Causes of rarity and range restriction of an endangered, endemic limpet, Siphonaria compressa

The pulmonate limpet Siphonaria compressa is South Africa's most endangered marine mollusc. It is endemic to just two localities: Langebaan Lagoon on the west coast, and Knysna Estuary on the south coast, and occurs only on the eelgrass Zostera capensis. In Langebaan Lagoon, eelgrass has fluctuated substantially over the last 34 years, and S. compressa has twice approached extinction. S. compressa is largely confined to the lower edge of the eelgrass beds there, being replaced higher up by another small gastropod, Assiminea globulus. We explored the physical and biological factors underlying the limpet's narrow habitat, using field observations, translocations, caging and transplant experiments. Abundance of S. compressa was positively correlated with Z. capensis cover and negatively correlated with shore height. When moved to the upper portions of the eelgrass bed, S. compressa had lower rates of persistence and survival than in the lowest zone. The lower limit of zonation for S. compressa was set indirectly by bioturbation by the sandprawn Callianassa kraussi, which excluded eelgrass from intertidal sandbanks. Transplants of eelgrass into the sandbanks proliferated provided C. kraussi was experimentally eliminated, and supported densities of S. compressa 20-fold greater than in control eelgrass beds, suggesting that high-shore eelgrass beds to which S. compressa is normally confined are suboptimal for the limpet. A. globulus showed patterns opposite to those of S. compressa: its persistence and survival were greatest in the upper zone and it actively avoided the lower sections of these beds and never colonised eelgrass transplanted into sandflats lower on the shore. There was no evidence that competition between S. compressa and A. globulus influenced the zonation or abundance of either species. Rarity of S. compressa and its endangered status seem dictated by its extremely narrow and temporally changeable habitat-range, which is defined by physical stress in the high-shore and bioturbation by C. kraussi in the low-shore. Fluctuations in eelgrass abundance and limitation of S. compressa to just two localities add substantially to the risks of extinction for this embattled stenotypic limpet.     

Influences of biological interactions on community structure within seagrass beds and sandprawn-dominated sandflats

The faunal composition of high-shore beds of eelgrass Zostera capensis versus unvegetated sandflats dominated by the sandprawn Callianassa kraussi was investigated by experimentally transplanting plots of eelgrass into sandflats where the sandprawn was either left unmanipulated or eliminated by prior faunal suffocation. After 12 months, multidimensional scaling defined five faunal clusters that were dictated primarily by the presence or absence of eelgrass rather than sandprawns. By 18 months, eelgrass and sandprawns played equal roles in defining faunal clusters. After 30 months, sandprawns had displaced the eelgrass transplants and faunal composition converged on that of sandflat control plots.Most species were either ‘eelgrass-associated’ or ‘sandflat-associated’, but three had unique responses to the treatments: two high-shore species (Assiminea globulus and Hydrobia sp.) that were abundant in eelgrass beds never appeared in the eelgrass transplants, whereas another eelgrass-occupant, Siphonaria compressa, became superabundant on the transplants for 18 months.Unexpectedly, the eelgrass fauna was never more rich or diverse than that of unvegetated sandflats, although abundance was greater in the eelgrass. As hypothesised, the sandflats were disproportionally populated by burrowers and eelgrass by non-burrowers, but a second hypothesis that these habitats should support mainly hard-bodied and soft-bodied animals respectively was rejected.The distinctively different faunas of eelgrass and unvegetated sandflats are seemingly respectively maintained by sediment stabilisation by eelgrass versus bioturbation by sandprawns. The transplants of eelgrass into sandflats developed an eelgrass-associated fauna provided sandprawns were excluded, but ultimately reverted to a sandflat fauna once sandprawns reinvaded.     

