Will providing a filamentous substratum in the water column and shell litter on the bottom increase settlement and post-larval survival of the scallop Argopecten purpuratus?

The marked variability in the natural recruitment of Argopecten purpuratus, a common characteristic for many marine invertebrates with a pelagic larval stages, with important consequences for community functioning, is a problem for the fishery on this species. We ran experiments in the subtidal zone in Tongoy Bay, Chile, to test whether providing a filamentous settlement substratum in the water column and shell litter on the bottom would increase the settlement and post-larval survival of scallops. We placed collectors made of Netlon® 50 cm above the sand and mud bottoms, and three and a half months later there were significantly more scallop spat on the bottom under the collectors (38.5 ind m^− 2), than in areas without collectors (0 ind m^− 2), or in controls where collectors were installed but a bag around the collector prevented the juveniles from falling to the bottom (4.8 ind m^− 2). Also, the addition of either entire or broken scallop shells to the bottom resulted in increased settlement of juveniles on the bottom (33.7 ind m^− 2 with entire shells and 48.1 ind m^− 2 with broken shells), compared to plots where no shell debris was added (0 ind m^− 2). The 2 week survival rate of juveniles (< 3 mm shell height) added to plots covered with entire scallop shells (12.4%) and to plots covered with broken shells (15.1%) was greater than in plots where we did not add shells (3.5%). These results suggest that substrate availability explains spatial variability of recruitment for this species, while temporal variability (between years) is mainly the consequence of larval supply. The manipulation of substrates can locally increase settlement, but will not remove the temporal variability. Whereas our experiments provide useful insights into strategies for managing or establishing local scallop populations, experiments over a longer term and at a large scale are needed to further understand the community functioning in order to develop a strategy for managing this fishery resource.     

Density of Monoporeia affinis and biogeochemistry in Baltic Sea sediments

This study focused on effects from Monoporeia affinis reworking and ventilation activities on benthic fluxes and mineralization processes during a simulated bloom event. The importance of M. affinis density for benthic solute (O₂, ΣNO₂⁻ + NO₃⁻, NH₄⁺ and HPO₄²⁻) fluxes and sediment reactivity (mobilization of NH₄⁺ and HPO₄²⁻) following additions of organic material to the sediment surface was experimentally investigated using sediment-water and closed sediment (jar) incubations. Three different densities of M. affinis were used to resemble a low, medium and high density situation (1300, 2500 and 6400 ind. m^− 2, respectively) of a natural amphipod community. The degradation of phytodetritus (Tetraselmis sp., 5 g C m^− 2) added to the sediment surface was followed over a period of 20 days. Benthic solute fluxes of O₂, ΣNO₂⁻ + NO₃⁻ and NH₄⁺ were generally progressively stimulated with increasing number of M. affinis, while no such correlation was found for HPO₄²⁻. Solute fluxes were initially enhanced 1 to 2 days after the addition of phytodetritius, caused by mineralization of the most labile organic material and a food-stimulated irrigation by the amphipods. There was no effect from the activity of M. affinis on total denitrification (Dtot = Dn + Dw) or denitrification utilizing nitrate from coupled nitrification/denitrification (Dn) for any of the densities examined. Denitrification utilizing overlying water nitrate (Dw) was only about 10% of Dtot. Dw was significantly enhanced for the highest M. affinis density investigated. The reactivity of the sediment decreased progressively with increasing density of M. affinis and with time of the experiment. However, enhanced ammonium production at least 6 days after the organic addition indicated excretion of N-containing organic compounds by M. affinis. In conclusion, large spatial and temporal variations in density of M. affinis may be of significant importance for benthic solute fluxes and overall mineralization of organic material in Baltic Sea sediments.     

Properties of shell-like assemblies of weakly charged Keplerate-type polyoxometalates

The Keplerate-type polyoxometalates (POMs) are known to self-assemble as single-layered hollow shells. It has been regarded that only when POMs carry moderate amount of charges, shells are observed in experiments. Using a coarse-grained molecular model for POMs and invoking patchy hydrogen bonding attractions, we show from Simulated Annealing simulations that patchy attraction alone is sufficient to stabilize sheet-like structure, which is the precursor for the formation of shells. The electrostatic interactions may play a role only in the folding of the sheet-like structure into a spherical shell. Simulation results suggest that shells can be formed even at low charge density. We report a theoretical model to predict the radius of the shell (R^∗) formed by weakly charged POMs in the limit of constant charge density. The model predicts that R^∗ scales with (i) dielectric constant of the solution (ε) as R^∗∝ε^1/3 and (ii) with charge density (σ) as R^∗∝σ^-2/3. This behavior is qualitatively different from the highly charged limit that is often encountered in these systems. Using the model, we find that the radius of the shell (20-120 nm) generally observed in experiments corresponds to charge densities in the range of 10⁻³-10⁻² nm⁻², and the number of charges per POM in the range of 0.02-0.2.     

