Bryoliths constructed by bryozoans in symbiotic associations with hermit crabs, Golfe d'Arguin, Mauritania

The Golfe d'Arguin offshore of northern Mauritania hosts a rare modern analogue for heterozoan carbonate production in a tropical marine setting. Dominated by ocean upwelling and with additional fertilisation by iron-rich aeolian dust, this naturally eutrophic marine environment lacks typical photozoan communities. A highly productive, tropical cosmopolitan biota dominated by molluscs and suspension-feeders such as bryozoans and balanids characterises the carbonate-rich surface sediments. Overall biodiversity is relatively low and the species present are tolerant against the eutrophic and low-light conditions, the strong hydrodynamic regime governed by ocean upwelling, and the unstable, soft-bottom seafloor with few hard substrata. Here, we describe an ectosymbiosis between the hermit crab Pseudopagurus granulimanus (Miers, 1881) and monospecific assemblages of the encrusting cheilostome bryozoan Acanthodesia commensale (Kirkpatrick and Metzelaar, 1922) that cohabits vacant gastropod shells. Nucleating on an empty gastropod shell, the bryozoan colonies form multilamellar skeletal crusts that produce spherical encrustations and extend the living chamber of the hermit crab through helicospiral tubular growth. This non-obligate mutualistic symbiosis illustrates the adaptive capabilities and benefits from a close partnership in a complex marine environment, driven by trophic conditions, high water energies and instable substratum. Sectioned bryoliths show that between 49 and 97 % of the solid volume of the specimens consists of bryozoan skeleton.

Initial vegetation, and spore density, diversity and evenness of different arbuscular mycorrhizal fungi morphotaxa 29,36 and 39 months after experimental vegetation removal, central Argentina

1. Dominant plant functional types (PFTs) are expected to be primary determinants of communities of other above- and below-ground organisms. Here, we report the effects of the experimental removal of different PFTs on arbuscular mycorrhizal fungi (AMF) communities in a shrubland ecosystem in central Argentina. 2. On the basis of the biomass-ratio hypothesis and plant resource use strategy theory, we expected the effect of removal of PFTs on AMF colonization and spores to be proportional to the biomass removed and to be stronger when more conservative PFTs were removed. The treatments applied were: undisturbed control (no plant removed), disturbed control (mechanical disturbance), no shrub (removal of deciduous shrubs), no perennial forb (removal of perennial forbs), no graminoid (removal of graminoids) and no annual forb (removal of annual forbs). AMF colonization was assessed after 5,17 and 29 months. Total density of AMF spores, richness and evenness of morpho-taxa, and AMF functional groups were quantified after 5,17,29,36 and 39 months. 3. Five months after the initial removal we found a significant reduction in total AMF colonization in all plots subjected to PFT removals and in the disturbed control plots, as compared with the undisturbed controls. This effect disappeared afterwards and no subsequent effect on total colonization and colonization by arbuscules was observed. In contrast, a significant increase in colonization by vesicles was observed in months 17 and 29, mainly in no graminoid plots. In general, treatments did not significantly affect AMF spores in the soil. On the other hand, no annual forb promoted transient (12-18 months) higher ammonia availability, and no shrub promoted lower nitrate availability in the longer term (24-28 months). 4. Synthesis. Our experiment, the first to investigate the effects of the removal of different PFTs on AMF communities in natural ecosystems, indicates that AMF communities are resilient to changes in the soil and in the functional composition of vegetation. Furthermore, it does not provide consistent evidence in support of the biomass-ratio hypothesis or differential trait-based direct or indirect effects of different PFTs on AMF in this particular system.