4.2 Growth features
Slow growth is one of the characteristics of plateau fishes (Chen et al., 2008; Duan, 2015; Zhu & Chen,2009). The samples of S. thermaliswere captured in Lake Amdo Tsonak Co, which is located at an altitude of 4,587 m on the Tibetan Plateau, and the average growth performance index was approximately 4.23. Hence, the growth rate of this species is also slow. This slow growth is related to both life history characteristics and the unique water chemistry caused by the very high altitude. In our study, the results of the ARSS test revealed significant differences in growth features between females and males in both the planktivorous morph and the benthivorous morph. There are two possible explanations for these differences. One is the widely divergent ratios of males to females (planktivorous morph: males:females =1:1.87; benthivorous morph: males:females =1:1.96). The other is the use of different age groups to fit the growth equations. For the planktivorous morph, the age of females ranged from 6-27 yr, while the age of males ranged from 7-22 yr. For the benthivorous morph, the age of females ranged from 6-30 yr, while the age of males ranged from 6-18 yr.
Different growth characteristics between morphs normally relate to feeding biology (Hindar & Jonsson, 1982). In this paper, disparate growth characteristics between morphs were shown via the ARSS test in both females and males. Overall, in comparison to the benthivorous morph, the planktivorous morph had a higher growth performance index (φ ) and a larger asymptotic value (L ) at a lower growth coefficient (k ). The distinct growth characteristics of the two morphs could be explained by dietary differences. Although both morphs represent typical omnivorous fish, their diet compositions are significantly dissimilar (He & Chen, 2006). The planktivorous morph predominately feeds on zooplankton and small fishes in the pelagic area, thus feeding partially on animals,while the benthivorous morph mainly feeds on periphytic algae and zoobenthos in benthic zones and the river, thus preferring a plant-based diet. Jonsson & Jonsson (2001) reported that the somatic growth rate and maximum size of fish generally depended on the quality and quantity of food. Therefore, planktivorous individuals with partially animal-based diets obtain more energy than benthivorous individuals with primarily plant-based diets. That is, the individuals of planktivorous fish have access to more higher-quality food than do those of benthivorous fish, which may facilitate nutrition acquisition during growth (Jonsson & Jonsson 1997, 1998). Our results are similar to those on the growth features of lake charr (S. namaycush ) in Rush Lake (Chavarie et al., 2016) and S. younghusbandi in Yarlung Zangbo River (Duan, 2015). In addition, Swanson et al. (2003) demonstrated that intraspecific competition was reduced in a sympatric population by a decrease in dietary overlap. Therefore, in our study, dietary divergence in S. thermalisfacilitates resource partitioning and helps maintain the population-level stability of different growth patterns.
4.3 Phylogenetic analyses
Resource polymorphism in fishes, such as salmonid species or cyprinid species, may be the result of phenotypic plasticity or trait heritability (Klemetsen, 2010; Seehausen et al., 2014; Skulason et al., 2019). In the African barb (L. gananensis ), mouth polymorphism is attributed to phenotypic plasticity (Levin et al., 2018), while in the European whitefish (C. lavaretus ), which exhibits differences in gill raker count, the trophic niches of littoral, pelagic, and profundal morphs are heritable (Praebel et al., 2013). In our study, phylogenetic analyses and AMOVA results did not support the two morphs being two different species. However, the genetic differentiation of the two morphs was significant (Fst = 0.0299, p < 0.05), which may explain the significant phenotypic differences between morphs. Compared withG. e. eckloni and G. e. scoliostomus, which are sympatric species of schizothoracins in Lake Sunmcuo, a small glacial lake on the Tibetan Plateau (Zhao et al., 2009), the S. thermalis in our study exhibited a low level of genetic differentiation. Our findings are similar to those of Chavarie et al. (2016), who described lean and huronicus morphs (S. namaycush ) in Rush Lake.
4.4 Ecological mechanisms of resourcepolymorphism in S. thermalis
Multiple factors could have resulted in the resource polymorphism observed in S. thermalis . First, Lake Amdo Tsonak Co is a high-altitude headwater lake in the Salween River system on the Tibetan Plateau that shows minimal interspecific competition due to scarce species (only one highly specialized schizothoracin species and three species of Triplophysa ) and a lack of predatory fish. In addition, Lake Amdo Tsonak Co has a discrete environment with more than two distinguishable habitats that have separated food resources. For example, the lake contains plentiful zooplankton in the pelagic area, abundant periphytic algae and zoobenthos in the benthic zone, and a large amount of aquatic plants on its shoals. Thus, the availability of vacant niches (separated food resources) and intense intraspecific competition might be fundamental prerequisites driving the differentiation in morphology. Ecological differences may establish barriers to gene flow, and reproductive isolation may occur when barriers are sufficient to prevent recent gene flow. During the field investigation in Lake Amdo Tsonak Co, we observed two different types of spawning: river spawning and lacustrine spawning. Because planktivores inhabit only the lake and benthivores are distributed in the lake and its outlet and tributaries, we speculate that the planktivores spawn only on shoals within the lake, which are rich in aquatic plants, while the benthivores may spawn both on lake shoals and in watercourses. This difference in spawning ground indicates a strong possibility for the existence of partial spatial isolation. Hence, the morphological differentiation of S. thermalis might be further maintained via this partial spatial isolation.
CONCLUSION
Two morphs, planktivorous and benthivorous, related to resource polymorphism were confirmed in S. thermalis in Lake Amdo Tsonak Co. The morphs differed in morphological characteristics (such as mouth position, jaw features, mucus cavity, and pharyngeal teeth), feeding habit and growth features. Although phenotypic plasticity representing the mechanism of resource polymorphism in S. thermalis cannot be excluded, the individuals of the two morphs exhibited a significant level of genetic differentiation. The pelagic-benthic resources and scarce species in the lake might have driven the initial morphological differentiation, and the dietary dissimilarity in the two morphs might be ultimately maintained and reinforced by partial spatial isolation in the spawning grounds in the freshwater environment. Resource polymorphism might be a common phenomenon in Tibetan Plateau freshwater lakes, which sheds light on intraspecific morphological polymorphism and speciation. However, our sampling was insufficient along the lake-depth gradient (e.g., littoral, pelagic and profundal) due to inconvenient traffic and extreme environmental conditions. Thus, the detailed distribution of the two morphs within lakes needs to be further investigated. It is also not clear whether the feeding difference between the two morphs developed a long time ago or only recently; hence, carbon and nitrogen stable isotope analysis, a more reliable method that could indicate individual long-term food resource use, should be conducted to explore the dietary differences between the two morphs in future studies. In addition, the putatively recent origin of the two morphs is also unclear; thus, more sensitive genetic markers, such as y-satellite or single nucleotide polymorphism (SNP) markers, should be used to assess the level of more recent divergence between the two morphs.