Functional morphology of an adaptive radiation – sailfin silversides from Sulawesi as a model system

Abstract

The adaptive fish radiation of sailfin silversides from Sulawesi serves as a model system for studying adaptive divergence. The phylogenetic lineage of “roundfins” is endemic to ancient Lake Matano and is composed of three morphospecies. These occupy different ecological niches and have evolved distinct morphological adaptations to exploit them. Since these adaptations are presumably facilitated by ecologically-based divergent selection, they are likely driven by ongoing ecological speciation processes. However, morphological and ecological variation is not limited to an interspecific level but is also present between sexes. In fact, roundfins were the first adaptive fish radiation in which sexual niche partitioning was detected, but its impact on the radiation remains unclear. While 14 of the 16 sailfin silversides from Sulawesi inhabit lakes, only two species – <em>Marosatherina ladigesi</em> and <em>Telmatherina bonti</em> – occur in flowing habitats. Contrary to the species of Lake Matano, the ecology and morphology of both riverine species remain unstudied. <br /> In chapter 2 of this thesis, I use the roundfins of Lake Matano to assess the benefits and drawbacks of 2D and 3D geometric morphometrics (GM) on laterally compressed fishes. We did not detect any differences in accuracy or measurement error between both analyses. Although the z-axis contained a significant amount of variance in the 3D approach, it was irrelevant for distinguishing species or sexes. Despite this limited additional value, the 3D data set required a substantially higher effort, processing time and data size than the 2D data set. Taken together, 2D GM was more efficient than 3D GM for discriminating roundfins. Further studies on a broad range of taxa are needed to project these findings on laterally compressed fishes in general. <br /> In chapter 3, I investigate if the sexual niche differentiation detected in roundfins has induced the evolution of adaptations in their heads. We detected sexual dimorphism in both morphospecies that show sexual niche partitioning while it was absent in the only morphospecies that does not show sexual niche differentiation. The revealed sexual differences are connected to the ecological niches and match the degree of sexual niche differentiation of each morphospecies. Since all analysed structures are ecologically relevant and partly situated internally, it appears likely that the identified sexual dimorphism has evolved due to ecologically-based divergent selection. This ecological character displacement presumably reduces ecological niche overlap and minimizes intersexual competition, which contributes to the adaptive spectrum of roundfins – and possibly other radiations. <br /> In chapter 4, I test if the stream-dwelling sailfin silversides, <em>T. bonti</em> and <em>M. ladigesi</em>, use different feeding modes and whether their feeding biomechanics differ from roundfins. Prey capture kinematics indicate that <em>T. bonti</em> is a ram feeder while <em>M. ladigesi</em> is a suction feeder. Intersexual variation in kinematics was significant in both species. Contradicting these findings, feeding biomechanics did not reveal any clear indications for alternative feeding modes or sexual dimorphism in both species. In conclusion, the lack of differentiation in biomechanics might be due to contrasting trophic niches and variable environmental conditions. However, this hypothesis needs to be treated with caution until the trophic ecology of both steam-dwelling species has been quantified.