / length of the depressed anterior part of the dorsal fin" ratio (PecF2/DFAd: 0.81–1.06 vs. 0.78–0.8). With the full size range of Lake Brienz specimens, C. albellus can be distinguished from C. alpinus by the smaller "predorsal length / lower jaw" ratio (PreD/LJ: 3.99–4.68 vs. 5.6–6.81), "erected anterior part of the dorsal fin / upper jaw" ratio (DFAe/UJ: 2.14–2.79 vs. 3.25–4.1), "head depth / upper jaw" (HD/UJ: 1.87–2.2 vs. 2.38–2.78) and a larger "lower jaw / interorbital width" ratio (LJ/IOW: 1.53–1.99 vs. 1.33–1.57). (Tables 3, 4, 11).

In Lake Thun C. albellus can further be distinguished from C. alpinus by having a less deep caudal peduncle (6.4–7.9% SL, mean = 7.1 vs. 7.6–8.9% SL, mean = 8.2) and a longer upper jaw (28.8–34.7% HL, mean = 31.2 vs. 24.3–30.1% HL, mean = 27.7). Based on pigmentation of the fins C. albellus can be distinguished from C. alpinus from Lake Thun by having translucent to weakly pigmented fins compared to strongly pigmented fins, respectively. In Lake Thun C. albellus can further be distinguished from C. alpinus by the smaller "caudal peduncle depth / upper jaw length" ratio (CD/UJ: 0.96–1.29 vs. 1.36–1.65) and "caudal peduncle depth / dorsal head length" ratio (CD/DHL: 0.44–0.54 vs. 0.54–0.62) (Tables 3, 4, 10).

Coregonus albellus-Coregonus fatioi

In Lake Brienz C. albellus can be distinguished from C. fatioi by having a larger head (16.1–23% SL, mean = 17.9, vs. 14.5–16.8% SL, mean = 15.7), a larger eye and eye cavity (eye diameter: 26.1–32% HL, mean = 29.4 vs. 21.2–27.6% HL, mean = 24.8; eye cavity: 30.4– 36.8% HL, mean = 33.3 vs. 25.3–33% HL, mean = 29; eye height: 26.5–30.6% HL, mean = 28.7 vs. 22.1–26.3% HL, mean = 24.4), a longer maxilla (22.6–26.9% HL, mean = 24.7 vs. 18.7–24.2% HL, mean = 21.7) and longer gill rakers (middle gill raker: 13.7–19.4% HL, mean = 16.5 vs. 10.5–15% HL, mean = 13.2; longest gill raker: 14.9–21.8% HL, mean = 18.2 vs. 12.3–16.4% HL, mean = 14.3). Based on ratios C. albellus smaller than 163.5 mm SL can be distinguished from C. fatioi by a larger "pectoral fin 2 length / preanal length" ratio (PecF2/PreA: 0.22–0.28 vs. 0.2–0.22), "upper jaw length / eye socket width" ratio (UJ/ES: 6.81–12.42 vs. 4.51–6.15) and "eye socket width / head length" ratio (ES/HL: 0.27–0.31 vs. 0.23–0.27). With the full-size range of Lake Brienz specimens (100–290 mm), C. albellus can be distinguished from C. fatioi by a smaller "prepelvic length / eye height" ratio (PreP/EH: 6.56–7.98 vs. 8.94–11.43) (Tables 3, 5, 11).

In Lake Thun C. albellus can be distinguished from C. fatioi by its live colouration above the lateral line on the dorsum ranging from a pale rose to a pale brown compared to a light to dark green colouration in C. fatioi. C. albellus can further be differentiated from C. fatioi by having no or few small pigmented dots on the edge of the scales or on the boundary of two scales on the flank and dorsum compared to moderate or many dots on the flanks and dorsum in C. fatioi.

