隐秘螳臂相手蟹攀爬芦苇行为的昼夜变化

蟹类是潮滩沼泽(本文简称潮沼)湿地生态系统底栖动物重要组成之一。蟹类对潮沼植物活体的牧食以及对潮沼植物上筑巢繁殖鸟类的巢捕食表明,潮沼蟹类会攀爬上潮沼植物。然而对于潮沼中蟹类攀爬植物的行为了解甚少。隐秘螳臂相手蟹(Chiromantes neglectum)是长江口及邻近潮沼生态系统中的优势底栖动物之一。本研究在长江口芦苇(Phragmites australis)潮沼中通过野外直接观测计数估计潮沼地表、潮沼植物(芦苇)上隐秘螳臂相手蟹密度的昼夜变化,来分析其攀爬芦苇行为的时间变化规律。攀爬芦苇隐秘螳臂相手蟹密度和地表隐秘螳臂相手蟹密度均具有显著的昼夜差异(所有P0.05),根据隐秘螳臂相手蟹攀爬密度、地表密度的昼夜变化结合野外行为观测发现,隐秘螳臂相手蟹一般在日落前后(17:00~19:00时)开始攀爬芦苇,在日出后(7:00时以后)大部分隐秘螳臂相手蟹都已经从芦苇上撤离回到潮沼表面。隐秘螳臂相手蟹攀爬比例也具有显著的昼夜变化(P0.05),在夜间时段(19:00~5:00时)攀爬比例高于白天时段(5:00~19:00时)(P0.05),夜间时段攀爬比例在0.16~0.34之间,白天时段攀爬比例在0.02~0.15之间。隐秘螳臂相手蟹相手蟹的总密度在各观测时间段间差异显著(P0.05),夜间时段总密度要高于白天时段(P0.05),表明隐秘螳臂相手蟹在白天时段会较多利用潮沼表面的洞穴,从而呈现出从潮沼植被、潮沼地表到潮沼表面洞穴间的昼夜移动。此外,不同观测日期之间隐秘螳臂相手蟹攀爬密度、地表密度、总密度和攀爬比例均存在显著差异(所有P0.05),但是规律性不明显。隐秘螳臂相手蟹的植物攀爬行为似乎与取食芦苇叶片有关系,同时又受到光照强度昼夜变化的影响,具体影响隐秘螳臂相手蟹攀爬芦苇行为的因素以及隐秘螳臂相手蟹攀爬芦苇行为对潮沼湿地生态过程的影响有待进一步研究。     

长江口盐沼湿地无齿螳臂相手蟹(Chiromantes dehaani)胃含物特征与取食偏好

无齿螳臂相手蟹(Chiromantes dehaani)是长江口盐沼湿地大型底栖动物的优势物种,其食性及行为特征对区域生态系统结构与功能的维系具有重要作用。在崇明北滩盐沼湿地取样调研的基础上,通过胃含物分析与室内喂食实验,分析了无齿螳臂相手蟹的食性及其影响因子。结果表明:植物叶片是无齿螳臂相手蟹的主要食物来源,沉积物次之;其对不同叶片的取食量有着明显的差异,具体表现为对芦苇嫩叶的取食量最高,占35.7%,对芦苇凋落叶片的取食量最少,占6.8%;不同样地间不同性别无齿螳臂相手蟹的食物组成也有明显的差异;在野外情况下,无齿螳臂相手蟹对植物的取食受其自身体重、壳宽、螯高的影响较大;室内投喂情况下,无齿螳臂相手蟹对新鲜芦苇嫩叶存在明显的取食偏好(P0.05),其对芦苇嫩叶的取食偏好受其性别的影响较大,而对芦苇凋落叶片的取食偏好受壳宽、体重、螯高的影响较大。无齿螳臂相手蟹的食性具有一定偏好,受食物及其自身生长特征等多种因素的影响。     

