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Functional Morphology and Evolutionary Pathways in the Locomotor System of Coral Reef Fishes

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Abstract

Coral reef fishes represent one of the most morphologically and functionally diverse vertebrate groups. Reef fishes contain more than 70 families of ray-finned fishes including speciose groups that are most abundant on reefs like wrasses, butterflyfish, damselfish, surgeonfish, and angelfish. However, also present in the reef fish assemblage are species with unique morphologies and highly specialized ecologies like frogfishes, pufferfish, triggerfish, and moray eels. Reef fish occupy all levels of the trophic pyramid from detritivores and algivores, microbial specialists, planktivores, invertivores, and carnivores. The trophic diversity is matched by diversity in habitat usage from species which are highly coral associated to those that exist exclusively in the open water above and near the reef. Further, reef fish are highly variable in their locomotor styles with species that swim exclusively using their body and caudal fin undulation to those that use numerous combinations of the median and paired fins for propulsion. Broadly, my dissertation uses phylogenetic comparative methods to explore the relationships between morphological, behavioral, and functional diversity in coral reef fishes, and to determine the evolutionary pathways reef fish have taken to diversify into novel conditions.

It is widely believed that because of biomechanical trade-offs, fish body shape and the mode of propulsion are strong predictors of swimming performance, with the best cruisers, maneuverers, and accelerators having different body forms and emphasizing different propulsion mechanisms. This paradigm is regularly projected onto routine swimming behavior and dominates the ecomorphological literature, despite the paucity of field measurements. In my first chapter, I measure variation in swimming behavior among 48 species of Indian Ocean coral reef fishes using recordings from a remote stereo video system. I measured average swimming speed, average swimming bout distance, frequency of turns, and percent of time spent station-holding and looked for the predicted trade-offs between them. I find little evidence of the expected relationships between swimming behaviors across species, little evidence that body shape affects swimming, and few differences between species that swim by undulating the body and those that emphasize the use of median and paired fins. Taxa widely thought of as archetypical maneuverers (Chaetodon) and cruisers (Caranx) were not outliers in any behaviors. My results indicate that swimming behavior is not easily predicted from simple measures of body shape and that alternative swimming modes can produce comparable behavioral profiles.

In my second chapter I further explore swimming diversity, though in this chapter I emphasize the effects of locomotor mode on fin shape evolution. In complex functional systems composed of many traits, selection can induce trait evolution by acting directly on individual components within the system, or indirectly through networks of trait integration. As a result, tension exists between the functional and developmental causes of trait integration and the capacity for diversification. I explore this dynamic in the evolution of fin shapes in 106 species from 38 families of coral reef fishes, a polyphyletic assemblage that shows exceptional diversity in locomotor function. Despite strong shared developmental pathways and expectations of a strong match between form and function, I find that species that share locomotor mode show substantial disparity in fin shape, and preferred locomotor mode is a poor predictor of fin shape. The evolution of fin shape is weakly integrated across the four functionally dominant fins and is weakened during transitions to derived swimming modes. As fin evolution is not strongly integrated there is substantial off-axis independent diversification, repeating a pattern found in many vertebrate systems. My study highlights the need for additional work on the functional consequences of fin shape in fishes and the diversifying impact of functions other than locomotion.

Trait integration and modularity have both been shown to be potent mechanisms in morphological evolution. While integration can provide access to morphologically novel phenotypes at the endpoints of axes of trait covariation, extreme phenotypes can also manifest via modular evolution producing novel trait combinations. It is unclear what the relative contributions of these two pathways are in accounting for the origins of novel or extreme forms. In my third chapter, I compare the importance of integration and modular evolution in generating novel forms in the body of coral reef fishes. I estimate novelty accommodating six morphological modules across the reef fish body and describe the evolutionary tendency of species to conform to axes of integration among modules. While just under a fifth of all reef fish species achieve extreme morphologies, evolution to the far reaches of integrated axes occurs slightly more often as the evolution of highly unique trait combinations. Through simulations, I show that strong evolutionary integration produces greater morphological novelty but only along axes of trait covariation, while weak integration allows species to enter the outer regions of morphospace with novel trait combinations but produces less extreme morphologies. Reef fishes closely match diversification under moderate integration, in which species evolve into novel regions of morphospace along integrated axes and through the evolution of extreme trait combinations facilitated by independence of modules.

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This item is under embargo until October 14, 2026.