Juvenile Clams Research Articles

Bedload Transport of Newly-Settled Juveniles of the Manila ClamRuditapes philippinarumObservedin situat Banzu Tidal Flat, Tokyo Bay

There have been numerous attempts to create and/or develop harvestable tidal areas where juveniles of the natural Manila clam Ruditapes philippinarum may be abundantly stocked. To prevent erosion and/or promote the colonization of natural juveniles through improving the physical stability of bottom sediment, it is important to understand the mechanisms that regulate dispersal and recolonization of benthic juveniles. The physical transport of bottom sediment is known to have a substantial effect on the spatial distribution patterns of infaunal bivalves in intertidal soft-bottom habitats. During the summers of 2004 and 2005, we conducted field experiments to identify the physical transport mechanisms for newly-settled postlarval Manila clams until growth to a size of 1 mm shell length at the Banzu tidal flat of Tokyo Bay in Japan. We used sediment traps to collect natural and released hatchery-reared juveniles, in parallel to acquiring measurements of seawater flow and observations of the spatial distribution of newly-settled natural juveniles. There was a sharp increase in the number of clams that were collected in the traps placed on the soft-bottom surface when τw (wave shear stress) exceeded 0.3 N m-2. Because the τc (advection shear stress) was far lower than τw (τc/τw =1/60–1/130), the initiation of juvenile transport appeared to depend primarily on wave-generated oscillatory flow. The number of trapped juveniles regressed linearly to the weight of the sediment that was simultaneously collected in the trap (R2 = 0.99, 0.81), which indicated that clams were transported in a similar way to that of sediment grains, despite juveniles and sand particles exhibiting different physical properties (size and specific gravity). Hence bedload transport may have resulted from the biologically induced adhesion of juvenile clams to sediment grains and/or their burrowing behavior. In the release-recovery experiment of marked juveniles, a larger number of clams were recovered from traps that had been placed downstream of the water current from the release point. A denser distribution of the natural Manila clam population settled in mid-July 2004, and subsequently moved several hundreds of meters inshore within a one month period. The concurrent monitoring of bottom flow during a total 4 wk period in the summer of 2004 indicated that τw frequently exceeded the incipient threshold of bedload transport (assumed to be 0.3 N m-2). Consequently, the bedload movement of Manila clam juveniles in the study area was expected to be initiated at a wave shear stress that was greater than the incipient threshold, and in a downstream direction of the advection current. Because juvenile clams in the summer population appeared to be frequently subjected to hydrodynamic stress, which forces juveniles to move and halt incidentally in the early benthic stages, physical transport is likely to contribute to the changing pattern of juvenile distribution at the Banzu tidal flat of Tokyo Bay in Japan.

Role of essential lipids in determining food quality for the invasive freshwater clam Corbicula fluminea

The invasive clam Corbicula fluminea has become a widespread benthic invertebrate in many freshwater ecosystems throughout Europe and North and South America. Invasive bivalves can dramatically alter the structure of native benthic communities, so understanding the factors responsible for successful invasion is important. We investigated C. fluminea nutritional requirements for essential lipids in a standardized growth experiment. Juvenile clams were fed different cyanobacteria (Aphanizomenon flos-aquae, Anabaena variabilis, Synechococcus elongatus) or eukaryotic algae (Scenedesmus obliquus, Cryptomonas sp.). Somatic growth rates were then correlated with elemental (C∶N and C∶P) and biochemical (sterol and fatty acid content) components of the food sources and clam tissue. Somatic growth rates were significantly higher when juveniles were fed eukaryotic algae than when fed cyanobacteria. Linear regression analyses revealed significant positive relationships between somatic growth rates and dietary sterol and polyunsaturated fatty acid content. Somatic growth rates also were highly correlated with the total sterol and α-linolenic acid content of clam tissues. This result suggests that the growth of C. fluminea is partially dependent on the availability of these essential lipids in the diet. Algal nutritional value may influence the successful geographic spread of this highly invasive species because food quality and quantity are changing as a result of global warming.

