Edwin Bourget

Geographical and seasonal variations in the relationship between trace metal content and body weight in Mytilus edulis

The relationships between trace metal content (Cd, Cu, Fe, Mn, Ni and Zn) and body weight of Mytilus edulis L. show that smaller mussels are richer in trace metal than larger ones. According to the metal, the season and the locality examined, 3 types of relationships were obtained: (1) straight lines over the whole size range of mussels; (2) curves with steeper slopes in larger mussels and (3) curves with lowering slopes in larger mussels. For most metals the slopes changed from one locality to another. A detailed analysis of the relationship between Cd content and body weight showed a gradual decrease in metal content until maturity was reached, thereafter the relationship varied significantly with the season, and 2 different types of curves were obtained. Thus, our results contradict the idea that slopes are uniform for a given metal and species. Multivariate analyses showed that weight accounted for most of the variance in metal content while age and gonad maturation explained little of the variance observed. We propose the metabolic changes associated with sexual maturity and season continually change the relative importance of the circulating and storage compartments of metals and may be responsible for the variations of slope observed.

Importance of physical and biological settlement cues used at different spatial scales by the larvae of Semibalanus balanoides

The settling cues used by larvae of Semibalanus balanoides (L.) were examined at large (1 m), medium (1 mm), and small (<300 μm) spatial scales, corresponding roughly to choices made during broad and close exploration and final inspection within the settling zone. The experiments were carried out at two locations in Canada in May and June 1984. Samples of substratum surfaces from above (n=87 sites unselected by cyprids) and below (n=84 sites selected by cyprids) the upper limit of the barnacle zone were characterized according to 13 quantitative and semi-quantitative variables (diatom cover, macroalgae, detritus) using scanning electron microscopy (SEM). The data were analysed using a stepwise logistic regression. At the large scale, the presence of an alga (Urospora wormskjoldii Mertens) in the upper uncolonized zone and a diatom (Achnantes parvula Kützing) within the settling zone are potential settlement cues. At the medium scale in the settling zone, larvae select clean sites, devoid of detritus and diatoms or on which detrital matter, when present, is of finer texture than on the unchosen sites within the settling zone. At the smallest scale (<300 μm, equal to the average maximum width of the larvae), we tested the hypothesis that the larva seeks optimal microheterogeneity (presumably providing good adhesion). Microheterogeneity of the surface was measured immediately under the larva, around the settling organs, the antennules. Our results show that such surface microheterogeneity is significantly greater and the vertical center of gravity of the surface significantly lower on the sites selected by the cyprids than on the unselected contiguous sites. Fractal measurements carried out on selected and contiguous unselected surfaces indicate that larvae discriminate microheterogeneity levels below a step length of 35 μm, a value approximately the size of the diameter of the antennular discs (40 μm), which are used to explore the substratum and attach.

Scales of substratum heterogeneity, structural complexity, and the early establishment of a marine epibenthic community

Abstract This study examines experimentally, in situ, the influence of four scales of heterogeneity (0, 1, 10, 100 mm) and the hierarchical combination of those scales of heterogeneity (0 + 1 + 10, 0 + 1 + 100, 0 + 10 + 100, 0+1 + 10+100 mm) on the early establishment of a sessile marine littoral epibenthic community. The study was carried out in St. Andrews, New Brunswick (Canada) from July to October 1989. Flat panels without and with crevices of various depths (1, 10, 100 mm) alternating with adjoining flat areas were moored according to a random design in the surface 2 m in the sublittoral zone. Species diversity and total percent cover increased from August to October. However, the ANOVA showed no significant effect of substratum heterogeneity nor complexity (the hierarchical combination of various scales of heterogeneity) on overall diversity (H′) in August and October nor on percent cover on the whole panels in August. However, one type of complex panel out of eight types tested did show a significantly lower percent cover than the others in October. Small scale patterns of distributions varied significantly among the different types of surfaces within each panel, however. In general, percent cover of organisms were greater in 1 mm crevices in August and in 10 mm crevices 2 months later, than on other types of surfaces. Local hydrodynamics and active larval choice may be involved. Our study shows that both substratum heterogeneity and complexity did not influence much the overall northern sessile community characteristics (diversity and abundance), but structured strongly the small scale distribution of its species during the early phases of development.

