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.