Title:
From microbes to whale sharks: how studying some of the smallest and largest organisms can inform elasmobranch biology and ecology
From microbes to whale sharks: how studying some of the smallest and largest organisms can inform elasmobranch biology and ecology
Author(s)
Perry, Cameron
Advisor(s)
Stewart, Frank J.
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Abstract
Whale sharks (Rhincodon typus) are the largest extant fish in the sea; however, there are still large knowledge gaps in their biology and ecology. Information about whale shark reproduction and mating has proven difficult due to logistical constraints of studying a large highly migratory pelagic species. Small oceanic islands can provide insight into these environments and ex-situ field research identified a unique and reliable aggregation of whale sharks in waters surrounding the remote South Atlantic Island of St. Helena. Sharks arrived seasonally in St. Helena waters from December to May each year, peaking in January. Using photo-ID, a total of 277 individual sharks were identified, consisting of a 1.1:1 sex ratio of male and female sharks ranging from 5-12 meters in total length, with 86% of males and 51% of females estimated to be mature. Eyewitness accounts of mating behavior were reported by reliable local observers on two separate occasions, which comprise the first observations of copulation in this species and are consistent with the size and sex demographics of the population. Acoustic telemetry showed that animals use the habitats around the entire island but are focused on the leeward side. Horizontal movements away from the island proved difficult to track, due to deep-diving behavior that either damaged or caused premature detachment of the archival satellite tags, however, some individuals showed large scale movement away from the island towards both Africa and South America. Deployment of CATS camera tags and MiniPAT tags allowed for exploration of the subsurface/diving behaviors and conspecific interactions of whale sharks in St. Helena. Deep dives (>100 meters) in St. Helena were dynamic, characterized by steep pitch angles and activity at depth. However, deep dives were not uniform suggesting that individual context and motivation are important for the function of the dives. Some evidence supports that deep dives may be associated with searching/foraging behaviors due to changes in activity and observed behaviors at depth. However, the cause of these behaviors is unknown and whether they are linked to reproductive behaviors cannot be ruled out. Pre-copulatory and social behaviors were observed on video further supporting that St. Helena is a unique location for whale shark reproductive ecology and conspecific interactions. Due to its likely role in the reproductive ecology of the whale shark, St. Helena represents a critical habitat for this endangered species.
Elasmobranchs (sharks, skates and rays) are of broad ecological, economic, and societal value. These globally important fishes are experiencing sharp population declines as a result of human activity in the oceans. Research to understand elasmobranch ecology and conservation is critical and has begun to explore the role of body-associated microbiomes in shaping elasmobranch health. There have been burgeoning efforts to understand elasmobranch microbiomes, exploring microbiome variation among gastrointestinal, oral, skin, and blood-associated niches. I identified major bacterial lineages in the microbiome, challenges to the field, key unanswered questions, and avenues for future work. There is a need to prioritize research to determine how microbiomes interact mechanistically with the unique physiology of elasmobranchs, potentially identifying roles in host immunity, disease, nutrition, and waste processing. Understanding elasmobranch–microbiome interactions may be critical for predicting how sharks and rays respond to a changing ocean and for managing healthy populations in managed care. Elasmobranchs are exposed to a plethora of microbes as they move through their aquatic environment in both horizontal and vertical dimensions. Therefore, understanding of environmental microbiomes is important for complete understanding of host-associated microbiomes in both natural and artificial settings. Longitudinal sampling of a large public elasmobranch aquarium exhibit revealed a dynamic and evolving water column microbiome across 18 months. Water column microbial dynamics were driven by a diverse nitrifying community likely linked to the initial seeding of the exhibit and waste products from the inhabitants. Few microbial members were observed across all sampling time points suggesting that there are significant selection processes that alter community composition, such as ozonation and treatment of the water. Addition of sharks and fish into the exhibit had small but measurable effects on community dynamics and composition. A final “stable” state was not reached over the course of sampling suggesting that microbial succession dynamics may still be occurring or that stable final community states may not be a reasonable expectation in managed care environments.
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Date Issued
2024-07-29
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Dissertation