The influence of bioturbation by the sandprawn Callianassa kraussi on feeding and survival of the bivalve Eumarcia paupercula and the gastropod Nassarius kraussianus

Field observations and experimental evidence have shown that bioturbation by the southern African sandprawn Callianassa kraussi may significantly influence the abundance and distribution of the filter-feeding bivalve Eumarcia paupercula and the grazing gastropod Nassarius kraussianus. It was hypothesized that (1) sediment reworking by C. kraussi negatively affects microalgal growth on the sediment surface, leading to a reduction in food intake by N. kraussianus, (2) sediment deposited by C. kraussi will also diminish the food uptake of E. paupercula by interfering with its filtration mechanism. To test these hypotheses, manipulative field and laboratory experiments were undertaken in which N. kraussianus and E. paupercula were added to treatments with and without C. kraussi, and their survival and gut chlorophyll-a content measured. The effects of C. kraussi on sediment erodability and on condition of E. paupercula (tissue mass/shell length) were determined in a second experiment. In the presence of C. kraussi, (1) microalgal consumption by both N. kraussianus and E. paupercula was halved; (2) condition and survival of E. paupercula were significantly reduced but survival of N. kraussianus was unaffected; and (3) sediment erodability was increased. A significant negative relationship was established between sediment erodability and condition of E. paupercula. Evidently C. kraussi exerts a strongly negative influence on the feeding of E. paupercula and N. kraussianus, and this may explain the scarcity of these organisms in areas containing high densities of C. kraussi.     

Experimental test of biodeposition and ammonium excretion from blue mussels (Mytilus edulis) on eelgrass (Zostera marina) performance

Blue mussels and eelgrass have been found to coexist in many locations. However, knowledge of the interactions between these species is limited. Two experiments were conducted in the laboratory, a “Deposit” and an “Epiphyte” experiment. The Deposit experiment examined possible effects of increasing load of blue mussel (Mytilus edulis) biodeposits on sediment biogeochemistry and eelgrass (Zostera marina) performance. Z. marina mesocosms received normal or high loads of mussel biodeposits (Normal and High), while no biodeposits were added to the Control. High dosage had overall negative effects on Z. marina, which was reflected as lower leaf numbers and biomass and accumulation of elemental sulphur in rhizomes. The sediment biogeochemical conditions were altered, as the mussel biodeposits enhanced sulphate reduction rates and increased sulphide concentrations in the porewater, which may result in sulphide invasion and reduced growth of Z. marina.In the Epiphyte experiment effects of mussel excretion, with particular emphasis on ammonium, on the growth of Z. marina and their epiphytes were examined. A thick cover of epiphytes developed on Z. marina growing together with M. edulis, and the relative growth rate was reduced with 20% compared to plants from control without mussels. Overall the experiments showed negative effects on Z. marina growing together with M. edulis, thereby supporting a preceding field study by Vinther et al. [Vinther, H.F., Laursen, J.S., Holmer, M. 2008. Negative effects of blue mussel (Mytilus edulis) presence in eelgrass (Zostera marina) beds in Flensborg fjord, Denmark. Est. Coast Shelf. Sci. 77, 91-103.].     

Effect of shading of Zostera marina (eelgrass) on sulfur cycling in sediments with contrasting organic matter and sulfide pools

Eelgrass Zostera marina was collected in spring and autumn from a light-saturated environment with low-organic sediments and a light-limited environment with organic-rich sediments in Denmark. The eelgrass and sediment responses to reduced light conditions were studied in 2-week shading experiments. Z. marina responded to reduced light conditions by decreasing growth rates and a loss of above-ground biomass. The spring plants were most sensitive to light reductions and the relative leaf elongation rates were reduced with up to 58% and the shoot densities with 33-36%. There was no difference in light response in relation to sediment organic matter contents. The sulfate reduction rates were reduced in the shaded low-organic sediments with up to 67%, whereas there was no effect of shading on rates in the organic-rich sediments. The lack of effect of shading in the organic-rich sediments was attributed to a limited coupling between Z. marina production and sediment bacterial carbon cycling. In contrast to the sulfate reduction rates, the pools of reduced sulfur were increased with up to 89% in the shaded, low-organic sediments, suggesting that the reoxidation of sulfides was reduced. Shading had no effect on the pools of sulfides in the organic-rich sediments due to much larger pools of sulfides. The enhanced sensitivity of spring plants to shading was probably due to a low above- to below-ground ratio compared to the autumn plants, which limited the plant-mediated oxidation of the sediments and thus the reoxidation of sulfides. The shaded plants were possibly more exposed to anoxic and sulfidic conditions affecting their growth and survival.     

