French oyster farming has been subject to severe mortalities during the summer months. Results from the research program “Morest”, which ran from 2000 to 2006 and examined the possible causes of these mortalities, led to the construction of a model to explain the interaction between environmental factors, oyster physiology and different opportunistic pathogens underlying oyster summer mortality. Temperature, food, reproduction and stress were the main factors required for oyster mortality. Genetically-based resistance (“R” oysters) or susceptibility (“S” oysters) to summer mortality was revealed by divergent selection. Building on these results, a literature search was made in 2007 on the molecular origin of genetic resistance to such a complex mortality risk. The objectives were to lay a foundation for the preparation and orientation of future research directions and to improve understanding of the underlying physiological mechanism leading to summer mortality. Three years later, the resulting conceptual analysis reported here was presented as an introductory lecture to Physiomar 2010, a conference where many new results contributing to this research field were also reported. The literature review highlighted two major review articles: the first dealing with nutrition and reproduction (Schneider 2004), the second with reproduction, temperature, oxidative stress and mortality (Heineinger 2002). The effect of nutrition level on energy orientation to growth or reproduction is controlled by endocrine factors. Among these, neuropeptide Y (NPY), ghrelin and leptin neuropeptides appeared to be potential candidates involved in germ-soma orientation in relation to trophic conditions. Depending on reproductive effort and temperature, a metabolic stress resulting from the germ-soma conflict can appear, characterized by mitochondrial reactive oxygen species (ROS) production. Such an excess of ROS induces perturbations in mitochondrial activity leading to cell death. Many organisms, such as annual plants or the Pacific salmon, do not survive their first reproduction. In contrast, others increase stress resistance by selection of antioxidant processes (superoxide dismutase SOD, catalase, etc.) through evolution, and survive first reproduction. A similar difference was observed in the comparison made between R and S oysters, which differed in ROS production, SOD and catalase levels. Such factors controlling reproduction and ROS detoxification processes could therefore provide new markers for selection of oysters with better resistance to non-specific pathogens, complementing other classic selection approaches against specific pathogens or for improved immunity. This antioxidant defence mechanism is found in many organisms including vertebrates and in some invertebrates, including oysters. Its role needs to be considered in pathology events involving other aquaculture species and it may also contribute to explaining the increase in marine pathologies under anthropogenic environmental changes.

A technique was developed for dissection and isolation of male germ cells in the oyster Crassostrea gigas. This procedure can provide cells for the exploration of processes involved in the reproductive physiology of bivalves. Spermatogonia were chosen because of their essential role in spermatogenesis and the impact of gonia proliferation on reproductive effort. A non lethal method for determining sex and reproductive cycle stage was first validated in oysters. This first step was essential in order to constitute a homogeneous pool of oysters at the same stages of gametogenesis. Germ cell fractions were then obtained from a density gradient, and enrichment of each fraction was ratified by electron microscopy and by means of a 2-parameter flow cytometry procedure (DNA and mitochondrial staining). A significant enrichment in spermatogonia and spermatocytes was confirmed by the increased expression of markers of proliferative cells (proliferative cell nuclear antigen, PCNA) and early germ cells (oyster vasa-like gene). A preliminary cell sorting procedure is also reported, which was applied to fractions enriched in spermatogonia.

In this paper, biochemical and physiological analyses were used to characterize changes associated with mortality event occurred during veliger development of American oyster, Crassostrea virginica. Biochemical analyses included the evaluation of lipid classes, fatty acid composition and total protein content. Larval physiology was evaluated by studying feeding activity, enzymes related to energy metabolism, oxidative stress levels and enzymatic antioxidant defenses. These analyses were complemented by bacterial community analyses as well as by measuring larval oyster performance. We observed that mortality events coincided with (1) strong changes in the surrounding bacterial community; (2) a progressive decrease in feeding activity; (3) higher levels of some lipid classes (free fatty acids, diglycerides, and acetone mobile phospholipids); (4) lower levels of phospholipids and protein; (5) higher contents of non-methylene interrupted dienoic fatty acids (22:2 NMI); (6) a decrease in energy metabolism activity (citrate synthase and cytochrome oxidase activities); (7) a higher oxidative stress (lipid peroxidation level); and (8) an activation of antioxidant defences before mortality (glutathione peroxidase and superoxide dismutase). We hypothesized that mortality emergence was related to higher energy consumption coupled with the progressive decline in feeding activity, lowered energy reserves and a decrease in energy metabolism activity. Thus, the low energy availability limited the efficiency of antioxidant defenses, resulting in a higher oxidative stress.