Coregonus albellus-Coregonus brienzii

Coregonus albellus from Lake Brienz differs from C. brienzii by having a longer longest gill raker (14.9–21.8% HL, mean = 18.2 vs. 12.1–16.8% HL, mean = 14.7), a longer maxilla (22.6–26.9% HL, mean = 24.7 vs. 15.4–24% HL, mean = 21), anterior a longer dorsal fin (anterior dorsal fin erected: 17.3–24.7% SL, mean = 19.7 vs. 15.5– 19.8% SL, mean = 17.9; anterior dorsal fin depressed: 18.3–26.6% SL, mean = 20.6 vs. 15.3–20.8% SL, mean = 18.6), a longer head (16.1–23.1% SL, mean = 17.9 vs. 14.6–16.8% SL, mean = 15.6) and a larger eye and eye cavity (eye diameter: 26.1–32% HL, mean = 29.3 vs. 23.1–28.3% HL, mean = 25.3; eye height: 26.5–30.6% HL, mean = 28.7 vs. 22–27.2% HL, mean = 24.4; eye cavity: 30.4–36.8% HL, mean = 33.3 vs. 25.6– 32.9% HL, mean = 29). Based on ratios C. albellus smaller than 163.5 mm SL can be distinguished from C. brienzii by a larger "maxilla length / eye socket width" ratio (M/ES: 5.35–9.76 vs. 3.31–4.37), "pectoral fin 2 length / predorsal length" ratio (PecF2/PreD: 0.36–0.45 vs. 0.29–0.32), "lower jaw length / eye socket width" ratio (LJ/ES: 9.62–17.28 vs. 6.01–6.49) and a smaller "predorsal length / lower jaw length" ratio (PreD/LJ: 3.99–4.68 vs. 5.05–5.57). With the full size range (100–290 mm) of Lake Brienz specimens, C. albellus can be distinguished from C. brienzii by a larger "eye height / head length" ratio (EH/HL: 0.27–0.31 vs. 0.22–0.27) and a smaller "predorsal length / eye height" ratio (PreD/EH: 6.1–7.58 vs. 8.12–10.32) (Tables 3, 7, 11).

Coregonus albellus-Coregonus steinmanni

Coregonus albellus from Lake Thun can be distinguished from C. steinmanni by having a longer longest gill raker (14.1–20.3% HL, mean = 17.2 vs. 10–14.4% HL, mean = 12.1), a longer maxilla (20.1–26.8% HL, mean = 22.4 vs. 18.1–21.8% HL, mean = 19.7), a less deep caudal peduncle (6.4–7.9% SL, mean = 7.1 vs. 7.5–8.6% SL, mean = 8.0) and a deeper adipose fin (4.5–7.7% SL, mean = 5.8 vs. 3.7–5.4% HL, mean = 4.5). Based on ratios C. albellus can be distinguished from C. steinmanni by a smaller "caudal peduncle depth / upper jaw length" ratio (CD/UJ: 0.96–1.29 vs. 1.36–1.55) (Tables 3, 6, 10).

Coregonus albellus-Coregonus profundus

Coregonus albellus from Lake Thun differs from C. profundus by having more and longer gill rakers (upper arch gill raker number: 9–17, mode = 13 vs. 5–10, mode = 9; lower arch gill raker number: 20–28, mode = 24 vs. 10–18, mode = 14; total number of gill rakers: 32–44, mode = 38 vs. 15–27, mode = 21; middle gill raker length: 11.7– 18.3% HL, mean = 15.6 vs. 7.6–11.7% HL, mean = 9.2; longest gill raker length: 14.1–20.3% HL, mean = 17.2 vs. 7.8–12.4% HL, mean = 10.1). Based on ratios C. albellus can be distinguished from C. profundus by a larger "caudal peduncle length / eye cavity length" ratio (CL/EC: 1.97–2.87 vs. 1.56–2.09) (Tables 3, 8, 10).

Coregonus albellus-Coregonus acrinasus

Coregonus albellus from Thun can be distinguished from C. acrinasus by having a deeper adipose fin (4.5–7.7% SL, mean = 5.8 vs. 3.7–6.2% SL, mean = 4.7), a thinner eye socket (ES: 2–4.9% HL, mean = 3.2 vs. 3.2–6.4% HL, mean = 4.7) and a longer longest gill raker (14.1–20.3% HL, mean = 17.2 vs. 11.4–16.9% HL, mean = 14.5) (Tables 3, 9).

General appearance is shown in Figure 4. Morphological and meristic characters of both sexes can be found in Table 3 and Suppl. material 1: Table S6 and first- and second-best ratios for both sexes combined can be found in Tables 10, 11. The description is valid for both sexes and both lakes; differences between the populations of lakes Thun and Brienz are mentioned.

