Wanted dead or alive : high diversity of macroinvertebrates associated with living and ’dead’ Posidonia oceanica matte

The Mediterranean endemic seagrass Posidonia oceanica forms beds characterised by a dense leaf canopy and a thick root-rhizome ‘matte’. Death of P. oceanica shoots leads to exposure of the underlying matte, which can persist for many years, and is termed ‘dead’ matte. Traditionally, dead matte has been regarded as a degraded habitat. To test whether this assumption was true, the motile macroinvertebrates of adjacent living (with shoots) and dead (without shoots) matte of P. oceanica were sampled in four different plots located at the same depth (5–6 m) in Mellieha Bay, Malta (central Mediterranean). The total number of species and abundance were significantly higher (ANOVA; P<0.05 and P<0.01, respectively) in the dead matte than in living P. oceanica matte, despite the presence of the foliar canopy in the latter. Multivariate analysis (MDS) clearly showed two main groups of assemblages, corresponding to the two matte types. The amphipods Leptocheirus guttatus and Maera grossimana, and the polychaete Nereis rava contributed most to the dissimilarity between the two different matte types. Several unique properties of the dead matte contributing to the unexpected higher number of species and abundance of motile macroinvertebrates associated with this habitat are discussed. The findings have important implications for the conservation of bare P. oceanica matte, which has been generally viewed as a habitat of low ecological value.peer-reviewe

Chemical changes in detrital matter upon digestive processes in a sesarmid crab feeding on mangrove leaf litter

Pathways and rates of decomposition of detrital matter partly depend on its chemical composition. Digestive processes of detritivores drive changes in the chemical composition of detritus, and these changes translate into the chemical composition of the organic matter sequestered into soils and sediments. The latter, in turn, determines how stable organic matter stocks are towards further decay and release of climate-active gases thereupon. We used metabolic fingerprinting to monitor changes in the chemical composition of mangrove detritus upon digestion by a mangrove crab. According to analyses through pyrolysis-GC/MS, the decaying leaf litter of three mangrove species of the Indo-West Pacific, Bruguiera gymnorhiza (L.) Savigny ex Lam. and Poiret 1798, Ceriops tagal (Perr.) C.B. Robinson 1908, and Rhizophora mucronata Lam. 1804, clearly differed from each other in their chemical signature. The feces of detritivorous crabs (Sesarma bidens de Haan 1835) feeding on these detrital sources differed from the source litter in their chemical composition, obviously owing to digestive processes. However, the chemical signatures of feces were more similar to those of their source litter than to those of feces from different litter sources, indicating that the origin of organic matter can be tracked in fecal material. Moreover, male and female crabs appear to exhibit sex-specific digestive processes, as they produced feces that clearly differed from each other in their chemical signature. The 15 chemical compounds most relevant for distinguishing litter sources and fecal material provide first hints on which compounds discriminate the different tree species and characterize digestion by S. bidens. For instance, coumaran (dihydro-benzofuran), indicative of certain carbohydrates, was abundant as a pyrolysis product of the litter of R. mucronata and, to a much lesser degree, C. tagal. Hence, the carbohydrates that were pyrolysed into coumaran seem to discriminate the former two litter sources. Similarly, a pyrolysis-derivate of plant phenolics or proteins, discriminated C. tagal from the other litter sources. From this, we conclude that even subtle differences in litter chemistry and digestive processes of detritivores can be characterized and followed with high resolution through (py-)GC/MS. Further, we propose that the origin of fecal material can be identified with the aid of this technique, and we are currently studying whether the origin of organic matter in the sediment can also be inferred from (py-)GC/MS-data.info:eu-repo/semantics/publishedVersio