Abstract
The Pacific oyster, Crassostrea gigas (Thunberg), is the main bivalve species cultivated in the world. With global warming enabling its reproduction and larval survival at higher latitudes, this species is now recognized as invasive and creates wild oyster reefs globally. In this study, the spatial distribution of photosynthetic assemblages colonizing the shells of wild C. gigas was investigated on both a large scale (two contrasting types of reefs found in mudflats and rocky areas) and a small scale (within individual shells) using a hyperspectral imager. The microspatial distribution of all phototrophs was obtained by mapping the Normalized Difference Vegetation Index (NDVI). Second derivative (δδ) analyses of hyperspectral images at 462, 524, 571 and 647 nm were subsequently applied to map diatoms, cyanobacteria, rhodophytes and chlorophytes, respectively. A concomitant pigment analysis was carried out by high performance liquid chromatography and completed by taxonomic observations. This study showed that there was high microalgal diversity associated with wild oyster shells and that there were differences in the structure of the phototropic assemblages depending on the type of reef. Namely, vertically-growing oysters in mudflat areas had a higher biomass of epizoic diatoms (hyperspectral proxy at δδ462 nm) and were mainly colonized by species of the genera Navicula, Nitzschia and Hippodonta, which are epipelic or motile epipsammic. The assemblages on the horizontal oysters contained more tychoplanktonic diatoms (e.g. Thalassiosira pseudonana, T. proschkinae and Plagiogrammopsis vanheurckii). Three species of boring cyanobacteria were observed for both types of reef: Mastigocoleus testarum, Leptolyngbya terrebrans, and Hyella caespistosa, but the second derivative analysis at 524 nm showed a significantly higher biomass for the horizontally-growing oysters. There was no biomass difference for the boring chlorophyte assemblages (δδ647 nm), with two species: Eugomontia testarum and Ostreobium quekettii observed for both types of reef. This study shows that oyster shells are an idiosyncratic but ubiquitous habitat for phototrophic assemblages. The contribution of these assemblages in terms of biomass and production to the functioning of coastal areas, and particularly to shellfish ecosystems, remains to be evaluated.
Highlights
Pacific oyster reefs are a growing habitat in temperate coastal areas, spreading in Europe and America, with a polarward expansion [1]
After the retrieval of their continuum, vertical and horizontal oyster reflectance spectra presented a similar shape in the near infrared (Fig 2B), but differences remained in the visible range
This study showed that high diversity of photosynthetic microorganisms was associated with wild oyster shells and that there were differences in the structure of the phototropic assemblages depending on the reef typology
Summary
Pacific oyster reefs are a growing habitat in temperate coastal areas, spreading in Europe and America, with a polarward expansion [1]. As a consequence of global warming, cultivated oysters began to reproduce at higher latitudes, with increasingly successful larval settlement leading to the development of these biogenic reefs [2]. These reefs are mainly known for the clusters of vertically-growing oysters, in soft-bottom environments such as tidal flats where they create three-dimensional hard-substrate structures [3]. Microspatial variations influence the nature of the biota colonizing hard surfaces [5], and oysters shells have long been known to host a large diversity of organisms [6]. Most studies have focused on colonization by metazoa and macrophytes [7,8] and less attention has been paid to microalgae and cyanobacteria
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