Sea urchins exposed to near-future elevated seawater temperature alter resource allocation under low-quality food conditions

The sea urchin Lytechinus variegatus inhabits seagrass meadows in the northern Gulf of Mexico where seasonal high temperatures are already capable of inducing physiological stress. Climate change will increasingly expose individuals to longer periods of elevated temperature. Sea urchins (n = 12 per treatment) were chronically exposed for a period of 90 days to ambient early summer temperature (26 degrees C) and a near-future predicted summer temperature (30 degrees C). Feeding and absorption efficiencies were measured in sea urchins fed a diet of the seagrass Thalassia testudinum embedded in agar. Test diameter and height, and total wet weight were recorded for each individual every 30 days. Mean spine length and diameter were also recorded for each individual at days 0 and 90. During the first 60 days, there were no significant differences in feeding rate, absorption efficiency, or growth of individuals held in the ambient (26 degrees) or elevated (30 degrees C) temperature treatment. However, after 90 days, sea urchins exposed to 30 degrees C exhibited reduced rates of feeding, lower levels of total organic content in the Aristotle's lantern, and thinner spines than those held at ambient temperature. Accordingly, despite the ability of L. variegatus to feed efficiently and sustain growth at predicted near-future seawater temperature for 60 days, after 90 days, individuals had significantly reduced their allocation of resources to skeletal components (spines and lantern). These findings suggest that in the near-future, chronic exposure to elevated temperature may compromise feeding through inadequate maintenance of the lantern and increase vulnerability to crushing predators through a reduction in spine diameter. Both factors could alter the role of L. variegatus as a key player in seagrass ecosystems.