Commentary

Abstract

<h3>Abstract</h3> The proliferation of sea urchins can decimate macroalgal forests in coastal ecosystems, leading to persistent barren seascapes. While kelp forests are among the most productive ecosystems on the planet, productivity in these urchin barrens is dramatically reduced (Filbee-Dexter and Scheibling 2014). Moreover, urchins inhabiting these food-depauperate barrens face starvation and many survive in these barrens for years or decades. Urchins in barrens can persist by eating food subsidies from drift algae (Rodríguez 2003, Vanderklift and Wernberg 2008, Britton-Simmons et al. 2009, Renaud et al. 2015, Quintanilla-Ahumada et al. 2018), pelagic salps (Duggins 1981), tubeworms (Spindel and Okamoto <i>personal observation</i>), as well as encrusting and filamentous algae, microbial mats, and slow-growing species resistant to herbivory (Ling and Johnson 2009, Filbee-Dexter and Scheibling 2014, Rasher et al. 2020). Despite both food from endogenous production and exogenous subsidies, many urchins in barrens likely experience prolonged food deprivation. This resource limitation may create a trade-off between reproduction and survival (Stearns 2000); for example, fecundity of purple sea urchins (<i>Strongylocentrotus purpuratus</i>) is 99.9% lower in barrens (Okamoto 2014). Despite food constraints, red sea urchins (<i>Mesocentrotus franciscanus</i>), the dominant urchin species at our study sites, can live in excess of 100 years (Ebert 2008) and barrens in Haida Gwaii, British Columbia (BC), Canada, have persisted for at least 143 years (Dawson 1880). While these phenomena are widespread and well documented, the bioenergetic adaptations that allow urchins to persist in these food-depauperate barrens remain poorly understood. Here we demonstrate that <i>M. franciscanus</i> in barrens versus kelp forests have substantially lower energy reserves (as measured by gonadal mass) and, importantly, also exhibit dramatic reductions in size-specific resting metabolic rates (RMR), even after standardizing by metabolically active body mass.