Coregonus acrinasus Selz, Doenz , Vonlanthen & Seehausen, 2020

Selz, Oliver M., Doenz, Carmela J., Vonlanthen, Pascal & Seehausen, Ole, 2020, A taxonomic revision of the whitefish of lakes Brienz and Thun, Switzerland, with descriptions of four new species (Teleostei, Coregonidae), ZooKeys 989, pp. 79-162 : 120-124

Coregonus acrinasus Selz, Doenz, Vonlanthen & Seehausen sp. nov.

Coregonus "Albock": Kirchhofer 1995 (see also synonymy of C. alpinus and C. steinmanni )

Coregonus "Albock", "THU1": Douglas et al. 1999; Douglas and Brunner 2002; Douglas et al. 2003 (see also synonymy of C. fatioi )

Coregonus fatioi : Hudson et al. 2011, 2013, 2016; Ingram et al. 2012; Vonlanthen et al. 2012

Coregonus sp. "Albock": Doenz et al. 2018

Material examined.

Holotype. NMBE-1077271 , Switzerland, Lake Thun (46°40'N, 7°46'E), 239.5 mm SL, male.

Paratypes. NMBE-1077238-1077240 , NMBE-1077268-1077270 , NMBE-1077272-1077290 , Switzerland, Lake Thun (46°40'N, 7°46'E), N = 25, 197-278 mm SL.

Diagnosis.

Coregonus acrinasus is a medium-sized whitefish with moderate pigmentation of all fins and body; dark greenish blue colour on the flanks above the lateral line; moderate to many pigmented small dots on the scales; tip of the snout pointy; long head; small eye with a thick and triangular shaped eye socket; many and moderately long gill rakers.

Differential diagnosis.

Coregonus acrinasus only occurs in Lake Thun and shows ancestry contributions from whitefish of Lake Constance, besides its Lake Thun ancestry. These derive from historically documented introductions of at least two whitefish species ( C. wartmanni and C. macrophthalmus ) into Lake Thun. Since, historically undocumented introductions of other whitefish from Lake Constance cannot be excluded and since there is no clear genetic assignment of C. wartmanni or C. macrophthalmus as likely source of the allochthonous introgression we compare the characters of this species with those of all whitefish species from Lake Constance and all other whitefish species from Lake Thun. The differential diagnoses against C. albellus , C. alpinus , C. fatioi , C. steinmanni and C. profundus are given under those species’ accounts.

Lake Constance comparison.

Coregonus acrinasus -all four Lake Constance species

The wider underjaw of C. acrinasus (9.2-14.3% HL, mean = 12.2) differentiates it from all other species from Lake Constance, C. gutturosus (6.8-9.9% HL, mean = 7.7), C. arenicolus (7.8-8.5% HL, mean = 8.1), C. macrophthalmus (6.4-8.8% HL, mean = 8) and C. wartmanni (8.1% HL) (Tables 9 , 12 ).

Coregonus acrinasus - Coregonus wartmanni

Coregonus acrinasus differs from C. wartmanni by having a larger eye and eye cavity (eye diameter: 21.6-25.5% HL, mean = 23.7 vs. 18.9% HL; eye cavity: 26-29.6% HL, mean = 27.7 vs. 23.9% HL; eye height: 21.7-24.8% HL, mean = 22.9 vs. 19% HL) (Tables 9 , 12 ).

Coregonus acrinasus - Coregonus macrophthalmus

Coregonus acrinasus differs from C. macrophthalmus by having a wider head (43.9- 56.2% HL, mean = 49.6 vs. 39.3-43.3% HL, mean = 41.6) (Tables 9 , 12 ).

Coregonus acrinasus - Coregonus gutturosus

Coregonus acrinasus differs from C. gutturosus by having more and longer gill rakers (upper arch gill raker number: 10-15, mode = 13 vs. 7-9, mode = 7; lower arch gill raker number: 20-26, mode = 24 vs. 9-12, mode = 10; total gill raker number: 30-40, mode = 36 vs. 16- 21, mode = 19; middle gill raker length: 9.1-16.6% HL, mean = 13.4 vs. 4.1-8.7% HL, mean = 6.9; longest gill raker length: 11.4- 16.9, mean = 14.5 vs. 6.7-10.6% HL, mean = 8.2), a longer lower jaw (38.6-47% HL, mean = 40.9 vs. 34.3-39.1% HL, mean = 36.6) and a shorter head (13.8-16.1% HL, mean = 15.2 vs. 15.4-18.1% HL, mean = 16.8) (Tables 9 , 12 , Suppl. material 1: Table S7).