长江口芦苇和互花米草盐沼湿地蟹类洞穴分布特征及主要影响因子

蟹类洞穴是蟹类在潮间带盐沼生存、繁衍的特征性结构,具有重要的生态功能。洞穴分布特征及其影响因子的分析,是深入探讨蟹类及其洞穴的生态系统功能的重要基础。2015年10月,在崇明北滩单一芦苇(Phragmites australis)群落,单一互花米草(Spartina alterniflora)群落和芦苇-互花米草混合群落3种典型生境中,对蟹类洞穴的分布特征及其相关的大型底栖动物、植被、沉积物等的特征参数进行了调研与分析。结果表明,生境类型差异对蟹类洞穴分布特征及相关生境因子具有重要影响。蟹类洞穴的分布密度和开口直径在不同生境间存在显著差异(P0.05),且单一芦苇群落生境内洞穴密度要显著高于单一互花米草群落生境(P0.05),洞穴开口直径在单一互花米草生境要显著高于单一芦苇生境(P0.05);大型底栖动物生物量、密度、植物地下部分生物量在不同生境间差异不显著(P0.05),而植株密度、活植株高度、植物地上部分生物量以及沉积物含水率、p H、氧化还原电位在不同生境间存在显著差异(P0.05)。沉积物中值粒径,总氮含量和总碳含量在不同生境间的差异随深度不同会发生变化。不同生境主要生境因子的差异是导致蟹类洞穴分布特征不同的根本原因;蟹类洞穴分布特征受多个生境因子的综合作用。筛选的生境因子的组合虽然与洞穴分布特征具有显著相关性,但相关系数较小。未来研究中需要拓展生境因子涵盖范围,加强多因子综合作用分析。     

长江下游狼山沙和新开沙中华绒螯蟹幼蟹栖息地评估

为探究中华绒螯蟹(Eriocheir sinensis)幼蟹栖息地适宜度状况,于2016年6月至2017年5月在长江下游狼山沙和新开沙放置定制网具,获取中华绒螯蟹幼蟹丰度分布特征及6个水环境因子,通过单因素方差分析、相关性分析和典范对应分析筛选出水深、透明度和无齿螳臂相手蟹(Chiromantes dehaani)丰度3个指示因子,建立栖息地适宜度曲线,计算栖息地适宜度指数(habitat suitability index,HSI)。结果表明:有水生植物站点的中华绒螯蟹幼蟹丰度较高;不同站点浊度、透明度和水深存在显著差异;中华绒螯蟹幼蟹丰度与无齿螳臂相手蟹丰度、水深极显著相关,与透明度显著相关;两沙洲有水生植物站点HSI值较高,均高于0.5,而其他站点普遍低于0.4,同时发现,中华绒螯蟹幼蟹偏好水深较浅、透明度10~20 cm、有水生植物覆盖的水域。研究表明,两沙洲适宜中华绒螯蟹幼蟹生长发育,应加大保护力度。     

滩涂湿地蟹类活动对表层土壤的扰动效应

为探究在不同植物群落中的大型底栖动物无齿螳臂相手蟹(Chiromantes dehaani)对滨海滩涂湿地表层土壤的生物扰动效应, 在江苏大丰糜鹿国家级自然保护区第三核心区的滩涂湿地潮坪区, 春季末期在潮沟附近的互花米草(Spartina alterniflora)群落、白茅(Imperata cylindrica)群落、芦苇(Phragmites australis)群落3个植物群落中, 分别设置多蟹围隔区、少蟹围隔区和无围隔区, 冬季采集并检测处理后围隔区蟹洞数量、土壤的理化性质及植物生物量等生态指标, 分析底栖动物对表层土壤的扰动效应。研究发现: 互花米草及白茅群落中土壤总有机碳(TOC)、总氮(TN)含量在多蟹区显著低于少蟹区, 蟹丘土壤的TOC及TN含量显著高于多蟹区表层土壤; 而在芦苇群落中则相反。互花米草群落多蟹区土壤C/N、白茅群落多蟹区土壤C/N及C/P显著低于少蟹区, 芦苇群落不显著。互花米草群落蟹穴数量与植物生物量呈显著正相关。研究结果显示, 相手蟹扰动降低了互花米草及白茅植物群落表层土壤的有机质稳定性, 加速碳氮分解, 降低表层土壤的碳氮含量, 并促进互花米草植物生物量的增长, 但对芦苇群落表层土壤扰动的影响则相反。     