Influence of substratum heterogeneity and settled barnacle density on the settlement of cypris larvae

On the Atlantic coast of Canada, Semibalanus balanoides (L.) is widely distributed in the mid-intertidal zone, whereas in the Gulf of St. Lawrence, this species is mostly limited to crevices. We tested the hypothesis of regional differences in microhabitat selection by barnacle larvae at settlement in 1984 and 1985 at St. Andrews, New Brunswick, Canada. Since larvae settle in microhabitats already colonized by adults, the relative influence of settled barnacle density and of different scales of substratum heterogeneity on settlement were evaluated experimentally at Capucins, Québec, (Gulf of St. Lawrence) and at St. Andrews, New Brunswick (Atlantic coast). On a large scale (>10 cm deep crevices) of heterogeneity, results show that, in the Gulf, cypris larvae settled nearly exclusively (93%) in natural crevices rather than on adjacent horizontal surfaces. On the Atlantic coast, settlement was more important outside than inside of crevices, when the substrata were either natural or artificial. This result is unique and contrasts sharply with all known reports on barnacle settlement in relation to surface contour. The influence of barnacle density on settlement was greater than that of large scale heterogeneity. On a small scale (<1.5 cm deep cracks), the presence of conspecifics had a stronger effect on settlement than heterogeneity in both regions. Field observations showed a relationship between larval settlement density and percentage of adult cover. Settlement increased up to 22 or 30% (Gulf and Atlantic coast) of adult cover and decreased afterwards. The results confirm the hypothesis of larval selection for cryptic habitats in the Gulf and the opposite behaviour (preferences for horizontal surfaces) on the Atlantic coast. This microhabitat selection is apparent at large scales of heterogeneity, whereas at small scales, the presence of conspecifics is the predominant factor.

Community development following removal of urchins, Strongylocentrotus droebachiensis , from the rocky subtidal zone of the St. Lawrence Estuary, Eastern Canada

SummaryThe role of sea urchins, Strongylocentrotus droebachiensis, in structuring the rocky subtidal community was examined at Anse aux Basques on the north shore of the St. Lawrence Estuary, Québec. In an experimental area, measuring 20x20 m and extending from 0 to 10 m in depth, we greatly reduced the intensity of urchin grazing by eliminating all urchins larger than 10 mm in test diameter. This area was observed for two years and compared to an adjacent control area. In the upper portion of the experimental area during the first month after urchin removal, mid-July to mid-August 1978, a dense diatom cover developed, and during the second month the diatoms were overgrown by Ulvaria obscura. After four months (November) an Alaria esculenta overstory was present from near low water level to 3 m deep. Community development was much slower at greater depths and it took a year for the Alaria zone to extend to 4–5 m deep, and two years to extend to 6 m deep. The low light penetration at this estuarine location was probably the main factor for the slow algal development at 6–10 m deep. At the end of the experiment Agarum cribrosum was second in importance after Alaria and was most common at 3 to 6 m in depth. Laminaria spp. was found in low numbers in the first year and did not show an increase during the second year. There was a dramatic increase in the number of species and abundance of algae in the experimental area. Also, there was a marked increase in many animal species, particularly Acmaea testudinalis, Mytilus edulis and Margarites helicinus, and a decrease in Metridium senile. By contrast, in the control area, the number of algal and invertebrate species remained low. In the experimental area a sharp increase in the growth rate of a cohort of very small urchins, which was not eliminated by our removal effort, demonstrated that there is strong intraspecific competition amongst urchins when the food supply is limited. In the St. Lawrence Estuary, there are few predators of urchins and the urchin dominated community appears to be a stable situation.

Influence of shoreline configuration on spatial variation of meroplanktonic larvae, recruitment and diversity of benthic subtidal communities

Abstract Recruitment, abundance (percentage cover, density) and diversity ( H ′) of a benthic subtidal community were examined in relation to large scale (⩾0.15 km) shoreline configuration in the St. Lawrence Estuary (Canada). Settlement panels were moored inside and outside bays of different sizes (0.15, 1.5, 4 and 7 km aperture) and along a portion of straight (≈12 km) coast. Only the largest bay was sampled in 1993, while all bays and the straight coast were sampled in 1994. In 1993, recruitment occurred only within the bay. No recruits were observed on moored panels from zones external to the bay. Within the bay, percentage cover on panels was greater than in zones external to the bay. In 1994, percentage cover and the density of recruits were also greater within each bay than outside bays. No differences in diversity, percentage cover and density of recruits were observed among zones along the straight coast. Diversity was greater in bays than along the straight coast and was greater inside bays of 1.5 and 7 km aperture than in zones external to the bays. Overall, diversity tended to increase with increasing size of bay, the straight coast exhibiting the smallest values. In contrast, the density of recruits and percentage cover tended to decrease with increasing size of bay; smallest values were from the straight coast. Abundance of recruits was not directly related to potential larval flux; current velocity alone explained 76% of the variation in density of recruits inside and outside of bays. Our study shows a relationship between shoreline configuration, the hydrodynamics, recruitment and benthic community characteristics. The generality of this relationship can be assessed by carrying out similar large-scale studies in other systems.