The physiological basis for inter-individual growth variability in the spat of clams ( Ruditapes philippinarum)

Juvenile (spat) clams ( Ruditapes philippinarum) reared under uniform hatchery conditions were selected for high and low rates of growth in two different ways: choosing large and small individuals from a same age group and choosing young and old individuals of similar size. These growth categories (F and S, for fast and slow growers, respectively) were compared in a series of experiments involving: Growth rate measurements at two different ration levels, physiological determinations of the energy budget and size determinations of gill and digestive glands as the main organs involved in food processing. Manipulation of growth using different food rations served to demonstrate the persistence of original differences between F and S clams, confirming the existence of an endogenous component to growth differentiation. The analysis of physiological traits indicate that high rates of growth are achieved through a combination of faster feeding and higher digestive performance, coupled to increased growth efficiency mainly based on reduced metabolic costs of growth. Significantly larger gill areas recorded for F clams appear to underpin their greater rates of food processing, representing one of several possible morpho-functional links involved in inter-individual growth variability.

Physiological energetics of juvenile clams Ruditapes decussatus in a high CO<sub>2 coastal ocean

Effects of coastal ocean acidification, other than calcification, were tested on juvenile clams Ruditapes decussatus during a controlled CO2 perturbation experiment. The carbonate chem- istry of natural (control) seawater was manipulated by injecting CO2 to attain 2 reduced pH levels (-0.4 and -0.7 pH units) as compared with the control seawater. After 87 d of exposure, we found that the acidification conditions tested in this experiment significantly reduced the clearance, ingestion and respiration rates, and increased the ammonia excretion rate of R. decussatus seeds. Reduced ingestion combined with increased excretion is generally associated with a reduced energy input, which will likely contribute to a slower growth of the clams in a future high CO2 coastal ocean. These results emphasize the need for management policies to mitigate the adverse effects of global change on aquaculture, which is an economically relevant activity in most coastal areas worldwide.

Growth of cultured giant clams (Tridacnaspp.) in low pH, high-nutrient seawater: species-specific effects of substrate and supplemental feeding under acidification

Four species of giant clams,Tridacna maxima, T. squamosa, T. derasaandT. crocea, were cultured in outdoor raceways for 364 days at the Waikīkī Aquarium and the Oceanic Institute on the island of O‘ahu, Hawai‘i, USA. Growth of each species was compared among individuals grown with and without supplemental phytoplankton feeding, and directly on the substrate or mounted on concrete plugs in low pH, high nutrient seawater. Among clams cultured with and without supplemental phytoplankton (Chaetocerosspp.), feeding resulted in significantly lower mortality in all species butT. deresa, whereas growth was significantly higher among fed clams for all species exceptT. squamosa. Tridacna derasashowed roughly a three-fold increase in growth when fed (88.5 g ± 4.4 SD) than when unfed (26.0 g ± 2.1 SD), whereasT. maximagrowth was substantially lower, but nearly 10-fold greater in response to feeding (9.0 g ± 1.9 SD). The overall mortality rate of juvenile clams was significantly lower in the fed (44.4 ± 10.0%) than the unfed (71.8 ± 9.6%) trials, with the greatest effect observed in mortality ofT. maxima(fed 15% versus unfed 80%) andT. squamosa(fed 65% versus unfed 95%). None of theT. squamosaremained on concrete plugs for the duration of the experiment. Among the remaining three species, there was no difference in either wet weight or shell length forT. maximaand for wet weight only inT. derasaon (186.5 g ± 16.1 SD) and off (147.0 g ± 6.0 SD) the concrete plugs. In contrast,T. croceahad significantly greater shell growth off the plugs (14.3 mm ± 1.0 SD versus 8.5 mm ± 1.7 SD) but significantly greater gain in wet weight on the concrete plugs (26.3 g ± 1.5 SD versus 58.5 g ± 2.5 SD). The seawater wells used for this study are well characterized with elevated levels of inorganic nutrients and higher pCO2relative to tropical ocean waters, roughly approximating predictions for future oceanic conditions under IPCC IS92a emission scenarios. In comparison to previous studies in natural seawater,T. derasahad a significantly higher shell growth rate in the high-nutrient, low-pH well water. In contrast,T. maximaandT. squamosahad significantly lower growth rates in low pH, whereas growth ofT. croceawas not significantly different between low pH and ambient seawater. These experiments demonstrate species-specific differences with each treatment, which cautions against making broad generalizations regarding the effects of substrate type, feeding effects, nutrient enrichment, and ocean acidification on tridacnid culture and survival.