Effects of population density on the sediment mixing induced by the gallery-diffusor Hediste (Nereis) diversicolor O.F. Müller, 1776

The aim of this work was to quantify the intensity of sediment mixing induced by the gallery-diffusor (functional bioturbation group) Hediste diversicolor as a function of density, using particles tracers (luminophores). In order to assess the impact of density on sediment reworking, a 1-D model was used to obtain sediment reworking coefficients such as Db (biodiffusion-like) and r (biotransport). Densities used in this experiment corresponded to population densities observed in the sampling area (Saint-Antoine Canal, Gulf of Fos, France): 144, 288, 577, 1153 indiv/m². At first, results showed that neither luminophore maximum burying depth nor the more marked tracer accumulation areas were influenced by density. Thus density did not seem to have any influence on size of galleries or complexity of structure. Then, density-dependent relations with Db (biodiffusion-like mixing) and r (biotransport) were highlighted with an observed process intensity rate twice as high at highest worm density. On the other hand, Db and r per capita coefficients were negatively influenced by density. Db and r per capita at highest density were equal to ∼ 20% of individual Db and r obtained at the lowest density. Finally, this study showed the importance of density which appears to be a key parameter in the functioning of the sedimentary ecosystem.     

Effects of Ilyanassa obsoleta (Say) on the abundance and vertical distribution of Corophium volutator (Pallas) on mudflats of the upper Bay of Fundy

Invertebrates in soft-bottom marine communities exhibit a range of responses to predators and competitors, including both emigration and avoidance. Corophium volutator (Pallas), a burrowing amphipod abundant in the upper Bay of Fundy, Canada, frequently interacts with the eastern mudsnail, Ilyanassa obsoleta (Say), as both species utilize similar habitats and food resources. Typically these interactions result in negative effects on C. volutator. There is evidence that Corophium species exhibit avoidance behaviour when interacting with other species, but whether such a response occurs in the presence of I. obsoleta is unknown. To investigate C. volutator response to I. obsoleta, total C. volutator abundance and vertical distribution within the sediment was examined in a field experiment involving I. obsoleta enclosures and exclosures at two mudflats in the upper Bay of Fundy. I. obsoleta densities were manipulated between June and August 2008 and their effects on C. volutator abundance and vertical distribution observed. Results varied between mudflats and C. volutator size classes. At Peck's Cove, total C. volutator abundance was largely unaffected by I. obsoleta. No size class exhibited a distribution shift in response to I. obsoleta, though both mid-sized individuals and adults occupied deeper layers of sediment late in the summer. Abundance of C. volutator in all size classes at Grande Anse was reduced in July in direct proportion to exposure to I. obsoleta, and adults occupied higher sediment layers in July when I. obsoleta were excluded, suggesting that I. obsoleta curtailed use of these upper layers. Juvenile abundance rebounded in August, possibly due to the mass dispersal of a newly produced juvenile cohort. Adults were reduced in all treatments in August, probably as a result of normal seasonal mortality. Differences in C. volutator responses between mudflats highlight the site-specific nature of such interactions.     

Experimental study on the influence of bioturbation performed by Nephtys caeca (Fabricius) and Nereis virens (Sars) annelidae on the distribution of dinoflagellate cysts in the sediment

Dinoflagellates include noxious microalgae responsible for the formation of toxic red tides and the poisoning of molluscs and crustaceans, resulting in important economic losses. As a consequence, the life cycle of these algae has been extensively studied, but the dormancy phase (cyst) in the sediment record is little known. In the intertidal zone, bioturbation, an important biological process resulting from the activities of benthic fauna, significantly influences the movement of particles in the sediments. Laboratory experiments have allowed comparing and quantifying the movements of fluorescent microspheres resulting from the activity of two polychaetes annelidae, Nereis virens and Nephtys caeca. The particles, which simulate 45 µm diameter dinoflagellate cysts, are deposited in flat aquaria at the surface or deep in the sediment. Photographs of the aquaria were taken at regular intervals, to observe, in a non-destructive manner, the movement of the particles and to calculate, using adapted software, the optical reworking coefficient (ORC) over time. A difference appears between the movements of the particles generated by both species of polychaetes. Nereis virens create “permanent” galleries that carry the microspheres deeply in the sediment during the digging, bioirrigation and feeding, and Nephtys caeca homogenize the particles in the first centimetres of sediment during its erratic movements. The study shows that the bioturbation generated by these organisms can modify the distribution of the 45 µm diameter dinoflagellate cysts in the sedimentary column, burying them or raising them back to the water-sediment interface.     

