Influence Of Habitat Heterogeneity Of Mangroves On Crabs' Species, Biomass And Food Sources

This study was designed to understand how the habitat heterogeneity of mangroves influence crabs' species, biomass and food composition. The study was carried out in Dongzhai Bay, Hainan, China. Four mangrove forests were chosen, including three monospecific forests dominated by Ceriops tagal, Avicennia marina, and .Rhizophora stylosa, respectively, and a mixed forests dominated by Bruguiera sexangula and Bruguiera gymnorrhiza. A pair sites was selected for each forests, One site was established inside the forest, and the other was on the mudflat near the forest. Species and biomass of crabs were investigated, andδ¹³C,δ¹⁵N values of mangrove leaves, sediments and muscle tissues of crabs were determined seasonally through2007 April to 2008, January.In Dongzhai Bay, 18 species of crabs belong to 5 Families were recorded during out investigation. Fiddler crabs and Sesarminae crabs were two major subfamilies. Most Fiddler crabs live on mudflat and Sesarminae crabs prefer inhabiting under canopy. Metaplax longipes (Grapsidae) was also a common species in the area, and distributed both on mudflat and under canopy. There were no significant differences in what among the four sites on mudflat near the forests. The biomass of crabs (25.24g/m²) inside the C. tagal forest was the lowest, significantly lower than that on the mudflat near the forest(105.13g/m²); the highest (166.73g/m²) was inside the R. stylosa forest, remarkably higher than that on the mudflat near the forest(89.28g/m²). The biomass of crabs in A. marina (102.57g/m²) is similar to that in B. sexangula + B. gymnorrhiza mixed forest (104.94g/m²), and there were no significant differences of biomass of crabs between the sites in the two forests and on the mudflat close to them. The biomasses of crabs in the four sites on mudflat were higher in wet season than in dry season, but reverse trend was found in forests.There were significant differences in theδ¹³C,δ¹⁵N values among mangrove species. Theδ¹³C values of C. tagal and A. marina were higher (-28.1 to -25.2‰), and that of R. stylosa and B. sexangula were lower(-30.7 to -27.8‰).δ¹⁵N values the four mangrove species can be listed as: C. tagal (3.8‰) < B. sexangula (5.0‰) < R. stylosa (6.3‰) < A. marina (7.7‰).δ¹³C values of C. tagal, B. sexangula and R. stylosa were higher in wet season than in dry season, butδ¹³C value of A. marina have inversed trend. However,δ¹⁵N values had no significant seasonal changes.Sedimentδ¹³C value in C. tagal forest was lower than C. tagal leafδ¹³C value. Sedimentδ¹³C value in R. stylosa forest was similar to R. stylosa leaves. The sediments in A. marina forest and B. sexangula + B. gymnorrhiza forest were about 2.0‰¹³C enriched than mangrove leaves, repectively. Sedimentδ¹³C values on mudflat near C. tagal, A. marina and B. sexangula + B. gymnorrhiza forests were similar to that in forests, but the sedimentδ¹³C value on mudflat near R. stylosa forest was about 3.0‰higher than that in the forest. Based on the analysis onδ¹³C values of the sediments and mangroves, we found that the sediments in different mangrove forests had different major contributions. The sedimentδ¹³C values in the four forests in wet season were lower than in dry season. The sedimentδ¹³C values on mudflat near C. tagal, R. stylosa and B. sexangula + B. gymnorrhiza forests were higher in wet season than in dry season. While, both sedimentδ¹³C values in A. marina forest and on the mudflat nearby were lower in wet season than in dry season. These seasonal changes were influenced by the changes of relative contribution of mangrove litters and microalgae.Based on analysis onδ¹³C,δ¹⁵N values of crab muscle tissues, we found differences in diets of crabs in different sites. Crabs in A. marina forest may mainly eat mangrove leaf litters and sediments, we estimated that the contributions were 26.7% and 73.3%, respectively. The major food for crabs in C. tagal and R. stylosa forests was probably macroalgae and sediments. Microalgae was possibly the major food of crabs in B. sexangula + B. gymnorrhiza forest. Crabs on mudflat near C. tagal and B. sexangula + B. gymnorrhiza forests may mainly eat macroalgae. Crabs on mudflat near A. marina and R. stylosa forests may mainly eat microalgae.