Getting a grip on the intertidal: flow microhabitat and substratum type determine the dislodgement of the crab Pachygrapsus crassipes (Randall) on rocky shores and in estuaries

Hydrodynamic forces can affect survival as well as limit the movement of motile benthic animals. An animal's danger of dislodgement depends on the hydrodynamic forces it experiences in its microhabitat relative to the force required to dislodge it (tenacity) from the substratum. We measured water flow and substratum characteristics in two different habitats of the shore crab Pachygrapsus crassipes: a wave-swept rocky shore and an intertidal mudflat. The maximum water velocities and accelerations in the microhabitats of the crabs at the wave-swept site were three times and two times greater, respectively, than at the mudflat site. In the laboratory, we measured the tenacity of crabs of various sizes on different substrata, and also measured their drag, lift and added-mass coefficients. Using these data, we calculated the flow conditions under which crabs would be overturned or sheared off the substratum in their two habitats. The net horizontal force (drag plus acceleration reaction) required to dislodge a crab on a rugose rock substratum was an order of magnitude greater than on smooth rock and two orders of magnitude greater than on mud. Our calculations indicate that, under non-storm conditions, crabs will not be dislodged from the substratum in either the mudflat or the wave-swept habitat when grasping the substratum with maximum tenacity. Moving crabs have lower tenacity and our calculations predict that hydrodynamic forces will restrict the mobility of large crabs more than that of small ones on smooth, but not on rugose rock.     

淡水排放对杭州湾湿地浮游动物群落分布的影响

于2010年4月至2011年1月对杭州湾南岸滩涂湿地5个断面(S1-S3为排水区, S4-S5为非排水区)的高潮位和中潮位分别进行浮游动物群落结构的周年调查, 共发现浮游动物38种(轮虫15种, 枝角类4种, 桡足类19种)。排水区浮游动物年平均密度88.89 ind./L, 生物量0.41 mg/L, 非排水区平均密度仅4.21 ind./L, 生物量0.10 mg/L。排水区轮虫和桡足类的第一优势种分别为萼花臂尾轮虫(Brachionus calyciflorus)和汤匙华哲水蚤(Sinocalanus dorrii), 而非排水区第一优势种为中华哲水蚤(Calanus sinicus)。S2-S3断面中潮位的水体营养盐浓度、浮游动物密度和生物量均明显大于高潮位。淡水排放、潮沟径流和潮汐决定了滩涂湿地群落的时空格局, 后两者还解释了排水区中、高潮位间群落结构组成的差异。       

Effect of the snail Ilyanassa obsoleta (Say) on dynamics of the amphipod Corophium volutator (Pallas) on an intertidal mudflat

We investigated interactions between two dominant invertebrate species of intertidal soft-sediment environments of the northwest Atlantic, the mud snail Ilyanassa obsoleta and the burrowing amphipod Corophium volutator, on a mudflat of the upper Bay of Fundy, Canada. Distribution of I. obsoleta on the mudflat was highly patchy and negatively correlated with density of C. volutator. Manipulation of snail density in cages showed that I. obsoleta influences C. volutator; specifically, increasing density of snails reduced density, increased patchiness in distribution, decreased recapture rates and decreased immigration of C. volutator. Ilyanassa obsoleta seems to be affecting C. volutator through an influence on survival rate and emigration rate, although temporal variation in these effects was observed. Given that both I. obsoleta and C. volutator show a preference for tide pools, an important microhabitat on mudflats, snails might have a profound impact on C. volutator population dynamics.     

红树植物对不同海滩面高度的适应性研究

通过研究华南沿海珠江口深圳湾红树植物无瓣海桑、海桑和秋茄在不同海拔高度的滩面中的生长适应性,揭示红树植物对海水淹浸的适生特性.研究结果表明:(1)在一定的滩面高度范围内,红树植物无瓣海桑(滩面高度为-12～35.5 cm)、海桑(滩面高度为-1～24 cm)幼树的保存率和生长量随着滩面高度的降低(即海水淹浸程度的加强)而逐渐减低,秋茄(滩面高度为12～24 cm)的保存率和生长量则无明显差异.(2)随生长期的延长,各树种幼树的保存率逐渐减低.当生长期未超过1年时,幼树生长和成活极不稳定,保存率骤减;当生长期超过1年后,无瓣海桑和海桑保存率趋于相对稳定,秋茄保存率继续下降,极难长久保存和定居.(3)无瓣海桑能够忍耐海水淹浸的限度是滩面高度最低为10 cm,海桑是滩面高度最低为6.5 cm,秋茄是滩面高度至少在24 cm以上,在低于此海拔高度的滩面上各树种极难长期保存和定居.为了保证各树种正常生长发育,种植红树林要选择高于此滩面高度的泥滩区域.     

