Bruno Vincent

Spatial and temporal variations of the reproduction cycle and energy allocation of the bivalve Macoma balthica (L.) on a tidal flat

In a tidal flat of the St Lawrence estuary, spawning of Macoma balthica (L.) occurred in the late spring (May–June) and lasted ≈ 1.5 months. At the upper (3 m above the lower-low water) and lower (1.5 m above the lower-low water) intertidal levels, individuals began spawning at the same moment but the spawning period began 2wk earlier in 1986 than in 1985. At the lower level only, the weight of sexual products increased linearly with shell length. The shell length at sexual maturity was the same at both intertidal levels but individuals at the lower level became sexually mature earlier due to their higher growth rate. At the upper level, a high proportion of individuals of ⩾9.5 mm contained no sexual products during the spawning period. Spatio-temporal variation of sexual-product and soft-tissue dry weights showed that individuals at both intertidal levels used an opportunistic reproductive strategy. This strategy allows investment of any surplus energy in gamete production when more favorable environmental conditions are encountered. Spatial variations in total biomass showed that gamete production for the whole population is largely assured by individuals from the lower level which live in a more stable environment. The important contribution of lower-level individuals to total fecundity and weak annual variations of the fecundity in the lower-intertidal zone allow regulation of recruitement for the whole population year after year.

Density, size distribution, energy allocation and seasonal variations in shell and soft tissue growth at two tidal levels of a Macoma balthica (L.) population

Abstract A 2-yr field study of growth in the bivalve Macoma balthica (L.) at two tidal levels in the St Lawrence Estuary, Canada, showed that in spring there is a time-lag of 2 wk in all growth parameters between the upper and lower tidal levels. The growth cycle could be divided in three phases, occurring in spring, summer and autumn, respectively. The zone of highest growth rate of both shell and soft tissue shifted from the upper tidal level during the spring phase to the lower tidal level during the summer phase. This resulted in individuals living at the lower level being longer and heavier than those from the upper level. The autumn phase was a period of nearly stagnant shell and soft tissue growth except for the older individuals from the upper level which showed a weight loss of the soft tissues. The first and the second phases would be linked to the temperature whereas the third one would be related to the food availability. Comparison of shell and soft tissue growth trajectories showed that at the two tidal levels tissue weight gains did not exactly coincide with increased shell growth rate. This study shows that any life cycle population study should include samples from a wide range of environmental conditions, covering the entire area of distribution of any local population.

Spatial variation in growth of the bivalve Macoma balthica (L.) on a tidal flat: effects of environmental factors and intraspecific competition

Abstract A descriptive field study was conducted on a tidal flat of the Lower St. Lawrence Estuary to examine spatial variations in shell growth of the infaunal bivalve Macoma balthica (L.) in relation to tidal level, sediment characteristics, and local density. This study was based on 83 0.3 × 0.3 m 2 sediment samples randomly located on a 1.6 km 2 tidal flat where maximal amplitude of tide was 4.8 m. Mean density of Macoma balthica for the whole sampling area was 1254 m −2 ( S x = 87 m −2 ) and densities were higher and more variable near the mean water level. Variations in the effects of density and environmental factors along the tidal gradient were studied after a stratification of the stations by immersion time. Overall, shell growth rate increased with immersion time, the latter accounting for about 5 to 63% of the spatial variation of mean shell length for the six youngest generations. Fifty-two to 61% of the adjusted annual shell growth was explained by a linear relationship with tidal level. A significant but weaker linear relationship was obtained between growth and density, and between growth and sediment grain size. For the whole sampling area, a linear model with tidal level and density as independent variables explained 37 to 80% of the spatial variability of mean shell length for each generation and 59 to 77% of the spatial variability in adjusted annual growth rates. The effect of density was not significant in the lower part of the tidal flat, however, it became significant when immersion time was ≈ 50%, and had a maximum effect when immersion time was about 40%. About 65 to 85% of the spatial variability of the growth rate along the tidal gradient can be explained by stepwise multiple regressions involving density as the first independent variable. Overall, the results suggested that intraspecific competition played a major role in ≈ 67% of the population of Macoma balthica within the study area.

Effets de la densité sur la croissance du bivalve Macoma balthica (L.) en zone intertidale

Abstract Experimental and descriptive field studies were conducted on an intertidal population of the infaunal bivalve Macoma balthica (L.) in the St Lawrence estuary in order to investigate the effect of density on growth and recruitment. The study of spatial variations in density and shell growth on the tidal flat was based on a stratified random sampling using three tidal level zones as strata. Mean population density over the whole area was 1302 · m −2 ; densities were higher and more variable near mean water level. Shell-growth rate and mean shell length of each generation increased with immersion time. There was a strong density effect on shell growth which increased with tidal level. The population was food limited in the upper two-thirds of the flat. The effect of density on shell growth was confirmed by a completely randomized block design experiment in the upper intertidal level. There was also a density effect on total tissue dry weight and on gonad size, but there was no significant effect of adult density on recruitment.

Dynamic skeletogenesis in fishes: insight of exercise training on developmental plasticity.

