Behavioral responses of Crassostrea gigas exposed to the harmful algae Alexandrium minutum
Introduction
Toxic algae blooms are a major problem in the world, in terms of aquatic ecosystem risks, human health, and economy (Bricelj and Shumway, 1998). Specifically, toxin accumulation in marine bivalves is a common phenomenon during algal blooming events that can lead to a closure of shellfish harvest for human consumption (Cembella and Todd, 1993).
Previous studies showed that filter-feeding bivalves exhibit different behavioral responses when exposed to a toxic algal bloom (Bricelj and Shumway, 1998). Responses may be related to the relative toxicity of compounds produced by the algae (Bricelj et al., 1996), toxins accumulated in the tissues of bivalves (Bricelj et al., 1991), and the history of harmful algal bloom exposure in any given ecosystem (Shumway and Cucci, 1987). Toxic algae of the genus Alexandrium are an important source of marine toxins in contaminated bivalves (Balech, 1990). Alexandrium minutum is found in coastal and estuarine waters. It is established in the coastal waters of Europe (Northeast Atlantic, North Sea, Baltic, Sea, Mediterranean and Black Sea), Southeast Asian Waters (South China Sea) and in parts of Southern Australia and New Zealand (Chang et al., 1995, Hallegraeff, 1993). This species also has been reported in North America (Page et al., 2001). The species A. minutum can reach up to 1.8 · 108 cells/L in natural marine European coastal waters (Belin and Raffin, 1998). Oysters, such as C. gigas, are known to reduce filtration rate when exposed to toxic dinoflagellate algae Alexandrium sp. (Bardouil et al., 1993, Lassus et al., 1999). Interestingly, C. gigas has been classified as having average sensitivity, in terms of response to paralytic shellfish poisoning toxins (PSP toxins) produced by toxic dinoflagellates. Indeed, oysters accumulate less PSP toxins than the mussel Mytilus edulis, described as a non-sensitive species (Bricelj and Shumway, 1998) that does not modify its nutrition activity. Oysters, such as C. gigas or C. virginica accumulate more toxin than the clam Mya arenaria, a highly sensitive species, which shows modified burrowing activity and reduces or even stops filtration activity under similar exposure. The scallop Argopecten maximus appears to be very sensitive to PSP toxins as well, increasing valve-clapping frequency and closing the shell under similar exposure conditions (Bricelj and Shumway, 1998).
In the present study, we describe the valve activity of Pacific oysters C. gigas during a 7-day exposure period to an ecologically relevant concentration of A. minutum. Valve activity was measured with a laboratory-made valvometer (Chambon et al., 2007, Tran et al., 2003; http://www.domino.u-bordeaux.fr/molluscan_eye). The original feature of this valvometer is that it uses lightweight electrodes with high sensitivity and with minimal experimental constraints. We report here different measures of valve-activity response, such as changes in daily valve-opening duration, number of micro-closures, or partial closures, and valve-opening amplitude. Our aim was to characterize, under simplified but well-controlled laboratory conditions, a putative behavioral alteration which might sign the impact of A. minutum on the oyster. The response to this toxic alga was compared to behavior of oysters in the presence of the non-toxic algae Isochrysis galbana clone Tahitian (T-Iso) or Heterocapsa triquetra.
Section snippets
Oyster characteristics and general conditions
The study was carried out in Brest (Brittany, France), in October–November 2007 (experiment 1) and December–January 2008 (experiment 2) with pacific oysters, Crassostrea gigas (Thunberg). Oysters were obtained from a shellfish farm in Kerner Island (Morbihan, France). Two homogenous groups of diploid oysters (n = 16/experiment) were chosen a priori according to the shell length (75 ± 8 mm shell length; 34.5 ± 7.6 g total fresh weight, shell plus flesh). During experiment 1, oysters were distributed
A. minutum exposure
During the exposure to A. minutum, the flow renewal of alga supply in the experimental tank of both experiments was 14 ml min− 1 from the supply tank at constant concentration of A. minutum, which was in average 4981 ± 253 cell ml− 1 (experiment 1 — 8 days, 2 samples in supply tank) and 4545 ± 323 cell ml− 1 (experiment 2–7 days, 2 samples in supply tank), i.e. equivalent to 6.7 ng ml− 1 STX eq (experiment 1) and 5.9 ng ml− 1 STX eq (experiment 2). In the experimental tank, the concentration of A. minutum
Discussion
The objective of this work was to test under laboratory conditions how an exposure to the dinoflagellate A. minutum in a bloom simulation could induce a change in valve activity behavior in Pacific oysters, C. gigas. Valve activity recording and behavioral parameters to test water quality have been available for years. The pioneering developments date back to the turn of the 20th century (Hopkins, 1931, Marceau, 1909, Nelson, 1921). This is not the place for an extensive review, but among the
Acknowledgements
The authors would like to thank Gilles Durrieu and Mohamedou Sow for their help and discussions about the HFNI valvometer and Christine Schwimmer for the correction of English language. Thank you to Gary Wikfors for stimulating and constructive remarks. All experiments presented in this paper complied with the law in effect in France, where they were performed.
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