REPRODUCTION IN A CHANGING ENVIRONMENT: MUSSELS, IMPOUNDMENTS, AND CONSERVATION

Abstract

As humans alter the environmental landscape, there is an increasing need to understand the relationship between species and the environment, how changes to the environment translate to populations and communities, and how to develop management practices that reduce or reverse our negative impacts. Different animals respond differently to environmental change as do different developmental life stages within the same species. Reproduction, for example, is often the most sensitive time period of an organism's life, yet has been largely ignored in conservation biology, partly due to the difficulties in studying reproduction (e.g. complex life cycles, migration, delayed reproduction). Nonetheless, it is vitally important that we have some basic understanding of the reproductive process in order to facilitate sound management of critically imperiled fauna. Freshwater mussels are one such globally imperiled group of invertebrates with over 70% of species considered threatened. However, very little is understood about their complex life cycle, particularly the portion of reproduction up to and including fertilization. The research detailed in my dissertation broadens our understanding of this portion of the mussel reproductive cycle and how it is being impacted by humans.

My first chapter explores the environmental variables that are important in regulating timing of gametogenesis in mussels. Using a year-long field sampling regime I found that water temperature, and in particular the number of degree days during which growth occurs, is an important correlate for the number of mature gametes present in adult mussel's gonadal tissue. Using a 3-month long laboratory study I confirmed these findings; however, I also discovered the potential for a food quality by temperature interaction in this study as mussels in my experiment were fed high quality food and had substantially more gametes present in their gonads than were ever observed in the field.

My second chapter explores how environmental variables affect the process of fertilization in freshwater mussels. I conducted a sperm viability experiment in which I manipulated water temperature (5, 15, 25, and 35oC) and measured the percentage of viable mussel sperm that were motile over time. I found that mussel sperm are viable for extensive periods of time, but that the highest motility was observed in the 15 and 25oC temperature range. I combined these data with a modeling approach to determine how mussel population dynamics and gene flow could be impacted by different thermal and flow regimes. I discovered that mussel sperm has the potential to move extremely long distances downstream, but that ultimately sperm transport is a function of stream velocity and height above the sediment at which sperm are released. Reproductive success, however, is a function of the proportion of sperm that have remained viable over time.

The research detailed in my third chapter examines the role of impoundments on the reproductive success and population attributes of freshwater mussels. Using data collected in my year-long field study, I found that mussels below a cold-water release impoundment had lower overall mussel densities, higher proportions of hermaphroditic individuals, higher prevalence of sterilizing trematodes, and lower body condition relative to mussels found above the impoundment. I also found that patterns in timing of gamete development were also unusual below the cold-water release dam. I outline a conceptual model by which alterations in temperature, stream flow, light, and food availability caused by impoundments could lead to overall negative density-dependence in mussel populations. These first three chapters illustrate the importance of natural temperature and flow regimes in maintaining healthy reproduction in freshwater mussel communities, information that is critical for managing rivers that provide habitat to mussels.

As humans continue to alter riverine landscapes, we are also likely to impact the evolutionary trajectories of species residing there. Unfortunately, another aspect of mussel biology that is also understudied is the evolution of the great diversity of freshwater mussels, particularly in North America. Several evolutionary hypotheses have been proposed for the evolution of these organisms, yet none have been tested. The goal of my fourth chapter was to address freshwater mussel evolution from the perspective of mechanisms of reproductive isolation, since barriers must exist between species to maintain distinct species identities. I examined the role that habitat use and timing of reproduction may play in isolating co-occurring, closely related mussel species of the genus Quadrula. I found that habitat overlap among closely related species varies (although is often high), but could be one isolating mechanism. Timing of reproduction, however, overlaps almost entirely among these species and is likely not a factor maintaining species identity in this genus. Further research into other isolating mechanisms is required to increase our understanding of reproductive barriers and evolution of freshwater mussels.