Background: Through developmental and evolutionary time, organisms respond variably to their environment not only in terms of size and shape but also in terms of timing. Developmental plasticity can potentially act on various aspects of the timing of developmental events (i.e., appearance, cessation, duration, sequence). In this study, we address the developmental plasticity of median fin endoskeleton by using exercise training on newly‐hatched Arctic charr (Salvelinus alpinus). Results: Developmental progress of cartilage formation (i.e., chondrification) in all fins is less influenced than ossification by an increase of water velocity. The most responsive elements, meaning those elements with greater onset plasticity owing to a water velocity increase, differ in terms of early versus late developmental events. The most responsive elements are those that chondrify and to a greater extent ossify later in the development. Conclusions: Plasticity is documented for the timing of appearance (i.e., onset) and the timing of transition from cartilage to bone (i.e., transitions of skeletal states) rather than the order of events within a sequence. Similarities of plastic response in developmental patterns could be used as a powerful criterion to strengthen the identification of phenotypic modules. Developmental Dynamics 241:1507–1524, 2012.

Orientation of the infaunal bivalve Mya arenaria L. in relation to local current direction on a tidal flat

Abstract The deep-burrowing and sedentary bivalve Mya arenaria L. has a preferential orientation on a tidal flat. Circular distribution of individuals is axial, the mean bearing of an organisms sagittal plane is perpendicular to the principal component of current direction, and variability in orientation is related to the variability of current direction. This mean orientation can be explained through optimization of energy acquisition during an entire tidal cycle. This position prevents recirculation during flood and ebb because exhalant and inhalant apertures are always in a plane normal to the tidal bidirectional current. No effects of individual orientation on shell growth, on somatic or reproductive tissue weights, and on gonadosomatic index could be demonstrated for individuals 6–19 yr old. This could result from a strong selective pressure of current direction on youngest individuals.

Spatial variability of length-specific production in shell, somatic tissue and sexual products of Macoma balthica in the Lower St. Lawrence Estuary

A field study on the spatial variability of production and some demographic parameters was conducted in 1988 in ten populations of Macoma balthica located on the north and south shores along the entire length (230 km) of the Lower St. Lawrence Estuary (LSLE, Canada) at the same intertidal level. Standard-length estimates (10 mm) of shell and somatic tissue production for the period of May to November were highly variable between stations and greater on the north shore than on the south shore. Standard-length estimates of the gamete production for the period of July to November were also highly variable between stations but there was no variation between shores. The inter-population variability of the standard-length estimates of production in shell, somatic tissue and sexual products was as large as the intrapopulation variability between both the upper and lower tidal levels measured in previous studies. There were no significant linear relationships between standard-length estimates of production and biotic (density) or abiotic (temperature, chlorophyll-a in top sediment, mean phi, water salinity) factors, but we observed some significant quadratic relationships between standard-length estimates of production and mean sediment-surface temperature during the growing season. The standard-length estimates of production were lower at the coldest and warmest stations than at the more temperature stations. There was also a significant negative linear relationship between mean sediment-surface temperature during the growing season and the grain size structure of sediment, indicating that the sediment texture, indirectly, largely influenced the inter-station and the inter-shore variability of production in shell, somatic tissue and sexual products of M. balthica in the LSLE.

Shape and surface variations of syphon openings during complete tidal cycles in Mya arenaria in the intertidal zone

Since the degree of syphonal opening can be an index of the pumping activity, the shape and the surface of syphon openings in Mya arenaria were filmed with an underwater camera at two stations in the middle of the intertidal zone. The individuals were perpendicularly oriented to the main current direction or parallel with the inhalant syphon upstream during flood tide, causing refiltration risks during ebb tide. The surface of the inhalant opening (SI) was strongly reduced with increasing current speeds. Its shape (XI) varied with the individuals orientation and had a tendency to become more circular with time. The surface of the exhalant opening (SE) decreased and its shape (XE) flattened with increasing current velocity and with time. However, variations of XI and XE were weak. Current direction had no significant effect on SI, SE, and XE, but did cause a strong decrease of the SE/SI ratio during ebb in individuals exposed to important refiltration risks during ebb tide. Significant negative correlations between stomach content in phaeopigments and SI and SE suggest that a syphonal constriction could contribute to more efficient feeding. For parallel oriented individuals, the decreases of SE, SE/SI, and XE during ebb can then be interpreted as an attempt to deviate or increase the excurrent velocity relative to the incurrent so as to limit refiltration. We suggest that decreases in opening surface and shape may serve, above all, to increase syphon current jet velocity in syphonate bivalves.

The Macrobenthic Fauna of the St. Lawrence Estuary

Most of the the studies on the macrobenthos of the St. Lawrence Estuary have been carried out since 1970, and these have been almost exclusively in the Lower Estuary. The intertidal zone fauna is species poor but densities are often elevated. The most common species occuring in the tidal flats are the bivalves Macoma balthica and Mya arenaria, the gasteropod Hydrobia totteni and the polychaete Nereis virens. The principal species of the rocky substrates of the intertidal zone are the barnacle Semibalanus balanoides at mid-tide levels and Mytilus edulis at lower levels, both species mainly colonizing crevices. The green sea urchin, Strongylocentrotus droebachiensis, is the dominant invertebrate in the rocky infralittoral zone and this species is important in determining the structure of the infralittoral community. The seastar Leptasterias polaris is also abundant in the infralittoral and the presence of this predator explains the rarity of mussels and barnacles at these levels. The circalittoral and bathyal zones display evident spatial variation in the structure of the benthic communities. The abundance and diversity of the communities, as well as their spatial heterogeneity, are greater at the head of the Laurentian Trough. Polychaetes (64.9%) and bivalves (15.8%) dominate the fauna of the Trough. The longitudinal differences in the community structure may be related to variations in the sand content of the sediments and to differences in the supply of nutrients falling from the upper layers of the water column.