Unfolding thermodynamics of the Delta-domain in the prohead I subunit of phage HK97: determination by factor analysis of Raman spectra

An early step in the morphogenesis of the double-stranded DNA (dsDNA) bacteriophage HK97 is the assembly of a precursor shell (prohead I) from 420 copies of a 384-residue subunit (gp5). Although formation of prohead I requires direct participation of gp5 residues 2-103 (Δ-domain), this domain is eliminated by viral protease prior to subsequent shell maturation and DNA packaging. The prohead I Δ-domain is thought to resemble a phage scaffolding protein, by virtue of its highly α-helical secondary structure and a tertiary fold that projects inward from the interior surface of the shell. Here, we employ factor analysis of temperature-dependent Raman spectra to characterize the thermostability of the Δ-domain secondary structure and to quantify the thermodynamic parameters of Δ-domain unfolding. The results are compared for the Δ-domain within the prohead I architecture (in situ) and for a recombinantly expressed 111-residue peptide (in vitro). We find that the α-helicity (∼ 70%), median melting temperature (T_m = 58 °C), enthalpy (ΔH_m = 50 ± 5 kcal mol^− 1), entropy (ΔS_m = 150 ± 10 cal mol^− 1 K^− 1), and average cooperative melting unit (〈n_c〉 ∼ 3.5) of the in situ Δ-domain are altered in vitro, indicating specific interdomain interactions within prohead I. Thus, the in vitro Δ-domain, despite an enhanced helical secondary structure (∼ 90% α-helix), exhibits diminished thermostability (T_m = 40 °C; ΔH_m = 27 ± 2 kcal mol^− 1; ΔS_m = 86 ± 6 cal mol^− 1 K^− 1) and noncooperative unfolding (〈n_c〉 ∼ 1) vis-à-vis the in situ Δ-domain. Temperature-dependent Raman markers of subunit side chains, particularly those of Phe and Trp residues, also confirm different local interactions for the in situ and in vitro Δ-domains. The present results clarify the key role of the gp5 Δ-domain in prohead I architecture by providing direct evidence of domain structure stabilization and interdomain interactions within the assembled shell.     

Community metabolism and buffering capacity of nitrogen in a ruppia cirrhosa meadow

Fluxes of oxygen, inorganic nitrogen (DIN) and denitrification (isotope pairing) were measured from January 1997 to February 1998 via intact cores incubation in a shallow brackish area within the eutrophic Valli di Comacchio (northern Adriatic coast, Italy). Rates were measured in the light and in the dark in sediments colonized by the rooted macrophyte Ruppia cirrhosa and in adjacent sediments with benthic microalgae. Ruppia biomass (25-414 g DW m^− 2) exhibited a seasonal evolution whilst that of microphytobenthos (12-66 mg chl a m^− 2) was more erratic. Net (NP) and gross (GP) primary productivity was 1.15 and 6.89 mol C m^− 2y^− 1 for bare and 25.4 and 51.7 mol C m^− 2y^− 1 for Ruppia vegetated sediments. Nitrogen pools in Ruppia standing stock varied from 43.6 to 631.4 (annual average 201.2) mmol N m^− 2; the macrophyte N content was correlated with DIN concentration in the water column. Estimated N pool in microphytobenthos was one order of magnitude lower (from 2.4 to 14.5 mmol N m^− 2, annual average 7.2). Theoretical DIN assimilation calculated from NP was 127.8 and 1112.6 mmol N m^− 2y^− 1 whilst that calculated from GP was 765 and 2282 mmol N m^− 2y^− 1 for microphytobenthos and Ruppia respectively. Measured annual fluxes of DIN were 974.6 and − 577 mmol N m^− 2y^− 1 in bare and Ruppia vegetated sediments meaning that the two sites were a source and sink for DIN and that from 25 to 50% of Ruppia annual DIN requirements came from the water column. During the period of this study total denitrification was lower in the macrophyte colonized (92.3 mmol N m^− 2y^− 1) compared to bare sediments (163.3 mmol N m^− 2y^− 1) as a probable consequence of higher competition between denitrifiers and phanerogams. At both sites the ratio between denitrification of water column nitrate (D_W) and denitrification coupled to nitrification (D_N) was >1.6 due to little oxygen penetration in reducing sediments (< 1.2 mm) and scarce nitrification activity. D_W (0-35 µmol N m^− 2h^− 1) was significantly correlated with water column NO₃⁻  (2-16 µM). Theoretical DIN assimilation to denitrification ratio varied from 12.0 to 24.8 for Ruppia vegetated and from 0.8 to 4.7 for unvegetated sediments.At Valle Smarlacca, Ruppia may influence nitrogen cycling by incorporating large DIN pools in biomass which is scattered in surrounding areas and fuels intense bacterial activity. With increasing anthropogenic nutrient input and insignificant organic matter export in the open sea the already severe eutrophic conditions are enhanced and may accelerate the decline of the macrophyte meadow.     

