Abstract
Among fishes, salmonids (family Salmonidae) have attracted a great deal of research attention focused on sexual dimorphism and associated selective forces. Most of this research has been directed toward anadromous and mostly semelparous salmon and trout (Oncorhynchus, Salmo), and comparatively little is known about intersexual variability in strictly iteroparous freshwater salmonids. We examined a comprehensive data set of 28 linear morphometric characters in 11 of 15 currently recognised species of grayling (Thymallinae, Thymallus), a genus consisting of iteroparous species only, to identify general patterns of intersexual morphological variability. Overall, we found that all grayling species show common sex-specific traits particularly relating to size dimensions of the dorsal, anal, pelvic and pectoral fins. Although the magnitude of sexual dimorphism differed among species, there was no significant phylogenetic signal associated with these differences across the genus. These results are discussed in terms of the assumed selection pressures driving sexual dimorphism in graylings and are compared to existing knowledge in Salmonidae as a whole where similarities and differences with both Salmoninae and Coregoninae exist. The present study provides the first detailed genus-wide comparison of sexually dimorphic phenotypic characters in graylings, and highlights the need for more large-scale comparative studies in multiple salmonid species to better understand general macroevolutionary trends among this important group of freshwater fishes.
Highlights
In many animal taxa, a key aspect of intraspecific variability is associated with sexual dimorphism (Andersson 1994), the differences in physiology, morphology and behaviour of conspecific males and females
Males were larger than females in the global data set (263.7 ± 73.7 mm vs. 256.9 ± 73.2 mm; two-way Analysis of Variance (ANOVA), F1,1517 = 8.208, P = 0.004; Welch, F1, 1513.03 = 3.938, P = 0.047), but this pattern was not consistent across all species reflected by the significant species x sex interaction (Table 2)
When species-specific pairwise differences were examined, predorsal length, length of dorsal-fin base, height of anterior part of dorsal fin, height of posterior part of dorsal fin, height of anal fin, and length of the pelvic and pectoral fins showed a uniform pattern of sexual dimorphism (Figs. 3, S1), these differences were not statistically significant for all Species x Sex
Summary
A key aspect of intraspecific variability is associated with sexual dimorphism (Andersson 1994), the differences in physiology, morphology and behaviour of conspecific males and females (sensu Punzalan and Hosken 2010). Semelparous Pacific salmon (Oncorhynchus), undergo dramatic phenotypic change during the reproductive period, which includes the development of an elongated snout, enlarged teeth, hooked jaws, dorsal hump, elongated fins, thickened skin, and bright colouration (Fleming and Gross 1994; Quinn and Foote 1994). These exaggerated traits are usually male-biased (i.e. larger, thicker or more pronounced in males) and are assumed to have evolved as a consequence of sexual selection where males compete for fertilization opportunities (Fleming and Reynolds 2004). The presence and degree of sexual dimorphism in these traits differ between taxonomic groups and show great intraspecific variability, which often is habitat associated (Johnson et al 2006; Oke et al 2019)
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