Effects of toxic Alexandrium tamarense on behavior, hemocyte responses and development of brown ring disease in Manila clams

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 430:35-48 (2011) - DOI: https://doi.org/10.3354/meps09111 Effects of toxic Alexandrium tamarense on behavior, hemocyte responses and development of brown ring disease in Manila clams V. M. Bricelj1,*, Susan E. Ford2, Christophe Lambert3, Annaick Barbou3, Christine Paillard3 1Institute of Marine and Coastal Sciences, Rutgers University, New Brunswick, New Jersey 08901, USA 2Haskin Shellfish Research Laboratory, Rutgers University, Port Norris, New Jersey 08349, USA 3Institut Universitaire Européen de la Mer, LEMAR, Technopôle Brest Iroise, 29280 Plouzané, France *Email: mbricelj@marine.rutgers.edu ABSTRACT: In this study of short-term exposure of Ruditapes philippinarum from Brittany, France, to an Alexandrium tamarense isolate that produces high concentrations of paralytic shellfish toxins (PSTs), the effects of the isolate on clam fitness, toxin uptake, and the response of hemocytes (responsible for internal defense in bivalves) were examined. Adults exhibited individual variability in ­resistance to the effects of PSTs, measured by their burrowing capacity. If genetically based, this may (1) indicate that resistance is under natural selection and (2) provide a new model to advance our understanding of the molecular basis for PST resistance in bivalves, so far determined only in Mya arenaria. Juvenile clams were more vulnerable to PSTs than adults, experiencing 100% burrowing incapacitation within 1 d of exposure to A. tamarense. They also experienced growth suppression and high mortalities following 4 d of toxification. Juveniles challenged with Vibrio tapetis, the cause of brown ring disease (BRD), and exposed to A. tamarense, developed significantly fewer BRD symptoms relative to controls fed non-toxic algae, but suffered higher mortality. Adult clams exposed to A. tamarense showed a significant increase in hemocyte concentrations and a small, but significant, decrease in phagocytic activity, and no effect on hemocyte viability or other functional parameters. We speculate that the inhibitory effects on BRD progression may be attributable to toxicity of PSTs to V. tapetis, inability of juveniles to activate the shell conchiolin-deposition response, and/or an overall increase in phagocytic cells induced by A. tamarense. Harmful algae and pathogens may thus ­interact and modulate the effects of disease in bivalve populations. KEY WORDS: Ruditapes philippinarum · Paralytic shellfish toxins · Vibrio · Burrowing · Hemocytes Full text in pdf format PreviousNextCite this article as: Bricelj VM, Ford SE, Lambert C, Barbou A, Paillard C (2011) Effects of toxic Alexandrium tamarense on behavior, hemocyte responses and development of brown ring disease in Manila clams. Mar Ecol Prog Ser 430:35-48. https://doi.org/10.3354/meps09111 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 430. Online publication date: May 26, 2011 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2011 Inter-Research.

Environmental stress and parasitism as drivers of population dynamics of Mesodesma donacium at its northern biogeographic range

Abstract Riascos, J. M., Heilmayer, O., Oliva, M. E., and Laudien, J. 2011. Environmental stress and parasitism as drivers of population dynamics of Mesodesma donacium at its northern biogeographic range. – ICES Journal of Marine Science, 68: 823–833. Mesodesma donacium is a commercially important bivalve in Chile and Peru. During strong El Niño events, populations at the northern end of its geographic distribution are wiped out, so to understand its threshold responses to biotic and abiotic factors, the population dynamics of one of the northernmost population remnants was analysed between 2005 and 2007. Strong interannual differences were found in abundance, body mass, growth rate, somatic production, and the prevalence of the parasite Polydora bioccipitalis. A Spearman rank correlation analysis showed that changes in beach slope, seemingly linked to repeated storm surges, negatively affected the clam's abundance and seemingly also affected growth, mortality, body mass somatic production, and parasite prevalence. Infestation by P. bioccipitalis was restricted to adult clams. Juvenile clams suffered high mortality because they inhabit the intertidal zone, where wave action is strong. Larger clams also showed high mortality, which seemed best explained by a synergistic effect of parasite load and environmental stress. This parasite-climate-driven mortality of larger clams had a strong impact on somatic production and implied a dramatic loss of fecundity (82%), which may significantly affect the ability of the species to recover its former abundance and distribution.