Effects of abiotic conditions on Phragmites australis along geographic gradients in Lake Burullus, Egypt

Stand structure and biomass production of Phragmites australis (Cav.) Trin. ex Steud. were analyzed along north-south and east-west transects in the Burullus coastal lagoon (N Egypt, 410 km²) at monthly intervals over a period of 1 year (February 2003 until January 2004). For this purpose, young and old stands were selected at eight different locations in the lagoon. It was found that the north-south transect mainly represented a fertility gradient (207-286 mg l⁻¹ TN, 30-106 mg l⁻¹ TP), while the east-west transect was associated with significantly decreasing salinity (7-4 ppt). All morphological and biomass variables of P. australis were significantly different between young and old stands. On average, the old (7.3 ± 0.2 kg DW m⁻²) accumulated three times more total above-ground biomass than the young stands (2.5 ± 0.1 kg DW m⁻²). Shoot height, diameter and shoot dry weight significantly increased by 25-50% with increasing fertility along the north-south transect. Shoot density significantly decreased from north to south, while it almost doubled in the north sites from 109 ± 6 to 216 ± 7 shoots m⁻² along the west-east transect. In separate stepwise multiple regressions, variation in water quality explained 34-63% of the variation in morphology and total above-ground biomass in the old stands (salinity and water level were most important for biomass, transparency also for height and density) while it explained 16-42% of variation in young stands (mainly transparency).     

Density dependent effects of an infaunal suspension-feeding bivalve (Austrovenus stutchburyi) on sandflat nutrient fluxes and microphytobenthic productivity

We examined in situ the density dependent effects of an infaunal suspension-feeding bivalve, Austrovenus stutchburyi (hereafter Austrovenus) on sandflat nutrient fluxes and microphytobenthic (MPB) production. Nine experimental plots (0.64 m^− 2) were established at two locations separated by 300 m. Ambient fauna was left intact and Austrovenus added to plots creating a density range from 20 to 2000 ind. m^− 2. Three weeks later, light and dark benthic chambers (area = 0.114 m^− 2) were deployed to measure MPB production and nutrient fluxes. Austrovenus density was positively correlated with organic content and porosity but did not affect other sediment properties (grain size, pigment content) or resident macrofauna. In dark chambers there was a net influx of oxygen (O₂) into the sediments which increased with Austrovenus density (from − 0.45 to − 1.21 mmol m^− 2 h^− 1) whereas in light chambers there was a net efflux from the sediments which decreased with density (from 0.90 to 0.31 mmol m^− 2 h^− 1). Significant (p < 0.01) multiple linear regression models explained respectively 42% and 72% of the variability in the dark and light chamber O₂ fluxes with Austrovenus density as the most important predictor variable. When the effects of significant co-variables (light intensity, grain size) were accounted for, the negative relationship between O₂ flux and Austrovenus density was less steep in light chambers (ANCOVA p < 0.001) suggesting a stimulation of MPB production at higher densities. Estimates of gross MPB primary production indicated a 30% increase in rates of carbon fixation with Austrovenus density (from 36 to 48 mg C m^− 2 h^− 1). Ammonium (NH₄⁺) was released from the sediments in both light and dark chambers and increased with Austrovenus density by a factor of 5.9-6.9×. Multiple linear regression models were significant for light and dark chambers (p < 0.001; r² 86-87%) with Austrovenus again as the most important variable influencing fluxes. ANCOVA results (p < 0.001) indicated that in dark chambers NH₄⁺ efflux increased with Austrovenus density at a rate 1.76× greater than in light chambers. These results indicate that the greater efflux of NH₄⁺ at high densities was being trapped by photosynthesising MPB at the sediment-water interface supporting higher rates of primary production. Our results suggest that a reduction in Austrovenus density will lower nutrient fluxes potentially influencing system productivity by reducing MPB production.     