Coregonus acrinasus - Coregonus arenicolus

Coregonus acrinasus can be differentiated from C. arenicolus by having more and longer gill rakers (lower arch gill raker number: 20-26, mode = 24 vs. 13-19; total gill raker number: 30-40, mode = 36 vs. 22-31; middle gill raker length: 9.1-16.6% HL, mean = 13.4 vs. 9.8- 10.6% HL, mean = 10.2; longest gill raker length: 11.4-16.9, mean = 14.5 vs. 10.9-12% HL, mean = 11.5), a larger eye (eye diameter: 21.6-25.5% HL, mean = 23.7 vs. 17.3-19.6% HL, mean = 17.7; eye cavity: 26-29.6% HL, mean = 27.7 vs. 24.1-25.7% HL, mean = 25; eye height: 21.7-24.8% HL, mean = 22.9 vs. 18.8-20.8% HL, mean = 19.6) and a shorter anal fin (9.2-13% HL, mean = 11.6 vs. 12.9-13.8 % HL, mean = 13.3) (Tables 9 , 12 , Suppl. material 1: Table S7).

Description.

General appearance is shown in Figure 10. Morphological and meristic characters of both sexes can be found in Tables 9 , 12 , and Suppl. material 1: Tables S6, S7 and first- and second-best ratios for both sexes combined can be found in Table 10 . The description is valid for both sexes.

Shape: Only slightly deep bodied with greatest body depth anterior of the dorsal fin. Dorsal and ventral profile equally arched such that both the dorsal profile from the tip of snout to the anterior origin of dorsal fin and the ventral profile from the interorbital area to the pelvic fin origin are moderately convex. Head long. Mouth (i.e., width of upper and lower jaw) is thick, moderately long and often sub-terminal and only rarely terminal. Rostral plate is mostly wider than deep, not strongly pronounced and the tip of the snout is often pointy in the sagittal plane. Eye-socket thick and triangular (i.e., sickle-shaped). Pectoral fin moderately tapered. Anterior unbranched ray of the erected dorsal fin has an approx. 40-60° angle to body axis and from the middle to the end of the ray it is moderately bent posteriorly. Caudal peduncle stout and moderately long. Caudal fin forked and sometimes slightly asymmetrical with the ventral part being longer. Unbranched ray of anal fin mostly straight and only sometimes slightly bent posteriorly. Anal fin is longest anteriorly and progressively shortening posteriorly with the outer margin of the anal fin slightly concave.

Meristics: Many and moderately long gill rakers.

Colour: Pigmentation of fins and body overall moderately strong in live specimens. Pectoral fin is mostly transparent to moderately pigmented with a yellowish faint pigmentation and only very rarely strongly pigmented. Dorsal, adipose, pelvic, anal, and caudal fins are moderately to strongly pigmented. Fish have a silvery ap pearance along the flanks with moderate to many pigmented small dots on the scales. Dots along the flank and the dorsum. Distribution of the dots is bound to the scale patterning such that the dots are found at the edge of the scales or at the boundary point of two scales. Dorsally above the lateral line the silvery appearance changes to dark greenish blue colour (e.g., RGB (7,168,125)). The snout around the nostrils is strongly pigmented with a gap of very little pigmentation posteriorly of the nostrils up to the height of the middle of the eyes. Pre-operculum and operculum are silvery with one black dot on the lower margin of the pre- operculum. For a comparison to the main colouration found in the other species see Suppl. material 1: Figure S8. Preserved specimens are pale in colouration with similar pigmentation as described for live specimens. 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)).

Distribution and notes on biology.

Coregonus acrinasus is found in Lake Thun (46°40'N, 7°46'E). Based on isotopic signatures C. acrinasus most likely feeds on a mix of benthic prey and zooplankton ( Selz 2008; Hudson 2011; Ingram et al. 2012) and based on the size-at-age data C. acrinasus must have a rather fast growth rate (Suppl. material 1: Figures S4-S6). The gill raker number and length of C. acrinasus (many gill rakers and moderately long gill rakers) suggests, based on the functional properties of the number of gill rakers on feeding on different prey items ( Lundsgaard-Hansen et al. 2013; Roesch et al. 2013), that C. acrinasus feeds more on zooplankton and less on benthic prey, but this assumption needs to be verified in the future with stomach content analyses. The relative species abundances in the pelagic and benthic habitat from a habitat-stratified random sampling of Lake Thun (mid-October 2013: Vonlanthen et al. 2015) also points to this. Coregonus acrinasus occupies only the shallow waters of the benthic habitat (15 m; N = 1) and the moderately deep pelagic waters (approx. 10-35 m; N = 9) ( Dönz et al. 2018). However, the habitat-stratified sampling needs to be treated with caution since it only shows a snapshot in time (one month) of the spatial distribution of this and the other species. Coregonus acrinasus phenotypically resembles to some extent C. alpinus and C. steinmanni . The average size (total length) at three years of age for specimens in this study of C. acrinasus is 304 ± 21 mm (mean and standard deviation, N = 9) (Suppl. material 1: Figures S4, S6).The average size at 3 years of age for the specimens of C. acrinasus from this study is similar to that for the years 2004-2005 (322.8 ± 18 mm, N = 50) (Bittner et al. unpublished; Vonlanthen et al. unpublished). The size of 3-year-old specimens of C. acrinasus is smaller to that of C. alpinus and C. steinmanni and considerably larger than that of C. albellus , C. fatioi , and C. profundus (Suppl. material 1: Figure S6). Coregonus acrinasus has a short spawning season in late December and very rarely have ripe individuals been caught in late autumn or winter (Suppl. material 1: Figure S3; Dönz et al. 2018). Coregonus acrinasus spawns mostly in moderately shallow waters of 10m down to approx. 100 m (Suppl. material 1: Figure S3; Bittner 2009; Dönz et al. 2018). The spawning season and depth of C. acrinasus overlaps largely with that of C. alpinus , C. steinmanni , and C. fatioi and to a much lesser extent with that of C. albellus and C. profundus .