Tomographic reconstructions of crab burrows from deltaic tidal flat: Contribution to palaeoecology of decapod trace fossils in coastal settings

Trace fossils play an important role in reconstructing palaeoecology and depositional environment. Their palaeoecological implications can be better understood when the trace makers are correctly identified. Neoichnology studies the burrow morphologies, behavioral ecology of modern organisms and their interaction with the environment. Neoichnology is of great significance in linking burrowing behaviors of extant organisms with interpretation of trace fossil palaeoecology. This paper reports a study on burrows produced by the ocypodid crab (Macrophthalmus japonicus De Haan) in the tidal region of the Yellow River Delta using CT scanning techniques. Our results show that M. japonicus produces abundant I-, U-, and Y-shaped burrows in a mudflat of the delta. These domicile structures are similar in morphological characteristics to the trace fossil Psilonichnus, suggesting M. japonicus as its possible additional trace maker. Our study supports the usage of Psilonichnus as a significant palaeoenvironmental indicator for coastal and shelf environments. Furthermore, this study highlights the great potential of applying CT techniques in future ichnological-related studies.     

Sarmatian (Late Middle Miocene) gastropod assemblages of the Central Paratethys

Summary A rather diverse gastropod fauna from Sarmatian deposits of the Austrian/Hungarian Eisenstadt-Sopron Basin was studied. The fauna derives from two layers of clay and silt within a siliciclastic section at St. Margarethen in Burgenland (Austria). These layers are interpreted as littoral mudflats which formed during the Sarmatian (Late Middle Miocene) along the western coast of the Central Paratethys. Strong shifts in the composition of the gastropod fauna, dominated by Potamididae (Cenogastropoda: Cerithioidea), within each layer indicate successions of limnic-fluvial to oligohaline, brackish-littoral, and marine-littoral environments. These shifts in facies are reflected by an alternational of thePotamides hartbergensis assemblage,Granulolabium bicinctum assemblage, and thePotamides disjunctus assemblage. The speciesJujubinus turriculus (Eichwald, 1850),Gibbula buchi (Dubois, 1831), andCylichnina elongata (Eichwald, 1830) are reported for the first time from the Sarmatian of the Paratethys.Mitrella agenta nov. sp. (Neogastropoda: Columbellidae) is introduced as a new species. These species might represent relics of the diverse Badenian fauna but could also prove a minor ingression of marine species from an adjacent bioprovince in the Late SarmatianMactra “Zone”.     

The impact of bait-pumping on populations of the ghost shrimp Trypaea australiensis Dana (Decapoda: Callianassidae) and the sediment environment

This study investigated the relative effects of bait-pumping and the associated disturbance of mudflats on populations of the ghost shrimp Trypaea australiensis and various sediment properties at Coronet Bay, Western Port, southern Australia. The experiment followed a BACI design, with 3 months of sampling before and after the simulated impact. Control plots were left undisturbed while two disturbance treatments (impact and procedural control) were bait-pumped. Ghost shrimp were removed from the impact plots, whereas ghost shrimp were allowed to re-burrow in the procedural control plots. The purpose of the procedural control was to test for the effect of the disturbance associated with bait-pumping. Initial destruction of burrows and compaction of sediment due to bait-pumping and trampling of the mudflat, reduced sediment porosity and created more reducing conditions to depths of 20 cm. The proportion of small particles (<250 μm) and abundance of benthic microalgae increased in surface sediments, whereas organic carbon content decreased. Ghost shrimp density decreased considerably and was slow to recover over the experimental period. Both disturbance treatments responded similarly indicating that the mudflat disturbance associated with bait-pumping was the major cause of the effects observed.     