红树林湿地相手蟹科动物摄食生态研究进展

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。     

长江口盐沼湿地不同亚生境的大型底栖动物群落组成和多样性差异

2015年7月—2016年6月每月在长江口盐沼湿地采集潮沟、光滩和盐沼(以海三棱藨草为主)三种亚生境的大型底栖动物样本,分析其群落组成和多样性差异。结果表明:共采集到大型底栖动物21种,隶属4门、5纲、15科,以软甲纲(7种)和多毛纲(4种)种类较多;潮沟和盐沼亚生境的优势种无差异;盐沼底栖动物物种数最高,潮沟次之,光滩物种数最低;盐沼底栖动物的密度和生物量均高于其他2种亚生境;三种亚生境的大型底栖动物密度两两间差异显著(P0.05),而生物量无显著差异(P0.05);盐沼底栖动物群落的Shannon多样性指数最高(1.08),光滩次之(1.03),而潮沟最低(0.63);光滩底栖动物群落的Simpson物种丰富度指数、Pielou均匀度指数值均高于潮沟和盐沼,反映光滩底栖动物群落种类密度的分布相比其他2种亚生境较为均匀;通过群落聚类和非度量多维标度分析发现,盐沼和潮沟亚生境的大型底栖动物群落结构较为相似,而与光滩差异较大,引起两者差异的主要贡献种是谭氏泥蟹、河蚬、背蚓虫等。     

翅碱蓬对盐沼沉积物微生物生物量及β-氨氧化细菌群落的影响——以双台河口为例

为了解翅碱蓬植被对盐沼沉积物微生物的影响,于2013年7月、8月、9月和11月对双台河口裸滩和翅碱蓬植被沉积物(10—15 cm)微生物生物量碳(MBC)、微生物生物量氮(MBN)、16S rRNA基因丰度、潜在硝化速率、β-氨氧化细菌(β-AOB)丰度及群落进行了调查。结果表明,不同采样日期裸滩沉积物MBC、翅碱蓬沉积物β-AOB amo A丰度和两种生境潜在硝化速率没有显著差异;而翅碱蓬沉积物MBC、裸滩沉积物β-AOB amo A丰度、两种生境MBN和16S rRNA基因丰度呈现时间波动。当所有采样日期的数据结合分析时,翅碱蓬植被显著影响沉积物MBC、MBN、细菌16S rRNA基因丰度、潜在硝化速率和β-AOB amo A丰度。从裸滩和翅碱蓬沉积物获得的β-AOB序列属于Nitrosospira和Nitrosomonas,翅碱蓬植被对β-AOB群落结构和多样性均有一定的影响。研究结果有助于了解翅碱蓬湿地中微生物的作用,为盐沼生境的生态修复技术提供参考。     

