Distribution Pattern And Its Influencing Factors Of Dominant Species Of Burrowing Crabs In The Intertidal Zone Along Zhangjiang Estuary,China

Burrowing crabs are important macrobenthic in the intertidal zone of the estuary.The study on the distribution pattern of burrowing crabs and its influencing factors will help to understand the structure and function of the intertidal ecosystem in the estuary.Due to the spatial scale,species,and the complexity of environmental factors among different studies,the distribution of burrowing crabs and their influencing factors are different.In addition,there are few studies on the distribution of burrowing crabs along estuarine gradient in China,and lack of manipulated experiments to disentangle the key factors of the distribution patterns.This study investigated the distribution of burrowing crabs and their main environmental factors in the vegetation habitat and unvegetated habitat of the intertidal zone which was in the up-section to down-section of Zhangjiang Estuary in Fujian,and selected four main dominant crab species(Chiromantes dehaani,Parasesarma plicatum,Metaplax longipes,Uca vocans borealis)and three key influencing factors(salt,particle size of sediments,interspecific competition)carrying out a series of manipulate experiments in the lab and field to explain the distribution pattern and its influencing factors of dominant species of burrowing crabs in Zhangjiang River.The main results are as follows1.According to the field survey,there are 12 species of burrowing crabs in the up-section to the down-section of Zhangjiang Estuary.The C.dehaani,P.plicatum,M.longipes of Ocypodidae and U.vocans borealis of Grapsidae were the dominant species of burrowing crabs.C.dehaani was distributed in the oligohaline area of the up-section of Zhangjiang Estuary which included the vegetation and the unvegetated mudflat habitat.The soil substrate was silty,co-occured with P.plicatum in the vegetation habitat between the up-section and mid-section.P.plicatum was distributed in the mesohaline and polyhaline areas near the up-section to down-section of Zhangjiang Estuary.It was only distributed in the vegetation habitat,and the soil substrate was silty,co-occured with M.longipes in the relatively lower elevation of mid-section and near vegetation habitats of down-section;M.longipes was distributed in the mesohaline and euhaline areas of the up-section to the down-section,mainly distributed in the habitat of the unvegetated mudflat.The soil substrate was silty sand and sandy,co-occur with U.vocans borealis in the sandy beach habitat of down-section;U.vocans borealis was only distributed in the sandy habitats in the down-section,and is not distributed in silty habitats.2.The near-surface seawater salinity and soil porewater salinity of the up-section to down-section of Zhangjiang Estuary increased gradually.The seawater salinity of the up-section,mid-section and down-section in spring and summer of 2018(May-August)were 7.7±1.6,14.7±0.7,19.6±0.3 PSU,respectively.In the summer and autumn(June-September),the up-section experienced a 3-10 days freshwater period per month.The soil porewater salinity of the up-section,mid-section and down-section were 11.3±0.9,24.0±2.3,31.3±2.5 PSU,respectively.Corresponding soil substrate types were sandy silt,silt and silty sand.3.The manipulated experiment of salinity tolerance in lab showed that the four burrowing crabs had a wide salinity tolerance range during the experimental time of one month,but the ability to withstand fresh water and high salinity was different.C.dehaani survived 24.61±2.9 days in 0-25 PSU seawater,significantly higher than 7.11±4.6 davs in 35,40 PSU seawater,with strong freshwater tolerance,available in fresh water(0 PSU)and survived for 24.6±2.9 days,significantly higher than the other three species of crabs in freshwater survival of no more than 3.4±0.6 days;The other three species of crabs had higher salinity tolerance and could survive up to 30.0±0.0 days in 5-40 PSU seawater,which was significantly higher than the maximum survival of 3.4±0.6 days in fresh water.4.The field manipulated experiment of salinity tolerance showed that C.dehaani survived in the up-and mid-section for up to 2 months,and survived for 55.0±5.0 days.The survival time was significantly higher than the lower 20.0±6.3 days,P.plicatum,M.longipes,V.vocans borealis had no significant difference in survival time in the up-,mid-and down-section,P.plicatum survived at least 47.5.0±9.8 days,M.longipes survived at least 27.5±9.8 days,U.vocans borealis survived at least 7.5 ± 7.5 days.Because the salinity(near-surface seawater and