Coregonus suspensus, 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 suspensus sp. nov.

Figs 5 , 14

Coregonus sp. 'pelagic intermediate’: Hudson et al. 2016.

Coregonus sp. ‘Schwebbalchen’: Vonlanthen et al. 2012; Alexander et al. 2017a (see also synonymy of C. intermundia ).

Material examined.

Holotype. Contemporary specimen (year: 2007): NMBE- 1078100, 258.5 mm SL, male; Switzerland: Lake Lucerne . Paratypes. All from Switzerland, Lake Lucerne: Contemporary specimens (years: 2007): NMBE-1078081, NMBE-1078082, NMBE-1078099, NMBE-1078101, N = 4, 255- 301.5 mm SL .

Diagnosis.

Coregonus suspensus is a medium-sized whitefish (standard length at 3 years of age: range = 266-315 mm, mean = 289 mm) with weak pigmentation of the pectoral fin and moderate pigmentation of all other fins and body; greenish blue colour on the flanks above the lateral line; none to a few pigmented small dots on the scales on the flanks; tip of the snout pointy; triangular eye socket; many and moderately long gill rakers (longest gill raker: 13.2-16.4% HL; total gill rakers number = 33-37).

Description.

Shape: Only slightly deep bodied with greatest body depth anterior of dorsal fin. Dorsal profile from the tip of snout to anterior origin of dorsal fin and ventral profile from interorbital area to pelvic fin origin is straight and only rarely is dorsal profile from tip of snout to interorbital area slightly convex. Head moderately short. Mouth moderately long and subtly sub-terminal. Lower jaw moderately wide. Rostral plate equally wide as deep, not strongly pronounced with tip of snout often more pointed than blunt. Eye-socket moderately thick and sickle cell-shaped. Pectoral fin moderately tapered and moderately short. Dorsal fin rather short with anterior unbranched ray of erected dorsal fin mostly 60-70° angle to body axis and slightly bent posteriorly. Dorsal fin longest anteriorly and progressively shortening posteriorly with outer margin of dorsal fin mostly concave and rarely straight. Caudal peduncle moderately stout with caudal fin moderately forked. Unbranched rays of anal fin moderately bent posteriorly. Anal fin is longest anteriorly and progressively shortening posteriorly with outer margin of anal fin mostly concave and only rarely straight. Meristics: Many and long gill rakers. Colour: Pigmentation of fins and body overall moderately strong in live specimens. Pectoral fin slightly pigmented at distal parts of fin. Pelvic fin moderately pigmented at median to distal parts of fin. All other fins strongly pigmented. Silvery appearance along flanks with few pigmented small dots (aggregation of melanophores) on scales along flank and dorsum. Distribution of dots bound to scale patterning such that dots are found at edge of scales or at boundary point of two scales. Dorsally above the lateral line silvery appearance changes to a pale greenish or dark greenish blue colour. Dorsal part of head and snout around nostrils moderately pigmented. Pre-operculum and operculum are silvery with one black spot on lower margin of pre-operculum. Preserved specimens pale in colouration with similar pigmentation as described for live specimens. Silvery, translucent, not coloured or unpigmented parts of the body brown-yellowish, whereas pigmented parts conserved and coloured parts (dorsally above lateral line) brownish.

Differential diagnosis.

The differential diagnoses against C. litoralis , C. intermundia , and C. sarnensis are given under those species’ accounts. Coregonus suspensus shows genetic ancestry contributions from whitefish of Lake Constance, besides its Lake Lucerne ancestry ( Douglas and Brunner 2002; Lundsgaard-Hansen 2009; Hudson et al. 2016; De-Kayne et al. 2022; this study). These seem to derive from historically documented introductions of fry of whitefish species from Lake Constance into Lake Lucerne ( Svarvar and Müller 1982). Due to the uncertainty of the species origin of the translocated fry and the possibility that there may have been more historically undocumented introductions of other whitefish from Lake Constance we compare the characters of this species with those of all whitefish species from Lake Constance and all other whitefish species in Lake Lucerne.

Coregonus suspensus - Coregonus nobilis

Coregonus suspensus differs from C. nobilis by having an anteriorly longer depressed dorsal fin (18.2-19.8% SL, mean = 19.1 vs. 16.7-18.8% SL, mean = 17.8), a shallower head (63.6-70.8% HL, mean = 67.5 vs. 65.9-77.8% HL, mean = 71.2) and snout (6.5-9.9% HL, mean = 8.1 vs. 7.9-12.4% HL, mean = 10.5) (Tables 4 , 5 ). Based on ratios Coregonus suspensus can be distinguished from Coregonus nobilis by having a smaller 'pectoral fin base / caudal peduncle depth’ ratio (PecFB/CD: 0.41-0.43 vs. 0.45-0.52) (Table 10 ).

