International Journal of Marine Science, 2025, Vol.15, No.4, 199-208 http://www.aquapublisher.com/index.php/ijms 203 observed, while many shrimps, crabs and other crustaceans are almost extinct. This shows that sea cucumbers can still live normally in a mild acidification environment (Wolfe et al., 2018). However, when the acidification is more severe, sea cucumbers may reduce their activity, hide in concealed areas to avoid adverse environments, and even escape areas with severe acidification in local areas. 5.3 The weakening of defensive behavior and environmental perception capabilities Sea cucumbers rely on touch and chemical sensations to perceive the environment and predators, and acidification may have an impact on their sensory and defensive behavior. Studies have shown that ocean acidification interferes with sensory functions and neural processes in marine animals. For sea cucumbers, low pH may weaken their sensitivity to chemical signals and tactile stimuli. For example, some sea cucumbers will spit out their internal organs or release sticky wires for defense when encountering predators, but the threshold for triggering such reactions under acidification stress may increase and slow down the reaction rate. Under moderate acidification conditions, studies have also found that the defense behavior of sea cucumbers has not changed significantly: in the experiment, adult sea cucumbers can still shrink their bodies normally and defend against foreign enemies under stimulation, showing that their behavior has certain resilience under moderate acidification (Yuan et al., 2018). Overall, the environmental perception and defense response of sea cucumbers under moderate acidification can remain normal, but stronger acidification may weaken their sensory and neurological functions, reduce their defense efficiency, and thus increase the risk of predation. 6 Chain Effects At the Ecosystem Level 6.1 The nutrient circulation effect of sea cucumbers in marine benthic ecosystems Sea cucumbers play an important role in the material circulation of marine benthic ecosystems through feeding and excretion. They swallow the organic matter of sediment and discharge the metabolic waste rich in nutrients, which promotes the regeneration of elements such as nitrogen and phosphorus from sediment to water. In tropical coral reef ecology, sea cucumbers are regarded as "scavengers" and "engineers" who maintain the health of the benthic environment: their feeding disturbances prevent sediments from silting, and the ammonium nitrogen in the feces can be used by algae and coral symbiotic algae to support primary production (MacTavish et al., 2012). It is worth noting that sea cucumbers also have a special contribution to the carbonate cycle - dissolving the swallowed calcium carbonate particles during digestion and releasing additional alkalinity, which can partially offset the acidification of coral reef water bodies due to biological respiration at night. Therefore, the reduction in the amount of sea parameters will weaken the above nutrient circulation and alkalinity buffering functions, which may lead to the accumulation of sedimentary organic matter and the aggravation of local hypoxia, and damage the quality of coral reef substrates (Figure 2) (Pan et al., 2024). 6.2 Effects of marine acidification on the release of heavy metals in sediment and decomposition of organic matter Ocean acidification not only affects organisms themselves, but also affects the sediment process by changing the chemical environment. The pH drop increases the release of many heavy metals at the sediment-water interface. Experimental research found that under CO₂-driven acidification conditions, metals such as Cd, Cu, Ni, Pb, Fe, Mn in the sediment are dissolution accelerated into the overburden water, and their release rate is controlled by the acidification intensity and metal chemical morphology (He et al., 2019). After being released from sediments, heavy metals can be ingested by benthic organisms or enter the food web, increasing the risk of ecological toxicity. In addition to heavy metals, acidification can also affect the decomposition efficiency of microorganisms on organic matter. The metabolic activities of many sedimentary microorganisms are sensitive to pH changes, and the acidity of seawater may reduce the activity of decomposing bacteria, resulting in slowing down the decomposition of organic residues in the sediment. Observations near the estuary show that excessive mineralization of organic matter caused by eutrophication will aggravate local seawater acidification, and the two promote each other to form a vicious cycle. If the function of the crumbled eaters such as sea cucumbers is weakened and the sedimentary organic load is further increased, it may induce hypoxia in the base and release more reducing substances, affecting the water quality and the stability of benthic communities.
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