Bruno Cognie

Selective feeding of the oystercrassostrea gigas fed on a natural microphytobenthos assemblage

In estuarine ecosystems, microphytobenthos resuspended by tidal currents often represents a large part of the food supply available to bivalves which feed selectively. This study investigated the feeding behavior of the Pacific oysterCrassostrea gigas (Thunberg) relative to a natural microphytobenthic assemblage and the effect on the structure of this assemblage. Oysters were fed only benthic microalgae collected on the intertidal mud flats of Bourgneuf Bay (France) at a suspended particulate matter concentration above the threshold of pseudofeces production. All species in the assemblage were endemic diatoms characteritic of tidal mud flat environments. Four dominant ones, which were all solitary cells with spear-shaped frustules [Navicula ammophila (Grunow),Navicula rostellata (Kützing),Plagiotropis lepidoptera (Kuntze), andStaurophora amphioxys (Mann)] represented more than 95% of the 16 species involved. Analysis of feeding processes showed that the retention of the four main diatoms was not significantly different, but that two species, the smallest (N. ammophila [22×4 μm]) and the largest (P. lepidoptera [60×15 μm]), were preferentially ingested. The study of post-ingestive selection revealed that these two species were also preferentially digeted, i.e., preferentially directed into the digestive diverticulum, when they passed through the gut ofC. gigas. Cell size and shape did not appear to account for pre-ingestive and post-ingestive selection. The composition of the assemblage was significantly modified by oyster filtration, although the retention rates of the four main species were not significantly different. The composition of microalgae in pseudofeces and feces as a result of pre-ingestive and post-ingestive selection differed from that in seawater.

Does bivalve mucus favour the growth of their main food source, microalgae?

Bivalves, by releasing inorganic nutrients, can stimulate the growth of the primary producers on which they feed. Dissolved excretion and mineralization of biodeposits (pseudofaeces and faeces), two indirect processes related to their feeding activity, have already been identified. This study investigated whether microalgal growth is also dependent on the mucus secreted by bivalves and rejected with their biodeposits. An experimental study based on algal growth tests was conducted to determine whether the mucus coating the pseudofaeces and faeces of the oyster Crassostrea gigas (Thunberg) could stimulate the growth of nine marine microalgae: a Prasinophyceae [Tetraselmis suecica (Butcher)] and eight Diatomophyceae [Amphora sp. (Ehrenberg), Chaetoceros sp. (Ehrenberg), Entomoneis alata (Ehrenberg), Haslea ostrearia (Simonsen), Nitzschia acicularis (Wm Smith), N. closterium (Wm Smith), N. gandersheimiensis (Krasske) and Skeletonema costatum (Cleve)]. Oysters were fed only inorganic particles at a concentration well above the threshold of pseudofaeces production. A fraction of pseudofaeces and faeces was then inoculated into glass tubes containing a single algal species. Algal growth tests showed that mucus stimulated the growth of four of the nine species studied: Chaetoceros sp., H. ostrearia, N. gandersheimiensis and T. suecica. The strongest responses were observed with H. ostrearia and Chaetoceros sp., for which mean algal biomass was significantly higher with mucus originating from pseudofaeces than faeces. Monitoring of the release of dissolved components from mucus showed significant enrichment in inorganic phosporus. The use of an antibiotic treatment suggested that this dissolved component is released by simple solubilization rather than by bacterial breakdown.

REVIVAL CAPACITY OF DIATOMS IN BIVALVE PSEUDOFAECES AND FAECES

The revival capacity of eight species of diatoms rejected in pseudofaeces and faeces of a filter-feeder, the Pacific oyster Crassostrea gigas Thunberg, was assessed by experimental culture tests. Isolated from oyster fattening ponds near Bourgneuf Bay on the French Atlantic coast (46–47°N, 1–2°W), the eight diatom species were: Amphora sp., Chaetoceros sp., Entomoneis alata, Haslea ostrearia, Nitzschia acicularis, Nitzschia closterium, Nitzschia gandersheimiensis and Skeletonema costatum. The results show that none of the diatoms were totally digested and that all underwent significant growth. Cell concentrations with pseudofaecal cultures were higher for all species excepted Haslea ostrearia for which the difference with faecal cultures was not significant. This result is consistent with the fact that diatoms rejected in faeces are processed through the bivalve gut, while pseudofaeces are simply formed on the gill and the palps. Nevertheless, the significant biomass obtained with faecal cultures compared to a control, confirms that diatoms have significant survival after extracellular digestion by the oyster. Survival was probably facilitated by the large amount of inorganic particles added to experimental diets to reproduce conditions characteristic of intertidal oysters cultivated on mudflats. This variable has probably more significance for the revival process than the size of the microalgae or its ecological type in such environment.

Does allelopathy affect co-culturing Haslea ostrearia with other microalgae relevant to aquaculture?

