Feeding ecology of sesarmid crabs in mangroves
- 环境生态－已发表论文 
相手蟹科物种是红树林湿地的主要底栖动物类群,在生态系统中起着重要的作用。我国大陆地区目前已记录的相手蟹科物种数量为12种,低于其它红树林地区(国内常用的采样方法会造成螃蟹物种数量和密度的低估),其中褶痕相手蟹(SESArMA PlICATA)、无齿相手蟹(S.dEAAnI)和双齿相手蟹(S.bIdEnS)等是常见种。红树植物叶片是相手蟹科动物的主要食物来源,相手蟹科动物通过地表摄食和洞穴贮存的形为消耗了大量的红树植物凋落叶,使这些凋落叶的有机质和营养元素得以保留在生态系统内,在凋落叶的周转和物质归还方面起到重要的作用。它们同时也摄食红树植物的繁殖体并且对不同物种的繁殖体具有摄食偏好,这可能对一些红树物种的植被更新能力和红树植被群落结构产生影响。相手蟹科动物对不同物种和不同状态的红树叶片也存在摄食偏好,通常对腐烂的叶片摄食偏好较强;螃蟹的摄食偏好与叶片的营养成份、粗纤维和单宁含量以及C/n比等性质有关;但在恶劣的野外环境下,螃蟹则会表现出随机性摄食。目前关于相手蟹科动物生态学作用的认识仍不充分,例如它们的种群大小和对凋落物的转化作用等,有待于进一步研究。Sesarmid crabs,common macro invertebrates in mangrove ecosystems,influence the structure and function of mangrove habitats through their burrowing activities and processing of leaf litter.A total of 12 sesarmid species have been recorded in mainland China,which may be an underestimate since more sesarmid species have been reported in Hong Kong and other tropical mangrove regions.( The reason for this underestimation could be due to the method commonly used to locate the crabs within mainland China.The most common method used is to place plot frame on the soil surface and simply dig to about 20—30 cm,and sift through the soil to locate crabs which could allow sesarmid crabs to escape deeper into the soil during the digging.) The most common sesarmid species in China are Sesarma plicata,S.deaani and S.bidens,while Neosarmatium meinerti is widely recorded in tropical areas of Australia and Kenya.Mangrove plant tissues,mainly leaves,are the main food source of sesarmid crabs,but crabs also feed on sediment detritus and faunal tissue to supplement the nutrient limited mangrove leaves.Some dietary specializations among different mangrove crabs have been recognized.Some crabs feed on only live leaves obtained by climbing into trees,while some species feed only on dead leaves on the mangrove floor.Crabs also have frequently been observed burying fallen mangrove leaves,to enhance leaf nutrition quality,prevent leaf litter from being removed by tidal flushing and avoid competition and predation when food or time available for collectingfood is limited,and predator abundance is high.The feeding preference of sesarmid crabs for mangrove leaves with different conditions or from different mangrove species also has been reported from the laboratory and the field research.For example,sesarmid crabs in China prefer leaves of Kandelia candel over those of Bruguiera gymnorrhiza and Aegiceras corniculatum.Leaf characteristics such as C /N ratio,tannin,water and crude fiber contents determine the feeding preferences.Generally,sesarmid crabs prefer decomposed leaves,ascribed to the decreased tannin content and C /N ratio,and increases in water during leaf decomposition.It also has been suggested that crabs need to consume more decomposed leaves to obtain adequate C and N since they have lower C and N assimilation rates from those leaves.Feeding preferences have been more often demonstrated in laboratory studies while some field studies have revealed a lack of selective feeding of different mangrove species.This is probably due to the environmental stresses in field like predation,limited feeding time,and limited food availability.The ability of sesarmid crabs to remove leaf litter has been studied frequently in the past decades,and crabs have been shown to consume a large proportion of annual leaf fall production,far in excess of the local production in some mangrove forests.The ability of crabs to remove litter seems to be stronger in tropical areas than in subtropical or warm temperate areas.In subtropical areas like China,removal of leaf litter is affected by temperature and showed significant seasonal variation.Not only sesarmid crabs but some snails( e.g.Terebralia palustris) and ocypodoid /grapsid crabs( e.g.Helograpsus haswellianus and Ucides cordatus) also have been reported to consume mangrove leaves.Direct grazing of leaf litter by crabs accounts for a small proportion of leaf litter removed from mangrove floor in China,while the stocking of leaves in crab burrows for later consumption is the key way by which leaf litter is retained in the ecosystem,preventing tidal export.Through shredding and grazing of leaf litter,crabs also initiate and enhance the breakdown of mangrove detritus and recycling of nutrients.Sesarmid crabs also have been found to graze mangrove propagules and seeds,thus play a critical role in determining seedling recruitment and vegetation regeneration.It also has been suggested that the predation preference on some non-dominant mangrove species regulates their distribution with the vegetation community.The feeding ecology of sesarmid crabs and its involvement of in mangrove restoration,their retention of C and nutrients in mangrove soils and predation of seeds /propagulse are worthy further studies.