1 Control of microbial populations
Prior improvements in rearing hatcheries concerned quality of food, larval handling and sanitary control. Such changes to rearing methods have decreased disease and improved survival. Some bivalve larvae are particularly vulnerable to certain marine bacteria but, in 1981, Prieur showed that bivalve larvae could tolerate a high concentration of bacteria. The objective of the present work is to stay below the bacterial threshold, whatever the rearing conditions (seawater renewal rate, larval density) or whatever system is used to treat the water in a RAS (UV, biofilter). Additionally, as some other bacteria are beneficial to cultured bivalves, e.g. microbial biofilms can improve settlement. REPROSEED will try to improve their development in RAS.
REPROSEED is making a study of the microbiota in larval rearing. Changes in microbiota may reveal the dysfunction of rearing methods and a deficit in sanitary management. Microbiota will be tentatively orientated by modification of parameters of rearing and preliminary test of probiotics will be performed before any further investigation.
2 Gene expression analysis of key biological questions in bivalve reproduction
Recent technological developments provide new possibilities for investigating differential gene expression using high-throughput methods. These new technologies have identified genes specifically related to innate immune defence in bivalve larvae. Incidentally, some of these genes play important roles in metal homeostasis, detoxification and the stress response.
Mussels and oysters have already been studied with some tools from these new technologies, and this research is being continued during REPROSEED.
For oysters, several immune-related genes active during ontogenesis have also been identified. Some studies demonstrated that the major maturation events leading to immuno-competence occur between the D-hinged larvae and veliger stages. However, other authors have also demonstrated that, during metamorphosis, there is an increase in immune genes associated with defences, particularly around the foot area of byssus-producing bivalves (Xu and Faisal, 2008).
In the case of mussels, some immune genes in M. galloprovincialis are expressed at early stages whereas in the other mussels, these genes are only expressed after larval settlement and metamorphosis, suggesting they are developmentally regulated (Mitta et al., 2000).
REPROSEED is looking at the immune system from larvae to post metamorphosis for all the studied species. More generally, it intends to identify genes involved in maturation of gametes, including their development in the gonads, competence, metamorphosis and immuno-competence. Moreover, the above-mentioned tools will be developed for the emerging species, scallop and clam. This approach will allow a better understanding of key stages and enable us to elucidate the role of specific factors, such as spawning and metamorphosis inducers.