Growth, secondary production and gonad development of two co-existing amphioxus species (Branchiostoma belcheri and B. malayanum) in subtropical Hong Kong

The present study investigated the growth, secondary production and gonad development of two co-existing amphioxus species Branchiostoma belcheri and B. malayanum in subtropical Hong Kong from June 2005 to June 2006. Based on the modal progression analysis, amphioxus populations were decomposed into separate cohorts. The von Bertalanffy growth models were also estimated according to the size incremental data. From the growth models, the size ranges of one, two and three-year-old B. belcheri were estimated to be 5-28 mm, 28-38 mm and 38-45 mm BL, respectively; while the one and two-year-old B. malayanum were estimated to be 7-30 mm and 30-35 mm BL, respectively. The secondary production was calculated at 1.15 g m^− 2 yr^− 1 DW or 0.63 g m^− 2 yr^− 1 AFDW for B. belcheri with density 424 ind m^− 2, and 0.51 g m^− 2 yr^− 1 DW or 0.40 g m^− 2 yr^− 1 AFDW for B. malayanum with density 121 ind m^− 2. The production to biomass ratio (P/B) was 1.13 for B. belcheri and 0.98 for B. malayanum. Changes in the gonad length index indicated that B. belcheri spawned mainly in June and July, while B. malayanum mainly in April and August. As compared with B. belcheri, B. malayanum was characterized by rapid growth, shorter life span, early maturity and lower population density. Such differences in population dynamics may allow both species to share a similar habitat and co-exist in subtropical waters of Hong Kong.     

The effects of algae species and densities on the population growth of the marine rotifer, Colurella dicentra

Colurella dicentra clones isolated from bay water in the Mississippi Gulf Coast were cultured with artificial seawater. Experiments were conducted to determine the effects of six algae species (Nannochloropsis oculata, Tetraselmis chuii, Chaetoceros gracilis, Rhodomonas salina, Isochrysis galbana, and Prorocentrum micans), six C. gracilis densities, and six N. oculata densities (25,000, 50,000, 100,000, 250,000, 500,000, and 1,000,000 cells ml^− 1) on C. dicentra population growth. Algae type influenced rotifer production (p < 0.0001). C. gracilis treatment (9120 ± 3351SD) produced the highest number of rotifers followed by N. oculata (5760 ±2232SD). P. micans had the lowest number of rotifers, although not significantly different from numbers in T. chuii, R. salina, and I. galbana treatments (p > 0.05).The population growth rate (r) varied with algae species treatment. The highest values were recorded for C. gracilis treatment (0.22 to 0.26 d^− 1), followed by N. oculata (0.21 to 0.24 d^− 1), and the lowest for P. micans (− 0.19 to 0.14 d^− 1). C. gracilis and N. oculata densities had significant effects (p < 0.0001) on C. dicentra population growth. The highest rotifer production was recorded at a C. gracilis density of 100,000 cells ml^− 1, followed by 250,000 cells ml^− 1 and 50,000 cells ml^− 1. Algae densities of 500,000 cells ml^− 1 and above produced the lowest rotifer numbers. Population growth rate (r) varied with C. gracilis densities. The highest values were observed for C. gracilis concentrations of 100,000 cells ml^− 1 (0.17 to 0.19 d^− 1), and the lowest for concentrations of 500,000 cells ml^− 1 and above (− 0.19 to 0.09 d^− 1). The 100,000 cells ml^− 1N. oculata density gave the highest rotifer production followed by 50,000, 250,000, 25,000, and 500,000 cells ml^− 1. Algae densities of 1,000,000 cells ml^− 1 produced the lowest rotifer numbers. Population growth rate (r) varied with N. oculata densities, with the highest values obtained for algae densities of 100,000 cells ml^− 1 (0.35 to 0.40 d^− 1), and the lowest for concentrations of 1,000,000 cells ml^− 1 (0.05 to 0.012 d^− 1). This is the first report of C. dicentra in Mississippi Coastal waters, and perhaps the smallest marine rotifer species (93 by 49 μm) ever cultured successfully.     

