Reciprocal Subsidies: Phytodetrital Influence on the Reproduction of Methanotrophic Mussels, and the Contribution of Mussel Larvae to Planktonic Food Webs in the Gulf of Mexico and Western Atlantic
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Date
2024-08-07
Authors
Plowman, Caitlin
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Publisher
University of Oregon
Abstract
There is growing evidence that chemosynthesis-based ecosystems in the deep sea influence the benthic and pelagic habitats around them, though the extent of these connections is unknown. Bathymodiolin mussels are among the dominant fauna at deep-sea chemosynthetic systems such as methane seeps and hydrothermal vents. Despite their symbiotic bacteria, many species in the subfamily have shown evidence of periodic reproduction driven by seasonal changes in surface productivity. In this dissertation, I explore the reproductive patterns of bathymodiolin mussels from methane seeps in the Gulf of Mexico and Northwestern Atlantic and how they contribute to connections between surface waters and the deep sea. In Chapter II, I use gonad histology to examine how the reproductive phenology changes with the season, ocean basin, and depth for three species of bathymodiolin mussels, Gigantidas childressi, Bathymodiolus brooksi, and Bathymodiolus heckerae. I find inter- and intra-specific variations across a bathymetric gradient and between the two ocean basins. The results further support that surface productivity plays a role in bathymodiolin reproduction. In Chapter III, I use carbon and nitrogen stable isotopes to investigate phytodetritus as a potential source of supplemental nutrition for reproduction. I also compare the phenology to satellite-derived measurements of surface productivity. Many of the sampled populations of bathymodiolin mussels incorporate phytodetrital nutrition, but it is not a necessary subsidy for reproduction. The onset of gametogenesis occurs near a peak in surface productivity, and spawning occurs during a change in surface productivity that is shortly followed by a peak. Thus, surface productivity may be an environmental cue that helps synchronize the gametogenic phenology of these mussels.
While Chapters II and III examine how the water column affects the reproduction of bathymodiolin mussels, Chapter IV examines how spawned mussel eggs influence the water column. For Gigantidas childressi from three seep sites, I used histology to estimate individual fecundity, bomb calorimetry to measure the energy contained in an egg, and video of the sites to determine the size of the mussel beds. All metrics vary among the sites; changes in fecundity correspond to changes in phenology, while the nutrient density of eggs increased with depth. Each factor affects the total energy released through spawned eggs, but the estimated values are on par with the photosynthetic production of oligotrophic surface waters. Most of the spawned eggs are eaten and thus may represent a critical trophic subsidy for the surrounding ecosystems.
The energy these mussels use to create gametes comes mostly from chemosynthetic sources. Seasonal pulses of phytodetritus may then trigger the spawning of these eggs, releasing much of that energy into the pelagic food web. This research highlights the intimate reciprocal connection between processes in the surface ocean and the deep seafloor. This dissertation includes previously unpublished co-authored material.
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Keywords
Bathymodiolus brooksi, Bathymodiolus heckerae, energy flux, Gigantidas childressi, methane seep, reproductive phenology