Resource allocation to growth and structure: The cost of mussel attachment in a dynamic coastal environment

Abstract

Specialized mechanical structures produced by organisms provide crucial fitness advantages, but the production of these materials can result in energy allocation trade-offs, affecting population and species distributions. Producing a higher quality or quantity of structure can result in less energy to invest in other processes such as growth. Energy budget models, including Scope for Growth (SFG), provide a framework with which to investigate energetic trade-offs in organisms. In wave-swept rocky shore habitats, attachment is key to survival. Mytilid bivalves produce byssus, a network of collagen-like threads that tethers individuals to hard substrate. In this dissertation I investigate linkages between energetics, mussel attachment, and growth from three different perspectives. In Chapter 1, I evaluate the linkage between energetics and mussel attachment by perturbing the energetic state of the mussels (M. trossulus and M. galloprovincialis) by exposing them to a range of temperature and food conditions and evaluating growth and attachment. In Chapter 2, I evaluate an energetic trade-off between byssal thread production and growth by combining a field manipulation with a Scope for Growth model of M. trossulus. Specifically, mussels are induced to produce a greater number of threads by severing the byssus, and the cost per thread produced is calculated from the relationship between byssal thread production and growth. Finally, in Chapter 3, I return to the linkage between energetic and mussel attachment, and address this question by calculating SFG, and exposure to other physiological stressors, across a range of natural seawater conditions at two depths over two years, for M. trossulus and M. galloprovincialis. I then use a stepwise multiple regression analysis to evaluate whether SFG, or other physiological stressors, are the best predictors of growth and attachment. Overall, I find no evidence for a relationship between energetics and mussel attachment for either species in the laboratory manipulation (Chapter 1) or in the field observation (Chapter 3). This work does provide evidence, however, for a trade-off between byssal thread production and growth, and using the SFG model I calculate that the cost of producing a byssal thread is approximately 1 J and that byssal thread production costs range up to 65% of the energy budget (Chapter 2). Together these results suggest that, unlike growth, byssal thread production is not an energetically-constrained trait, and that there is an energetic trade-off between byssal thread production and growth. Field observations (Chapter 3) also suggest that other physiological stressors, co-occurring low dissolved oxygen and pH that are predicted to be exacerbated by global change, affect growth but not byssal thread production. Overall this work suggests that in the field, growth (which includes growth of reproductive tissue) is constrained by both energetic resources and physiological stressors, and that induction of byssal thread production (i.e. cued by waves, etc.) has a cost on growth. Anthropogenic global change is projected to affect seawater conditions (i.e. lowered pH, dissolved oxygen) and increase ocean wave power. This investigation of the linkages between energetics, growth and investment in a structural material suggests that more severe abiotic stressors may increase energetic constraints, either directly by affecting feeding rates and costs or indirectly through trade-offs. Greater energetic constraints can affect organism size and reproductive output, and may ultimately affect organism fitness and population dynamics.