Effects of stocking density on intermediate culture of the razor clam Ensis arcuatus (Pharidae: Bivalvia)

Commercial production of most bivalve species involves a phase of intermediate cultivation during which juveniles are grown under protected conditions until they can be transferred to the final grow-out location. Consequently, the aim of this study was investigating the effect of density on growth and survival in the intermediate culture of the razor clam Ensis arcuatus in raft. Two series of experiments were performed, using in the first experiment two holding systems: 5-L plastic bottles and PVC cylinders, both perforated, covered with a 1 mm mesh net to prevent razor clam escape and a 10-cm layer of coarse grained sand (300–1200 lm grain diameter). In this trial, two stocking densities were tested: 0.15 and 0.30 kg m-2. In the second one-two densities were essayed (0.62 and 1.24 kg m-2) in the PCV cylinders. In the first experiment after 27 of trial higher growth in cylinders at low density was observed. At the end of this trial all juveniles died in the 5-L plastic bottles and higher growth and survival were recorded in the cylinders at low density. In the second experiment, significant differences in growth and survival were found on day 70, with the best results at low density. Five-L plastic bottles must be rejected as holding system for the intermediate culture of E. arcuatus and the PVC cylinders at low densities could be a good alternative. At the end of the experiment razor clams reached the adequate size for their transfer into the grow-out system

Essential Fatty Acid Assimilation and Synthesis in Larvae of the Bivalve Crassostrea gigas

Essential fatty acids (EFA) are important for bivalve larval survival and growth. The purpose of this study was to quantitatively assess for the first time through a mass-balance approach dietary EFA incorporation and synthesis within Crassostrea gigas larvae. A first experiment was carried out using two microalgae, Tisochrysis lutea (T) and Chaetoceros neogracile (Cg), as mono- and bi-specific diets. A second experiment using a similar design was performed to confirm and extend the results obtained in the first. Flow-through larval rearing was used for accurate control of food supply and measurement of ingestion. Non-methylene-interrupted fatty acids were synthetized from precursors supplied in the diet: 16:1n-7 and 18:1n-9, mediated by Delta 5 desaturase. Moreover, this Delta 5 desaturase presumably allowed larvae to convert 20:3n-6 and 20:4n-3 to 20:4n-6 and 20:5n-3, respectively, when the product EFA were poorly or not supplied in the diet, as when larvae were fed T exclusively. Under our experimental conditions, none of the diets induced 22:6n-3 synthesis; however, 22:6n-3 incorporation into larval tissues occurred selectively under non-limiting dietary supply to maintain optimal levels in the larvae. This combination of flow-through larval rearing and biochemical analysis of FA levels could be applied to additional dietary experiments to precisely define optimal levels of EFA supply

Effects of microalgal diet on growth, survival, biochemical and fatty acid composition of Ruditapes decussatus larvae

Successful bivalve larval growth and survival depend on stored reserves provided by diet. The aim of this study was to evaluate the effects of microalgal diet on growth, survival, together with biochemical and fatty acid composition during larval development in grooved carpet shell (Ruditapes decussatus). Four mono- and multi-species diets were tested: (1) Isochrysis galbana, Pavlova lutheri and Chaetoceros muelleri (1:1:1, IPC); (2) I. galbana, P. lutheri and C. muelleri (1:1:2, IP2C); (3) I. galbana and P. lutheri (1:1, IP); (4) C. muelleri (C). Our results showed that feeding regime greatly influenced larval growth and survival, as well as biochemical and fatty acid composition. Higher growth and survival rates were observed in the IPC, IP2C and C diets, which all include C. muelleri. Despite the fact that the larvae fed with IP exhibited the lowest length at settlement and survival, they stored more total lipids than any of the diets including C. muelleri. The IP and IPC diets exhibited a higher percentage of total lipids, whereas IP2C and C showed a higher proportion of proteins. Lipid and carbohydrate concentration in these diets negatively affected growth and growth and survival of the larvae, respectively. Protein content of the larvae was however positively related to larval survival, whereas lipid content of the larvae negatively affected growth. Fatty acid (FA) profile of neutral lipids (NL) and polar lipids (PL) of 22-day-old larvae clearly reflected the FA proportion of each diet. Dietary DHA and DHA in NL of the larvae were negatively related to larvae growth. DHA incorporation into PL in larvae fed with DHA-deficient diets was compensated by an enrichment in EPA, 18:0dma, 22:5(n-3) and 22:2 non-methylene-interrupted dienoic FA. According to these results, the presence of C. muelleri in the diet seems to promote better growth and survival of R. decussatus larvae