Crawling and righting behavior of the subtropical sea star Echinaster (Othilia) graminicola: effects of elevated temperature

Abstract

Sea stars exhibit discrete neuromuscular-mediated behaviors, such as crawling and righting, which are critical to daily functioning and survival. As global seawater temperatures rise, subtidal asteroids such as Echinaster (Othilia) graminicola on the Gulf Coast of Florida are increasingly subjected to elevated temperatures, which may compromise their neuromuscular health. To better understand the neuromuscular biology of E. graminicola and the effects of elevated seawater temperature on behavior, we first describe their righting mechanism and identify the arms used in righting and crawling. Together, this allowed us to determine if they exhibit any bilateral tendencies, related to their anterior–posterior ancestry, by measuring the frequency with which each arm led in righting and crawling. To determine the effect of elevated seawater temperature on their described righting behavior, we tested the effects of acute (1 day) and chronic (7 day) exposure to seawater temperatures ranging from 28 to 36 °C on righting time in a laboratory experiment. We found that E. graminicola rights by somersaulting, but exhibits no lead arm preference during this behavior. In crawling, Arm E, located adjacent to the madreporite, led most frequently, indicating a partial tendency toward bilateralism. Sea stars righted significantly faster with increasing temperature and exposure time as temperatures increased from 28 to 34 °C, suggesting some capacity for acclimatization to elevated temperatures. However, at 36 °C, righting time increased dramatically and sea stars experienced high mortality, suggesting E. graminicola may be living near the upper limits of its thermotolerance. As seawater temperatures rise beyond an organism’s thermal tolerance, their neuromuscular responses may become critically impaired, reducing their capacity to survive in a warming ocean.