Coregonus acrinasus appears to be a species of partially allochthonous origin, closely related to the radiation of Lake Constance with genetic contributions from Lake Thun. Indications of this situation were seen in several earlier genetic studies ( Douglas and Brunner 2002; Douglas et al. 2003; Bittner 2009; Hudson et al. 2011) and this was clearly confirmed with large sample sizes recently ( Hudson et al. 2016; Dönz et al. 2018). Historical records mention the stocking of alevins of the Lake Constance endemics C. wartmanni and C. macrophthalmus into Lake Thun. To fully understand the relationship of C. acrinasus to the Lake Constance species, we compared the morphology of C. acrinasus with that of all four described species of Lake Constance, C. wartmanni , C. macrophthalmus , C. arenicolus , and the now extinct C. gutturosus . Our data clearly reveal C. acrinasus as distinct from all Lake Constance species based on morphological characters. Historical records from Fatio (1890) in his book on Swiss fish ( Fatio 1890: Page 123) and from Heuscher (1901) in his report on the biology of lakes Thun and Brienz ( Heuscher 1901: Pages 69-70, 103) report several incidences of introductions of whitefish from other lakes. Evidence for additional introductions comes from historical records from a fisheries club that was responsible for the propagation of whitefish in lakes Thun and Brienz before stocking with allochthonous fish was forbidden in Lake Thun (nothing is stated regarding Lake Brienz) in 1946 by the local fisheries authorities ( Douglas et al. 2003; Dönz et al. 2018). Since 1991 such introductions were banned in all of Switzerland through federal law (BGF 6 I b). These historical records reveal that in 1888, 1889, and 1934 in Lake Thun and 1892 in Lake Brienz between 20'000 and 750'000 (Lake Thun) and once 39'000 (Lake Brienz) fry of either C. macrophthalmus (only Lake Thun) or C. wartmanni (both lakes) were stocked. Heuscher (1901: Page 70) further noted that the introductions of 1888, 1889, and 1892 were unsuccessful in both lakes, as fishermen did not catch adult fish of either of the Lake Constance species ever after those introductions. Steinmann (1950) in his monograph on Swiss whitefish diversity did not mention any species from Lake Constance to be present in Lake Thun or in Lake Brienz. Dönz et al. (2018) could recently show with genetic data from scales dating back to 1972 and earlier that C. acrinasus was completely absent in catches of that period. The first qualitative reports of this species in spawning fisheries catches are from around the year 2000 (Douglas et al. 1999; Bittner 2009), and our own genetic data from samples of more than 2000 whitefish from Lake Thun confirm the presence of the species. Based on a recent lake-wide quantitative survey in 2015 Dönz et al. (2018) showed that this species accounts for ca. 10% of all whitefish in Lake Thun in abundance when based on genetic assignments. Several independent multilocus microsatellite and AFLP data sets suggest that it has genetic contributions from the endemic Lake Thun species and cannot clearly be designated genetically to one of the Lake Constance species ( Douglas and Brunner 2002; Douglas et al. 2003; Bittner 2009; Hudson et al. 2011; Hudson et al. 2016; Dönz et al. 2018). This suggests that some individuals of one or several of the introduced species from Lake Constance must have successfully reproduced in Lake Thun and hybridized with one or several of the local species.

Etymology.

The name C. acrinasus is a combination of the ablative case of the Latin adjective acer resulting in acri, which means pointed and the noun nasus for nose. The name acrinasus refers to a phenotypic feature of this species, which often has a pointed snout when viewed in the sagittal plane.

Common name.

Albock