Biodiversity of benthic invertebrates and organic matter processing in shallow marine sediments: an experimental study

The main objective of this study was to measure the impact of benthic invertebrate diversity on processes occurring at the water-sediment interface. We analyzed the effects of interactions between three shallow water species (Cerastoderma edule, Corophium volutator, and Nereis diversicolor). The impacts of different species richness treatments were measured on sediment reworking, bacterial characteristics, and biogeochemical processes (bromide fluxes, O₂ uptake, nutrient fluxes, and porewater chemistry) in sediment cores. The results showed that the three species exhibited different bioturbation activities in the experimental system: C. edule acted as a biodiffusor, mixing particles in the top 2 cm of the sediments; C. volutator produced and irrigated U-shaped tubes in the top 2 cm of the sediments; and N. diversicolor produced and irrigated burrow galleries in the whole sediment cores. C. edule had minor effects on biogeochemical processes, whereas the other species, through their irrigation of the burrows, increased the solute exchange between the water column and the sediment two-fold. These impacts on sediment structure and solute transport increased the O₂ consumption and the release of nutrients from sediments. As N. diversicolor burrowed deeper in the sediment than C. volutator, it irrigated a greater volume of sediments, with great impact on the sediment cores.Most treatments with a mixture of species indicated that observed values were often lower than predicted values from the addition of the individual effects of each species, demonstrating a negative interaction among species. This type of negative interaction measured between species on ecosystem processes certainly resulted from an overlap of bioturbation activities among the three species which lived and foraged in the same habitat (water-sediment interface). All treatments with N. diversicolor (in isolation and in mixture) produced similar effect on sediment reworking, water fluxes, nutrient releases, porewater chemistry, and bacterial characteristics. Whichever species associated with N. diversicolor, the bioturbation activities of the worm hid the effect of the other species. The results suggest that, in the presence of several species that use and modify the same sediment space, impact of invertebrates on ecosystem processes was essentially due to the most efficient bioturbator of the community (N. diversicolor). In consequence, the functional traits (mode of bioturbation, depth of burrowing, feeding behaviour) of an individual species in a community could be more important than species richness for some ecosystem processes.     

The SW Atlantic burrowing crab Chasmagnathus granulatus Dana affects the distribution and survival of the fiddler crab Uca uruguayensis Nobili

In this paper, we address the question of whether the presence of the burrowing crab Chasmagnathus granulatus affects the habitat use of the fiddler crab Uca uruguayensis. Field samples showed that the species have a disjoint spatial distribution. Male fiddler crab density decreased in zones with C. granulatus, however, female density increased. Male fiddler crabs avoided feeding on sediment affected by C. granulatus and were more preyed. Predation was higher during the fiddler crab reproductive season and, probably due to predation risk, males showed lower reproductive display in shared zones. Field experiments shows that when C. granulatus were excluded, densities of U. uruguayensis increased mainly due to an increase in density of males. Habitat differentiation of these species may be because C. granulatus affects U. uruguayensis in several ways, including direct predation, disturbance and behavioural changes associated to predation risk. Males and females are affected differentially probably because of the extreme sexual dimorphism of this crab species. Coloration on enlarged claw and waving activities are all factors that increase predation risk for male and the presence of only one feeding claw may increase sediment-mediated effects.     

Botryosphaeriaceae species overlap on four unrelated,native South African hosts

Botryosphaeriaceae represents an important and diverse family of latent fungal pathogens of woody plants. We address the question of host range of these fungi by sampling leaves and branches of four native South African trees, including Acacia karroo (Fabaceae), Celtis africana (Cannabaceae), Searsia lancea (Anacardiaceae), and Gymnosporia buxifolia (Celastraceae). Two new species of the Botryosphaeriaceae, namely Tiarosporella africana sp. nov. and Aplosporella javeedii sp. nov. were identified, together with five known species, including Neofusicoccum parvum, Neofusicoccum kwambonambiense, Spencermartinsia viticola, Diplodia pseudoseriata, and Botryosphaeria dothidea. Most Botryosphaeriaceae occurred on more than one host. With the exception of S. lancea, which was infected by A. javeedii all the hosts were infected by more than one Botryosphaeriaceae species. Collectively, the results suggest that some intrinsic host factors, possibly combined with local environmental conditions, affect the distribution and co-infectivity of various hosts by the Botryosphaeriaceae. This would counteract the general ability of a species in the Botryosphaeriaceae to infect a broad range of plants. The combination of host and environmental factors might also explain why some Botryosphaeriaceae with apparently broad host ranges, are found on different suites of hosts in different areas of the world.     