Seed banks of Phragmites australis-dominated brackish wetlands: Relationships to seed viability, inundation, and land cover

In tidal wetlands of the eastern United States, buried seeds of the non-native haplotype of Phragmites australis may be a source of propagules for re-establishment after eradication efforts but factors controlling the development and expression of seed banks in non-native Phragmites stands have not been examined. We sampled surface soil at four Chesapeake Bay brackish tidal wetlands dominated by the non-native (European) haplotype M of Phragmites and used the seedling emergence method to quantity species of seedlings emerging under flooded and non-flooded soil conditions. Within each subestuary, one site was dominated by Phragmites that produced viable seeds (high viability) and the other by Phragmites that did not (low viability). We also described standing vegetation in plots, measured soil salinity, analyzed soil characteristics, and described surrounding land cover. Based on number of emerging seedlings, we found that 284 and 698 Phragmites seeds m⁻² occurred at the two high-viability sites, which was significantly higher than seed densities at the low-viability sites (10 seeds m⁻²), and greater than densities reported elsewhere. We also found that emergence of Phragmites seedlings from soil samples was prevented by continuous flooding of 3.5 cm of standing water, suggesting that colonization of deep water areas is due to vegetative clonal expansion from Phragmites in adjacent higher elevations. The density of Phragmites seeds was not related to soil salinity or abundance of other species in the seed bank or vegetation, but instead was positively related to greater wave energy disturbance (much longer fetch and more open water) and lower area of wetlands nearby. The seed bank was more species-rich (15-22 species observed) than standing vegetation (3-15 species) at all sites, meaning that the dominance of Phragmites in vegetation does not prevent the development of a diverse seed bank and implying that a species-rich community may establish rapidly following control efforts. Based on these results and our findings in related studies, we postulate that wave energy disturbance generates repeated opportunities for seedling recruitment by Phragmites, which creates stands of Phragmites with higher genotypic diversity. In turn, genetically diverse stands favor greater cross-pollination and production of viable seed. These findings suggest that, in North America, targeting control efforts on non-native Phragmites patches in areas of higher exposure to wave energy may be more effective in reducing source populations than efforts in more protected locations.     

Shell recruitment in the Mediterranean hermit crab Clibanarius erythropus

Gastropod shells are vital for the majority of hermit crab species, being essential for their survival, growth, protection, and reproduction. Given their importance, shells are acquired and transferred between crabs through several modalities. We conducted observations and experiments at the Asinara Island (Sardinia, Italy) to investigate the efficacy of the different behavioral tactics adopted by the hermit crab Clibanarius erythropus to acquire shells, such as: (1) locomotion and activity at different tidal phases; (2) attendance at shell-supplying sites (simulated predation sites with five different odors: live and dead gastropods, live and dead crabs, predator); and (3) interactions with conspecifics in aggregations on simulated gastropod predation sites. In each tidal phase, locomotion was slow (0.7 cm min^− 1) and, as a consequence, the probability of encountering empty shells and conspecifics was low. Simulated gastropod predation sites quickly attracted a larger number of hermit crabs than the other sites tested. Aggregations seemed to function as shell exchange markets, as previously suggested for other species: the first attendant took the experimental shell and a chain of shell exchanges among conspecifics followed. Our results show that, in C. erythropus, aggregation is the most efficient tactic for the acquisition of new shells, whereas in other species, such as Pagurus longicarpus, it is associated with exploitation ability due to the intense locomotion. The interspecific plasticity in hermit crabs' behavior is confirmed.     

Regulation of feeding mode by the sand dollar Dendraster excentricus in a shallow estuarine habitat

The sand dollar Dendraster excentricus is a facultative suspension feeder, which is relatively rare within the Order Clypeasteroida. Field studies of regulation of its feeding mode have been mostly conducted in exposed coastal habitats, where they are typically observed in an inclined position and oriented parallel to each other during suspension feeding. Physical (current speed and direction, reduction of drag and lift) and biological factors (increased efficiency of capture of particulates, and density) have been associated with regulation of its feeding mode in exposed coastal systems. We simultaneously measured the role of physical and biological factors in regulating the selection mode for feeding under varying tidal conditions in a shallow estuary in Baja California, Mexico. We used photographic records and direct sampling in fixed plots to determine the relationship between feeding behavior and environmental conditions. Current direction and speed, tidal level, density and content of organic matter in the water column and sediments were measured with respect to feeding mode (prone or inclined) and orientation relative to prevailing currents during spring and neap tides. Multiple regression analysis indicated that the percentage of inclined sand dollars was strongly and positively correlated only with tidal level at the densities found in the estuary (mean < 180 individuals m^− 2); there was no relationship with current velocity, density, and organic matter content of the water. The prone position, indicative of deposit feeding, was largely limited to low tidal levels. We used circular statistics to determine whether the orientation of inclined sand dollars was correlated with current direction and speed. Sand dollars were only oriented parallel to the prevailing currents during the strongest currents of spring tides (> 20 cm s^− 1). We did not observe the predominant oral:aboral configuration found in exposed coastal systems, which may be attributed to the relatively low densities of sand dollars in the exposed coastal environment (30-180 individuals m^− 2), compared to protected coastal habitats (up to 1000 individuals m^− 2). Our results suggest that regulation of the feeding mode of sand dollars in shallow and hydrodynamically complex estuarine systems differs from the feeding mode found in exposed coastal environments.     

