Date of Award

Fall 10-1-2021

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Ecology and Evolutionary Biology

First Advisor

Dunn, Casey

Abstract

The open-ocean midwater is the largest habitat for animal life on Earth, and one of the most homogeneous environments in space and time. However, these waters harbor complex food webs involving predatory animals from manifold phyla, presenting unique adaptations to capturing the scarce prey resources. These conditions create a natural laboratory, with minimal abiotic disturbances, where interspecific interactions such as predation play a major role in shaping community dynamics and the evolution of form and function. Siphonophores play a central role in midwater food webs feeding across multiple trophic levels on different prey phyla (i.e. jellyfish, crustaceans, worms, fishes). Like other cnidarians, siphonophores capture prey using stinging capsules (nematocysts) in their tentacles, but unlike other cnidarians, their prey-capture nematocysts are often organized in complex prehensile batteries (tentilla) specialized for this function. Tentilla and their nematocysts come in a wide variety of shapes and sizes across species. This dissertation explores how tentacle and nematocyst morphology coevolved with prey-type specialization and selectivity in siphonophores, and how the role of siphonophore diversity in the midwater food web is determined by their evolved specializations and the prey encountered across their depth ranges in the water column. In order to collect siphonophore specimens, I used a combination of blue water SCUBA diving and Remotely Operated Vehicles (ROVs) during offshore expeditions. I imaged and measured several morphological characters of siphonophore tentilla and nematocysts using specimens deposited at the Yale Peabody Museum, and reconstructed the evolutionary history of these characters and diet on an expanded molecular phylogeny. Contrary to most theoretical expectations, I found that siphonophore predatory specialists can evolve into generalists, and that specialists can shift their specific prey type. My results indicate that tentilla and nematocyst morphology evolved in correlation with evolutionary shifts in prey-type specialization. Dietary shifts are associated not only with the character states, but also with the mode of character evolution, and the patterns of correlations among characters. Moreover, I found evidence that distantly related small crustacean prey specialists, as well as fish prey specialists, have converged in the evolution of nematocyst shape and tentillum size. In addition, using multivariate discriminant analyses, I was able to generate dietary predictions for understudied species using tentilla morphology alone. Reviewing the literature on siphonophore feeding, I identified large gaps in knowledge and reported biases associated with visual methods. In order to bridge these gaps, I used ROV observation data to determine the diets and selectivities of deep-sea siphonophores, and DNA metabarcoding to detect prey types overlooked by visual methods. By comparing the vertical distributions of siphonophore and potential prey together with feeding observations, I found that most deep midwater siphonophore species are specialized and strongly selective for specific prey types. Using custom-built universal primers, I amplified and sequenced prey DNA from freshly-collected siphonophore gut contents. In addition, I collected and sequenced barcodes for several open-ocean species missing from public repositories to enhance the accuracy and resolution of my DNA-based gut content identifications. The metabarcoding results were largely congruent with previously published findings. In addition, they validated some of the dietary hypotheses generated from the morphology-based analyses, and revealed that small-crustacean specialists can occasionally prey on small soft-bodied animals. This dissertation advances our understanding on how open-ocean food web structure emerges from community composition and organismal traits. Incorporating explicit phylogenetic comparative methods into trophic ecology will enhance the value of descriptive systematic and oceanographic work, enabling its use to predict food web structure, nutrient flow, and ecosystem dynamics.

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