黄河河口盐沼植被分布、多样性与土壤化学因子的相关关系

以黄河河口盐沼实地调查的67个样地19个植物种的重要值,运用双向指示种分析将黄河河口盐沼植被分为柽柳-翅碱蓬+补血草+碱蓬群落、翅碱蓬群落、柽柳-芦苇+翅碱蓬群落、芦苇群落、穗状狐尾藻+芦苇群落和旱柳-芦苇+荻群落.利用典范对应分析(CCA)分析了土壤盐分、pH、Na+、K+、Ca2+、Mg2+、Cl-、全氮、全磷、全碳、有机质等11个土壤化学因子与黄河河口盐沼植被分布的相关关系.CCA排序结果表明,土壤盐分和土壤pH是影响黄河河口盐沼植被分布的主要土壤化学因子.CCA排序第一轴与土壤盐分呈显著正相关关系(P<0.001),第二轴主要和土壤pH呈显著负相关关系(P<0.001).柽柳-翅碱蓬+补血草+碱蓬群落排列于排序图的右上部,与生境土壤中高含盐量及中等pH相对应,穗状狐尾藻+芦苇群落和芦苇群落分布于第一排序轴的最左端,主要与土壤低含盐量相关,翅碱蓬群落位于第二排序轴的下侧, 与较高的土壤pH相关,柽柳-芦苇+翅碱蓬群落和旱柳-芦苇+荻群落,土壤pH和盐分均较低.通过物种丰富度R、香农维纳多样性H′、物种均匀度E等指数与土壤化学因子的相关分析,发现黄河河口盐沼植物物种多样性与土壤盐分没有呈现出与土壤盐分很好的负相关关系,物种丰富度R、香农维纳多样性H′、物种均匀度E均与土壤盐分呈现显著正相关关系;R、H′与土壤pH呈现出十分显著的负相关关系; R、H′与土壤Ca2+、Mg2+含量呈现极为显著正相关关系.     

上海郊区螃蜞的洞穴及其对农田水利的危害性

一.前言螃蜞隶属节肢动物门(Arthropoda),甲壳纲(Crustacea),十足目(Decapoda),方蟹科(Grapsidae),相手蟹属(Sesarma),俗称的螃蜞,就是相手蟹属的这类动物。上海郊区广泛分布的称为无齿相手蟹Sesarma(Holometopus)dehaani H.Milne-Edwarls(图1),该     

The validity of burrow counting in estimating the densities of Chiromantes dehaani and Ilyoplax deschampsi in tidal flat at Yangtze Estuary

It has been confirmed that the crabs play significant roles in the structure and function of coastal wetland ecosystems, such as mangroves and salt marshes. However it is not easy to estimate the abundance and density of burrowing crabs effectively, thus a further understanding of roles of crabs in these ecosystems has been lagged. Some studies have discussed the suitability of several census techniques, such as burrow counting method in estimating crab density in mangroves. The validities of burrow counting method and other census techniques in estimating crab density, however, has not been tested in salt marshes, especially where vegetation are dense. In this study, we tested the validity of burrow counting method in estimating the densities of Chiromantes dehaani and Ilyoplax deschampsi in tidal flat with dense vegetation of Phragmites australis and Zizania aquatica at Yangtze Estuary through comparing densities estimated by the burrow counting method and the excavation. Burrow counting averagely underestimated the density of C. dehaani by 15% and the degree of underestimation varied among vegetations and habitats (from an overestimate by 23% to underestimate by 41%). Burrow counting averagely overestimated the density of I. deschampsi by 43% and the degree of overestimate varied from 0% to 133% depending on the vegetations and habitats. The percentage of occupied burrows and the number of crabs sharing one burrow were important factors influencing the validity of estimating crab density through burrow counting method.     

The validity of burrow counting in estimating the densities of Chiromantes dehaani and Ilyoplax deschampsi in tidal flat at Yangtze Estuary

It has been confirmed that the crabs play significant roles in the structure and function of coastal wetland ecosystems, such as mangroves and salt marshes. However it is not easy to estimate the abundance and density of burrowing crabs effectively, thus a further understanding of roles of crabs in these ecosystems has been lagged. Some studies have discussed the suitability of several census techniques, such as burrow counting method in estimating crab density in mangroves. The validities of burrow counting method and other census techniques in estimating crab density, however, has not been tested in salt marshes, especially where vegetation are dense. In this study, we tested the validity of burrow counting method in estimating the densities of Chiromantes dehaani and Ilyoplax deschampsi in tidal flat with dense vegetation of Phragmites australis and Zizania aquatica at Yangtze Estuary through comparing densities estimated by the burrow counting method and the excavation. Burrow counting averagely underestimated the density of C. dehaani by 15% and the degree of underestimation varied among vegetations and habitats (from an overestimate by 23% to underestimate by 41%). Burrow counting averagely overestimated the density of I. deschampsi by 43% and the degree of overestimate varied from 0% to 133% depending on the vegetations and habitats. The percentage of occupied burrows and the number of crabs sharing one burrow were important factors influencing the validity of estimating crab density through burrow counting method.     

