Isolation of microsatellite loci and cross‐species amplifications in three gobiid fish of the genus Pomatoschistus

Gobiids of the genus Pomatoschistus are increasingly being investigated as models for adaptation to coastal environments and for mating system studies. Among the dozen currently analysed species, microsatellite primers have been characterized only for Pomatoschistus minutus. This paper describes seven new polymorphic loci isolated from Pomatoschistus marmoratus and Pomatoschistus microps, two species that hybridize. Cross‐species amplification was tested for these new loci, together with seven already published P. minutus loci. Systematic amplification of samples of each of the three species provided a first indication of their polymorphism.     

Interactions between fauna and sediment control the breakdown of plant matter in river sediments

1. A substantial portion of particulate organic matter (POM) is stored in the sediment of rivers and streams. Leaf litter breakdown as an ecosystem process mediated by microorganisms and invertebrates is well documented in surface waters. In contrast, this process and especially the implication for invertebrates in subsurface environments remain poorly studied. 2. In the hyporheic zone, sediment grain size distribution exerts a strong influence on hydrodynamics and habitability for invertebrates. We expected that the influence of shredders on organic matter breakdown in river sediments would be influenced strongly by the physical structure of the interstitial habitat. 3. To test this hypothesis, the influence of gammarids (shredders commonly encountered in the hyporheos) on degradation of buried leaf litter was measured in experimental systems (slow filtration columns). We manipulated the structure of the sedimentary habitat by addition of sand to a gravel‐based sediment column to reproduce three conditions of accessible pore volume. Ten gammarids were introduced in columns together with litter bags containing alder leaves at a depth of 8 cm in sediment. Leaves were collected after 28 days to determine leaf mass loss and associated microbial activity (fungal biomass, bacterial abundance and glucosidase, xylosidase and aminopeptidase activities). 4. As predicted, the consumption of buried leaf litter by shredders was strongly influenced by the sediment structure. Effective porosity of 35% and 25% allowed the access to buried leaf litter for gammarids, whereas a lower porosity (12%) did not. As a consequence, leaf litter breakdown rates in columns with 35% and 25% effective porosity were twice as high as in the 12% condition. Microbial activity was poorly stimulated by gammarids, suggesting a low microbial contribution to leaf mass loss and a direct effect of gammarids through feeding activity. 5. Our results show that breakdown of POM in subsurface waters depends on the accessibility of food patches to shredders.     

Influence of side-arms on aquatic macroinvertebrate drift in the main channel of a large river

1. Drift from lotic and lentic side-arms to the main channel was studied over 2 years in different sections of the Upper Rhône River (France). The hypothesis of an influence of drift on the main channel was tested by measuring drift in the main river at sites downstream and upstream from the confluences with two side-arms, as well as in the two side-arms themselves.
2. Side-arms were differentiated from the main channel by having a typical composition of macrofauna with a high propensity to drift, particularly in spring–summer and during spates. Drift densities in side-arms averaged more than twice those measured in main channel sites, but these inputs did not significantly affect taxon richness and total drift density in the main channel.
3. Nevertheless, some taxa found in the more lotic of the two side-arms were more abundant downstream of the confluence than upstream, indicating that they had drifted into the main channel; no such direct effect was found throughout the sampling period for the more lentic side-arm. The densities of two competitive filter feeders ( Hydropsyche and Simuliidae) were significantly higher below the confluences than above, suggesting that side-arms were also contributing substantial amounts of seston to the main channel.
4. Therefore, side-arms influence the community structure of drifting macroinvertebrates in the Upper Rhône River, both directly through immigration—depending on the degree of connectivity to the river—and indirectly via food supply.     

Ion-Exchange Properties of Isolated Cell Walls of Brown Algae: The Interstitial Solution

Kloareg, B., Demarty, M. and Mabeau, S. 1987. Ion-exchange propertiesof isolated cell walls of brown algae: the interstitial solution.—J.exp. Bot. 38: 1652–1662. Isolated cell walls of Pelvetia canaliculata (Dene) et Thur.,Laminaria digitata (L.) and other intertidal brown algae wereequilibrated in seawater and various mixtures of sodium andcalcium chlorides. After elution with distilled water, the interstitialsolution was assayed for ionic composition. With respect tothe external solution, it was less concentrated and containedlower proportions of divalent cations. Such modifications wererelated to the divalent ionic fraction and the total anion concentrationof the external solution. No significant differences were foundamong the brown algae investigated in spite of the differingcomposition of their walls. Results were analysed accordingto the polyelectrolyte condensation model. When the concentrationof the external solution was high (1000 meq dm³), experimentaldata were qualitatively in good agreement with the predictionsof the model. Key words: Cell walls, ion-exchange, Phaeophyta.