Hydrobia

Abstract: In the large archipelago area of the northern Baltic Sea, increasing occurrences of drifting benthic macroalgae have been recorded in the subtidal zone. Their role as a structuring factor on the zoobenthic community has been altered from inducing occasional small-scale disturbances to inducing large-scale mortality of macrobenthic populations. A controlled field experiment was conducted on a sandy bottom in order to test for temporal responses of benthlc invertebrate populations to severe stress imposed by the algal mats. Algae corresponding to amounts recorded in the field were enclosed In netbags and attached to the bottom. Population abundance of the zoobenthic species under algae were compared w ~ t h control plots for 5 ivk with weekly sanipl~ng. Mass~ve die-offs of benthic populatlons were recorded in both the experiment and under natural occurrences of drift algal mats. The community dominants in abundance (mudsnails Hydrobia spp.) and biomass (bivalve Macoma balthica) exhibited strong population reductions after 9 d of algal stress. Within 5 wk, populat~on crashes were recorded for the sedentary po1ychaett.s Manayunkia aestuanna and Pygosplo elegans, ivh~le populations of the errant polychaete Nereis diverslcolor and tubificid oligochaetes remained stable under the algal mats. Initial short-term recovery after the algae were removed was rapid and do~nlnated by Hydrobia spp. The strong negative effects on key species such as M. balthica may have severe effects on entire food-web dynamics in Baltic coastal ecosystems. KEY MIORDSMacrofauna . Drifting macroalgae Disturbance . Baltic Sea . Macoma balthica . Hydrobia spp

Abstract: Infraspecific relationships and population parameters are largely unknown in the ecologically significant mud snail genus Hydrobia s.I. We therefore studied infraspecific variation, population structure and gene flow in two Hydrobia species with different life history strategies: the marine, planktonic H. ulvae and the brackish-water, directly developing H. ventrosa. Based on sequencing data of a 638 bp fragment of the mtDNA gene for cytochrome oxidase 1, we found considerable differences between the two species. H. ulvae shows high average pairwisc nucleotide diversity, low population level differentiation (Fst). AND high average gene flow (Nm) between populations. Dispersal appears to accord with Wright's island model. In contrast, many populations of H. ventrosa have high population level differentiation and low gene flow. The average pairwise nucleotide diversity is relatively low; this species disperses according to Wright's isolation by distance model. Differences in dispersal modes and gene flow could be partly due to differences in type of early ontogeny and quantitative differences in passive dispersal. However, the fact that H. ulvae is a marine species with high tolerance to environmental stress and therefore less sensitive to migration barriers than H. ventrosa may better explain these differences. The extant lineages of H. ulvae and H. ventrosa most likely evolved in the northeastern Atlantic during the Pleistocene.

Abstract: The macrobenthic fauna community of a 70-km 2 Zostera noltii seagrass bed (Arcachon bay, France) was studied by sampling 49 stations systematically. A total of 126 taxa were identified. Cluster Analysis based on χ 2 distance showed that in this apparently homogeneous habitat, four distinct macrobenthic communities could be identified. Multiple Discriminant Analysis highlighted the major contribution of the overlying water mass as a forcing variable, and, to a lesser extent, of tidal level and Z. noltii 's below-ground parts. Seven stations did not constitute any conspicuous group, and were characterized by a low biomass of leaf ( −2 ), considered as the lowest value to constitute a Z. noltii community. Less than 24% of the seagrass bed was situated in more oceanic waters and at a quite low tidal level. In this relatively stable environment, the macrofauna community was characterized by a high species richness (mean = 39) and a moderate density and high biomass (12 638 individuals m −2 and 25 g AFDW m −2 , respectively). Annelids dominated, particularly the oligochaetes. When physical constraints increased (emersion or brackish water conditions), diversity decreased, abundance and biomass increased. The seagrass bed (55%) was flooded with highly fluctuating waters in term of temperature and salinity, here species richness was low (mean = 27) but abundance and biomass were high (24 384 individuals m −2 and 28 g AFDW m −2 , respectively), with a dominance of molluscs. The meadow (7%) was in external waters but at a higher tidal level (2.4 m vs 1.8 m above medium low tide level). This community was characterized by the particularly high density (41 826 individuals m −2 ) and dominance of oligochaetes (79% of total abundance). Species richness was high (mean = 37) here. A fourth community, extending over 12% of the meadow was dominated by the gastropod Hydrobia ulvae but could not be linked to a specific forcing variable. This study confirmed the almost complete replacement of the native clam Ruditapes decussatus by the Manila clam Ruditapes philippinarum since its introduction in the 1980s.