Light-induced conformational change and transient dissociation reaction of the BLUF photoreceptor Synechocystis PixD (Slr1694)

The light-induced reaction of the BLUF (blue light photoreceptor using flavin adenine dinucleotide) photoreceptor PixD from Synechocystis sp. PCC6803 (Slr1694) was investigated using the time-resolved transient grating method. A conformational change coupled with a volume contraction of 13 mL mol^− 1 was observed with a time constant of 45 ms following photoexcitation. At a weak excitation light intensity, there were no further changes in volume and diffusion coefficient (D). The determined D-value (3.7 × 10^− 11 m² s^− 1) suggests that PixD exists as a decamer in solution, and this oligomeric state was confirmed by size-exclusion chromatography and blue native polyacrylamide gel electrophoresis. Surprisingly, by increasing the excitation laser power, we observed a large increase in D with a time constant of 350 ms following the volume contraction reaction. The D-value of this photoproduct species (7.5 × 10^− 11 m² s^− 1) is close to that of the PixD dimer. Combined with transient grating and size-exclusion chromatography measurements under light-illuminated conditions, the light-induced increase in D was attributed to a transient dissociation reaction of the PixD decamer to a dimer. For the M93A-mutated PixD, no volume or D-change was observed. Furthermore, we showed that the M93A mutant did not form the decamer but only the dimer in the dark state. These results indicate that the formation of the decamer and the conformational change around the Met residue are important factors that control the regulation of the downstream signal transduction by the PixD photoreceptor.     

Land hermit crab (Coenobita clypeatus) densities and patterns of gastropod shell use on small Bahamian islands

Aim To examine patterns of abundance, density, size and shell use in land hermit crabs, Coenobita clypeatus (Herbst), occurring on three groups of small islands, and to determine how these variables change among islands. Location Small islands in the Central Exuma Cays and near Great Exuma, Bahamas. Methods Land hermit crabs were captured in baited pitfall traps and were separately attracted to baits. A mark–recapture technique was used in conjunction with some pitfall traps monitored for three consecutive days. The size of each crab and the type of adopted gastropod shell were recorded, along with physical island variables such as total island area, vegetated area, island perimeter, elevation and distance to the nearest mainland island. Results Relative abundances, densities and sizes of crabs differed significantly among the three island groups. Densities of land hermit crabs were as high as 46 m⁻² of vegetated island area. In simple and multiple linear regressions, the only variable that was a significant predictor of the abundance of hermit crabs was the perimeter to area ratio of the island. Patterns of gastropod shell use varied significantly among the island groups, and the vast majority of adopted shells originated from gastropod species that inhabit the high intertidal and supratidal shorelines of the islands. Main conclusions Although densities of land hermit crabs varied, they were relatively high on many islands, and land hermit crabs may play an important role in these insular food webs. Patterns of shell use may be strongly restricted by island geomorphology: irregular shorelines provide relatively more habitat for the gastropod species that account for the majority of adopted shells and the steep sides of the islands prevent the accumulation of marine gastropod shells. The size of adult hermit crabs appears to be limited by the relatively small gastropod shells available, while the abundance of hermit crabs may be limited by the number of shells available.     

Ecological relations between hermit crabs and their shell-supplying gastropods: Constrained consumers

Hermit crabs are critically dependent upon gastropod shells for their survival and reproductive fitness. While anecdotal reports have suggested that hermit crabs may be capable of removing live gastropods from their shells to access the essential shell resource, no systematic experiments have been conducted to investigate this possibility. This paper reports experiments on both marine (Pagurus bernhardus) and terrestrial (Coenobita compressus) hermit crabs in which crabs were paired in the laboratory with the gastropods whose shells they inhabit in the field. Pairings included both shelled and naked crabs and spanned the full range of the gastropod life cycle. Neither marine nor terrestrial hermit crabs were successful at removing live gastropods from their shells. Furthermore, only a small fraction of the crabs (5.7%) were capable of accessing shells in which the gastropod had been killed in advance, with its body left intact inside the shell. Finally, although hermit crabs readily entered empty shells positioned on the surface, few crabs (14.3%) were able to access empty shells that were buried just centimeters beneath them. These results suggest that hermit crabs are constrained consumers, with the shells they seek only being accessible during a narrow time window, which begins following natural gastropod death and bodily decomposition and which typically ends when the gastropod's remnant shell has been buried by tidal forces. Further experiments are needed on more species of hermit crabs as well as fine-grained measurements of (i) the mechanical force required to pull a gastropod body from its shell and (ii) the maximum corresponding force that can be generated by different hermit crab species' chelipeds.     

