Larvae of Caribbean Echinoids Have Small Warming Tolerances for Chronic Stress in Panama

In species with complex life cycles, early developmental stages are often less thermally tolerant than adults, suggesting that they are key to predicting organismal response to environmental warming. Here we document the optimal and lethal temperatures of larval sea urchins, and we use those to calculate the warming tolerance and the thermal safety margin of early larval stages of seven tropical species. Larvae of Echinometra viridis, Echinometra lucunter, Lytechinus williamsi, Eucidaris tribuloides, Tripneustes ventricosus, Clypeaster rosaceus, and Clypeaster subdepressus were reared at 26, 28, 30, 32, and 34 degrees C for 6 days. The temperatures at which statistically significant reductions in larval performance are evident are generally the same temperatures at which statistically significant reductions in larval survival were detected, showing that the optimal temperature is very close to the lethal temperature. The two Echinometra species had significantly higher thermal tolerance than the other species, with some surviving culture temperatures of 34 degrees C and showing minimal impacts on growth and survival at 32 degrees C. In the other species, larval growth and survival were depressed at and above 30 or 32 degrees C. Overall, these larvae have lower warming tolerances (1 to 5 degrees C) and smaller thermal safety margins (-3 to 3 degrees C) than adults. Survival differences among treatments were evident by the first sampling on day 2, and survival at the highest temperatures increased when embryos were exposed to warming after spending the first 24 hours at ambient temperature. This suggests that the first days of development are more sensitive to thermal stress than are later larval stages.