3.3 Growth
Back-calculation
of otolith length was performed for males, females, and both sexes of
each morph. We did not find a significant difference in the relationship
of SL with otolith radius (ANCOVA after log transformation)
between males and females in the planktivorous morph (N = 66,
males:females: 1:1.87, F = 0.382, p = 0.539 >
0.05) or benthivorous morph (N = 74, males:females: 1:1.96,F = 3.297, p = 0.073 > 0.05). Therefore, the
back-calculated SL of all ages was obtained using a modified
Frase-Lee function for both sexes (males and females combined) of each
morph, as follows:
planktivorous morph:
logeLi = 5.0789 +
(logeLc-5.0789)(logeOi /logeOc )
(R2 = 0.9241)
benthivorous morph:
logeLi = 5.1594 +
(logeLc -
5.1594)(logeOi /logeOc )
(R2 = 0.8436)
The back-calculated SL of the samples (Table S4) was used to fit
a VBGF for females, males and both sexes of each morph (Figure 8).
For the samples of the planktivorous morph, we found that females
exhibited a larger asymptotic SL (L∞ = 408.23),
lower growth rate (k = 0.104) and higher growth performance index
(φ = 4.24) than males
(L∞ = 372.22, k = 0.12 and φ =4.22). However, for the benthivorous morph, the results differed. Males
presented a larger asymptotic SL (L∞ = 371.88),
higher growth rate (k = 0.139) and higher growth performance index
(φ = 4.29) than did females (L∞ = 371.23,k = 0.12 and φ = 4.22). According to the VBGF test
performed via ARSS, we found a significant difference between males and
females for each morph (planktivorous morph:F 0.01(65,64) < 4.45, p<0.01; benthivorous morph: F 0.01(73,72)< 681.74, p < 0.01).
Females of the planktivorous morph showed a larger asymptotic SL
(L∞ = 408.23), lower growth rate (k =
0.104) and higher growth performance index (φ = 4.24) than those
of the benthivorous morph (L∞ = 371.23, k =
0.12 and φ = 4.22). Males of the planktivorous morph exhibited a
larger asymptotic SL (L∞ = 372.22), lower growth
rate (k = 0.12) and lower growth performance index (φ =4.22) than those of the benthivorous morph (L∞ =
371.88, k = 0.139 and φ = 4.23). A significant difference
was observed for females (F 0.01(91,90)< 115.86, p <0.01) and males
(F 0.01(47,46) < 47.82, p<0.01) of the two morphs. In the samples of both sexes,
individuals of the planktivorous morph exhibited a larger asymptotic SL
(L∞ = 405.14), lower growth rate (k =
0.102) and higher growth performance index (φ = 4.22) than
individuals of the benthivorous morph (L∞ =
374.22, k = 0.116 and φ = 4.21). The significantly
different VBGFs between morphs were supported by the ARSS results
(F 0.05(139,138) = 1.64, p <
0.05). The detailed results are shown in Table 2 and Table 3.
3.4 Phylogeneticanalyses
Phylogenetic analyses of the
concatenated sequence data (Cyt b + D-Loop) based on ML and BI analysis
produced similar topologies (Figure 9). With respect to outgroups and
other closely related taxa, all
analyses showed that the individuals of the two morphs were genetically
mixed and did not support the monophyly of the forms (bootstrap support
(BS) = 100; Bayesian posterior probability (PP) = 1.00). In
addition, AMOVA revealed that most
of the genetic variation (97.01%) was found within morphs, while little
genetic variation (2.99%) was present between the two morphs. These
results indicate that the two morphs of S. thermalis are
significantly genetically differentiated (Fst = 0.0299, p< 0.05).
DISCUSSION
To utilize the available separated food resources in environments,
species often display morphology with a certain degree of polymorphism
(Jonsson & Jonsson, 2001; Smith & Skulason, 1996). Resource
polymorphism is a ubiquitous
phenomenon in vertebrates and may represent a critical intermediate
stage in speciation. In our study, we demonstrated the existence of two
forms (planktivorous and
benthivorous) of S.
thermalis . Both morphological and ecological analyses showed that the
two morphs of S. thermaliscoexist in Lake Amdo Tsonak Co, which is likely caused by their
different feeding habits. The two morphs of S. thermalis might
also represent an important intermediate stage in ecological speciation.