Coregonus suspensus - Coregonus muelleri

Coregonus suspensus differs from C. muelleri by having strong pigmentation of all fins vs. none or very little pigmentation of the pectoral, anal, caudal and dorsal fin in C. muelleri , a deeper caudal peduncle (7.1-7.5% SL, mean = 7.3 vs. 6.3-7.4% SL, mean = 6.9), a shorter head (dorsal head length: 12.8-14.5% SL, mean = 13.8 vs. 14.4-17% SL, mean = 15.7; head length: 18.7-20.4% SL, mean = 19.6 vs. 20.3-23.5% SL, mean = 22), smaller eye (eye diameter: 21.2-22.3% HL, mean = 21.8 vs. 22.2-26% HL, mean = 24.1; eye cavity: 25.1-26.4% HL, mean = 25.6 vs. 26.4-31.1% HL, mean = 27.7; eye height: 20.8-23.3% HL, mean = 22.2 vs. 22.3-26.2% HL, mean = 24.3), differently shaped eye socket (sickle cell-shaped vs. roundish), a longer postorbital length (52.1-54.2% HL, mean = 53 vs. 48.8-52.6% HL, mean = 50.6), a shorter lower jaw (37.1-41.6% HL, mean = 40.1 vs. 40.3-45.8% HL, mean 42.7), a wider head (45.5-50% HL, mean = 48 vs. 37.2-48.7% HL, mean = 43.8) and a wider internarial width (12.1-13.6% HL, mean = 12.9 vs. 9.6-12.3% HL, mean = 11) (Tables 4 , 6 ). Coregonus suspensus can be distinguished from C. muelleri by having a larger 'predorsal length / eye diameter’ ratio (PreD/ED: 10.73-11.4 vs. 8.17-9.63) and a larger 'body depth / lower jaw’ ratio (BD/LJ: 2.71-3.33 vs. 2.01-2.63) (Table 10 ). Also, the average standard length at 3 years of age differentiates C. suspensus (range = 266-315 mm, mean = 289) from C. muelleri (160-232 mm, 194 mm) (Suppl. material 1: table S3).

Lake Constance comparison

Coregonus suspensus - all four Lake Constance species

The shorter dorsal head length of Coregonus suspensus (12.8-14.5% SL, mean = 13.8) differentiates it from all species of Lake Constance, C. wartmanni (14.5% SL), C. macrophthalmus (14.4-16.5% SL, mean = 15.7), C. gutturosus (15.4-18.1% SL, mean = 16.8) and C. arenicolus (14.8-15.3% SL, mean = 15.1).

Coregonus suspensus - Coregonus wartmanni

Coregonus suspensus differs from C. wartmanni by having a longer adipose fin base (4.2-5.5% SL, mean = 5 vs. 4% SL), a larger eye (eye diameter: 21.2-22.3% HL, mean = 21.8 vs. 18.9% HL; eye cavity: 25.1-26.4% HL, mean = 25.6 vs. 23.9% HL; eye height: 20.8-23.3% HL, mean = 22.2 vs. 19% HL), a shorter lower jaw (37.1-41.6% HL, mean = 40.1 vs. 43.5% HL), a shorter maxilla (18.3-21% HL, mean 19.9 vs. 22% HL), longer gill rakers (middle gill raker: 11.5-14.3% HL, mean = 13.1 vs. 10.8% HL; longest gill raker: 13.2-16.4% HL, mean = 14.7 vs. 11.3% HL) and fewer branched pelvic fin rays (10-11, mode = 10 vs. 12) (Tables 4 , 9 , 14 ).

Coregonus suspensus - Coregonus macrophthalmus

Coregonus suspensus can be differentiated from C. macrophthalmus by having a smaller eye (eye diameter: 21.2-22.3% HL, mean = 21.8 vs. 21.3-26.1% HL, mean = 24.1; eye cavity: 25.1-26.4% HL, mean = 25.6 vs. 25.4-30.8% HL, mean = 28.9), a shorter lower jaw (37.1-41.6% HL, mean = 40.1 vs. 40-44.4% HL, mean = 42.2), a shorter maxilla (18.3-21% HL, mean 19.9 vs. 20.1-24.7% HL, mean = 23.1), a wider head (45.5-50% HL, men = 48 vs. 39.3-43.3% HL, mean = 41.6) and more lateral line scales (80-91 vs. 73-80, mode = 80) (Tables 4 , 9 , 12 ).