Effects of depth, sediment and grazers on the degradation of drifting filamentous algae (Cladophora glomerata and Pilayella littoralis)

Eutrophication in the northern Baltic Sea promotes growth of annual filamentous algae. The algae detach, gather at the bottom and give rise to algal mats of varying size, density, composition and condition. Dense mats of filamentous algae induce anoxia, which in turn leads to faunal mortality. By a set of field experiments, we have studied the fate of the abundant Cladophora glomerata after detaching from the rocky substrate, and the effect of water depth and sediment on its decay. Further, we have studied the importance of common mesograzers (Gammarus and Idotea) on the rate of degradation of C. glomerata and Pilayella littoralis.Our results show that loose algae at shallow sites (8 m) decompose faster than algae in deeper (18 m) areas. Drifting C. glomerata on the sediment is more rapidly broken down and dissolved than algae floating in the water column, which depends on higher microbiological activity. Dominant amphipods (Gammarus spp) colonise near-shore drift algae quickly, and juvenile bivalves (Cerastoderma glaucum) utilise algae in the water column for settling. Moderate natural densities of grazers (Gammarus spp and Idotea baltica) in the drifting algae did not increase the degradation rates of C. glomerata and P. littoralis. C. glomerata was completely decomposed in 4 months.Our experiments demonstrate the effects of position (depth, water/sediment) and grazing on the degradation of drifting filamentous algae. Mass developments of opportunistic algae occur annually in the study area, and information on the destiny of detached drift algae may help us to predict their longevity and the damage they cause, and hence, to decide on long-term measures needed to improve environmental conditions.     

Genetic characterization of Zostera asiatica on the Pacific Coast of North America

We gathered sequence information from the nuclear 5.8S rDNA gene and associated internal transcribed spacers, ITS-1 and ITS-2 (5.8S rDNA/ITS), and the chloroplast maturase K (matK) gene, from Zostera samples collected from subtidal habitats in Monterey and Santa Barbara (Isla Vista) bays, California, to test the hypothesis that these plants are conspecific with Z. asiatica Miki of Asia. Sequences from approximately 520 base pairs of the nuclear 5.8S rDNA/ITS obtained from the subtidal Monterey and Isla Vista Zostera samples were identical to homologous sequences obtained from Z. marina collected from intertidal habitats in Japan, Alaska, Oregon and California. Similarly, sequences from the matK gene from the subtidal Zostera samples were identical to matK sequences obtained from Z. marina collected from intertidal habitats in Japan, Alaska, Oregon and California, but differed from Z. asiatica sequences accessioned into GenBank. This suggests the subtidal plants are conspecific with Z. marina, not Z. asiatica. However, we found that herbarium samples accessioned into the Kyoto University Herbarium, determined to be Z. asiatica, yielded 5.8S rDNA/ITS sequences consistent with either Z. japonica, in two cases, or Z. marina, in one case. Similar results were observed for the chloroplast matK gene; we found haplotypes that were inconsistent with published matK sequences from Z. asiatica collected from Japan. These results underscore the need for closer examination of the relationship between Z. marina along the Pacific Coast of North America, and Z. asiatica of Asia, for the retention and verification of specimens examined in scientific studies, and for assessment of the usefulness of morphological characters in the determination of taxonomic relationships within Zosteraceae.     

Repellent activity of alligator pepper, Aframomum melegueta, and ginger, Zingiber officinale, against the maize weevil, Sitophilus zeamais

The repellent activity of alligator pepper, Aframomum melegueta, and ginger, Zingiber officinale (Zingiberaceae), against the maize weevil, Sitophilus zeamais (Coleoptera: Curculionidae), was investigated in four-way olfactometer bioassays. Results showed that vacuum distilled A. melegueta and Z. officinale extracts were repellent towards adult S. zeamais both in the absence and the presence of maize, Zea mays, grains. Bioassay-guided liquid chromatographic fractionation of the distillates showed that fractions containing oxygenated compounds accounted for the repellent activity. Coupled gas chromatography-mass spectrometry (GC-MS), followed by GC peak enhancement and enantioselective GC using authentic compounds, identified 3 major compounds in the behaviourally active fractions of A. melegueta and Z. officinale to be (S)-2-heptanol, (S)-2-heptyl acetate and (R)-linalool in a ratio of 1:6:3, and 1,8-cineole, neral and geranial in a ratio of 5.48:1:2.13, respectively. The identification of these behaviourally active compounds provides the scientific basis for the observed repellent properties of A. melegueta and Z. officinale, and demonstrates the potential for their use in stored-product protection at the small-scale farmer level in Africa.     