Calcification, growth and mortality of juvenile clams Ruditapes decussatus under increased pCO2 and reduced pH: Variable responses to ocean acidification at local scales?

We investigated the effects of ocean acidification on juvenile clams Ruditapes decussatus (average shell length 10.24mm) in a controlled CO2 perturbation experiment. The carbonate chemistry of seawater was manipulated by diffusing pure CO2, to attain two reduced pH levels (by −0.4 and −0.7 pH units), which were compared to unmanipulated seawater. After 75days we found no differences among pH treatments in terms of net calcification, size or weight of the clams. The naturally elevated total alkalinity of local seawater probably contributed to buffer the effects of increased pCO2 and reduced pH. Marine organisms may, therefore, show diverse responses to ocean acidification at local scales, particularly in coastal, estuarine and transitional waters, where the physical–chemical characteristics of seawater are most variable. Mortality was significantly reduced in the acidified treatments. This trend was probably related to the occurrence of spontaneous spawning events in the control and intermediate acidification treatments. Spawning, which was unexpected due to the small size of the clams, was not observed for the pH −0.7 treatment, suggesting that the increased survival under acidified conditions may have been associated with a delay in the reproductive cycle of the clams. Future research about the impacts of ocean acidification on marine biodiversity should be extended to other types of biological and ecological processes, apart from biological calcification.

Overwinter mortality and spring growth in selected and non-selected juvenile Mercenaria mercenaria

Juvenile cultured northern quahogs or hard clams Mercenaria mercenaria can experi- ence significant mortalities during their first winter, and cultured M. mercenaria var. notata clams, selected for fast growth, may be more susceptible. In the present study, selected and non-selected juvenile clams were overwintered off the north shore of Long Island, NY from November 2007 to June 2008 in order to understand the physiological, genetic, and environmental mechanisms con- tributing to overwinter mortality. Selected clams had a cumulative mortality of approximately 47% while the non-selected clams experienced 25% mortality. A decrease in carbohydrate reserves coin- cided with the mortality in both clam strains, but time-specific loss of carbohydrates was greater in the selected clams. Metabolic activity, as estimated by the activity of the electron transport system (ETS), was high during the winter-to-spring transition and was later reduced, presumably when clams became acclimated to the warmer temperature in May. Growth occurred after the observed reduction in ETS activity and was greater in the selected clams. This reduction in metabolic activity may allow for the balance of available energy to switch from maintenance to growth. These results support previous work suggesting a genetic component to overwinter mortality and demonstrate the significant effects of seasonal transitions on hard clam growth and energetics.

Experimental evaluation of the pathogenicity of Perkinsusolseni in juvenile Manila clams Ruditapes philippinarum

We evaluated the pathogenicity of Perkinsus olseni towards the Manila clam, Ruditapes philippinarum, by an experimental challenge. For production of prezoosporangia of P. olseni, we injected uninfected Manila clams with cells of a pure strain of P. olseni and reared them for 7 d. Prezoosporangia were isolated from the soft tissue of the injected clams after culturing in Ray’s fluid thioglycollate medium. Hatchery-reared, uninfected juvenile clams (3–10 mm shell length) were challenged by immersion in one of two concentrations of a prezoosporangial suspension of P. olseni for 6 d. The challenged clams had significantly higher mortality at both the concentrations than the unchallenged clams. The mortality due to infection dose-dependently began approximately 4 weeks and 7 weeks after challenge in the higher and lower concentrations, respectively. This is the first experimental evidence that P. olseni causes direct mortality in Manila clams. The lethal level of infection was estimated at approximately 10 7 pathogen cells/g soft tissue weight.

