Can sea urchins beat the heat? Sea urchins, thermal tolerance and climate change

The massive die-off of the long-spined sea urchin, Diadema antillarum, a significant reef grazer, in the mid 1980s was followed by phase shifts from coral dominated to macroalgae dominated reefs in the Caribbean. While Diadema populations have recovered in some reefs with concomitant increases in coral cover, the additional threat of increasing temperatures due to global climate change has not been investigated in adult sea urchins. In this study, I measured acute thermal tolerance of Diadema antillarum and that of a sympatric sea urchin not associated with coral cover, Echinometra lucunter, over winter, spring, and summer, thus exposing them to substantial natural thermal variation. Animals were taken from the wild and placed in laboratory tanks in room temperature water (similar to 22 degrees C) that was then heated at 0.16-0.3 degrees C min(-1) and the righting behavior of individual sea urchins was recorded. I measured both the temperature at which the animal could no longer right itself (T-LoR) and the righting time at temperatures below the T-LoR. In all seasons, Diadema antillarum exhibited a higher mean T-LoR than Echinometra lucunter. The mean T-LoR of each species increased with increasing environmental temperature revealing that both species acclimatize to seasonal changes in temperatures. The righting times of Diadema antillarum were much shorter than those of Echinometra lucunter. The longer relative spine length of Diadema compared to that of Echinometra may contribute to their shorter righting times, but does not explain their higher T-LoR. The thermal safety margin (the difference between the mean collection temperature and the mean TLoR) was between 3.07-3.66 degrees C for Echinometra and 3.79-5.67 degrees C for Diadema. While these thermal safety margins exceed present day temperatures, they are modest compared to those of temperate marine invertebrates. If sea temperatures increase more rapidly than can be accommodated by the sea urchins (either by genetic adaptation, phenotypic plasticity, or both), this will have important consequences for the structure of coral reefs.