Jassa herdmani (Walker 1893), a tube-building amphipod typical of hard substrates, was found in large densities on shipwrecks from the Belgian part of the North Sea, in association with the hydrozoan Tubularia indivisa. In this area, shipwrecks only represent the source of hard substrates in an environment dominated by soft sediments. Nevertheless, the long-distance dispersal of Jassa species has never been investigated. Therefore, we tested the hypothesis of dispersal with currents, by investigating the behaviour of J. herdmani in the laboratory. Size distribution revealed that newly released juveniles (1 mm) predominated throughout the year and the lower frequency of the size class 1-3 mm indicates a high mortality or a dispersal at this life stage. Individuals of J. herdmani may initiate actively the transport by tidal or surface currents by swimming to the surface of the water or by floating at the surface, as suggested by behaviours noted in the laboratory. The reaction of the amphipods to a current was investigated in the laboratory. We could not detect any sexual function associated with drifting and there was also no preponderance of a size class in the drifting individuals. When testing different substrates, we observed a significant influence of the substrate type on the frequency at which drifting occurred: J. herdmani showed a better adherence on T. indivisa compared to other substrates. Finally, the amphipod showed also a preference for its host compared to other substrates, which suggests a possible detection mechanism. © 2007 Springer-Verlag.
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Placer extraction in Puck Bay, a shallow (3 m depth) area of the Baltic Sea, resulted in the formation of post-dredging pits. Such dredging activities led to a considerable local disturbance of the soft-bottom communities. Topography and sedimentary characteristics of the disturbed area have been radically changed. It is therefore a matter of concern as to whether these alterations to the environment resulted in any serious permanent changes to the biological communities in the affected areas. Benthic copepod assemblages were examined 10 years after termination of placer digging in one of the post-dredging pits and compared with the fauna in a natural depression (Kuźnicka Hollow) and on the shallow sandy bottom surrounding the pits. Samples were collected on four occasions in 2001–2003. This study has generated ecological information on the status of harpacticoid species inhabiting the dredged and undredged areas in the vicinity. Analyses of data showed that the sampling stations differed significantly, during all the sampling events, in harpacticoid abundance, taxonomic composition and Shannon–Wiener diversity index. The natural depression and the shallow sandy bottom of Puck Bay were found to support specific harpacticoid assemblages. Interstitial and sand-burrowing species (e.g. Paraleptastacus spinicauda) dominate the shallow sandy bottom, and the Kuźnicka Hollow is inhabited mainly by epibenthic and silt-burrowing species (e.g. Halectinosoma curticorne, Tachidius discipes, Microarthridion littorale). The post-dredging pit assemblage showed the presence of epibenthic species, e.g. Tachidius discipes, Dactylopusia euryhalina, and Stenhelia palustris, passively transported into the pit by wave action and currents, possibly with algal mats and/or plant remains. The taxonomic composition and occurrence of harpacticoid species in the post-dredging area is therefore random and accidental.
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