Regional Differences in Initial Settlement and Juvenile Recruitment ofMya arenariaL. (Soft-Shell Clam) in Maine

Clam landings have dramatically decreased in eastern Maine since 1982. Earlier studies demonstrated that densities of 0-y-old individuals of Mya arenaria L. were lower in Washington County on the eastern Maine coast than in Cumberland County on the southern Maine coast. Reduced juvenile recruitment could result from reduced larval abundances, high postsettlement mortality, or a combination of these factors. Attempts to assist recovery of populations experiencing reduced recruitment would vary depending on which combination of factors inhibited recruitment. In this study we measured initial settlement, short-term (<4 wk) postsettlement densities, and longer term (several months) postsettlement densities of 0-y-old juvenile M. arenaria in both eastern and southern Maine. Although there were seasonal and spatial differences, densities of juvenile clams were significantly greater in southern Maine over eastern Maine at each of these life history stages (initial settlement, and early and later postsettlement), generally by one or more orders of magnitude. These results suggest the low number of M. arenaria recruits in eastern Maine is a consequence primarily of reduced densities of larvae in near-shore waters rather than processes affecting postsettlement survival differentially between southern and eastern Maine. Because of the very low abundance of competent clam larvae in eastern Maine, restoration of clam populations there will require intensive efforts, such as seeding mudflats with hatchery-reared juvenile clams and managing subsequent clam survival.

The importance of turbulent kinetic energy on transport of juvenile clams ( Mya arenaria)

Soft-shell clam, Mya arenaria, culture on the east coast of Canada is characterized by high loss following seeding. To evaluate the importance of passive transport due to currents, an experimental flume study was designed. The purpose was to measure the effects of hydrodynamic conditions, substrate, and clam size on dispersal in controlled laboratory condition and to interpret these results in relation to field measurements. Unidirectional currents with gradual increasing velocities (0 to 60 cm s − 1 ) were applied to three substrates (muddy sand, medium sand, and coarse sand) in which clams from one of three size classes (10, 15, and 20 mm) had burrowed. We also examined the resulting effects of turbulent kinetic energy on the erosion of medium-grain sand and clams from the three size classes. Turbulent energy was created with a homemade device acting on the unidirectional currents. Nearly 95% of buried clams (all substrates and size classes together) were unaffected by unidirectional currents of up to 60 cm s − 1 , but only 10% withstood turbulent kinetic energy of 10.1 J m − 3 , a level that is lower than that measured in the field during an autumnal storm. The transport of clams was found to be directly related to substrate erosion-levels.

Quantification of water squirting by juvenile fluted giant clams (Tridacna squamosa L.)

The giant clam species Tridacna crocea, T. deresa, T. gigas, T. maxima and Hippopus hippopus are known to exhibit squirting behaviour, ejecting a stream of water either from their exhalant or inhalant siphon. Here, for the first time, squirting in juvenile fluted giant clams, T. squamosa, was measured. By analysing stills from video recordings it was possible to determine the horizontal and vertical distances travelled by each squirt above the water line, the cross-sectional area of the water jet, and the angle of squirt perpendicular to the horizontal axis of the giant clam. The weight of each “aerial squirt” was measured by collecting the displaced water. Using these parameters, the initial velocity, force and pressure exerted by each squirt on an object was calculated. Strong positive correlations were observed between shell length and weight of seawater and force exerted by aerial squirts. We also modelled the pressures that would be experienced by underwater targets. The simulated “underwater squirts” indicate the pressure produced rapidly decreases with distance from the clam.

Growth promotion of the juvenile clam, Ruditapes philippinarum, on sugars supplemented to the rearing water

Soft-bodied marine invertebrates are known to have the ability to uptake dissolved organic matter, but actual benefits to growth of these organisms have not been demonstrated. In the present study, the effects of sugars supplemented to the rearing water were tested on juveniles of the clam, Ruditapes philippinarum. Culture water supplemented with glucose at 10 and 100 mg l − 1 significantly promoted soft tissue growth of the clam. In contrast, maltopentaose and pullulan (a polysaccharide composed of maltotriose with average M.W. 10 5) did not show any promoting effect, suggesting that only low molecular weight sugars can be incorporated through the epidermal pathway and contribute to clam growth. Soaking of adult clams in water supplemented with glucose at 100 mg l − 1 for 24 h resulted in 1.5 times higher quantity of total organic acids in the soft tissues compared with clams soaked in water without glucose, suggesting that the absorbed glucose is easily transformed into organic acids through the glycolysis pathway and makes an energetic contribution to clam growth. Here, we suggest a novel clam rearing method with “sugar supplements”, which can be used to promote clam growth through epidermal uptake.