surface soil porewater)of Zhangjiang Estuary during the field manipulated experiment of salinity tolerance was 5-35 PSU,therefore,the variation of the four crabs along the salinity gradient was consistent with the manipulated experiment of salinity tolerance in lab.5.Particle size of sediments selection experiments showed that C.dehaani tended to choose silty(10%sand)habitat,the selection proportion was 85.8±5.6%,and the burrow density was 44.4±6.1 pcs/m2,which was significantly higher than those of sandy(60%sand)14.2±5.6%and 5.6±3.1 pcs/m2;P.plicatum tended to choose silty(10%sand)habitat,the selection proportion is 73.8.8±5.1%,and the burrow density was 23.6±7.5 pcs/m2,which was significantly higher than those of sandy(60%sand).M.longipes had no preference for particle size of sediments.The frequencies of silty(10%sand)and sandy(60%sand)habitats were 53.3±11.9%and 46.7±11.9%,respectively,and the burrow density were 8.3±3.1.Pcs/m2 and 11.1±5.0 pcs/m2;U.voca6s borealis tended to choose sandy(60%sand)habitat,the selection proportion was 96.7±1.8%,and the burrow density was 68.1±11.6 pcs/m2,higher than 3.3±1.8%and 16.7±3.5 pcs/m2 of the selected silty sand(10%sand).6.Soil substrate competition experiment,seted 3 combinations of crab of single,2 individuals of the same species,and mixed.C.dehaani was not affected by P.plicatum,M.longipes and U.vocans borealis,and tended to choose sandy habitat(100%sand)with a selectivity of at least 85.4±2.3%,which was significantly higher than that of selected sandy(100%sand)habitats for 14.6±2.3%;P.plicatum was not affected by C.dehaani,M.longipes,U.vocans borealis,and tended to choose sandy habitat(100%sand)with a selectivity of at least 77.8±9.5%,which was significantly higher than that of selected sandy(100%sand)habitats for 22.2=9.5%;M.longipes competeed only with P.plicatum and was less competitive than P.plicatum.M,longipes tended to choose silty(10%sand)habitats in non-mixed 2 crab combinations with a selectivity of 77.8±7.7%.Above,when combined with P.plicatum,it tended to choose sandy(100%sand)habitat with a selectivity of 65.3±6.9%;U.vocans borealis competed only with P.plicatum and was less competitive than P.plicatum.In the non-mixed 2 crab combinations,U.vocans borealis tended to choose silty(10%sand)habitat with a selectivity of 75.0 ±12.5%,when combined with P.plicatum,tended to choose sandy(100%sand)habitat with a selectivity of 83.3±9.1%.7.Vegetation habitat competition experiment,seted 3 combinations of crab of single,2 individuals of the same species.P.plicatum was not affected by M.longipes and tends to choose vegetation habitats.The selection proportion is 83.7±4.3%,which is significantly higher than 16.3±4.3%of selected unvegetated habitats.M.longipes competed with P.plicatum,and its competitiveness was weaker than that of P.plicatum.In non-mixed crab treatments,it tended to choose vegetation habitats with a selection proportion of up to 90.3±3.0%,and combined with P.plicatum.When combined with P.plicatum,it tended to choose the unvegetated habitat,and the habitat selection proportion was 69.1±8.8%.The above results indicated that C.dehaani,P.plicatum,M.longipes,and U.vocans borealis were the dominant burrowing crab species in the intertidal zone along Zhangjiang Estuary.C.dehaani was mainly distributed in the up-section of Zhangjiang Estuary mainly due to freshwater tolerance,while not distributed in the down-section is mainly affected by high salt stress and particle size of sediments;P.plicatum was mainly distributed in near the up-to down-section of Zhangjiang Estuary,and were not distributed in the up-section mainly affected by freshwater stress,and were not distributed in the down-section is mainly affected by particle size of sediments;M.longipes was mainly distributed in the area from the up-to down-section of Zhangjiang Estuary.The up-section was not affected by freshwater stress,and was rarely distributed in the vegetation habitat mainly by competition of P.plicatum:U.vocans borealis was only distributed in the down-section sandy habitat.and the not distribution in the up-section was mainly affected by salinity and soil particle size.The absence of dominant distribution in the mid-section was mainly affected by soil particle size.This research acts as a case study of the distribution of burrowing crabs in the intertidal zone along the estuary and the impact of key environmental factors on it,can help to enrich the theory of species distribution along environmental gradient in coastal wetland.It can also provide a scientific support for understanding the distribution patterns of burrowing crabs in Zhangjiang Estuary,China.