Combined effects of thermal stress and mortality of long-spined sea urchins (Diadema savignyi) from Gulf of Aqaba, Haql, Saudi Arabia

The level of vulnerability to thermal stress can profoundly affect sea urchin physiology and may also influence overall fitness, particularly by increasing mortality rates. The biological performance of sea urchins, including their viability, functional health, and reproductive output, is strongly determined by the prevailing ecological conditions. High temperatures can increase metabolic rates, thereby imposing metabolic constraints that place additional pressure on reproduction and growth, ultimately reducing overall fitness. Furthermore, under extreme conditions, these heightened temperatures can lead to die-offs that affect local marine ecosystems, in which sea urchins play a vital role in by grazing on algae and helping to maintain ecological balance. With climate change continuing to drive ocean warming, understanding these repercussions is becoming increasingly important for conserving sea urchin populations and maintaining marine ecosystem health. This study aimed to evaluate environmental factors, particularly heat stress, and their possible effects on the survival of sea urchins ( Diadema savignyi ). Specifically, the study investigated the impact of different temperature levels on the mortality rate of sea urchins ( Diadema savignyi ). Sea urchins were exposed to temperatures of 22, 25, 28, and 31C for up to 10 days. Mortality rates were assessed on days 2, 4, 6, 8, and 10. The findings indicated a mortality rate of 8.89% at 22C, which increased to 15.56% at 25C. At 28C, the mortality rate was 93.33%, and at 31C, it reached 100%. The environmental parameters of the organism's natural habitat were thoroughly assessed using a comprehensive range of physicochemical and biological measurements. This study confirmed the absence of other environmental stressors that could potentially have influenced sea urchin the survival rates. These findings provide a clearer understanding of the relationship between temperature change and species resilience, namely D. savignyi , in marine ecosystems such as the Gulf of Aqaba. The findings demonstrated that raising the temperature by 3C above the ambient level increased the mortality rate by 1.75-fold. The highest mortality rates were observed at 28C and 31C, demonstrating the high sensitivity of sea urchins to elevated temperatures. The current research showed that survival declined significantly at temperatures of 28C or higher, suggesting that future ocean warming may surpass the species' thermal tolerance thresholds, increase population susceptibility, and potentially disrupt coral reef ecosystem dynamics. These results highlight the importance of maintaining stable environmental conditions to preserve healthy sea urchin populations.