Understanding the potential effects of water regime and salinity on recruitment of Melaleuca ericifolia Sm.

Although many emergent wetland plants may readily tolerate rapid changes in flooding and drying under freshwater conditions, their tolerance to dynamic water regimes may be compromised by salinity. Melaleuca-dominated woodlands occur naturally in Australia, south-east Asia and New Caledonia. Coastal wetlands dominated by Swamp paperbark (Melaleuca ericifolia) (Myrtaceae), native to south-east Australia, are commonly degraded as a consequence of altered water regime and salinity. This study simulates the release of M. ericifolia seeds from the aerial canopy under a range of water regime and salinity scenarios to determine conditions limiting sexual recruitment. Plant growth and survival were examined following seed release under two static water regimes (moist and flooded sediment) and two dynamic water regimes (simulated drawdown—“flooded-moist” and simulated re-flooding—“moist-flooded”). All water regimes, excluding the continuously flooded regime, were examined at three salinities: 0.1 dS m⁻¹ (fresh), 8 dS m⁻¹ and 16 dS m⁻¹, over a 50-day period commencing 44 days after the seeds were sown. The flooded treatment was examined at 0.1 dS m⁻¹ only, to confirm that flooding prohibits establishment of M. ericifolia. Seed and seedlings were positively buoyant and establishment was limited to moist soil. Flotation of seedlings in the flooded-moist treatment, however, did not inhibit subsequent establishment upon moist soil, even at the highest salinity of 16 dS m⁻¹. Growth, but not survival, was reduced by salinities of 8 dS m⁻¹ and 16 dS m⁻¹ in the moist treatment. Flotation of seedlings in saline water in the flooded-moist treatment did not reduce growth or survival compared with fresh water. Survival of seedlings in the moist-flooded treatment was lower in the freshwater and 16 dS m⁻¹ treatment compared with the moist treatment, but not at 8 dS m⁻¹. These findings suggest that water regime influences establishment of young M. ericifolia plants more strongly than does salinity, at least up to ∼1/3 seawater and in the short term (<2 months). Seedlings are likely to establish during a drawdown where the soil is exposed at salinities of ≤16 dS m⁻¹. In contrast, premature re-flooding of seedlings, even with fresh water, will compromise survival.     

Primary production,respiration and calcification of the temperate free-living coralline alga Lithothamnion corallioides

Calcification and primary production responses to irradiance in the temperate coralline alga Lithothamnion corallioides were measured in summer 2004 and winter 2005 in the Bay of Brest. Coralline algae were incubated in dark and clear bottles exposed to different irradiances. Net primary production reached 1.5 μmol C g⁻¹ dry wt h⁻¹ in August and was twice as high as in January–February. Dark respiration showed significant seasonal variations, being three-fold higher in summer. Maximum calcification varied from 0.6 μmol g⁻¹ dry wt h⁻¹ in summer 2004 to 0.4 μmol g⁻¹ dry wt h⁻¹ in winter 2005. According to P–E curves and the daily course of irradiance, estimated daily net production and calcification reached 131 μg C g⁻¹ dry wt and 970 μg CaCO₃ g⁻¹ dry wt in summer 2004, and 36 μg C g⁻¹ dry wt and 336 μg CaCO₃ g⁻¹ dry wt in winter 2005. The net primary production of natural L. corallioides populations in shallow waters was estimated at 10–600 g C m⁻² y⁻¹, depending on depth and algal biomass. The mean annual calcification of L. corallioides populations varied from 300 to 3000 g CaCO₃ m⁻². These results are similar to those reported for tropical coralline algae in terms of carbon and carbonate productivity. Therefore, L. corallioides can be considered as a key element of carbon and carbonate cycles in the shallow coastal waters where they live.     