The Contribution of Crab Burrow Excavation to Carbon Availability in Surficial Salt-marsh Sediments

Geomorphology, vegetation and tidal fluxes are usually identified as the factors introducing variation in the flushing of particulate organic matter (POM) from tidal marshes to adjacent waters. Such variables may, however, be insufficient to explain export characteristics in marshes inhabited by ecosystem engineers that can alter the quantity and quality of POM on the marsh surface that is subject to tidal flushing. In this study we evaluated the balance between transfer of buried sedimentary organic carbon (C) to the marsh surface due to crab excavation (measured from the mounds of sediment excavated from burrows) and outputs of C from the surface due to sediment deposition within crab burrows (estimated from sediment deposited within PVC burrow mimics), in a Southwestern Atlantic salt marsh supporting dense (approximately 70 ind m⁻²) populations of the crab Chasmagnathus granulatus. C excavation by crabs was much greater than deposition of C within crab burrow mimics. Per area unit estimates of the balance between these two processes indicated that crabs excavated 5.98 g m⁻² d⁻¹ and 4.80 mg m⁻² d⁻¹ of total and readily (10 d) labile C, respectively. However, sediments excavated by crabs showed a significantly lower content of both total and readily-labile C than sediment collected in burrow mimics. This indicates that ecosystem engineering by burrowing crabs causes a net decrease in the concentration of C in the superficial sediment layers and, thus, an overall decrease in the amount of C that can be washed out of the marsh by tidal action. Incorporating the in situ activities of ecosystem engineers in models of marsh export should enhance understanding of the function of marshes in estuarine ecosystems.     

Bioturbation of Burrowing Crabs Promotes Sediment Turnover and Carbon and Nitrogen Movements in an Estuarine Salt Marsh

Ecological functions of bioturbation in ecosystems have received increasing attention over the recent decades, and crab burrowing has been considered as one of the major bioturbations affecting the physical and chemical processes in salt marshes. This study assessed the integrated effects of crab excavating and burrow mimic trapping on sediment turnover and vertical C and N distributions in a Chinese salt marsh in the Yangtze River estuary. Crab burrowing increased soil water content and the turnover of carbon and nitrogen and decreased bulk soil density. Vertical movement of materials, nutrient cycling and reuse driven by crab burrowing might be obstructed by vegetation (Phragmites australis and Spartina alterniflora communities). The amount of soil excavated by crab burrowing was higher than that deposited into burrow mimics. In Phragmites marshes, Spartina marshes and unvegetated mudflats, net transport of soil to the marsh surface was 171.73, 109.54, and 374.95 g m⁻² d⁻¹, respectively; and the corresponding estimated soil turnover time was 2.89, 4.07 and 1.83 years, respectively. Crab burrowing in salt marshes can mix surface and deeper soil over a period of years, accelerating litter decomposition and promoting the efficient reuse of nutrients by plants. Therefore, bioturbation affects soil physical processes and functioning of ecosystems, and needs to be addressed in ecosystem management.     