Haslea ostrearia is a marine diatom known to produce marennine, a water-soluble blue-green pigment responsible for the greening of oysters in ponds along the French Atlantic coast. This phenomenon occurs seasonally when H. ostrearia blooms in oyster ponds, and it increases the economic value of cultured oysters. From an ecological perspective, H. ostrearia blooms are accompanied by a decrease in the abundance of other microalgae, suggesting that this diatom produces allelochemicals. Recent studies showed that purified marennine has other biological activities, for instance antioxidant, antibacterial, and antiviral activities, which could be used in aquaculture to promote this pigment as a natural antipathogen agent. One important issue regarding the possible use of H. ostrearia in aquaculture as a mixed algal diet, however, is the importance of marennine allelopathy. In this study, we investigated the allelopathic effect of H. ostrearia on the growth of five microalgal species relevant to aquaculture: Chaetoceros calcitrans, Skeletonema costatum, Phaeodactylum tricornutum, Tetraselmis suecica, and Tisochrysis lutea. Allelopathic tests were realized by co-culturing these microalgae with H. ostrearia in batch and in semi-continuous mode, based on initial biovolume ratios. Our findings showed that inhibition of the growth of microalgae due to the presence of H. ostrearia and marennine was species dependent. Skeletonema costatum, C. calcitrans, and T. lutea were significantly more sensitive, whereas T. suecica and P. tricornutum appeared to be more resistant. Growth irradiance significantly influenced the allelopathic effect against the sensitive species S. costatum, and the H. ostrearia production of marennine increases with irradiance. Data presented in this study partly support the hypothesis that marennine released into the culture medium possibly acts as an allelochemical compound, thus explaining the dominance of H. ostrearia and the loss of sensitive algae in oyster ponds, but also that some species are insensitive, which allows co-culturing and use in a mixed algal diet in aquaculture.

Utilisation of intertidal mudflats by the Dunlin Calidris alpina in relation to microphytobenthic biofilms

AbstractThe small-scale spatial distribution of Dunlin Calidris alpina staging and wintering in Bourgneuf Bay (France) was investigated on two different mudflats colonized by either epipsammic- or epipelic-dominated microphytobenthos assemblages. Shorebird counts were conducted monthly at ebb tide from October 2011 to May 2012 and from November 2012 to May 2013. Multiple linear regressions followed by hierarchical partitioning of variance showed that microphytobenthos biomass was not a significant factor to explain Dunlin densities. However, on epipelic-dominated mudflats, Dunlins did not show their typical “tide follower” behaviour and instead significantly selected the highest microphytobenthos biomass zones. The biomass of a gastropod predator of an important local Dunlin prey, Retusa obtusa, was negatively correlated with Dunlin densities. This paper provides new suggestions in the ways that biofilms on mudflats affect small shorebird foraging.ZusammenfassungDie Nutzung von Wattflächen durch AlpenstrandläuferCalidris alpina im Hinblick auf mikrophytobenthische Biofilme Die kleinräumige Verteilung von Alpenstrandläufern Calidris alpina, die in der Bucht von Bourgneuf (Frankreich) rasten und überwintern, wurde auf zwei verschiedenen Wattflächen untersucht, die von Mikrophytobenthosgemeinschaften besiedelt wurden, welche entweder von epipsammischen oder epipelischen Arten dominiert wurden. Watvogelzählungen wurden monatlich bei Ebbe von Oktober 2011 bis Mai 2012 durchgeführt. Multiple lineare Regressionen, gefolgt von der hierarchischen Partitionierung der Varianz, zeigten, dass die Biomasse des Mikrophytobenthos keinen signifikanten Einfluss auf die Dichte der Alpenstrandläufer hatte. Auf von epipelischen Arten dominierten Wattflächen zeigten Alpenstrandläufer jedoch nicht ihr typisches “Gezeitenfolgeverhalten”, sondern wählten stattdessen die Zonen mit der höchsten Mikrophytobenthosbiomasse. Die Biomasse einer Schnecke, die eine wichtige lokale Beute des Alpenstrandläufers frisst (Retusa obtusa), zeigte eine negative Korrelation mit der Alpenstrandläuferdichte. Diese Studie liefert neue Hinweise, dass Biofilme auf Wattflächen das Nahrungsverhalten kleiner Watvögel beeinflussen.

Optimizing metamorphosis in Paracentrotus lividus aquaculture using alternative macroalgae species to Corallina sp.

Metamorphosis induction cues (by chemical mediation or direct contact) were tested in Paracentrotus lividus using three different macroalgae treatments: Corallina sp., Palmaria palmata, and Laminaria digitata. Higher percentages of metamorphosis were reached in Paracentrotus lividus larvae by direct contact with a Palmaria palmata substrate.

Cell size-based, passive selection of the blue diatom Haslea ostrearia by the oyster Crassostrea gigas

Pre-ingestive selection has been identified as a feeding mechanism of oysters that may influence their uptake of particles and microalgal cells. Oysters can feed specifically on the pennate diatom Haslea ostrearia, which produces the blue pigment marennine that is responsible for the greening of oysters. Because the size of particles or cells plays a significant role in the selection process, and given that diatoms experience a decrease in size as a consequence of vegetative reproduction, H. ostrearia consumption and marennine uptake might be influenced by pre-ingestive selection. We examined the role of H. ostrearia cell size in the selective feeding of Crassostrea gigas. Individual flow-through chambers were used to deliver mixtures of H. ostrearia of varying cell length to oysters. Inflow, outflow and pseudofaecal samples were collected from chambers during oyster feeding. Video-endoscopy was used to sample material in the dorsal and ventral particle tracts. Diatom cells counts showed that pseudofaeces contained on average larger cells than the ambient medium. However, proportions of the different populations of H. ostrearia in pseudofaeces were identical to those in the ventral tracts, indicating that no selection was performed by the labial palps. Video-endoscopy, plus imaging by scanning electron microscopy, of gills and labial palps revealed that only those larger H. ostrearia that were orientated dorsoventrally could enter the principal filaments (pfs) and then access the dorsal acceptance tract. These results show that for particles like Haslea cells with only one axis exceeding the width of the pfs, the selection on the oyster gills is passive and based on cell size.