Carter R. Newell

The effects of velocity and seston concentration on the exhalant siphon area, valve gape and filtration rate of the mussel Mytilus edulis

With the inhalant siphon facing into the flow, and with adequate seston levels, water velocity has a significant negative linear effect on mussel exhalant siphon area, but no significant effect on valve gape. Mussel filtration rates of polystyrene beads, measured by ingestion, were three times less at 30 cm s−1 than at 10 cm s−1, and they mirrored the trends observed with the exhalant siphon area. However, with the inhalant siphon oriented downstream of flow direction at the two higher flow speeds (20 and 30 cm s−1), there was no significant effect of velocity on exhalant siphon area. There was also a significant positive effect of particle concentration on mussel exhalant siphon area. In contrast to the effects of velocity, mussel valve gape responses to increasing particle concentrations mirrored the responses of the exhalant siphon aperture. The thresholds for the initiation of active pumping, opening the valve gape, extending the mantle and opening the exhalant siphon are at minimum seston levels of about 4×103 particles ml−1, or particle volumes of about 1.0 mm3 l−1. Thus, the closure of valves in the relatively non-turbid waters of Maine indicates insufficient food particle concentrations for feeding. Increases in exhalant siphon area caused by particle volume increases from 1 to 5 mm3 l−1, or particle concentrations from 6×103 to 4×104 particles ml−1, indicate that blue mussels respond to increasing ambient food concentrations by increasing their pumping rates. These results demonstrate that exhalant siphon area has potential for the quantitative remote sensing of feeding rate, and that valve gape is a more indirect measure of activity with respect to minimum concentrations for the initiation of feeding. Maintaining an open valve gape with partial or full closure of the exhalant siphon under high flow conditions is similar to the response of blue mussels to changes in salinity, and is interpreted with respect to increased oxygen diffusion for respiration.

Hydrodynamically induced synchronous waving of seagrasses: ‘monami’ and its possible effects on larval mussel settlement

Underwater observations of eelgrass (Zostera marina L.) beds at the mouth of the Jordan River, Maine, USA, indicated that the eelgrass blades gently undulated with low-amplitude movements under low current speeds. When the above-canopy speeds exceeded 10 cm s−1, dramatic large-amplitude waving of many blades in synchrony occurred. The eelgrass waving caused wide variability in horizontal water current speeds measured above the canopy. During three summers (1986, 1989, 1990), the blade tips (distal 30 cm) had an average of about 3 × more recently settled (plantigrade) blue mussels (Mytilus edulis) compared to regions lower (30–90 cm) on the blade. Because waving of seagrass blades results in the blade tips moving through much more of the water column than lower regions of the blade and in enhanced turbulent mixing above the plant canopy, we hypothesize that such movements increase the likelihood of blade encounter with mussel larvae, and explain enhanced mussel abundances on blade tips. We further hypothesize that the enhanced mixing may direct larvae into seagrass beds generally. Large-amplitude, synchronous waving of terrestrial grasses has been termed ‘honami,’ (Japanese: ho = cereal; nami = wave) and has been shown to dramatically alter aerodynamical conditions within and above the grass canopy. We suggest that ‘monami’ (mo = aquatic plant) is important in coastal hydrodynamics and has major implications for larval settlement and recruitment.

The effects of sediment type on growth rate and shell allometry in the soft shelled clam Mya arenaria L

Abstract Hatchery-reared juvenile Mya arenaria L. were grown for 11 wk in replicated gravel, sand, mud, and pearl net treatments under flow-through sea-water conditions in Maine. Analyses of variance showed significant differences between sediment treatments for final shell length, dry meat weight, chondrophore growth increment, and percent shell weight. Growth of juvenile M. arenaria was more rapid in fine sediments than in coarse sediments or nets. Regression slopes of shell length-shell height and shell length-shell depth varied significantly between sediment treatments. Slower-growing clams from nets and gravel were more globose than clams from sand or mud treatments. Clams grown in sand were longer and narrower than those from mud. Differences in growth rates and shell form were attributed primarily to the physical properties of the substrata, and their effects on the scope for growth of M. arenaria .