Coregonus suspensus - Coregonus gutturosus

Coregonus suspensus differs from C. gutturosus by having a longer pelvic fin base (3.3-3.8% SL, mean = 3.7 vs. 3.7-4.4% SL, mean = 4.1), a shorter pelvic fin (15-16.1% SL, mean = 15.3 vs. 15.4-19.1% SL, mean = 17.1), a shorter pectoral fin (PecF2: 15.3-17.2% SL, mean = 16.4 vs. 16.8-20.3% SL, mean = 18.2), an anteriorly shorter erected dorsal fin (16.9-17.8% SL, mean = 17.6 vs. 17.6-21.6% SL, mean = 19.3), a shorter prepelvic length (47.9-51% SL, mean = 49.2 vs. 50.4-54.1% SL, mean = 52.7), a shallower head (63.6-70.8% HL, mean = 67.5 vs. 69.9-80.6% SL, mean = 74.2), a longer lower jaw (37.1-41.6% HL, mean = 40.1 vs. 34.3-39.1% HL, mean = 36.6), a shallower snout (6.5-9.9% HL, mean = 8.1 vs. 9.3-11.9% HL, mean = 10.2), longer gill rakers (middle gill raker: 11.5-14.3% HL, mean = 13.1 vs. 4.1-8.7% HL, mean = 6.9; longest gill raker: 13.2-16.4% HL, mean = 14.7 vs. 6.7-10.6% HL, mean = 8.2) and fewer gill rakers (33-37 vs. 16-21, mode = 17, 18, 19) (Tables 4 , 9 , 13 ).

Coregonus suspensus - Coregonus arenicolus

Coregonus suspensus can be differentiated from C. arenicolus by having a shorter pelvic fin base (3.3-3.8% SL, mean = 3.7 vs. 3.9-4.6% SL, mean = 4.4), a shorter pelvic fin (15-16.1% SL, mean = 15.3 vs. 16.8-18.1% SL, mean = 17.3), a shorter pectoral fin base (3-3.2% SL, mean = 3.1 vs. 3.2-3.5% SL, mean = 3.4), an anteriorly shorter erected and depressed dorsal fin (erected dorsal fin: 16.9-17.8% SL, mean = 17.6 vs. 18-20.3% SL, mean = 19.2; depressed dorsal fin: 18.2-19.8% SL, mean = 19.1 vs. 19.3-21.9% SL, mean 20.5), an anteriorly shorter anal fin (11.6-12.4% SL, mean = 11.9 vs. 12.9-13.8% SL, mean = 13.3), a shallower caudal peduncle (7.1-7.5% SL, mean = 7.3 vs. 7.7-8.2% SL, mean = 8.1), a shorter dorsal head length (12.8-14.5% SL, mean = 13.8 vs. 14.8-15.3% SL, mean = 15.1), being shallower bodied (21.5-25.1% SL, mean = 23.6 vs. 24.4-27.1% SL, mean 26.2), a larger eye (eye diameter: 21.2-22.3% HL, mean = 21.8 vs. 17.3-19.6% HL, mean = 17.7; eye height: 20.8-23.3% HL, mean = 22.2 vs. 18.8-20.8% SL, mean = 19.6), a shorter mouth width (8.9-9.7% HL, mean = 9.2 vs. 10-11% HL, mean = 10.5), a shallower snout (6.5-9.9% HL, mean = 8.1 vs. 9.7-12.3% HL, mean = 10.9), a less wide head (45.5-50% HL, mean = 48 vs. 50.5-51.8% HL, mean = 50.8), a less wide interorbital width (26.5-28.6% HL, mean = 27.5 vs. 28.8-30.8% HL, mean = 29.7), a less wide lower jaw (22.1-24.5% HL, mean = 23.1 vs. 24.9-27.2% HL, mean = 26.4), longer gill rakers (middle gill raker: 11.5-14.3% HL, mean =13.1 vs. 9.8-10.6% HL, mean = 10.2; longest gill raker: 13.2-16.4% HL, mean = 14.7 vs. 10.9-12% HL, mean = 11.5), fewer predorsal scales (31-34, mode = 32 vs. 36-44) and more gill rakers (33-37 vs. 22-31) (Tables 4 , 9 , 12 , 13 ).

Distribution and notes on biology.