Particle reworking and solute transport by the sediment-living polychaetes Marenzelleria neglecta and Hediste diversicolor

This experimental study quantified and compared particle-mixing and solute transport by the polychaetes Marenzelleria neglecta (2 g ww, 3200 ind. m^− 2) and Hediste diversicolor (2 g ww, 800 ind. m^− 2) in Baltic Sea sediments. Particle tracers (luminophores) were added to the sediment surface and their vertical distribution in the sediment was measured after 10 d. The rate of particle mixing was quantified using a gallery-diffusion model calculating the biodiffusion coefficient D_b and the non-local transport parameter r. Bioirrigation was measured by adding an inert solute tracer (bromide) to the overlying water 1, 1.5 and 2 d before the end of the experiment, and quantified by calculating the net bromide flux and fitting the bromide profiles to a 1D diffusion model providing an apparent biodiffusion coefficient D_a. The two polychaete worms displayed similar particle-mixing and solute transport efficiencies (based on total biomass) despite different modes of bioturbation. However, H. diversicolor was a more efficient particle-reworker and M. neglecta a more efficient bioirrigator, on an individual level. H. diversicolor buried a higher percentage (13%) of luminophores below the top 0.5 cm surface layer than M. neglecta (6%). D_b did not differ between the two species (2.4 × 10^− 3 cm² d^− 1) indicating a similar rate of diffusive mixing of the top sediment, however, the non-local transport parameter r was 2.5 y^− 1 for H. diversicolor and zero for M. neglecta, suggesting no significant particle-transport below the biodiffusive layer by M. neglecta. The average individual net bromide fluxes obtained were ca. 0.01 mL min^− 1 for H. diversicolor and 0.003 mL min^− 1 for M. neglecta, corresponding to an area-specific rate of ca. 12 L m^− 2 d^− 1 at the used densities. D_a did not differ between the two polychaetes, suggesting a higher individual solute exchange efficiency of M. neglecta considering the much higher ventilation rates reported for H. diversicolor than for Marenzelleria sp. The ongoing colonization of Baltic Sea sediments by M. neglecta at high densities may thus lead to an enhanced soluble release of both nutrients and contaminants. These results add information to the understanding of the potential effects of the invasion of M. neglecta on sediment biogeochemistry when competing with and/or replacing native species.     

Ventilatory and cardiovascular modulation associated with burying behaviour in two sympatric crab species, Cancer magister and Cancer productus

Physiological parameters associated with burying were investigated in the Dungeness crab, Cancer magister, and the red rock crab, Cancer productus. Ventilation frequency of the branchial chambers increased while the crabs were burying, this was associated with the greater oxygen demand of the tissues. The number of ventilatory reversals in C. magister increased in number as well as in duration and magnitude when the crabs were buried, which functioned to clear the branchial chambers of sediment. In contrast, the number of ventilatory reversals in C. productus decreased. On the surface of the sand, cardiac parameters (heart rate, stroke volume, cardiac output) of both species remained stable. During the burial process, there was a large increase in cardiac output which was afforded primarily by an increased stroke volume of the heart. Once buried, cardiac output declined in both species; this was due to a decrease in stroke volume in C. productus, but a decreased heart rate in C. magister. There were also differences in haemolymph flows through each arterial system. During the burying process, both species increased haemolymph flow to the muscles of the limbs via the sternal artery. Once buried haemolymph flows to the limbs decreased, and increased flow to eyestalks and antennae via the anterior aorta occurred. Perfusion of the digestive organs via the anterolateral and hepatic arteries did not change when the crabs were buried. There was an increase in flow through the posterior aorta, of C. magister, but flow through this artery did not change in C. productus. Periods of spontaneous cardiac arrest were observed in both species while resting on the surface. These increased in duration in C. productus when buried, but there was no change in C. magister. Changes in ventilatory and cardiac variables were closely linked on the surface, but tended to uncouple when the animals were buried. The physiological responses of C. magister resembled those of true sand-dwelling crabs, whereas the responses of C. productus paralleled those of crabs that only bury occasionally in the substrate. Although these two species often occur sympatrically, they employ different physiological mechanisms when buried in the sediment.     

