Distribution Characteristics And Influence Factors Of Crabs Among Mangrove Communities At Dongzhaigang Bay, Hainan

As an important member of macrobenthos,mangrove crabs play a dominant role among mangrove wetland macrobenthos and go by the name of "ecology engineer" in mangroves.Predecessors almost researched crabs together with other zoobenthos or only researched several species,wherein effects of singular factors on crab diversity were overemphasized but some rules concerning mangrove crabs were concealed.Exploration of distribution characteristics and influential factors of mangrove crabs facilitates further acquaintance of the maintenance mechanism of mangrove crabs.With Dongzhaigang Bay National Natural Reserve in Hainan as the research sample site,the paper conducted crab survey at Sanjiang(pioneer species-Sonneratia apetala),Houpai(pioneer species-Avicennia marina and Ceriops tagal on rear edge)and Tashi(pioneer species-Avicennia marina and Rhizophora stylosa on the rear edge)during December 2014(winter),March 2015(spring),June 2015(summer)and September 2015(autumn).Samping method and centipede net method were combined in the survey.Mangrove crabs were taken as a separate research object to study crab distribution between pioneer mangrove communities,and between pioneer mangrove plants and mangrove communities on the rear edge.In addition,key factors influencing crab distribution were analyzed.Analysis results are as follows:1.Thirty eight crab species subordinate to 1 class and 13 families were recorded in the survey.Dominant crab species in the mangrove area were Ilyoplax tansuiensis and Cleistostoma dilatatum;common species were Metopograpsus quadridentatus,Pachygrapsus planifrons,Sesarma plicata,Metaplax elegans,Macrophthalmus definitus and Macrophthalmus banzai.S.apetala community had the most crab species at Sanjiang,namely 21 species;17 crab species of A.marina community were recorded at Houpai;16 crab species of C.tagal community were recorded at Houpai;18 crab species of A.marina community were recorded at Tashi;15 crab species of R.stylosa community were recorded at Tashi.1553 crabs of Ocypodoidea were captured,occupying 87.89%of the total capture amount.The capture amount of the Portunoidea ranked the second place.The capture amount of the Grapsoidea ranked the third place.Among the five mangrove communities under survey,crab density and crab biomass in the S.apetala community were higher than those of other mangrove communities.2.Distribution characteristics and influence factors of crabs between diferent mangrove communities were different.Details as follows:(1)The distribution characteristics of crabs between pioneer mangrove communities at Sanjiang,Houpai and Tashi:21 crab species were captured in Sanjiang;17 crab species were captured at Houpai;18 crab species were captured at Tashi.Crab density and biomass decreased progressively from Sanjiang to Tashi.Dominant crab species(I.tansuiensis and C.dilatatum)were affected by salinity;they like inhabiting in argillaceous sediment.Therefore,crab density decreased progressively from Sanjiang to Tashi under the influence of seawater salinity and sediment type.Root structure and ground elevation were also influence factors of crab density and biomass.(2)Crab density and biomass in A.marina community of Houpai were higher than those A.marina community of Tashi.Due to the large span between the sample sites of Houpai and Tashi,salinity was one of factors influencing the distribution of dominant crab species(I.tansuiensis and C.dilatatum).Apart from salinity,root structure features and ground elevation were also related to distribution of crab species,density and biomass.Within a certain range,the higher root density was,the higher its crab density and biomass would be;the higher ground elevation was,the lower its crab density and biomass would be.(3)On the same section,distribution characteristics and influential factors of crabs were different between pioneer mangrove communities and rear-edge mangrove communities.At Houpai,Species,density and biomass of crabs of A.marina community were higher than those of C.tagal community.Distribution differences of crabs between A.marina community and C.tagal comunity were significantly correlated with tannin content and protein content of litters,root structure,ground elevation and coverage at the Houpai.Within a certain range,higher complexity of root structure led to higher density of crabs.Lower tannin content and higher protein content of litters were corresponding to higher nutritive values and higher density of crabs.Distribution of dominant crab species(I.tansuiensis and C.dilatatum)were influenced by ground elevation;coverage was also correlated with distribution of crabs.At Tashi,species,density and biomass of crabs in A.marina community(pioneer species)were higher than those of the stylosa community.At Tashi,crabs tended to distribute in A.marina community with high root system biomass and low ground elevation.The projects of increasing ground elevation might lead to crab distribution tendency in A.marina community with low coverage.In conclusion,distribution of crabs between different mangrove communities was characterized in that:species,density and biomass of crabs in pioneer mangrove communities were higher than those of mangrove communities on the rear edge;species,density and biomass of crabs in pioneer mangrove communities(bottom of the bay)exceeded those of pioneer mangrove communities at top of the bay.Comprehensive factors,involving biological and abiotic ones,influenced the distribution of mangrove crabs.