Impacts of stage-specific acute pesticide exposure on predicted population structure of the soft-shell clam, Mya arenaria

A combined laboratory and modeling approach was used to assess the impact of selected pesticides on early life stages of the soft-shell clam, Mya arenaria. Clams were exposed for 24h as veligers or pediveligers to the broad-spectrum herbicide hexazinone [3-cyclohexyl-6-(dimethylamino)-1-methyl-1,3,5-triazine-2,4(1h,3h)-dione; Velpar®], the phenoxyacetic acid herbicide, 2,4-D (2,4-dichlorophenoxyacetic acid; Agway® Super BK 32), or phosmet (Imidan®). In addition, juvenile clams were exposed for 24h to 2,4-D and their growth monitored for 21 months. Laboratory experiments indicated veligers were more sensitive to acute pesticide exposure than pediveligers, with 2,4-D exposed veligers exhibiting the lowest survival among all treatments. Relative to controls, juvenile clams exposed to 0.5ppm 2,4-D had enhanced survival following the initial 3 months of grow out. Juveniles exposed to 0.5, 5 and 10ppm 2,4-D showed an initial growth delay relative to control clams, but at 21 months post-exposure these clams were significantly larger than control clams. Data from the larval and juvenile exposures were used to generate a stage-specific matrix model to predict the effect of pesticide exposure on clam populations. Impacts on simulated clam populations varied with the pesticide and stage exposed. For example, 2,4-D exposure of veligers and pediveligers significantly reduced predicted recruitment as well as population growth rate compared to controls, but juvenile exposure to 2,4-D did not significantly reduce population growth rate. With the exception of veligers exposed to 10ppm, hexazinone exposure at the both veliger and pediveliger stages significantly reduced predicted recruitment success compared to 0ppm controls. Hexazinone exposure also reduced modeled population growth rates, but these reductions were only slight in the pediveliger exposure simulations. Veliger and pediveliger exposure to phosmet reduced modeled population growth rate in a dose-dependent fashion. Changes in modeled population stable stage distributions were also observed when veligers were exposed to any pesticide. These results suggest that both the stage of exposure and the specific toxicant are important in predicting effects of pesticide exposure on soft-shell clam populations, with earlier life stages showing greater sensitivity to the pesticides tested.

Effects of Water Flow and Density on Early Survivorship and Growth of the Northern QuahogMercenaria mercenariaL.

ABSTRACT Clam aquaculturists have suspected low water flow over clam grow-out areas as a principal explanation for decreased growth and yield of bivalves; however, empirical data from field studies to support these observations are rare and in some cases contradictory. I conducted two experiments in Back Sound, NC, to examine the effects of water flow and juvenile density on hard clam survivorship and growth. The first experiment assessed differences in early growth and survivorship of juvenile seed clams under three manipulated water flows (enhanced [0–32 cm sec-1], reduced [0–12 cm sec-1], and ambient tidal flows [0–22 cm sec-1]) and three initial stocking density (250, 500, or 1000 m-2) of juvenile clams (shell length [SL] = 9 mm). The second set experiment examined the effects of two different water flow regimes (ambient [0–22 cm sec-1] and low [ 0–11 cm sec-1]), and three stocking densities (250, 500, or 1000 m-2) on growth of seed clams (SL = 12 mm) to market size. Unlike the first experiment, in wh...

