Multilevel effects of long-term elevated temperature on fitness related traits of the sea urchin Strongylocentrotus intermedius

The Intergovernmental Panel on Climate Change predicts that global mean seawater temperature will increase 2.0–4.5 °C by 2100. Therefore, it is essential to study fitness-related consequences of ecologically important marine organisms exposed to high temperature. Here, we investigated the effects of approximately 4 and 10 mo of exposure to elevated temperature (approximately 3 °C) on morphological, behavioral, and physiological fitness related traits of the sea urchin, Strongylocentrotus intermedius (A. Agassiz, 1864). Elevated temperature significantly increased mortality of S. intermedius after approximately 4 mo. Test diameter, body weight, and test weight significantly decreased after approximately 10 mo of exposure to elevated temperature, but not after 4 mo of exposure. This indicates that the effect of ocean warming on growth of S. intermedius is likely to be underestimated by the investigation of relatively short (e.g., approximately 4 mo) exposure. A significantly higher test height/diameter was found in S. intermedius exposed to elevated temperature for both 4 and 10 mo. Lantern length/test diameter was significantly lower in the sea urchins exposed to elevated temperature for approximately 4 mo, while significantly higher in those exposed to elevated temperature for about 10 mo. These results enrich our understanding of phenotypic plasticity of sea urchins. Elevated temperature significantly impacted gut weight of S. intermedius. Further, elevated temperature significantly increased Hsp70 expression, but decreased foraging behavior of S. intermedius. Surprisingly, we found no significant difference on the reproductive traits (gonad weight, gonad index, crude protein concentration of gonads, and gametogenic development) between S. intermedius exposed to ambient and elevated temperatures. The present study provides new insights into the thermal responses and acclimation of sea urchins to ocean warming.