The role of kelp availability and quality on the energetic state and thermal tolerance of sea urchin and gastropod grazers

Widespread decline in underwater forests, formed by kelps and other macroalgae, is occurring in all oceans. Loss of the vulnerable surface canopy typically leads to alternate rocky reef states dominated by substrate-near, understory vegetation, or reefs without any large fleshy macroalgae such as sea urchin barrens. Such alternate reef states represent a major shift in food availability and quality, and may impact kelp-preferring grazers forced to rely on less nutritious foods. Here we investigate the physiological responses of grazers to changes in food quality and quantity. We quantify the mass-independent oxygen consumption (ṀO2), as a proxy of whole-organism physiology, of four grazing invertebrate species (two sea urchins: Mesocentrotus franciscanus and Strongylocentrotus purpuratus; and two gastropods: Pomaulax gibberosus and Tegula pulligo) provisioned with one of three diet treatments over seven weeks: 1) higher quality, canopy kelp, 2) lower quality, substrate-near kelp, or 3) restricted: no kelp. We further test for differences in the heat resistance of four grazers using an acute, near-lethal heat exposure after completion of the provisioning period. Food restrictions had the strongest influence on oxygen consumption in the two urchins (mass-independent ṀO2 was reduced by 26–78%). By contrast, the two gastropod grazers did not display metabolic depression under food restrictions. Although kelp restrictions had clear physiological consequences in sea urchins, overall, the type of kelp provisioned (Macrocystis and Saccharina) did not relate to oxygen consumption after seven weeks. While heat resistance was also similar across food treatments, unfed individuals showed a slight tendency for lower heat tolerance compared to fed individuals. These results suggest a weak coupling between heat resistance and food quality and quantity, at least for these species and at short time spans. Our work highlights the importance of diet diversity and metabolic depression as strategies to cope with the energetic costs of warming.