Development of the mussel aquaculture lease site model MUSMOD©: a field program to calibrate model formulations

Abstract Field observations were used along with mathematical modelling to develop model formulations to represent mussel (Mytilus edulis) growth at several shallow subtidal study sites along the Maine coast, USA. In order to match predicted growth from the model MUSMOD© (D.E. Campbell, C.R. Newell, 1997. MUSMOD©: A mussel production model for use on bottom culture lease sites. J. Exp. Mar. Biol. Ecol., in press.) with observed growth rates in the field, modifications were made in the model with respect to: (a) factors affecting the supply of food to the mussel feeding zone (vertical mixing, settling, resuspension and particle depletion); (b) short-term variability in mussel feeding and respiration over a tidal cycle; (c) the effects of seasonal variations in food quality on assimilation, scope for growth, and observed growth. Surface and bottom water samples taken from 1989 to 1991 revealed daily fluctuations in SPM, POM, chlorophyll a, particulate carbon and nitrogen, and phytoplankton carbon which were similar to annual ranges. Maine waters are typified by a spring diatom bloom, followed by a rise in detritus in early summer. Successful representation of mussel growth was obtained when food concentration was expressed as phytoplankton and detrital carbon, and assimilation was modelled as a function of food type (phytoplankton or detritus), and detritus quality (as percent of maximum annual N/C ratio of the detritus). By expressing particle depletion as a percent reduction from conditions at the edge of the lease site, scope for growth was modelled over an annual period. Observations with a time-lapse benthic video monitor (TLBVM) and flow-through physiological chambers demonstrated tidal variations in filtration rate, respiration rate and shell gape in subtidal mussels, with periods of valve closure correlated with low particle concentrations. These observations support the hypothesis that mussel energy gain is maximized during fluctuating food availability by the control of pumping rate via the shell gape response. This may explain why field observations of filtration rate are sometimes lower than maximum published values, and allows the mussels to maintain a slightly positive scope for growth by reducing respiratory losses when food particles are limiting. The settling flux of phytoplankton and detritus, while poorly characterized in coastal New England waters, provides over 30% of the estimated food supply to subtidal mussel cultures and may explain, in part, the patterns of shell gape observed in this study.

MUSMOD©, a production model for bottom culture of the blue mussel, Mytilus edulis L.

Abstract A mussel production model, MUSMOD© was developed to seed bottom culture lease sites in Maine to their carrying capacity. The process of model development is demonstrated with three models: (a) an initial conceptual model, (b) an aggregated model driven by the tidal exchange of food particles and (c) MUSMOD©, the final model driven by food supplied in the tidal flow of water across a site. The final model predicts mussel production using the concentrations of phytoplankton and detritus in the surface water, detritus quality, tidal current speed, water depth and temperature. Field measurements of several quantities (e.g., clearance, respiration, growth rates for shell and meat, food concentration gradient, and temporal feeding pattern, (Newell et al., 1997, Development of the mussel aquaculture lease site model, MUSMOD©: a field program to calibrate model formulations, J. Exp. Mar. Biol. Ecol., this volume) were obtained to evaluate and calibrate the final model. Model refinement using iterations of modeling and field work demonstrated the importance of food quantity and quality in explaining the observed patterns of mussel growth. Food quantity explained the first-order growth pattern, but it was necessary to account for the quality of the food to explain the second-order details of growth. Vertical mixing supplied the majority of new food particles, however, particles settling over the mussel bed during slack water accounted for 33% of the phytoplankton and 45% of the detritus entering the feeding layer from above. A sensitivity analysis of the effects of seed density on mussel growth using MUSMOD© identified the optimum carrying capacity for three Maine lease sites. Seeding mussels during the optimum time period (May to early July) resulted in the harvest of marketable mussels from 40 mm seed in 8 months for a high food year and in 13 months when the food supply was low. Characterizing the food supply using particulate organic matter, POM, alone was not sufficient to explain mussel growth in the detail necessary to answer many farm management questions.