Macrofaunal recolonization on intertidal mudflats: effect of sediment organic and sand content

Following concerns over how differences in dredged material properties affects recovery following dredged material deposition on mudflats, a field manipulation experiment was conducted to investigate the effects of increased sediment organic content (from 0.9% to 2.8%) and sand content (from 12.0% to 47.0%) on macrofaunal recolonization and sediment properties on an intertidal mudflat, Crouch Estuary, Essex, UK. The manipulated sediments were placed in 1-m² plots, 10 cm deep, in a randomised block design and macrofauna and sediments sampled after 1 week, 1, 3, 6, and 12 months. The results suggested that while increased sand content had no discernible effect on sediment property changes, increased organic content resulted in marked reductions in redox potential at 1-, 2- and 4-cm sediment depths and increased shear strengths. These sediments exhibited a distinct ‘cracking’ appearance on their surfaces. Similarly, while macrofaunal recovery was only affected by increased sand content in the short-term (1 month), total numbers of individuals, number of species and diversity took longer to recover in sediments with increased organic contents. Recolonization proceeded via a gradual increase in the abundance of those taxa present in ambient sediments rather than via a distinct successional sequence. Recovery was species-specific with two species (Hediste diversicolor (Müller) and Hydrobia ulvae (Pennant)) recovering to ambient levels within the first week of the experiment. Multivariate analyses indicated that the community assemblages of the low-organic content treatments had recovered after 12 months while those of the high-organic content treatments were still significantly different to those of the controls at the end of the experiment. We propose that the relatively rapid recovery and lack of successional sequence generally observed in all sediment types of this study can be explained by the opportunistic nature of the ambient assemblage and by the dispersive nature of the dominant species. Reduced sediments and/or increased shear strengths led to a slow recovery in high-organic content treatments. These results indicate that a careful consideration of the organic content of fine-grained dredged material must be made during the licensing process of beneficial use schemes.     

Foraging strategies involved in habitat use of shorebirds at the intertidal area of Chongming Dongtan, China

Dense flocks of migratory shorebirds from diverse species often concentrate in the intertidal areas for stopover. Trophic structure, food partition, prey availability and selectivity, predation risk, and abiotic factors are often used to explain the differences in habitat use of coexisting shorebirds. We sampled the macrobenthos and surveyed the distribution of shorebird populations to study the effects of foraging strategies on the habitat use of shorebirds at Chongming Dongtan, an important stopover site for shorebirds on the East Asian–Australasian Flyway. Results show that the relative abundance of epifaunal macrobenthos in salt marshes was much higher than that in the bare flats, whereas the relative abundance of infaunal macrobenthos in salt marshes was much lower than that in bare flats. The relative abundance of two life forms of macrobenthos was similar in the transitional zones between the salt marshes and the bare flats. Shorebirds with different foraging strategies exhibited different habitat uses. Pause-travel shorebirds mainly utilized the salt-marsh fringes, while tactile continuous shorebirds relied heavily on the bare flats. There was no significant difference in habitat use for visual continuous shorebirds. The density of tactile continuous shorebirds was positively correlated with bivalve density, and that of visual continuous shorebirds positively with crustacean density. Meanwhile, the relative abundance of pause-travel foraging shorebirds was positively correlated with the relative abundance of epifaunal, but negatively with infaunal macrobenthos. In contrast, the relative abundance of tactile foraging shorebirds had a positive correlation with infaunal but a negative one with epifaunal life form. Therefore, foraging strategies may play important roles in shorebirds’ habitat use in intertidal areas.     

优化滩涂养殖水体生态结构和调节水质的研究

采用在滩涂鱼塘和虾池中接种小球藻和有益微生物的方法,探讨了调控滩涂养殖水体生态结构的浮游生物种群、数量、生物量及化学环境的可行性。结果表明,虾池和鱼塘中接种小球藻后,小球藻数量均大幅度增加,变为优势种群,分别是其对照的16.92倍、4.76倍;浮游动物生物量为4.32mg·L^-1和2.84mg·L^-1,分别比对照增加19.3%、2.5%,同样,光合细菌、酵母菌及硝化细菌显著地改变了水体藻类和浮游动物种群的组成、数量、比例及生物量等,“酵母菌＋硝化细菌”的处理,使水体氨态氮浓度下降最明显,为对照的44％,虾池生化耗氧和化学耗氧,为对照的56.5%和38.4%,增加了水体溶解氧和初级生产力。