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

Elizabeth Sherman

doi:10.7717/peerj.1006

Abstract

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 D. 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 (∼22 °C) that was then heated at 0.16–0.3 °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 (TLoR) and the righting time at temperatures below the TLoR. In all seasons, D. antillarum exhibited a higher mean TLoR than E. lucunter. The mean TLoR of each species increased with increasing environmental temperature revealing that both species acclimatize to seasonal changes in temperatures. The righting times of D. antillarum were much shorter than those of E. 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 TLoR. The thermal safety margin (the difference between the mean collection temperature and the mean TLoR) was between 3.07–3.66 °C for Echinometra and 3.79–5.67 °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.

Highlights

Adult sea urchins are grazers and have been dominant herbivores in many Caribbean reefs for decades and perhaps millennia (Lessios, Garrido & Kessing, 2001; Levitan, Edmunds & Levitan, 2014)
The proximate cause of the Diadema die-off was likely a disease agent that has never been identified (Beck, Miller & Ebersole, 2014), but additional environmental stressors such as increasing ocean temperature due to global climate change may be deleterious for marine ectotherms such as echinoids in intertidal and subtidal tropical habitats (Nguyen et al, 2011)
Collection site Sea urchins of both Diadema antillarum and Echinometra lucunter were collected, by snorkeling in a small rectangular inlet south of George Town harbour, Grand Cayman, located at 19◦16′42.69′′N,81◦23′34.79′′W, (in the Marine Protected Area between study sites GCM9 and GCM5 of McCoy, Dromard & Turner (2009))

Summary

Introduction

Adult sea urchins are grazers and have been dominant herbivores in many Caribbean reefs for decades and perhaps millennia (Lessios, Garrido & Kessing, 2001; Levitan, Edmunds & Levitan, 2014). The subsequent overgrowth of macroalgae resulted in decreased coral cover and recruitment and concomitant biodiversity losses from which reefs have not fully recovered (Jackson, 2001; Levitan, Edmunds & Levitan, 2014; Miller et al, 2009). The proximate cause of the Diadema die-off was likely a disease agent that has never been identified (Beck, Miller & Ebersole, 2014), but additional environmental stressors such as increasing ocean temperature due to global climate change may be deleterious for marine ectotherms such as echinoids in intertidal and subtidal tropical habitats (Nguyen et al, 2011)

Methods

Results

Discussion

Conclusion