Evidences of habitat displacement between two common soft-bottom SW Atlantic intertidal crabs

The intertidal burrowing crab Chasmagnathus granulatus Dana is the dominant species in soft sediments and vegetated intertidal areas along the SW Atlantic estuaries (southern Brazil 28°S to the northern Argentinean Patagonia 41°S) where it produces dense and extensive burrowing beds. The mud crab Cyrtograpsus angulatus Dana coexists with Ch. granulatus in this area, but it also inhabits areas to the south (northern and central Argentinean Patagonia). A survey covering both areas showed that C. angulatus rarely live in burrows when coexisting with Ch. granulatus, but form large burrowing beds when not coexisting with Ch. granulatus. When both species coexisted, burrowing beds of C. angulatus are restricted to sandy-muddy areas. Only rarely are burrows of C. angulatus found within Ch. granulatus beds. However, when Ch. granulatus were experimentally excluded within their burrowing beds, new settlers of C. angulatus made burrows and maintained them until they reached large size. Paired (inside and outside Ch. granulatus burrowing bed) sampling during high tide using beach nets showed that C. angulatus rarely venture inside the Ch. granulatus crab beds. Other field experiments showed that adults Ch. granulatus always displace C. angulatus from burrows. Furthermore, in several sites located south of the limit of distribution of Ch. granulatus at the Patagonian coast, soft bare intertidals are dominated by burrowing beds of C. angulatus mixed with the congener C. altimanus Dana. Together, these evidences suggest that the mud crab C. angulatus is displaced from soft bottom areas by the burrowing crab Ch. granulatus. It is an example of competitive exclusion through aggressive interference in soft-bottom habitats when the shared resource is the access to sediment surface, a two-dimensional well-defined resource.     

Strategies of burrowing in soft muddy sediments by diverse polychaetes

Muddy sediments are elastic solids through which morphologically diverse animals extend burrows by fracture. Muddy sediments inhabited by burrowing infauna vary considerably in mechanical properties, however, and at high enough porosities, muds can be fluidized. In this study, we examined burrowing behaviors and mechanisms of burrow extension for three morphologically diverse polychaete species inhabiting soft muddy sediments. Worms burrowed in gelatin, a transparent analog for muddy sediments, and in natural sediments in a novel viewing box enabling visualization of behaviors and sediment responses. Individuals of Scalibregma inflatum and Sternaspis scutata can extend burrows by fracture, but both also extended burrows by plastic deformation and by combinations of fracture and plastic deformation. Mechanical responses of sediments corresponded to different burrowing behaviors in Scalibregma; direct peristalsis was used to extend burrows by fracture or a combination of plastic deformation and fracture, whereas a retrograde expansive peristaltic wave extended burrows by plastic deformation. Burrowing speeds differed between behaviors and sediment mechanical responses, with slower burrowing associated with plastic deformation. Sternaspis exhibited less variability in behavior and burrowing speed but did extend burrows by different mechanisms consistent with observations of Scalibregma. Individuals of Ophelina acuminata did not extend burrows by fracture; rather individuals plastically deformed sediments similarly to individuals of the related Armandia brevis. Our results extend the range of natural sediments in which burrowing by fracture has been observed, but the dependence of burrow extension mechanism on species, burrowing behavior, and burrowing speed highlights the need for better understanding of mechanical responses of sediments to burrowers.     

The influence of habitat, season and tidal regime in the activity of the intertidal crab Neohelice (=Chasmagnathus) granulata

The activity pattern of intertidal crabs is influenced by factors that usually change rhythmically following tidal and/or diel cycles, and is often associated with the use of refuges. The movement activity of the burrowing crab Neohelice granulata was compared among three populations from SW Atlantic coastal areas where they face different tidal regimes, water salinities, substrata and biological factors. At each site, we examined the seasonal activity of the crabs (individuals collected in pitfall traps) in two types of habitat: mudflat and salt marsh. The working hypothesis is that the activity would vary according to the diverse environmental conditions encountered at geographical and local scales. Crab activity varied between sites and seasons showing to be more intense when habitats were covered by water. The most active groups were large males, followed by large non-ovigerous females. Ovigerous females were almost inactive. Most crabs were near or inside burrows at low tides in Mar Chiquita and Bahía Blanca, but they were active at both low and high tides in San Antonio during spring and summer. N. granulata were active in a wide range of temperatures: from 10 to 37 °C at low tides and at temperatures as low as 2 °C when covered by water. Differences of activity between mudflat and salt marsh varied among sites depending on flooding frequencies. Movement activity of N. granulata varied both in space and in time; crabs move under very different abiotic conditions (e.g., low or high tide, daylight or night, low and high temperature) and their movement may also be prevented or elicited by biotic conditions like burrow complexity, food quality and predation pressure. The wide set of conditions under which N. granulata can be active may explain why this is the only semiterrestrial crab inhabiting latitudes higher than 40°S in South America.     