Sexual maturation as a source of variation in the relationship between cadmium concentration and body weight of Mytilus edulis L.

Abstract In order to refine the utilization of Mytilus edulis as a pollution indicator species, the relationship between cadmium concentration and mussel body weight was investigated at different sites in the Estuary and Gulf of the St. Lawrence. Our data show that the regression coefficients become considerably more variable when animals enter adulthood and that the variations are probably related to biochemical changes occurring during the sexual cycle. A substantial improvement of the applicability of mussels for monitoring marine pollution can be achieved by using only immature animals. This should reduce the influence of metabolism on the metal content of mussels and thereby give a more precise indication of ambiant pollution levels.

Behavioural responses of competent lobster postlarvae to odor plumes

We determined the ability of competent laboratory-reared Homarus americanus postlarvae to use odor plumes as a potential mechanism of orientation during the habitat selection process at settlement. In a Y-maze apparatus, both swimming direction (upstream or downstream) and arm selection (control or experimental) were strongly influenced by the nature of the chemical stimulus tested. In control conditions (unconditioned water in both proximal arms of the experimental device), postlarvae swam predominantly upstream (61:39) but displayed no arm preference (48:52). When submitted to a metabolite solution of adult conspecifics or macroalgae, postlarvae swam consistently upstream (≥73%) and showed a significant preference for the arm providing these stimuli (≥69%). In contrast, postlarvae submitted to cunner (Tautogolabrus adspersus) metabolites swam predominantly downstream (68:32), and those that did swim upstream significantly avoided the experimental arm (30:70). In repeated trials with the same individuals, postlarvae behaved similarly in ≥80% of the assays, suggesting a low intra-individual variability in behavioural responses. It is concluded that distance chemoreception may play a role in habitat selection by lobster postlarvae at settlement.

Barnacle Larval Settlement: the Perception of Cues at Different Spatial Scales

I review here some recent work carried out (1) to characterize differences in habitat selection by the larvae of the barnacle, Semibalanus balanoides , in two regions, the Gulf of St. Lawrence and the Bay of Passamaqquoddy on the Atlantic coast of Canada, and (2) to examine the mechanism used by the larvae to select a microhabitat to settle. The focus is placed on the influence of scales of substratum heterogeneity, scales in the exploration of substratum characteristics by the larvae, and its perception of cues. The spatial scales considered range from 1 m down to a few urn. A model summarizing the sequence of events taking place during the settlement process is presented.

Scale of observation and distribution of adult conspecifics: their influence in assessing passive and active settlement mechanisms in the barnacle Balanus crenatus (Brugière)

Abstract The settlement behaviour of Balanus crenatus (Brugiere) was examined in field experiments in relation to different patterns of distribution of adult conspecifics. The study was carried out in August 1994 in Anse du Petit Mitis, Quebec, Canada. Flat panels supporting random, aggregated, and uniform distributions of adult conspecifics or ceramic mimics were moored vertically on 3 lines. Panels were placed at random 2.5 m below the water surface. Settlement only occurred on panels supporting barnacles. Over 30% of the observed settlement occurred on adult conspecifics. ANOVAs showed no significant effects of distribution on number of settlers at the scale of the panel. Settlement tended to become homogenized over the panel with uniform distributions, while it became aggregated on panels with random and aggregated adult distributions. Mean nearestneighbour distances of spat did not vary among spatial patterns tested. When settling on adults, cyprids settled preferentially on the rostrum, facing current flow. A nested ANOVA carried out on the number of spat per adult on a given panel showed no effect of adult distribution. Differences in spat distribution among replicate panels, barnacle individuals, and positions were observed, suggesting that settlement of B. crenatus was affected by variations of the hydrodynamic environment at the scale of the adult barnacle. Flume experiments using inert particles and the same field panels were carried out to determine the position of initial contact on the panel. Comparison of field and flume results suggests that it is the small-scale behaviour taking place after initial contact on the panel and prior to permanent attachment that determines the patterns of spat distribution.