Thymallus arcticus

Taylor Elsa Froufe, Steven J. Weiss Duarte V. Gonçalves Giulia Secci-Petretto Gernot K. Englmaier André Gomes-Dos-Santos Gael P. J. Denys Henri Persat Alexander Antonov Christoph Hahn Eric B., 2021, Global systematic diversity, range distributions, conservation and taxonomic assessments of graylings (Teleostei: Salmonidae; Thymallus spp.), Organisms Diversity & Evolution (New York, N. Y.) 21 (1), pp. 25-42 : 36-38

Thymallus arcticus s.l. —Arctic grayling

Occurs from just east of the Urals in Russia to Hudson Bay, Canada; a disjunct population in the Big Hole and Red Rock river drainages in Montana, USA ( Fig. 1 ). In our analysis, T. arcticus s.l. is 2.6% divergent from its sister taxon T. baicalolenensis . Phylogeographic structure across the Arctic is weak; for example, the haplotype MT063012 near the type locality (Sob River, Ob) groups closely with haplotypes from the Lena River drainage and the Okhotsk Sea catchment in far eastern Russia. Haplotype MT063010 from the presumed type locality of T. pallasii (see Dyldin et al. 2017) (Appendix I) in eastern Siberia is intermediate between most North American haplotypes and those from Kamchatka, which some authors assign to T. mertensii .

Taxonomic validity

A species based on its clear genetic divergence to other taxa, morphological distinctiveness especially in the dorsal-fin size and colouration (albeit with regional variation), and confirmed reproductive isolation to T. baicalensis ( Weiss et al. 2007) and T. baicalolenensis ( Weiss et al. 2006) . See comments in Appendix I concerning potential recognition of additional taxa, herein treated as T. arcticus s.l.

Conservation remarks

The global population of T. arcticus s.l. is listed by the IUCN as a species of Least concern (LC). Numerous reports exist of population size declines for the species locally, both in North America and Russia. Suggested Global Status: Least Concern.

Thymallus baicalensis —Baikal black grayling

Occurs throughout the Enisei River drainage including Lake Baikal and its major tributary the Selenga River ( Fig. 1 ); also in some right-hand tributaries of the Ob River drainage (Mrassu and Kabyrza rivers) represented by haplotypes MT063026 and MT063025 (Fig. 2). Displays a net divergence between 1.6 and 1.9% to four species comprising its sister clade ( T. nikolskyi , T. svetovidovi , T. brevicephalus and T. brevirostris ). Based on Koskinen et al. (2002), multiple samples throughout Lake Baikal were in Hardy-Weinberg Equilibrium reflecting a single taxon occupying Lake Baikal; this inference is also supported by morphological and genetic data from Knizhin et al. (2006b). All species delineation approaches allocated both T. baicalensis and T. nigrescens as a single MOTU.

Taxonomic validity

A species based on clear genetic divergence to other taxa, distinct dorsal-fin colouration and multiple contact zones with little to no gene flow with other species ( T. arcticus , T. baicalolenensis ) (see Knizhin et al. 2006b; Weiss et al. 2007). Thymallus brevipinnis is suggested to be a synonym of T. baicalensis (Appendix I).

Conservation remarks

Local population declines and extinctions reported; threats include hydropower development, overfishing and pollution. However, the species has a very large distribution range and occupies many habitats that are in pristine or near-pristine condition. Suggested Global Status: Least Concern.

Thymallus baicalolenensis —Baikal-Lena grayling

Small-bodied grayling occurring throughout the Lena River drainage ( Fig. 1 ). Also occurs in Lake Baikal tributaries, most notably the Barguzin River drainage, the Tiya River and Yakchinskie Lakes of the upper Angara River ( Knizhin et al. 2006c, 2008; Kirillov and Knizhin 2014). Also found in the Uda River drainage, Sea of Okhotsk ( Antonov and Knizhin 2011) and upper Amur River drainage ( Antonov and Knizhin 2011; Antonov and Mikheev 2016), including the upper Bureya River, together with T. burejensis and T. grubii , where it could be diagnosed with 100% accuracy based on morphological characters ( Weiss et al. 2020b). Its morphological and genetic distinction from T. arcticus s.l. is shown in Weiss et al. (2006) (therein referred to as T. a. lenensis) and Koskinen et al. (2002) (therein referred to as T. arcticus, Lena basin). Both its body and dorsal-fin colourations are highly distinct ( Knizhin et al. 2008; Knizhin and Weiss 2009) from T. arcticus s.l. as well as all other members of the genus ( Dyldin et al. 2017). Thymallus baicalolenensis is reciprocally monophyletic to T. arcticus s.l. with a net divergence of 2.6% (Fig. 2).

Taxonomic validity

A species showing relatively strong reproductive isolation to four species to which it comes into contact; T. baicalensis , T. arcticus s.l., T. burejensis and T. grubii .

Conservation remarks

Very large distribution area, occupies both large rivers and headwaters including small lakes. Found in numerous relatively remote and/or pristine systems. Suggested Global Status: Least Concern.