Assessing the potential benthic-pelagic coupling in episodic blue mussel (Mytilus edulis) settlement events within eelgrass (Zostera marina) communities

Coastal marine seagrass ecosystems are important nursery grounds for commercially and recreationally important species, and they serve as key settlement and recruitment sites for other species. We investigated several years (2001-2003) where episodic settlement events of blue mussels (Mytilus edulis) occurred in Barnegat Bay, NJ, USA. Population assessment indicated that blue mussels settled in eelgrass beds (Zostera marina) in late spring with peak densities exceeding 170,000 m⁻². Based on calculated filtration rates of M. edulis, we determined that for at least 53 days in 2001, the density and size distribution of M. edulis were sufficient to filter the water column volume in excess of twice a day, with maximum calculated filtration rates exceeding 8 m³ water m⁻² day⁻¹. While the settlement event in 2001 was very localized, in 2003, the settlement event was considerably more widespread throughout the bay, with maximum settling densities exceeding 175,000 individuals m⁻². Associated with these high densities, maximum calculated filtration rates exceeded 15 m³ water m⁻² day⁻¹. This filtration potential may have impeded the localized development of a brown-tide (Aureococcus anophagefferens) bloom in 2001, which occurred in other regions of the bay, but the widespread settlement event seen in 2003 may have impeded the development of any brown-tide blooms in Barnegat Bay during that summer. The decline in mussel densities throughout the summer may be a result of elevated water temperatures in this back bay, but at one site, the high settlement of M. edulis was followed by a substantial migration (>40 individuals m⁻²) of small sea stars (Asterias forbesii). In 2001, A. forbesii was a significant factor in reducing M. edulis density by the end of the summer at the Barnegat Inlet site and a community level assessment showed significant positive correlations between mussel aggregations and sea star densities (r=0.68-0.73, P<0.001). At this same site in 2003, the sea stars were again present in high densities (26 m⁻²) and were a potential mechanism for mussel decline. In other regions of the bay, sea star densities are very low, but numerous other predatory species exist, including blue crabs (Callinectes sapidus), green crabs (Carcinus maenus), spider crabs (Libinia spp.), and several Xanthid crabs. Given the high mussel densities seen in this study and the considerable predation by sea stars and other benthic predators, the benthic-pelagic coupling which these mussels provide in this system contributes to the high secondary production in these grass beds.     

Environmental and behavioural factors affecting activity in the intertidal gastropod Hydrobia ulvae

Laboratory microcosms were used to investigate the mud snail Hydrobia ulvae (Pennant) bioturbation activities and behavioural changes in response to snail density, algal food, sediment moisture content, light regime and water cover conditions. Density-dependent kinetics of bioturbated muddy areas were described by von Bertalanffy equations, which provided reliable estimates of mud surface covering rates by snail tracks (m² h⁻¹ snail⁻¹). Snails need a wet habitat to be active either covered by seawater or by moving in fluid layers for low-tide conditions. Light and microphytobenthic biomass, which are less potent to affect snail activity, are positively interrelated to increase covering rates in the tested chl a concentrations within the range of 1-15 μg g⁻¹. Experimental results suggested us the relevance of microphytobenthos migration processes in affecting crawling activities of H. ulvae that appeared to adjust their foraging efforts in response to benthic algal biomass. Behavioural processes of H. ulvae, in terms of floating, crawling, burrowing and inactive snails, were described using a Markov model. Finally, an empirical model based on von Bertalanffy equations was proposed to describe kinetics of sediment covering by snail tracks under the influences of snail density, sediment moisture content, chl a concentrations and the four combinations of presence/absence of light and seawater. This model should provide a base for further development of a hydrosedimentary model to simulate the effects of H. ulvae bioturbation activities on the resuspension of the intertidal cohesive sediment-water interface for various in situ conditions.