Heavy Metal Contents And Bioaccumulation Characteristics Of Acanthaster Planci In Coral Reef Of South China Sea

As the natural enemy of corals,the periodic outbreaks of Acanthaster planci have caused significant harms to the coral reef ecosystem.The current research on Acanthaster planci mainly focuses on biology and ecological hazards.As an important chemical component of seawater,heavy metals are closely related to the physiological processes of marine organisms.However,the heavy metal contents and bioaccumulation characteristics of Acanthaster planci have not been reported.In this research,the Acanthaster planci collected from the coral reef area of Xisha Islands and Nansha Islands in the South China Sea was taken as the research object.The contents of V,Cr,Mn,Fe,Co,Ni,Cu,Zn,As,Mo,Cd,and Pb were determined by inductively coupled plasma mass spectrometry(ICP-MS)and atomic absorption spectrometer(AAS)in the pyloric caecum,stomach,gonad,body wall,spine and skeleton of Acanthaster planci.The contents characteristics of the above heavy metals were analyzed by Kruskal-Wallis test and Mann-Whitney U test.The main conclusions are as follows:(1)There are significant differences in the contents of heavy metals in different parts of Acanthaster planci,which might be closely related to the physiological functions of heavy metals.The contents of V,Cr,Cu,As,Cd,Fe,Zn,and Mo are high in pyloric caecum and stomach,speculating that they are involved in regulating metal absorption and metabolic activities of the digestive system.The content of Zn in the gonads is significantly higher than that in other parts,with an average of 92.78 mg/kg,indicating that Zn is an important element to promote the development of gonads in Acanthaster planci.The contents of Mn,Co,Ni,and Pb are higher in skeleton,spines,and body walls.The enrichment of these elements may be related to their functions in connective tissue and skeleton development of Acanthaster planci.(2)The heavy metal contents of Acanthaster planci are generally correlated with diameter and weight.The content of As is positively correlated with the diameter and weight of Acanthaster planci.However,the contents of most of the essential heavy metals including V,Cr,Fe,Co,Ni,and Mo are negatively correlated with the diameter and weight of Acanthaster planci,which means the heavy metal content of the tiny Acanthaster planci is generally higher than the large individual.This may be related to the more vigorous metabolism and higher absorption and utilization of heavy metals of the tiny Acanthaster planci.(3)There is a spatial difference in heavy metal contents of the Acanthaster planci in the South China Sea.The average content of heavy metals in Xisha Islands and Nansha Islands is V(Xisha Islands:2.9±1.75 mg/kg;Nansha Islands:0.79±0.21 mg/kg),Cr(1.5±0.84 mg/kg;0.88±0.4 mg/kg),Zn(57.07±14.67mg/kg;45.55±6.96 mg/kg),Cd(20.54±10.23 mg/kg;8.69±0.85 mg/kg).The contents of V,Cr,Zn,and Cd of Acanthaster planci in Xisha Islands are significantly higher than those in Nansha Islands,which may be related to the contents of some heavy metals in Xisha Islands being higher than Nansha Islands.(4)The outbreak of Acanthaster planci may be related to the increase of heavy metal contents in coral reef areas.On the one hand,as a nutrient reserve element for the development of gonads,Zn is conducive to the production of sperm and eggs,which can improve the development rate of larvae and the ability to adapt to the environment.On the other hand,Fe,Zn,Cd,Co,and Mn can promote the growth of medium-sized phytoplankton such as Chaetoceros and Dunaliella tertiolecta,which can provide food for larvae of Acanthaster planci.In addition,the enrichment of non-essential heavy metals such as Cd in Acanthaster planci may affect the feeding preferences of predators and increasing their survival chances.(5)Heavy metals enriched by Acanthaster planci may increase the ecological risk of coral reef areas at the later stages of population outbreaks.The outbreak of Acanthaster planci eventually led to population collapse due to the diseases among high-density of populations.Acanthaster planci has accumulated high contents of heavy metals such as Cd,As,and Zn through bioaccumulation.According to the outbreak threshold of Acanthaster planci(1000 ind/km~2),the average weight of adult Acanthaster planci(0.72 kg/km~2),and the average contents of heavy metals measured in this paper,the total amount of heavy metals(g)released into seawater in a short time after the death of the Acanthaster planci population per square kilometer is about V(1.16),Cr(0.59),Mn(0.97),Fe(46.1),Co(0.09),Ni(1.44),Cu(3.31),Zn(22.68),As(4.64),Mo(0.17),Cd(8.17),Pb(0.15).These elements may have ecotoxic effects on corals through biochemical cycles.