Adaptation and challenges for giant clam species under marine heatwaves

Abstract

Amid ocean warming, giant clams face serious threats, yet our understanding of their adaptation to marine heatwaves is limited compared to corals. In the South China Sea, a 9-year record of Tridacna gigas surviving recurrent heatwaves provides valuable insights. Our results indicate that the daily growth bands in T. gigas shell effectively track their growth responses to heatwaves. The T. gigas shell exhibits abnormal growth bands under mild to moderate cumulative heat stress caused by heatwaves but recovers as temperatures return to normal. However, following the severe cumulative heat stress in 1998, shell growth took 2.5 years to gradually return to pre-heat stress levels, indicating a significant negative impact. Unexpectedly, T. gigas has developed some degree of heat tolerance with repeated exposure to heatwaves. However, its ability to withstand heat stress increases non-linearly, and recovery slows significantly after exposure to severe heat stress. This raises concerns about its ability to adapt to future more frequent marine heatwaves.