THE PENNSYLVANIA STATE UNIVERSITY SCHREYER HONORS COLLEGE DEPARTMENT OF BIOLOGY THE EFFECT OF A NUTRIENT GRADIENT ON THE PORITES-LITHOPHAGA SYSTEM IN THE PHOENIX ISLANDS GAVIN VANSTONE SPRING 2017 A thesis submitted in partial fulfillment of the requirements for a baccalaureate degree in Biology with honors in Biology Reviewed and approved* by the following: Iliana Baums Associate Professor of Biology Thesis Supervisor Benoît Dayrat Associate Professor of Biology Honors Adviser * Signatures are on file in the Schreyer Honors College. i ABSTRACT Excess nutrients on coral reefs can lead to increased bioerosion by boring animals, such as mollusks and sponges. Bioerosion leads to weaker coral skeletons that are susceptible to fragmentation by both abiotic and biotic factors. The Phoenix Islands are an isolated archipelago found in the central Pacific Ocean in which the northern islands, Kanton and Enderbury, lie in the path of the Equatorial Undercurrent (EUC) creating periods of upwelling around the islands. This creates a nutrient gradient across the northern group and the southern group of islands. Two morphologically similar reef building corals, Porites evermanni and Porites lobata, were differentially susceptible to bioerosion by Lithophaga mussels, which can increase asexual reproduction rates by fragmentation. Nine of the ten islands studied here exhibited mainly sexual reproduction. Our results suggest a different trophic interaction in the Phoenix Islands than seen in the Eastern Tropical Pacific, where fragmentation by triggerfish leads to increased asexual reproduction. ii TABLE OF CONTENTS Acknowledgements ...................................................................................................... iii INTRODUCTION ....................................................................................................... 1 MATERIALS AND METHODS ................................................................................. 4 Study Sites ........................................................................................................................ 4 Sampling Method ............................................................................................................. 4 Genotyping ....................................................................................................................... 5 Analysis of Multi-Locus Genotypes (MLGs) .................................................................. 6 Mussel Count Data ........................................................................................................... 7 Population Structure Analysis .......................................................................................... 8 RESULTS .................................................................................................................... 9 Species Identification ....................................................................................................... 9 Mussel Count Data ........................................................................................................... 9 Multi-Locus Genotype Data ............................................................................................. 10 Population Structure Data ................................................................................................ 11 DISCUSSION .............................................................................................................. 12 Appendix A Tables ..................................................................................................... 15 Appendix B Figures .................................................................................................... 21 BIBLIOGRAPHY ........................................................................................................ 26 iii ACKNOWLEDGEMENTS First, I would like to thank Dr. Iliana Baums for all her help and support through this entire project. I would not be the scientist I am today without her guidance. I would also like to thank Dr. Benoît Dayrat and Dr. Michael Axtell for acting as my honors advisors over the past two years. They provided valuable help throughout the entire process. Secondly, I would like to thank the rest of the members of Baums Lab. Meghann was an absolutely amazing resource and I would not have gotten this project done without her help. Andie was the perfect person to turn to whenever I had questions or needed direction. Thanks also to Sam and Sheila, who provided help when I had nowhere else to turn. Thanks also to Caitlyn Kupp who was responsible for helping calculate and assemble the mussel boring data. I also want to thank my family and friends who helped me get through the entire project. This project was possible because of Randi Rotjan and her leadership over the PIPA 2012 Research Expedition. Thank you to Jenny Boulay and Hanny Rivera for helping collect these samples on the expedition. Without funding from Penn State’s Eberly College of Science, this project would not have been possible. 1 INTRODUCTION Coral reefs are delicate ecosystems that thrive in nutrient poor tropical and subtropical environments. Even though corals are adapted to thrive under low nutrient concentrations, collectively they create the three-dimensional structure that provides for one of the most productive ecosystems in the world and can support a wide array of marine organisms (Hallock and Schlager, 1986). Unfortunately, small changes in nutrient availability can swing the equilibrium out of favor of corals into favor of other organisms. While there is evidence that increased nutrients could be beneficial to coral under certain conditions, increased nutrient concentrations often favor macroalgae and filter feeders (D’Angelo and Wiedenmann, 2014). Increased populations of filter feeders are especially threatening to coral reefs because it can lead to increased rates of bioerosion on the reef. Bioerosion occurs when the filter feeders, such as mussels, bore into the coral skeleton (Szmant-Froelich, 1983). Some amount of bioerosion is natural as it allows for another form of asexual reproduction called fragmentation. However, large populations of filter feeders can jeopardize the integrity of the reef by weakening the reef building corals, also known as hermatypic corals. Porites is one of the most common species of hermatypic corals throughout the Eastern and Central Tropical Pacific. The Phoenix Islands have two common Porites corals, P. lobata and P. evermanni. Both species are morphologically similar, but are genetically different (Boulay et al. 2013). Scott and Risk (1988) showed that P. lobata is among the weakest hermatypic corals and its skeleton is often heavily bored by Lithophaga mussels. In the Eastern Tropical Pacific (ETP), P. evermanni is also bored by Lithophaga mussels (Boulay et al. 2013). Boring by the mussels weakens the Porites already relatively weak skeleton. Reefs with 2 increased nutrient availability, either through natural or anthropogenic causes, have higher rates of bioerosion (D’Angelo and Wiedenmann, 2014). Lithophaga are preyed upon by triggerfish that fragments the coral when it consumes the bored mussel. This is a naturally occurring process and Porites fragments have a 30-50% chance of survival when fragmented from the parent colony (Guzmán and Cortés, 1989). However, excessive fragmentation can be detrimental to the long-term health of the reef due to a decrease in genetic diversity. Decreased diversity of foundation species can make reefs less resilient to changes in the environment, making them more susceptible to disease and disturbance (Reusch et al. 2005). The Phoenix Islands is an archipelago found in the south central Pacific Ocean. It consists of eight islands and two submerged reefs. The islands are spread over an area that covers more than 100,000 square kilometers of open ocean, meaning there is varying oceanic conditions at each island. The islands also create the Phoenix Island Protected Area (PIPA), which is the largest protected marine area in the world and has helped to limit the amount of local anthropogenic disturbances. Even though there are few local anthropogenic disturbances, the reefs in the Phoenix Islands still must deal with natural fluctuations in the environment. The Phoenix Islands lie at a similar latitude to Jarvis Island, which experiences large amounts of upwelling caused by the Equatorial Undercurrent (EUC) (Gove et al. 2006). This cold, nutrient rich and eastward flowing current creates a north to south nutrient gradient in the Phoenix Islands, where there are higher levels of nutrients in the northern Phoenix Islands compared to the southern Phoenix Islands. Due to natural upwelling events, we would predict a higher degree of Lithophaga boring and thus higher rates of asexual reproduction in the nutrient rich northern Phoenix Islands as compared to the southern Phoenix Islands. Based on previous work in the Eastern Tropical 3 Pacific (ETP), we would also predict that P. evermanni will experience higher rates of mussel boring and therefore higher rates of asexual reproduction as compared to P. lobata (Boulay et al. 2013). 4 MATERIALS AND METHODS Study Sites This study analyzed 193 samples collected in 2012 from the Phoenix Islands. There were 121 samples of P. lobata and 72 samples of P. evermanni. The Phoenix Islands are an archipelago found in the central Pacific Ocean containing eight islands and two submerged reefs. Five of the eight islands within the Phoenix Islands archipelago