Seasonal variations of immune parameters in diploid and triploid Pacific oysters, Crassostrea gigas (Thunberg)
Introduction
The production of the Pacific oyster, Crassostrea gigas, is the highest in global and French shellfish production (FAO, 2003). Oysters are reared on coastal producing sites and therefore are exposed to seasonal environmental changes throughout the annual cycle. Natural individuals are diploids but triploids′ production has been introduced in aquaculture in the two last decades in order to enhance growth versus reproduction in fish and in shellfish. The reproductive cycles of diploids and triploids differ largely. Many comparative studies have hence been performed on growth enhancement, reproduction (Allen and Downing, 1990), and feeding performance (Kesarcodi-Watson et al., 2001a, Kesarcodi-Watson et al., 2001b). However no converging results have been published on the efficiency of the immune system towards pathogens (Nell, 2002). Furthermore no literature exists on the comparison of the immune parameters of triploids and diploids.
The immune system of shellfish and molluscs has been studied in many bivalve species of economic value such as clams (Fournier et al., 2001, Ballarin et al., 2003, Soudant et al., 2004), mussels (Pipe et al., 1995, Cartier et al., 2004) and oysters (Auffret and Oubella, 1995, Lambert et al., 2003, Goedken and De Guise, 2004). It is an innate immune system composed of specific cells, the haemocytes (Auffret, 1988) and some humoral effectors (Mitta et al., 2000) in the circulating fluid, the haemolymph. As the circulatory apparatus is semi open to the media, the physical and chemical parameters of the water column interferes with the physical and chemical parameters of the haemolymph and various external agents may enter the organism (Hine, 1999). The status of the immune system can be estimated by assessing a number of immune parameters which indicates whether the components of the immune system are structurally (number of haemocytes, haemocytes′ mortality, aggregation) capable of performing immune functions (phagocytosis, lysosomal system, oxidative burst) efficiently (Auffret, 2005). Many studies have shown that this multiparametric approach can discriminate different polluted or rearing sites (Auffret et al., 2004, Soudant et al., 2004, Auffret et al., 2006). Laboratory studies have revealed that these parameters can be modulated by chemicals (Pipe et al., 1999, Fournier et al., 2001, Fournier et al., 2002). But knowledge on natural seasonal variations of immune parameters is still lacking (Petrovic et al., 2004, Leinio and Lehtonen, 2005) and is needed to better understand biological critical periods of local species. A further deficiency is the absence of data of factors, like age, sex, gametogenic status, nutrition, which can be considered as confounding factors interfering with the immune system and which may induce the high inter individual variability observed in Bivalve statistical populations (Selgrade, 1999, Galloway and Depledge, 2001, Auffret, 2005, Selgrade, 2005). Seasonal variations of biological parameters have two origins: exogenous and endogenous. Firstly, exogenous factors such as temperature and salinity (Gagnaire et al., 2006, Hégaret et al., 2003a, Hégaret et al., 2003b, Gagnaire et al., 2006), pH, carbonates, amount and quality of feeding particles (Delaporte et al., 2003, Hégaret et al., 2004, Delaporte et al., 2005) or bacteria or viral concentration (Paillard et al., 2004), modulate structurally and functionally the immune system of bivalves. Secondly, there is increasing evidence that the immune status of molluscs is influenced by annual reproductive cycles. Hemocyte migration toward interstitial tissues and finally, gonadic follicles, has been extensively observed during gamete atresy (Cajaraville et al., 1996). This phenomenon leads to variations in circulating haemocyte counts. Other interactions may arise from molecular regulation pathways. Indeed, neuro-endocrino-immunology is by far a less documented field of knowledge. The occurrence of hormonal receptors in haemocytes has been recently observed (Canesi et al., 2004a). Furthermore, alterations of haemocyte functions appear to be modulated in mussels exposed to endocrine disruptors depending on gender (unpublished data).
Therefore the aim of this study was to compare the annual pattern of immune parameters in diploid and triploid Pacific oysters, C. gigas (Thunberg), and the role of the reproductive cycle (sex and gametogenic status) on this pattern, considering triploids as altered-reproduction treated individuals.
Section snippets
Animal handling
Diploid and triploid Pacific oysters from the farm “Huîtrerie de la pointe du château”, (Belon, Brittany, France) were reared according to local techniques, pouches full of oysters of similar age were fixed on tables on the foreshore. Diploid spat had been collected in the South-West of France (Gujan-Mestras) and were settled at the farm in April 2002. The triploid spat had been produced by the company SATMAR and triploidy was checked to be over 99% successful. Triploids were settled at the
Immunopathology variations
THC varied along the sampling period in diploid individuals. The values varied between a minimum (below 0.5 × 106 cell ml− 1), during winter months, and a nonsignificant maximum in May (1.3 × 106 ± 9.6 × 105 cell ml− 1; N = 28). Values remained high during summer. A significant isolated peak value was observed in March (1.6 × 106 ± 1.2 × 106 cell ml− 1; N = 10). From May to September, THC values remained constant (9.3 × 105 ±7.1 × 105 cell ml− 1; N = 73) with high individual variations before decreasing (5.7 × 105 ± 5.0 × 105
Discussion
The aim of this study was firstly to describe the variation of selected immune parameters of diploids oysters along the year and secondly, to compare immune parameters of triploid and diploid individuals.
Conclusion
This study has shown that immune parameters, especially phagocytosis, drawn marked seasonal variations, in relation with reproductive cycle in C. gigas, in diploid and triploid individuals. Triploids were shown to respond less to environmental variations than diploids. Further experiments should now focus on the relationship between reproduction and immune function and on the toxic effects of xenobiotics on triploids.
Acknowledgements
The authors would like to thank Sorcha Nì Longphuirt, for English revising, Alain Marhic for its valuable technical help as well as the team of “Huîtrerie du Chateau” for animal delivery throughout the experiment. This study was part of the CONCHPOL project funded by the French ′‘Réseau d’innovation et technologie de la mer’’ (RITMER). Contribution N° 1022 of the IUEM, European Institute for Marine Studies (Brest, France).
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