Pelagic–littoral resource polymorphism in Hovsgol grayling Thymallus nigrescens from Lake Hovsgol, Mongolia

Abstract

Resource polymorphism is a widespread phenomenon in post-glacial fishes where multiple morphotypes of a species occur sympatrically and exhibit distinct resource use. Availability of open niches and high levels of within and among species competition are thought to drive differences in morphology and may provide insights into early stages of speciation. Hovsgol grayling (Thymallus nigrescens) are endemic to Lake Hovsgol, a lake colonised by fish following the Pleistocene, and are threatened with habitat loss due to climate change and illegal harvest. Previous analysis of Hovsgol grayling diet inferred through C and N stable isotopes suggested the presence of littoral and pelagic foraging groups. We hypothesised that morphology of the two foraging groups would reflect predictions of functional morphology, indicating the presence of resource polymorphism. To test this hypothesis, we evaluated evidence from C and N stable isotopes, stomach contents, fish ages, capture location and morphology. Two foraging groups of Hovsgol grayling were identified through stomach content and C and N stable isotope analysis. Individuals with greater zooplankton consumption were more frequently captured in the pelagic zone, exhibited higher gill raker counts, larger orbit (eye) sizes, longer paired fins, narrower head width, larger maxilla and smaller size at age than the group with greater reliance on benthic invertebrate prey. These differences were generally consistent with those described in other fish species exhibiting littoral-pelagic resource polymorphism.Our study provides the first example of littoral-pelagic resource polymorphism in the salmonid subfamily Thymallinae and highlights the potential influence of competition on fish evolution. K E Y W O R D Sfisheries conservation, functional morphology, Lake Hovsgol, resource polymorphism ACK N OWLED G EM ENTSWe are very grateful to Mfor their assistance with this research. We also thank J. Hoffman for providing his invaluable expertise in stable isotope analysis.

Pelagic-benthic resource polymorphism in Schizopygopsis thermalis Herzenstein 1891 (Pisces, Cyprinidae) in a headwater lake in the Salween River system on the Tibetan Plateau

“…For example, planktivorous morphs often have robust heads and long lower jaws, while benthivorous morphs are characterized by blunt round heads and short lower jaws. These findings are similar to those for European whitefish (C. lavaretus ) in Lake Ruskebukta and Lake Skrukkebukta and Thymallus nigrescens in Lake Hovsgol (Amundsen et al, 2004;Olson et al, 2019).…”

Global systematic diversity, range distributions, conservation and taxonomic assessments of graylings (Teleostei: Salmonidae; Thymallus spp.)

“…Next-generation sequencing RAD data supported a 15-30 thousand year divergence between the two taxa (Roman et al 2018). The taxon is morphologically distinct from T. baicalensis based on a small dorsal fin and a higher number of gill rakers (Knizhin and Weiss 2009;Olson et al 2019), and thus, its occupation of a distinct ecological niche supports its status as a distinct species despite relatively low genetic divergence. Mean pairwise distances among species (3.7%) conformed to minimum thresholds applied in other studies.…”

“…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.…”

Building the Climate Change Resilience of Mongolia’s Blue Pearl:

“…At Khuvsgul Lake, such changes may result in the extinction of species and also affect food supply for other species. For the fish in Khuvsgul Lake, climate change is identified as a key threat (Olson et al 2019;Tsogtsaikhan et al 2017). The ecology and daily and seasonal movements of fish in the lake are linked with the distribution of their invertebrate prey (Ahrenstorff et al 2012), and some species spawn upstream (Sideleva 2006).…”

“…Combined with lake pollution, this may result in the extinction of species that can no longer survive or compete under the changed conditions. Because of the isolated nature of the lake, its plants and animals have few options for adaptation, e.g., shifting northward to cooler waters (Olson et al 2019;Tsogtsaikhan et al 2017). This emphasizes the importance of management actions that address the existing threats to the lake (Chapter 6).…”