The Effects of Ambient and Aquaculture Structure Hydrodynamics on the Food Supply and Demand of Mussel Rafts

ABSTRACT A field and modeling study of the food supply and demand of mussel (Mytilus edulis) rafts in Maine established the hydrodynamic and particle consumption characteristics of shellfish aquaculture structures. Mussels on rafts filtered about 8 × 106 L/h and consumed about 40 g chlorophyll a (chl a)/h under favorable conditions. Because of the drag of the culture ropes and predator nets, velocity inside the rafts was reduced by 75%–80% in relation to ambient conditions. Chlorophyll consumption by mussels increased with increased food (chl a) concentration and also with increased water velocity inside the rafts. Clearance rates per raft also increased with food concentration. Model results allow for an estimation of water flux and seston depletion within the rafts through the use of point measurements and correction factors. Water velocity measurements taken mid depth in the middle of the rafts underestimated the mean flow through the raft by 10%. Measurements of current velocity and chl a concentration taken mid depth in the middle of the rafts underestimated the mean particle consumption rates by 13%. Model results and field data indicate that mussel raft hydrodynamics are a function of raft orientation to current direction, mussel raft size, raft aspect ratio, the presence of predator nets, the presence of multiple rafts, rope spacing, and rope diameter. Mussel raft design, placement, and biomass may be adjusted to optimize hydrodynamics and conditions favorable for improved mussel growth rates. Recommended flow speeds through experimental mussel rafts with a cross-sectional area of 121 m2 require a minimum outside flow speed of 14–23 cm/sec.

Scanning Electron Microscopic Aids for Identification of Larval and Post-Larval Bivalves

ABSTRACT The identification of bivalve larvae and early postlarvae in plankton and benthic samples has long been a challenge, hampering both basic and applied research efforts in marine, estuarine, and freshwater environments. The usefulness of published optical micrographs of the early life-history stages of bivalves is limited because of the great morphological similarity of the imaged articulated shells, particularly at the early (straight-hinge) developmental stages. While a number of techniques have been refined in recent years and show promise for use in routine identifications of larval and post-larval bivalves (e.g., single-step nested multiplex polymerase chain reaction; in situ hybridization protocols through color coding with taxon-specific, dye-labeled DNA probes; coupled fluorescence in situ hybridization and cell sorting; and image analysis techniques using species-specific shell birefringence patterns under polarized light), no adequate comprehensive reference source exists that accurately depicts the morphology and morphometry of the shells of larval and post-larval stages of target bivalve species in a consistent format to assist in identification of such stages. To this end, scanning electron micrograph (SEM) sequences are presented of the disarticulated shell valves of laboratory-reared larval and post-larval stages of 56 species of bivalve molluscs from a wide spectrum of marine, estuarine, and freshwater habitats. Emphasis is placed on the usefulness of the morphology and morphometrics of consistently-oriented, disarticulated shell valves and associated hinge structures in discriminating the early life-history stages of these various bivalve species. Although the scanning electron micrograph sequences presented accurately depict the gross morphologies/ morphometrics and hinge structures of the disarticulated shell valves of the larvae and/or postlarvae of the 56 species of bivalves, it is important to emphasize that a scanning electron microscope is not necessary to observe even fine hinge structures associated with the early ontogenetic stages of these species. Such structures are readily visible using a wide range of optical compound microscopes equipped with high-intensity reflected light sources, although the disarticulated shell valves must be viewed in several planes of focus to discern the often subtle details seen clearly in the scanning electron micrographs. These morphological characters provide researchers with invaluable aids for the routine identification of the early life-history stages of these species isolated from plankton and benthic samples.