UC Davis

Though environments change across space and time, living organisms must always find ways to succeed under the mechanical constraints posed by the world around them. The nearly 25,000 species of teleost fishes inhabit nearly every imaginable aquatic habitat and have amazing diversity of shapes, sizes, and color patterns. Yet all fishes must eat, and almost every living teleost fish is capable of using ‘suction feeding’ for prey capture. Suction is a dynamic feeding mechanism that leverages the density and viscosity of water to pull in water and a prey item via rapid expansion of the head. But there are many common prey, like algae, sponges, or coral, that are not easily suctioned in by a hungry critter: and thus, some fishes use ‘biting’ to graze, scrape, or yank prey from an attachment to the substrate. My dissertation explores how the interactions between animals and their physical world shape evolutionary processes over millions of years, seeking to understand the drivers of diversity in form and function.

In my first chapter, I explored the potential for a fundamental mechanical trade-off to constrain the evolution of biters. Fishes that use biting for prey capture can also use suction, but this may create a trade-off between cranial expansion for suction and force transmission for biting. I studied how this mechanical trade-off affects diversification of both head shape and feeding kinematics. I filmed high-speed videos of feeding strikes of fishes of both groups, then used geometric morphometrics to calculate the amount of motion of the anatomy of the head during feeding, which I refer to as “cranial mobility.” I then compared rates of evolution of cranial mobility and of head shape between biters and suction feeders. I demonstrated that the trade-off results in less diversity of kinematics, which evolve more slowly in biters, but accelerates evolution of morphology, as biters evolve head shape more rapidly than suction feeders. These results show the potential for trade-offs to provide differing evolutionary constraints on the evolution of morphology and kinematics, and to create a mismatch between the adaptive landscapes of form and function.

In my second chapter, I reconstructed the history of feeding mode among 1,530 species of reef fishes and found that prior to the end-Cretaceous mass extinction, over 96% of reef-associated lineages were suction feeders. But I found that there has been a trophic revolution among fishes since then, and benthic grazers make up fully 40% of modern reef fish species diversity. Alongside ecological shifts in the structure of reefs, innovations for improved biting by fishes at the dawn of the Cenozoic may have provided opportunities for biters to thrive and resulted in a trophic revolution among reef fishes. Using body shape data for all 1,530 species, I showed that benthic grazers are evolving body shapes 1.7x faster than suction feeders. These results suggest that benthic grazing may inherently elevate evolutionary potential by providing access to attached prey with diverse functional properties. This study demonstrates that there has been a transformation the trophic makeup of reef fishes in the last 65 million years, and that biting has been a major contributor to the ecological and phenotypic diversity of reefs.

In my third chapter, I leveraged body shape data from 5,940 species of teleost fishes across 392 families from a dataset that I helped collect at the National Museum of Natural History to understand evolution of extreme body shapes in fishes. Fishes range from pancake-like, dorsoventrally flattened goosefishes to species that have been laterally compressed into a disc-like shape, such as surgeonfishes, and even to slender, elongate eels. Yet fishes of all these shapes are subject to the same physical constraints of life underwater. How, then, is it possible for some lineages to “break the rules” and evolve extreme body shapes? One intuitive possibility is that fishes are concentrated in shape space because it is hard to evolve or function with these odd shapes, and so evolution towards extreme shapes is a long, slow battle against the physics of aquatic life. Yet this work found that species further in shape space from the average body shape are evolving more rapidly, suggesting that the evolution of an extreme body shape requires the release of the constraints on the formation of a typical body shape, such as a major developmental or ecological shift, likely paired with a major shift in adaptive zone. Such a shift in adaptive zone may result in dramatic evolution of body shapes and allow the evolution of extreme or unique body shapes.