Preliminary Studies On The Hatchery,Settlement And Predation On Two Reef-forming Oysters

Oyster reef is a biogenic reef formed by the aggregation of oysters gathering on hard substrate.In the past centuary,more than 85% of oyster reefs have been disappear or degraded around the globe due to the overfishing,habitat destruction,disease infection and environment pollution.There is an urgent need for restoration and conservation of oyster reefs.Seed breeding is one of the key technology for oyster reef restoration.At present,several oysters have been successively bred for aquaculture in China,such as Pacific oysters(Crassostrea gigas),Hong Kong giant oysters(Crassostrea hongkongensis),Kumamoto oysters(Crassostrea sikamea)and Portuguese oysters(Crassostrea angulata).The oysters C.ariakensis and C.sikamea commonly coexist and are reef-building species.However,few information are available on the artificail hathery of the two species.The hatchery-reared technique of the two oysters can provide large number of spat supplement to the restored reef to preceed the efforts on oyster reef restorations.Substrates types directly affect the oyster population establishment and reef development as well as reef-associated community and ecosystem function of oyster reefs.This study examined the effect of shell types(old shells vs new shells)on oyster larval settlement,and further explored whether biofilm induced the settlement of oyster larvae on oyster shells.Predation is among the important biotic factors regulating the post-settlement mortality of oyster populations.This study determined the predation efficiency of three invertebrate predators on C.ariakensis and C.sikamea.The main results are as follows:1.The artificial breeding of C.ariakensis and C.sikamea This study carried out the artificial breeding of C.ariakensis and C.sikamea simultaneously,recorded the growth and development process of oyster larvae,and identified the critical stage for larvae growth and development during the artificial breeding.The study found that the salinity for breeding C.ariakensis and C.sikamea ranged from 20 to 22,the average salinity was 20.65 ± 0.20.The seawater temperature ranged from 23.0? to 27.0?,with the average values of 25.0 ± 0.37 ?.The planktonic larvae of the two oysters experienced the same four stages,including D-shaped larvae,umbo larvae,eyepoint larvae,and settlement and metamorphosis.Settlement and metamorphosis is the key stage of artificial breeding.Two species of oysters were artificially bred simultaneously,C.sikamea first completed the artificial breeding process.2.Effects of biofilm and substrate types on the larval settlements of oyster Substrate types and biofilms have signifciant effects on larval settlement of oysters.Oyster shells are by far the most common substrate type.An oyster shell forms a biofilm on the surface of the oyster shell by soaking it in unfiltered circulating seawater for at least 72 hours.This experiment uses a two-factor random block design.The two independent variables are oyster shell type(new oyster shell,old oyster shell)and biofilm(soaked and unsoaked).The dependent variable is the number of seedlings on the oyster shell.The old oyster shell is abbreviated as "FS",the new oyster shell is denoted as "NS",the biofilm is denoted as "B",and the non-biofilm is denoted as "C".Through lab and field experiments,the effects of oysters attachment with and without biofilm on old and new oyster shells were compared,and the differences in the effects on the settlement of oyster larvae with different age of oyster shells and the importance of biofilms on oysters with different ages were explained.A two-factors ANOVA showed that the shell type(F = 12.443,P = 0.002)and biofilm(F = 5.508,P = 0.031)significantly affected the larval settlement of oysters in lab experiment,but there was no significant interaction effect between the shell type and the biofilm(F = 3.208,P = 0.090).The results of Duncan's multiple comparisons indicated that there were significant greater abundances of the settled oyster spats in new oyster shell with biofilm than the other three treatments(P <0.05).The abundances of oyster larvae on new oyster shell was greater than that on the old oyster.It was found that the biofilm treatment significantly improved the larval settlement than the non-biofilm treatment,for each of the two shells..In the field experiment,a two-way ANOVA showed that biofilms had significant effects on the the settlement of oyster larvae(F = 4.312,P <0.05),while the larval settlement had no difference between the two shells(F = 1.933,P> 0.05).There was no interaction effect between shell type and biofilm(F = 3.668,P> 0.05).Duncan multiple comparison results showed that the old oyster shell without biofilm had lower larval settlement than the other three groups(P <0.05).3.Predation of Crassostrea ariakensis and C.sikamea by three invertebrate species in lab experiments Predation is one of the most important biotic factors regulating oyster population establishment and oyster reef development.The present study examined the predating preference and efficiency of a stone crab(Charybdis japonica)and two oyster drillers(Rapana venosa,Thais luteostoma)on the four size class(W1:10-20mm;W2:20-30mm;W3:30-40mm;W4: >40mm)of C.ariakensis and C.sikamea through lab experiments.The 2-way ANOVA indicated that Charybdis japonica showed no discriminatory predation on C.ariakensis and C.sikamea(P > 0.05),while the oyster size significantly influenced the predating efficiency of the stone crab.The stone crab had significantly greater predating efficiency on W1 group of C.ariakensis than W2 and W4 group of the conspecific,and there were intermediate predation efficiency on the W3 group(P> 0.05).The W1 group of C.sikamea was predated at significantly greater ratios than W2 and W3 groups of the conspecific,while the W4 group of C.sikamea had similar predating efficiency with the other 3 groups(P<0.05).The driller Rapana venosa had consistently low predating efficiency on the two oyster species(P = 0.590)and the four size class of oysters(P = 0.357).The driller Thais luteostoma had consistent predating efficiency between C.ariakensis and C.sikamea(P = 0.917),but the oyster size significantly affected the predating efficiency of Thais luteostoma(P = 0.035).Duncan' multiple comparison indicated that Thais luteostoma had similar predating efficiency among the four size class of C.ariakensis(P > 0.05),but the drillers had greater predating efficiency on W1 group of C.sikamea than the other 3 groups of the conspecific(W2,W3 and W4)(P < 0.05).Significantly positive correlations were found between the shell height and the shell thickness for each of C.ariakensis and C.sikamea(P < 0.001).It was concluded that the three invertebrates showed no discriminatory predation on C.ariakensis and C.sikamea,while the predating efficiency on different size class of oysters had interspecies variations.