Shape: Body elongate, slender. Greatest body depth anterior of the dorsalfin. Ventral profile and dorsal profile similar and slightly arched. Dorsal and ventral profile from tip of snout to interorbital mostly straight and then slightly convex to dorsal and pelvic fin origin respectively. Head long. Snout often 40–50° angle to the body axis anterior of the eye, such that the profile from the tip of the snout to the vertical projection where the anterior part of the eye crosses the dorsal profile is straight and afterwards slightly convex. Mouth (i.e., width of upper and lower jaw) wide, long and often terminal and only rarely slightly sub-terminal. Snout mostly wider than deep, not strongly pronounced, since the tip of the snout is often fleshy and roundish. Large eye, which is more pronounced in specimens from Lake Brienz. Individuals from both lakes have a thin and roundish eye-socket from the middle to the outer margins. Pectoral fin long and moderately tapered. Anterior unbranched ray of the erected dorsal fin range from almost vertically straight to an approx. 70–80° angle to body axis and only bent slightly posteriorly at the end of the ray. Caudal peduncle narrow and elongated with caudal fin forked and sometimes moderately to strongly asymmetrical with either the ventral or dorsal part being longer. Unbranched ray of anal fin straight and rarely bent posteriorly at the end of the ray. Anal fin longest anteriorly and progressively shortening posteriorly with the outer margin of the anal fin slightly concave.

Meristics : Many and long gill rakers.

Colour: Pigmentation of fins and body overall weak in live specimens. In specimens from Lake Thun the pectoral fin is translucent, sometimes yellowish with faint pigmentation at the median to distal parts of the fin. Pelvic fin is translucent and only weakly to moderately pigmented. Dorsal, adipose, anal and caudal fins are moderately pigmented. In specimens from Lake Brienz all fins are translucent, with the dorsal, anal and caudal fins sometimes showing some very faint pigmentation. In both lakes fish have a silvery appearance along the flanks and dorsally above the lateral line the silvery appearance changes to a pale rose colouration (e.g., RGB (247, 187, 175)) and then to a pale brown (e.g., RGB (230, 202, 110)). In specimens from Lake Thun the flanks very rarely have few pigmented small dots on the scales. Distribution of dots are bound to the scale patterning (i.e., at the edge of the scales or at the boundary point of two scales. In specimens from Lake Brienz the upper dorsum ranges from pale brown (e.g., RGB (230, 202, 110)) to a light green colouration (e.g., RGB (136, 245, 205)) and sometimes has pigmented dots more or less in a row on the upper dorsum that are rather large ("cheetah look") (Suppl. material 1: Figure S7). Distribution of the dots not restricted to the scale patterning (i.e., at the edge of the scales or at the boundary point of two scales), as can be found for the species of C. alpinus, C. steinmanni, C. brienzii and C. fatioi. For a comparison to the main colouration found in the other species see Suppl. material 1: Figure S8. Dorsal part of head of specimens of Lake Brienz is weakly pigmented, whereas that of specimens from Lake Thun is moderately pigmented. Snout around the nostrils is weakly (Lake Brienz) to moderately (Lake Thun) pigmented with a gap of little pigmentation posteriorly of the nostrils up to the height of the middle of the eyes. Operculum and pre-operculum are silvery with one black dot on the lower margin of the pre-operculum. Preserved specimens are pale in colouration with similar pigmentation as described for live specimens. The silvery,translucent,not coloured or unpigmented parts of the body become brown-yellowish (e.g., RGB (239, 210, 40)), whereas the pigmented parts are conserved and the coloured parts (dorsally above the lateral line) become brownish (e.g., RGB (186, 140, 100)).

