Martin Desmalades

Reproduction, recruitment and larval metamorphosis in the serpulid polychaete Ditrupa arietina (O.F. Müller)

Abstract Reproduction, larval development and settlement were investigated in the serpulid polychaete Ditrupa arietina. Sexes are strictly separated. Sperm shows a spherical head, characteristic of broadcast spawning species, and the average number of oocytes per female before spawning is close to 9000. The survey of the presence of oocytes in the coelomic cavity indicates that the breeding period in D. arietina from the bay of Banyuls-sur-Mer lasts from November to June. The monitoring of new recruits within the sediment indicates that recruitment begins as early as January, peaks in April–May, and ends in July. The fecundity as well as the spawning frequency per female depends on body size. The planktonic life is close to 3 weeks and completion of metamorphosis occurs very rapidly after larval settlement. Competent larvae do not show any selective behaviour relative to sediment particle size. This confirms that distribution of the species within the bay, spatial heterogeneity in the structure and in the density of adult populations mainly results from post-settlement processes. The potential effect of some environmental factors (such as food availability or meteorological conditions) on the first benthic stages is discussed briefly.

Spatial and Temporal Variations in Stable Carbon (δ13C) and Nitrogen (δ15N) Isotopic Composition of Symbiotic Scleractinian Corals

Tropical scleractinian corals are considered autotrophic as they rely mainly on photosynthesis-derived nutrients transferred from their photosymbionts. Corals are also able to capture and ingest suspended particulate organic matter, so heterotrophy can be an important supplementary trophic pathway to optimize coral fitness. The aim of this in situ study was to elucidate the trophic status of 10 coral species under contrasted environmental conditions in a French Polynesian lagoon. Carbon (δ13C) and nitrogen (δ15N) isotopic compositions of coral host tissues and photosymbionts were determined at 3 different fringing reefs during wet and dry seasons. Our results highlighted spatial variability in stable isotopic compositions of both coral host tissues and photosymbionts. Samples from the site with higher level of suspended particulate matter were 13C-depleted and 15N-enriched relative to corals and photosymbionts from less turbid sites. However, differences in both δ13C and δ15N between coral host tissues and their photosymbionts (Δhost-photosymbionts 13C and Δhost-photosymbionts 15N) were small (0.27 ± 0.76‰ and 1.40 ± 0.90‰, respectively) and similar at all sites, thus indicating no general increases in the heterotrophic pathway. Depleted δ13C and enriched δ15N values of coral host tissues measured at the most turbid site were explained by changes in isotopic composition of the inorganic nutrients taken up by photosymbionts and also by changes in rate of isotopic fractionation with environmental conditions. Our results also highlighted a lack of significant temporal variations in δ13C and δ15N values of coral host and photosymbiont tissues and in Δhost-photosymbionts 13C and Δhost-photosymbionts 15N values. This temporal stability indicated that corals remained principally autotrophic even during the wet season when photosymbiont densities were lower and the concentrations of phytoplankton were higher. Increased coral heterotrophy with higher food availability thus appears to be species-specific.

Semi-automatic analysis and interpretation of sediment profile images

Sediment Profile Images (SPIs) are widely used for benthic ecological quality assessment under various environmental stressors. The processing of the information contained in SPIs is slow and its interpretation is largely operator dependent. We report here on a new software: SpiArcBase, which allows for a semi-automatic analysis of SPIs and facilitates the interpretation of observed features. SpiArcBase enhances the objectivity of the information extracted from SPIs, especially for the assessment of the apparent Redox Potential Discontinuity (aRPD). This new software also allows the user to create and manage a database containing original SPIs and corresponding derived pieces of information. Examples of the use of SpiArcBase for SPIs collected during a case study carried out within the Rhone River Prodelta are provided. Correlations between: (1) visually and automatically assessed aRPD and Benthic Habitat Quality Index (BHQ), and (2) automatically assessed aRPD and BHQ and surface sediment organic carbon support the use of this new software. SpiArcBase is the software that allows for a semi-automatic analysis of SPIs.SpiArcBase facilitates the interpretation of observed features via a database.Sediment-water interfaces and some biogenic structures can be automatically drawn.We present a new based on knowledge procedure for the drawing of aRPD.SpiArcBase enhances the objectivity of the information extracted from SPIs.

Can UV radiation affect benthic deposit-feeders through biochemical alteration of food resources? An experimental study with juveniles of the benthic polychaete Eupolymnia nebulosa.

The growth, tentacle development and feeding activity of the benthic polychaete Eupolymnia nebulosa were examined to determine whether UV might affect marine deposit-feeders indirectly through the modification of the nutritional quality of their resources. Since marine invertebrates have higher nutritional requirements during the period following settlement, we tested the effect of UV-altered phytodetritus on freshly settled juveniles of E. nebulosa. Phytodetritus was prepared from cultures of the diatom Skeletonema costatum either grown under or sheltered from UVB radiation. Sterol content of phytodetritus was unmodified by UV radiation. Conversely, phytodetritus was noticeably depleted in polyunsaturated fatty acids. Growth and tentacle development of juveniles fed on altered phytodetritus were reduced by 35% and 15% respectively, suggesting potential deficiencies in essential nutrients. In response to the lower quality of the phytodetritus, juveniles explored a wider area as they search for food, a strategy that could compensate for low food quality.