Coregonus suspensus occurs in all basins of Lake Lucerne (Fig. 2 ) except possibly in Lake Alpnach. It has been caught during the spawning season in all basins except for Lake Alpnach, where no nets were set by Hudson et al. (2016). There is no isotopic or stomach content data available for C. suspensus to infer the prey spectrum of this species. Coregonus suspensus is a medium-sized whitefish species and the size (i.e., standard length) at 3 years of age of C. suspensus is smaller than that of C. litoralis , considerably larger than that of C. muelleri and almost equal to that of C. intermundia (Suppl. material 1: table S3). We cannot compare its size to C. nobilis since only older individuals of the latter species have been caught ( Hudson et al. 2016). Coregonus suspensus shows genetic ancestry contributions from whitefish of Lake Constance, besides its Lake Lucerne ancestry ( Hudson et al. 2011, 2016). We therefore also compared it to the four described species from Lake Constance, namely C. wartmanni Bloch, 1784, C. macrophthalmus Nüsslin, 1882, C. arenicolus Kottelat, 1997, and the extinct C. gutturosus Gmelin, 1818. Early indications that C. suspensus may be of partially allochthonous origin, closely related to the radiation of Lake Constance with genetic contributions from Lake Lucerne, were seen in work by Douglas and Brunner (2002). They named a population from Lake Lucerne simply as ‘Blaufelchen’ (i.e., the local name for the species C. wartmanni from Lake Constance), which grouped in their study closest to Lake Constance taxa and another species of partially allochthonous origin from Lake Thun, C. acrinasus . More recent work confirmed these findings, showing that C. suspensus has a genetic affinity (based on pairwise DAPC, neighbour-joining tree and private allele analyses) with C. wartmanni from Lake Constance ( Hudson et al. 2016; Suppl. material 1: fig. S3). Interestingly, C. suspensus may not just share a genetic affinity with C. wartmanni but also a phenotypic affinity. During the spawning season sexually mature individuals of C. suspensus were caught exclusively in the open water, which may suggest that they also spawn in the open water. This unique spawning behaviour has so far only been documented for individuals of C. wartmanni from Lake Constance ( Brehm 1884; Fatio 1890; Steinmann 1950). Further research is needed to resolve if the same spawning behaviour is present in C. suspensus . It cannot be inferred solely based on the catch locality, although all ripe individuals of C. suspensus were caught with pelagic nets in the pelagic habitat ( Hudson et al. 2016). This is not unique to this species though, as also C. litoralis and C. intermundia can be caught during the spawning season in the pelagic waters. However, in contrast to C. suspensus they seem to move from the pelagic into the benthic habitat at the time around sunset and are thus caught in the benthic habitat during spawning ( Hudson et al. 2016). Coregonus suspensus has most likely a short spawning season in winter around the month of December. The spawning season lasts for approx. two weeks. Since the 1940s whitefish fry have been stocked from Lake Constance (among other lakes, see discussion) into Lake Lucerne, specifically in Lake Alpnach ( Svarvar and Müller 1982; E. Odermatt and J. Muggli, pers. comm.). Nothing is written about the time span of these introductions nor which quantity of fry from Lake Constance were introduced into Lake Lucerne. Steinmann (1950) reported individuals not corresponding to any of the known Lake Lucerne whitefish species and questioned if additional unknown species were residing in Lake Lucerne. Specifically, Steinmann (1950) mentions an increase of whitefish individuals in 1949-1950 that have been caught mostly in the basins ‘Gersauerbecken’, ‘Urnerbecken’ and Lake Alpnach (Fig. 2 ), which resemble in gill raker number individuals of C. nobilis , but which spawn in December in the upper water column (i.e., in German ‘oberflächlich’). These individuals have been named by the commercial fishermen after the colouration of their dorsum, which was blueish, as ‘Blaufelchen’ and were even noted in the commercial fisheries statistics of Lake Lucerne for a certain time period (J. Muggli, pers. comm.). This is the same local name that is given to C. wartmanni from Lake Constance. This could possibly be the first mention of C. suspensus in the scientific literature. However, already in the year 1661 Cysat described three forms of large-type whitefish in Lake Lucerne, the ‘Krautbalchen’, the ‘Schwembalchen’ and the ‘Steinbalchen’ and a local fisherman, J. Blättler, mentioned in 1908 that he was able to distinguish between three forms of large-type whitefish in the lake ( Muggli 2015). It is thus unclear, if C. suspensus derived from recent allochthonous stocking and/or if one of these three forms may partially be what we know today as C. suspensus .

Etymology.

The specific name suspensus in Latin means being ‘suspended’ or ‘levitating’. The name refers to the fact that this species has only been caught during the spawning season in the pelagic water column. An adjective.

Common names.

This species was not recognised by the local fisheries, fisheries authorities, researchers, or the public before the work by Lundsgaard-Hansen (2009) and Hudson et al. (2016). Lundsgaard-Hansen (2009) and Hudson et al. (2016) named the species as Coregonus sp. ' pelagic-intermediate ', in German 'pelagischer Schwebbalchen’.

Figures

Figure 2. Map of Switzerland with the lakes studied; 1. Lake Constance, 2. Lake Sempach, 3. Lake Zug, 4. Lake Lucerne, 5. Lake Sarnen. On the right is a bathymetric map of Lake Lucerne with the German names of each basin (modified from Alexander et al. 2017 a).

Figure 5. Coregonus suspensus, Lake Lucerne, Switzerland A holotype, NMBE- 1078100, 258 mm SL, male, freshly caught specimen, (right side, reversed) B, C holotype, NMBE- 1078100, preserved specimen D non-type, Eawag- 124305, Lake Lucerne, 300 mm SL, male. 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.