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.