Laboratory experiments on the effects of variable suspended sediment concentrations on the ecophysiology of the porcelain crab Petrolisthes elongatus (Milne Edwards, 1837)

The porcelain crab Petrolisthes elongatus is a particulate suspension feeding species common to coastal areas of New Zealand (NZ). Consistent with the responses of other suspension feeding species, it is likely to be negatively influenced by elevated suspended sediment concentrations. Laboratory experiments were conducted to quantify the effect of temperature (12 °C, 15 °C and 18 °C) and suspended sediment concentration (total particulate matter (TPM): low < 100 mg L^− 1; medium 100-1000 mg L^− 1; high > 1000 mg L^− 1) on the clearance rate (CR in L h^− 1), oxygen uptake rate (VO₂ in mL h⁻¹), net absorption efficiency (AE), and net energy budget (NEB in J h^− 1) of P. elongatus across a range of sizes. Variation in CR and AE was independent of temperature and of body size, but were significantly different (P < 0.05) at low and medium suspended sediment concentrations compared with high suspended sediment concentrations. CR responded in a non-linear manner to changes in TPM, increasing with TPM up to a maximum value at medium-low concentrations (approximately 250 mg L^− 1) and then decreasing thereafter. CR had almost completely shut down at TPM concentrations of > 1000 mg L^− 1 and at particulate organic matter (POM) concentrations of > 250 mg L^− 1. AE was zero at approximate TPM and POM values of 1200 mg L^− 1 and 300 mg L^− 1, respectively. VO₂ was positively correlated with body size and with temperature, but was independent of TPM. NEB values for P. elongatus were low (approx 110 J g^− 1 h^− 1) at low sediment concentrations, were high (approx 320 J g^− 1 h^− 1) at medium sediment concentrations, and were negative (approx − 114 J g^− 1 h^− 1) at high sediment concentrations. These findings indicate that P. elongatus is likely to be food-limited at sediment concentrations of < 100 mg L^− 1, and severely negatively affected at sediment concentrations of > 1000 mg L^− 1, at least for the duration of such events which may persist for 2-3 days in coastal environments where this crab occurs.     

Simultaneous study of particle reworking, irrigation transport and reaction rates in sediment bioturbated by the polychaetes Heteromastus and Marenzelleria

The present study employed simultaneously methods to investigate particle and solute transport and reaction rates in sandy sediments inhabited by two worms (2500 m^− 2) with different feeding modes. Heteromastus filiformis is a head-down deposit-feeder and the main activities exerted by this worm are transport of particles as faecal pellets from subsurface to surface sediments and burrow ventilation. Marenzelleria viridis is a surface deposit-feeder that actively searches for food by burrowing near the sediment surface, producing a network of ventilated galleries in this zone. M. viridis exhibited 1.5 to 2.2 times higher particle mixing rates (D_b = 3.3 to 4 × 10^− 3 cm^− 2 d^− 1) compared to H. filiformis. In M. viridis treatments, continuous advection (eddy diffusion) was the major factor influencing solute transport resulting in apparent diffusion rates (D_a = 2.2 cm^− 2 d^− 1), which were 3 times higher than molecular diffusion within the sediment. In H. filiformis inhabited sediments, the transport of solutes was discontinuous and driven by a surprisingly high nonlocal exchange (α = 1.1-1.3 d^− 1), emphasizing its strong irrigation effects. Accordingly, the enhancement of solute fluxes was more pronounced for H. filiformis compared to M. viridis. Depth integrated TCO₂ production derived from diagenetic modelling, which takes into account three reaction zones, is in good agreement with rates obtained from measured fluxes, indicating the applicability of both approaches to get reliable rates. However, the reaction rates showed that the presence of animals had a modest effect on microbial carbon oxidation. The results proved that transport conditions are deeply related to feeding modes. Exchange of solutes was the most important transport process by H. filiformis, while M. viridis affected both mixing and solute transport.     

Denitrification, nitrate turnover, and aerobic respiration by benthic foraminiferans in the oxygen minimum zone off Chile

Population density, nitrate turnover, and oxygen respiration of benthic foraminiferans were investigated in the oxygen minimum zone (OMZ) off the Chilean coast. Live foraminiferans were found predominantly in the upper 3 mm of the sediment, and the nitrate accumulating species Nonionella cf. stella and Stainforthia sp. dominated with a combined standing stock of 2.0 × 10⁶ Rose Bengal stained specimens m^− 2. The rate of denitrification in cells of N. cf. stella analyzed with nitrous oxide microsensors during acetylene inhibition was 84 ± 33 pmol C individual^− 1 d^− 1. Multiplied with the standing stock of N. cf. stella and Stainforthia sp. this yielded a minimum benthic denitrification rate of 173 µmol N m^− 2 d^− 1 by foraminiferans. Foraminiferal denitrification, which seemed to account for almost all benthic denitrification at the investigated site will be overlooked by most conventional methods measuring benthic denitrification. Compared to the denitrification rates, the potential rates of nitrate accumulation and oxygen respiration by N. cf. stella were an order of magnitude higher (864 pmol N individual^− 1 d^− 1 and 760 ± 87 pmol C individual^− 1 d^− 1, respectively), which seems an adaptation to the infrequent availability of nitrate and oxygen in the sediment surface.