Ecosystem engineers activate mycorrhizal mutualism in salt marshes

Theory predicts that ecosystem engineers should have their most dramatic effects when they enable species, through habitat amelioration, to live in zones where physical and biological conditions would otherwise suppress or limit them. Mutualisms between mycorrhizal fungi and plants are key determinants of productivity and biodiversity in most terrestrial systems, but are thought to be unimportant in wetlands because anoxic sediments exclude fungal symbionts. Our field surveys revealed arbuscular mycorrhizal associations on salt marsh plant roots, but only in the presence of crabs that oxygenate soils as a by-product of burrowing. Field experiments demonstrate that fungal colonization is dependent on crab burrowing and responsible for nearly 35% of plant growth. These results highlight ecosystem engineers as ecological linchpins that can activate and maintain key mutualisms between species. Our findings align salt marshes with other important biogenic habitats whose productivity is reliant on mutualisms between the primary foundation species and micro-organisms.     

Commensalism used by freshwater crayfish species to survive drying in seasonal habitats

Abstract. Gramastacus insolitus is a very small non-burrowing Australian freshwater crayfish with a restricted distribution, occurring almost exclusively in seasonal habitats throughout its range. It is listed as a threatened species but its strategy for surviving dry periods was unknown. Eight seasonal surveys of crayfish distribution showed that members of G. insolitus were never found at sites that were outside the distribution of two larger burrowing freshwater crayfish species, Geocharax falcata and Cherax destructor. Excavation of 80 burrows of members of G. falcata and C. destructor in three different seasonal habitats in the Grampians National Park, Victoria, Australia, revealed that individuals of G. insolitus found refuge from drying by estivating in cracks and shallow depressions at the side of the main burrow tunnels constructed by larger species. Members of G. insolitus were not found estivating at the surface, such as under fallen wood, nor was it usually found in crayfish burrows unoccupied by the host crayfish. This study indicates that members of G. insolitus are commensal upon larger crayfish species, using their burrows to survive the seasonal drying of their habitat. Conservation strategies for populations of G. insolitus will need to consider co-existing species of burrowing crayfish.     

Crab Burrowing Limits Surface Litter Accumulation in a Temperate Salt Marsh: Implications for Ecosystem Functioning and Connectivity

Burial of aboveground plant litter by animals reduces the amount available for surface transport and places it into a different environment, affecting decomposition rates and fluxes of organic matter to adjacent ecosystems. Here we show that in a Southwestern Atlantic salt marsh the burrowing crab Neohelice granulata buries aboveground plant litter at rates (0.5–8 g m^?2 day^?1) comparable to those of litter production (3 g m^?2 day^?1). Buried litter has a low probability (0.6%) of returning to the marsh surface. The formation of burrow excavation mounds on the marsh surface is responsible for most litter burial, whereas litter trapped in burrows was an order of magnitude lower than rates of burial under excavation mounds. Crab exclusion markedly increased surface litter accumulation (3.5-fold in just 21 days). Tides with the potential to transport significant amounts of surface litter are infrequent; hence, most litter is buried before it can be transported elsewhere or decomposes on the surface. Crab litter burial can account for the observed low levels of surface litter accumulation in this ecosystem and likely drives organic matter transformation and export. The impacts of ecosystem engineering by this crab species are therefore substantial and comparable in magnitude to the large effects found for tropical crabs and other litter-burying organisms, such as anecic earthworms.