Thymallus brevicephalus —Shorthead or Markakol grayling

Reported endemic to Lake Markakol ( Dyldin et al. 2017); probably not limited to this lake. Haplotypes LC168675 and MT063035 (Fig. 2) stem from samples in the upper Irtysh River drainage; population genetic analysis shows close affinity with samples from the Kara-Kaba River ( Weiss et al. 2020a) ( Fig. 1 ). Both our mitogenome analysis and a population genetic analysis in Weiss et al. (2020a) show a very close (0.4%) sister clade relationship to T. brevirostris (Fig. 2). More data concerning morphology and ecology is needed.

Taxonomic validity

Although closely related genetically to T. brevirostris , viewed as species based on highly distinct morphology (short jaws, no dentation) and ecology (predominantly benthivore), as well as strict allopatric occurrence to T. brevirostris .

Conservation remarks

Populations within Lake Markakol are in serious decline due to overfishing (perhaps> 50% across recent decades, M. Baimukanov, pers. comm.); listed as endangered in Kazakhstan. Its strict area of aquatic occupancy (Markakol Lake) is <700 sq. km, but its distribution is likely considerably larger. Suggested Global Status: Data deficient.

Thymallus brevirostris —Mongolian grayling

Distributed across the large endorheic basin of Western Mongolia, extending into Tuva Republic, Russia ( Fig. 1 ). Considered a large-growing, piscivorous grayling, with pronounced dentation on both jaws and vomer ( Knizhin et al. 2008). Displays considerable phenotypic variability concerning head and jaw size and dentation ( Knizhin et al. 2008; Weiss et al. 2020a). Displays a close (0.4%) sister clade relationship to T. brevicephalus from Lake Markakol (Fig. 2).

Taxonomic validity

Although closely related to T. brevicephalus , viewed as a species based on highly distinct morphology (large jaws, significant dentation), ecology (predominantly piscivorous) and strict allopatric occurrence to T. brevicephalus (see also Appendix I).

Conservation remarks

Overfishing (primarily illegal), hydropower development and spawning ground deterioration are a major concern. Suggested Global Status: Vulnerable.

Thymallus nigrescens —Chovsgul grayling

Endemic to Lake Chovsgul in Mongolia. The single mitogenome (MT063028) in our analysis is 0.3% divergent from T. baicalensis (Fig. 2). Traditional genetic markers (microsatellites and single mtDNA sequences) allow no distinction between T. nigrescens and T. baicalensis ( Koskinen et al. 2002; Kaus et al. 2019).

Taxonomic validity

Treated as a species despite its very close relationship to T. baicalensis . Morphologically distinct from T. baicalensis based on a small dorsal fin and a high number of gill rakers ( Knizhin et al. 2008; Olson et al. 2019), occupies a distinct allopatric ecological niche.

Conservation remarks

Listed as endangered in the Mongolia Red List ( Ocock et al. 2006). Its habitat comprises the 2760 sq. km Lake Chovsgul. Substantial illegal harvest via gillnetting in the littoral zone has led to dramatic declines in population sizes ( Free et al. 2015). Suggested Global Status: Vulnerable.

Thymallus nikolskyi —Upper Ob grayling

Originally reported from the upper Ob River drainage; exact distribution is unclear. May occur together with T. baicalensis in the Mrassu and Kabyrza rivers of the Ob River drainage ( Fig. 1 ). Our analysis reveals a close (0.6%) sister relationship to T. svetovidovi from the upper Enisei River drainage, and considerably more divergence from the two other taxa, T. arcticus s.l. and T. brevicephalus , in the Ob River drainage (3.0% and 1.7%, respectively). Population genetic analysis shows no gene flow between T. nikolskyi and T. brevicephalus ( Weiss et al. 2020a) .

Taxonomic validity

Treated tentatively as a species whereby the distributions and genetic relationships of all grayling in the Altai-Sayan mountain region require further investigation ( Weiss et al. 2020a).

Conservation remarks

Insufficient data on its diagnosis and distribution. Suggested Global Status: Data deficient.

Thymallus svetovidovi —Upper Enisei grayling

Recently described from the Sharga Gol River in Mongolia ( Knizhin and Weiss 2009); occurs in headwater reaches of the Enisei River in Mongolia and possibly Tuva Republic ( Fig. 1 ). Bright yellow caudal peduncle and fin is highly characteristic; a close (0.6%) sister taxon to T. nikolskyi from the upper Ob River drainage.

Taxonomic validity

A species based on a unique phenotype and genetic divergence from T. baicalensis . See Appendix I for comments on potential synonymies.

Conservation remarks

Known distribution range is limited (but uncertain), populations are reportedly dense and the river systems where this taxon is found are pristine. Thus, there are currently no threats to this taxon. Suggested Global Status: Data deficient.

Figures

Fig. 1 Map showing the global distribution range of Thymallus species. Information on sampling sites and species is given in Table S1. Numbers in the map refer to known contact zones of the following species: 1 = T. arcticus s.l. and T. baicalensis in the lower Enisei River; 2 = T. arcticus s.l. and T. baicalolenensis in the lower Lena River; 3 = T. nikolskyi and T. baicalensis in tributaries of the upper Ob River; 4 = T. baicalolenensis and T. baicalensis in tributaries of Lake Baikal; 5 = T. grubii, T. tugarinae