Research progress of factors influencing the growth of Tridacna and its artificial cultures
Background,aim,and scope Giant clams(Tridacna spp.)are the largest marine bivalves of any ocean.They stay in one location and live in symbiotic relationship with zooxanthellae.Many environmental factors can affect their growth over their life time.In recent years,the number of Tridacna individuals have decreased dramatically,and some species have come close to extinction due to overfishing by humans.Artificial breeding and release of giant clams have become important elements in giant clams'conservation.In addition,the carbonate shells of giant clams can provide valuable high-resolution paleoclimate research archives,however,the full environmental implications of shell geochemical proxy values are still to be fully understood.Laboratory controlled culture methods to reveal the environmental implication of geochemical proxy formation is an essential part of increasing our understanding.In recent years,some culture experiments have been conducted to investigate the effect of environmental factors on growth of the giant clams.Here,the influencing factors on Tridacna growth are reviewed,providing reference points for further culture,conservation,and geochemistry of giant clam shell carbonate properties.Materials and methods Here,the culture modes of Tridacna are illustrated and the environmental factors,such as temperature,light,salinity,pCO2,selected metal ions are reviewed.Results Studies have shown that the Tridacna culture modes can be divided into primary environment culture:semi-natural culture,and laboratory culture.The Tridacna survival rate under the first two culture modes and the influence of various environmental factors on the third mode are analyzed.Prolonged temperatures above 30℃can cause damage to the Tridacna and its symbiotes.When the luminous flux exceeds 1061 μmol ∙ m-2 ∙ s-1,the Tridacna shell will dissolve more than calcium.Tridacna gigas could adapt to lower water salinity by adjusting its respiration rate and zooxanthellae density and volume.Discussion Primary environment culture may be the most suitable mode to quantify Tridacna growth,but the survival rate is hardly guaranteed due to the presence of predators and unsuitable climatic conditions.Laboratory culture can solve this problem.However,it must be noted that the environmental factors still influence the growth of Tridacna.Studies have found that the Tridacna have the ability to adapt the changes of environment in the short term,and maintain physiological balance by changing respiratory rate,reducing metabolism,and adjusting zooxanthellae density.Some studies have also explored Tridacna genetics.It was found that Tridacna may control the up-regulation or down-regulation of differentially expressed genes,and adjust amino acid composition to adapt to the environment.Conclusions Although Tridacna can adapt to the changes of surrounding environment;if changes of environmental factors exceed certain thresholds,the adaptive capacity of Tridacna decreases.Eventually,when the growth of Tridacna is interrupted,the shell dissolves,the mantle is bleached and can be life-threatening.Different species or sizes of Tridacna have different tolerance ranges,and may have different responses to the same environmental conditions.Recommendations and perspectives In laboratory seawater tanks,it is necessary to introduce normally co-habiting animals to help the growth of Tridacna,including mint shrimp to remove algae,sea cucumbers to help turn sand,and trochids to resist Oncomelania.Thus,in addition,it is important for researchers to recognize that in nature other organisms are essential for healthy Tridacna populations.
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