Reproduction, settlement, and phenology of intertidal barnacles: Implications for larval dispersal

Abstract

Knowledge of the consequences of ocean warming on marine populations and communities is urgent. Warming oceans are predicted to result in changes to the seasonal timing of reproduction and settlement (phenology); faster development rates and, for crustaceans, smaller larvae; reduced larval dispersal distances; and reduced connectivity between coastal populations. However, these predictions are largely based on laboratory and modelling studies, with little observational research to explore how these interactions unfold in natural ecosystems where temperature variability is pervasive. In this thesis, I investigate the links between reproduction and settlement timing of intertidal barnacles, and I explore the extent to which the timing of these events is explained by environmental and astronomical cycles and by water column conditions. In Chapter 2, I assess the cycles driving Chthamalus fissus reproduction and settlement in Southern California, and I offer a first order estimate of alongshore larval transport. I found that barnacles were reproductively active almost year-round, with clear lunar cyclicality and modest seasonality. Conversely, settlement exhibited little cyclicality on any timescale. Chapters 3, 4, and 5 focus on the effects of temperature on Semibalanus balanoides early life history along a steep temperature gradient in the northwest Atlantic over twenty years of warming. In Chapter 3, I investigate the effects of intertidal temperature on reproduction timing, analyzing separately the processes of fertilization, embryonic brooding, and larval release. In Chapter 4, I estimate larval duration in natural populations, and I measure the impact of temperature on larval duration in the laboratory and field. In Chapter 5, I investigate the effects of water temperature on larval size at settlement. I found that warmer nearshore temperatures significantly correlated with shorter brooding times of developing embryos, shorter field-estimated larval duration, and smaller larval settlers. Notably, the interplay between benthic reproduction, pelagic development, and temperature variability across space and time created counter-intuitive patterns in larval duration, size, and likely dispersal. Together, these findings point to the importance of reproductive timing in determining dispersal and population connectivity, and they highlight the need for extensive field measurements to quantify phenology and phenology shifts in benthic systems.