We examined whether escape response performance and mitochondrial capacities could reveal differences created by feeding scallops, Argopecten purpuratus, mono-specific algal diets composed of either Chaetoceros calcitrans or Isochrysis galbana (variety T. iso) hereafter T. iso. Before and after feeding scallops with these diets, we assessed force production in vivo to evaluate escape response performance (initial and repeat). We measured oxidative capacities of mitochondria isolated from the adductor muscle and from the male and female portions of the gonad. Initial escape response performance was reduced more by the C. calcitrans diet than by the T. iso diet. Repeat escape responses, which require aerobic recuperation, were reduced by both treatments. The oxidative capacity of mitochondria isolated from muscle and female gonad was markedly lower in scallops fed C. calcitrans than in those fed T. iso. Flux through complex I–IV and through complex IV was also lower in mitochondria from muscle of scallops fed C. calcitrans than in those fed T. iso. Muscle aerobic capacity, assessed by citrate synthase activity, was lower in scallops fed C. calcitrans than in those fed T. iso. Despite the marked differences in fatty acid (FA) composition of the algal diets, the FA composition of mitochondrial phospholipids differed little between scallops fed C. calcitrans and those fed T. iso. Both escape response behaviour and mitochondrial capacities changed with feeding mono-specific diets. The simplicity of measurements of scallop escape responses suggests this as a practical means of evaluating the status of scallops for the two monoalgal diets tested.

The bivalve’s aquaculture industry is an important component of the economy in Eastern Canada. Seed collection is an initial and critical activity in most bivalve aquaculture industries including mussel farming in Prince Edward Island, production is entirely dependent on natural spat collection. Although seed supply is not a concern from a quantitative standpoint, there are growing concerns about the quality of natural seed. The general objective of this study was to identify and assess mussel seed quality criteria on the basis of physiological and biochemical status under laboratory and field conditions. The performance, as estimated by metabolic measurements, lipid class composition, multi-locus heterozygosity (MLH) and survival to stressful environment of seed from 6 different stocks sources was first compared under laboratory conditions at 12 °C and 25 °C. Results showed that MLH varied among the six sources of mussels in a way which is consistent with the physiological and biochemical indicators of seed quality. Mussels from Shippagan (New Brunswick) and Tracadie (Prince Edward Island) were found to have the highest quality scores and the best adaptive capacity to extreme water temperature under laboratory conditions. The results of the stock-site reciprocal field studies are in general agreement with those of the laboratory experiments with higher survival of mussels from Shippagan, Tracadie and St. Peters Bays in the various study sites. Our results suggest that the measure of MLH and survival curves at stressful temperature could be a good criteria combination to identify the improved survival potential of mussels stocks.

Mussel culture relies on the mussel self-attachment capacity through byssus production. By doing so, no cages or containment devices are needed. It has been previously suggested that thread production requires a non-negligible part of the energy expenditure in blue mussels Mytilus edulis. Therefore our work investigates the relationships between byssal thread production, mussels energetic reserves and phospholipids profiles in the foot. The relationship between thread production and heterozygosity was also examined. The study was realized in a small lagoon of the Îles-de-la-Madeleine, southern Gulf of St. Lawrence, Canada, on four sampling dates between June and September encompassing the pre- to post-spawning periods. Our results show a significant relationship between the thread numbers and attachment strength. However, no correlations were found between energy reserves, phospholipids composition of the foot or heterozygosity level and byssal production. Our results suggest that mussels in suspension culture in this lagoon were not energy-limited so that the energy reserves did not influence the byssal thread production.

The purpose of this study was to determine biochemical changes occurring in blue mussels (Mytilus edulis) fed effluent from an Atlantic cod (Gadus morhua) aquaculture facility over a period of ten weeks, compared to those in mussels fed a commercial shellfish diet and those supplied only filtered seawater. The total lipid and fatty acid content (mg g-1 wet weight) significantly decreased for mussels fed effluent during the experiment. The only change in the lipid class composition (% total lipid) at the end of the experiment was a significant increase in the proportion of acetone mobile polar lipids. There were several significant changes in the fatty acid composition (% total fatty acid) including an increase in the proportion of 18:1ω9, 18:2ω6, 20:4ω6, 21:5ω3 and the dienoic non-methylene-interrupted fatty acids 20:2a and 22:2b and significant decreases in the proportions of 16:0, 18:4ω3 and 20:5ω3. The increase in non-methylene interrupted dienes suggests that the amount of essential fatty acids in the effluent may be insufficient for optimal mussel growth. The presence of the terrestrial plant marker 18:2ω6 in both the fish feed and the effluent and its increased proportion in mussels fed effluent suggest that this fatty acid may have potential as a marker for aquaculture wastes.