Coregonus albellus is found in the lakes Thun (46°40'N, 7°46'E) and Brienz (46°43'N, 7°57'E) that are connected by the short river Bödeli Aare at Interlaken. It is believed to have been endemic to these lakes yet,individual fish have been caught in Lake Biel (47°05'N, 7°10'E) in recent years (since 2005), after it was artificially connected with Lake Thun through the river Aare during the Jura water correction project dating back to 1868–1878. Individuals of C. albellus were first identified by local fishermen and fisherwomen, which reported that they had caught small, ripe fish during the summer months (Bittner 2009). The native whitefish species of Lake Biel only spawn in the winter months (Fatio 1885; Steinmann 1950; Rufli 1978). Genetic analysis has shown that these summer-ripe individuals belong to the species C. albellus (Bittner 2009). We show for two ripe specimens caught in summer in Lake Biel, genetically assigned based on the assignment method of Dönz et al. (2018) to C. albellus with 84% and 94% probability, that they can also be assigned to C. albellus based on their morphology (gill raker number, morphological characters) (Suppl. material 1: Figure S9). The species may have established an independent population in Lake Biel, since ripe fish have now been caught for several years in reasonable numbers during the usual spawning period known for this species from Lake Thun (Bittner 2009; Vonlanthen and Périat 2018). Coregonus albellus feeds predominantly on zooplankton (stomach content for Lake Brienz: Maurer and Guthruf 2005; Müller et al. 2007; isotopic signature for both lakes: Selz 2008; Hudson 2011; Ingram et al. 2012) and has a slow growth rate (Kirchhofer 1995; Müller et al. 2007; Bittner et al. unpublished). The gill raker number and length of C. albellus (many and long gill rakers) also suggests that, based on the functional properties of the number of gill rakers (experimentally tested with specimens of this species and other whitefish species from lakes Thun and Lucerne) (Lundsgaard-Hansen et al. 2013; Roesch et al. 2013), that C. albellus feeds predominantly on zooplankton. Habitat-stratified random sampling of lakes Thun (mid-October 2013: Vonlanthen et al. 2015) and Brienz (mid-September 2011: Vonlanthen et al. 2013) show for a snapshot of a few months in summer, that C. albellus in Lake Thun occupies the moderately shallow to the deepest benthic waters (approx. 30–217 m; N = 29) and the moderately shallow to moderately deep pelagic waters (approx. 10–70 m; N = 44) (Dönz et al. 2018). In Lake Brienz C. albellus occupies the very shallow (few meters) to the deepest waters (260 m) of the benthic habitat (N = 78) and the very shallow to the deeper waters of the pelagic habitat (few meters down to approx. 60 m and exceptionally down to 130 m; N = 47) (Dönz et al. 2018). It is to note that the habitat-stratified random sampling data for both lakes only covers a short period of time (one month in late summer) and it is thus not clear how the species are distributed spatially through the rest of the year. Furthermore, the habitat-stratified random sampling in both lakes did not distinguish between ripe and unripe specimens, and thus in the case of C. albellus the distribution pattern along the depth in the benthic zone is biased by the spawning aggregation of this species since the sampling period in both lakes coincides with the main spawning season of this species. Most of the whitefish that were phenotypically assigned as C. albellus and that were caught in deeper waters during habitat stratified sampling of lakes Brienz and Thun were ripe (PV pers. obs.). In Lake Thun C. albellus phenotypically resembles C. fatioi and to some extent C. profundus. Interestingly, Steinmann (1950) already mentioned for Lake Thun that C. albellus (Steinmann, 1950: Coregonus lavaretus L. nat. arurenis, oekot. nanus; common name: "Zwergalbock" or "Brienzlig") resembles morphologically C. fatioi (Steinmann, 1950: Coregonus lavaretus L. nat. arurenis, oekot. pelagicus; common name: "Schwebalbock" or "Albock"). The average size (total length) at 3 years of age for specimens in this study is 258±13 mm (mean and standard deviation, N = 9) and 152±8 mm (N = 14) for lakes Thun and Brienz, respectively (Suppl. material 1: Figures S4–S6). In Lake Brienz the size of 3-year old specimens of C. albellus is considerably smaller than that of the other three whitefish species (C. alpinus, C. brienzii, C. fatioi), whereas in Lake Thun it is similar to that of C. profundus and C. fatioi (Suppl. material 1: Figure S6) and smaller than that of C. alpinus, C. steinmanni, and C. acrinasus. Coregonus albellus has a long spawning season with two peaks. The main spawning peak is in late summer/early autumn from August to October (Locally known as "Sommer-Brienzlig") and the second peak is in early to late winter from December to March (locally known as "Winter-Brienzlig") (Suppl. material 1: Figure S3; Bittner 2009; Dönz et al. 2018). Spawning depth varies with spawning season and can range from approx. 30 m to max. lake depth at 217 m in Lake Thun and approx. 50 m to max. lake depth at 261 m in Lake Brienz (Suppl. material 1: Figure S3; Bittner 2009; Dönz et al. 2018). The spawning season and depth of C. albellus partially overlaps with that of C. steinmanni, C. fatioi,and C. profundus in Lake Thun and with that of C. brienzii and C. fatioi in Lake Brienz.

Brienzlig, Brienzling; often the time of the year the fish is caught on the spawning grounds is added to the name and shows that this species has a very wide spawning season: Sommer-Brienzlig (for summer) or Winter-Brienzlig (for winter). This species was historically known by local fishermen and fisherwomen as white whitefish (German: "Weissfelchen", but also Albele and Albuli). The common name for this species today is Brienzling which has an ending that is known as a diminutive suffix.