Physical limitations of dissolved methane fluxes: The role of bottom-boundary layer processes

In situ methane emission measurements from sediments are combined with water column backscatter anomalies recorded with an Acoustic Doppler Current Profiler (ADCP) integrated on a benthic observatory. During cruise SO191 to the Hikurangi Margin (New Zealand), the Fluid Flux Observatory (FLUFO) was deployed at a cold seep site at Omakere Ridge. The sediments incubated in the two benthic chambers of FLUFO contained seep-associated fauna, including small and larger tubeworms, juvenile bivalves of the genus Acharax and some juvenile clams. The first 26 h of in situ incubation revealed low to moderate methane fluxes of 0.01 to 0.4 mmol m − 2 d − 1 into the overlying water of the backup and flux chamber, respectively. In the following sampling sequence, however, the methane concentration in the flux chamber reached 3-fold higher concentrations whereas the methane concentration in the backup chamber remained low and unchanged. Simultaneous to the sudden methane increase, a significant backscatter anomaly was recorded and persisted for 30 min and covered the entire depth range (100 m) of the upward looking ADCP. Data analyses revealed that a single-phase plume (no bubbles) outburst likely occurred during this time. While bubbles appeared to be present during some periods, plume simulations revealed that the volume of gas required (rate of 8 ton/day) does not support a bubble plume. A second data set was obtained during lander deployments at Rock Garden where visual observations by ROV confirmed the transient pattern of free gas injection into the water column. Acoustic flares and methane concentration increase in the bottom water hint towards a pressure (tidal) induced discharge mechanism. The presented data demonstrate the temporal and spatial variability of seabed methane emission, and very short methane signal lifetime in the water column (hours to a few days) due to turbulent diffusion. Both have to be considered when methane budgets are extrapolated from single methane emission rates.

Settlement and Recruitment Patterns of the Soft-Shell Clam, Mya arenaria, on the Northern Shore of the Bay of Fundy, Canada

To examine the roles of settlement and early postsettlement processes in the recruitment of the soft-shell clam, Mya arenaria, abundance of recent settlers and juveniles was monitored over two field seasons at four locations on the northern shore of the Bay of Fundy in New Brunswick, Canada. Results showed great spatial and temporal variability in patterns of settlement. M. arenaria appeared to become highly vulnerable to postsettlement processes at a shell length of approximately 2 mm. Postsettlement processes drastically altered patterns of settlement less than 1 year after they were established. Results suggest that local factors at specific sites within the Bay of Fundy, such as hydrodynamics, larval behavior, and early postsettlement events, likely control the abundance of juvenile clams. Additionally, postsettlement events are extremely important in shaping M. arenaria populations in this area. Very few mature adult clams greater than 50 mm in shell length were found at any sampling sites, and no relationship was found between abundance of setters and density of juveniles and adults.

Estimating clam recruitment potential in a nursery area in the Sacca di Goro lagoon (Italy)

The assessment of clam recruitment potential is a necessary step to improve the economic and environmental sustainability of the exploitation activities in a regulated fishery. We discuss the application of a method for estimating the clam recruitment potential in a natural nursery area (the so‐called Bassona) of a northern Adriatic lagoon (Sacca di Goro, Italy). Larvae in the plankton were collected at weekly intervals, while post‐larvae that settled were analysed monthly. Planktonic larval and benthic post‐larval cohorts were determined, and growth and mortality rates were calculated. The theoretical formulation derived allows the estimate of the future abundance of juvenile clams during their first year of benthic life.

Can artificial substrates enriched with crustose coralline algae enhance larval settlement and recruitment in the fluted giant clam (Tridacna squamosa)?

Habitat recognition and selection can greatly increase the early-life survival of sessile reef organisms. This study describes the settlement and recruitment responses of the fluted giant clam, Tridacna squamosa, to concrete tablets and tiles containing different concentrations of crustose coralline algae covered coral rubble (CCACR). Crustose coralline algae is known to induce settlement in a variety benthic animals, but it has not been used previously as an aggregate in concrete—potentially a way of encouraging colonization of man-made structures erected on or near coral reefs. After being given the choice of small tablets made with 0%, 30% or 60% CCACR for 4 days, 11 days old larvae preferred the substrate containing the most CCACR. Recruitment responses of juvenile clams to larger tiles made with the same three CCACR concentrations were also tested. These tiles were further divided into rough and smooth surface textures. After 6 weeks, more juvenile clams had recruited to the rough surfaced tiles than the smooth ones, but no significant differences among the CCACR treatments were found. Thus, even though concrete made with CCACR is initially attractive to larvae, it has no effect on recruitment of juvenile T. squamosa.