The impact of an alien species is correlated with its abundance and potential to colonise new environments. Consequently, the crucial aspects that give a mytilid species invasive potential include its strength and capacity for adhesion to a wide variety of substrates and its ecological tolerance to environmental conditions. The alien black pygmy mussel Xenostrobus securis settles together with the indigenous and raft cultured mussel Mytilus galloprovincialis on hard substrates of the inner coastline of the Ría de Vigo (Atlantic Coast NW Spain). Key parameters for the ecological success of these species could include their byssus structure and ecological plasticity. In this study, we assessed their comparative ability to attach to different substrates under different environmental conditions, using both in situ and laboratory approaches. Byssus threads secreted by X. securis were found to be extremely thin and their thickness did not vary with mussel size. This thinness of byssus filaments in X. securis was compensated by the secretion of a huge number (up to thousand of byssus filaments). Although no increase in attachment strength was found with increasing mussel size in this species, multiple byssus thread secretion might be a key factor in the successful invasion of inner areas of the estuary. Furthermore, X. securis exposes a much lower shell area to lift and drag forces in mussel conglomerates. By contrast, the indigenous species M. galloprovincialis secretes thicker byssus threads, which provide stronger in situ attachment displaying a linear increase in strength relative to the size of mussels. The exposure of both mytilid species to different abiotic factors in the laboratory confirmed the weaker byssus secretion and limited variation in attachment strength in the alien species. On one hand, X. securis seemed to be better adapted to life on soft bottoms and not capable of much plasticity when attached to different substrates. However, X. securis might be also well adapted to variable environmental conditions, e.g., salinity fluctuations, with no need to modify its byssus properties to ensure secure attachment. Attachment strength of M. galloprovincialis was higher than X. securis in all abiotic conditions tested. Results are discussed in relation to the ecological tolerance and impact of these mytilid species.

The common whelk Buccinum undatum (Neogastropoda, Buccinidae) is widespread along the north Atlantic coasts of France. In west Cotentin (Normandy), an important local fishery exists that depends exclusively on this marine resource and represents about 90% of French whelk production. Despite successive restrictive measures, fishing yields have decreased since 2000. Shell length frequency distributions of Buccinum undatum caught in west Cotentin were studied in 2005 (N = 2910), in 2007 (N = 2032) and in 2009 (N = 4145) ranging from 7 to 85 mm. In order to preserve this fishery and to characterise the whelk stock at this site, the reproductive biology of this species was studied in relation to age, sex and size, for whelks of commercial size (45 to 61 mm) over the complete annual cycle in 2008. Age was determined by reading the number of operculum striae: animals between 45 and 59 mm had 2 to 4 striae. Four reproductive stages were defined for males and females on the basis of histological analysis. The time course of reproductive events was described according to size and sex: the highest percentages of ripe stages were found in October for both sexes. Spawning events occurred between October and December. The shell length that corresponded to 50% of animals having mature gonads (49 mm and 52 mm for males and females, respectively, corresponding to 3 and 4 years old animals) exceeded the minimal landing size (45 mm). Fifty percent maturity was reached for males in their third year and for females in their fourth year. Reproductive investment was evaluated by quantitative analysis of the gonadal thickness or surface and was found to be proportional to shell size. We found that less than 20% of females under 49 mm in length were sexually mature and that these produced only a few eggs. This information, collected in a major fishing area, should assist fishery management decisions.

Determining age is an important step when assessing growth, mortality, and yield of cultivated and wild populations, but studies linking shell growth marks in the pearl oyster Pteria sterna with the age of individuals are lacking. Thirty juveniles (20.0 ± 1.2 mm shell height), collected from a winter spatfall, were marked with the fluorochrome calcein and kept in the field in culture containers. After day 16, the juveniles were cleaned and their shells cut along the sagittal axis to determine periodicity of micro growth bands formed in the inner shell layers and to estimate age. During this trial, fluorescent calcein marking succeeded in individuals larger than 20 mm shell height; these formed an average of 15 micro growth bands over the 16 days, representing 1 band per day. The marker created a wide fluorescent band containing three micro growth marks, suggesting that calcein was incorporated into the shell over the first three days. The use of calcein was found to be an accurate method for validating the micro growth band frequency of formation in P. sterna juveniles, which in turn can help to estimate age.