ALBERT

Description: Extracellular enzymes contribute substantially to the remineralisation of organic matter in aquatic systems. Marine invertebrates release endogenous enzymes through activities such as sloppy feeding or egestion, but the significance of such processes is widely unexplored. We compared functional key properties such as activity, stability, and apparent molecular masses of extracellular phosphatases from faeces of the marine crustaceans Idotea balthica, Palaemon varians, and Homarus gammarus with those of their digestive organs. A fluorescent substrate was used to examine enzyme activity qualitatively on agarose plates and quantitatively in microplate assays. Apparent molecular masses and enzyme stability were examined by native substrate gel electrophoresis (NSGE). Active extracellular phosphatase were present in the faeces of all tested species. NSGE activity band patterns were similar in digestive tissue extracts and faeces extracts. The initial enzyme activity retained for about 2 days, but thereafter rapidly decreased. Activity band patterns of digestive organs and faeces of I. balthica remained consistent for up to 72 h. Antibiotic treatment did not reduce phosphatase activity in the faeces of I. balthica and H. gammarus but in the faeces P. varians. Because of the comparatively high activities in the faeces, the electrophoretic similarity between faeces and digestive organs, and the limited durability of the enzymes in faeces extracts, we conclude that the studied crustaceans release predominantly endogenous, rather than bacterial phosphatases through faeces. These can substantially contribute to the pool of active extracellular phosphatases and the recycling of phosphorus in aquatic systems. The wider physiological and ecological context is discussed.

Description: The Asian shore crab Hemigrapsus sanguineus (De Haan, 1853) has recently established populations in the North Sea and now occurs within the native ranges of the green crab Carcinus maenas (Linnaeus, 1758). To determine potential competitive effects and to assess the progress of the invasion, species-specific population characteristics (numerical abundances, biomasses, and size distributions) of the two species around the island of Helgoland (German Bight, southern North Sea) were compared for surveys conducted in 2009 and 2014. Sampling sites were chosen based on accessibility and differed in their topography and wave exposure, which allowed testing for the influence of these factors on the establishment success of H. sanguineus. The numerical abundance and biomass of H. sanguineus increased markedly and approached those of C. maenas in 2014. At a sheltered site, H. sanguineus even outnumbered C. maenas, whereas the converse was observed at a site exposed to strong winds and waves. Although such contrasting abundance patterns between the native and the introduced shore crab may be the result of direct interference, the dominance of H. sanguineus at the sheltered site may also be explained by enhanced larval settling rates caused by odors of conspecifics. The results suggest that the invasion of H. sanguineus has not yet reached its equilibrium, and population abundances in the North Sea are expected to further increase in the future.

Description: The Asian shore crab Hemigrapsus sanguineus (De Haan, 1853) has recently established populations in the North Sea and now occurs within the native ranges of the green crab Carcinus maenas (Linnaeus, 1758). To determine potential competitive effects and to assess the progress of the invasion, species-specific population characteristics (numerical abundances, biomasses, and size distributions) of the two species around the island of Helgoland (German Bight, southern North Sea) were compared for surveys conducted in 2009 and 2014. Sampling sites were chosen based on accessibility and differed in their topography and wave exposure, which allowed testing for the influence of these factors on the establishment success of H. sanguineus. The numerical abundance and biomass of H. sanguineus increased markedly and approached those of C. maenas in 2014. At a sheltered site, H. sanguineus even outnumbered C. maenas, whereas the converse was observed at a site exposed to strong winds and waves. Although such contrasting abundance patterns between the native and the introduced shore crab may be the result of direct interference, the dominance of H. sanguineus at the sheltered site may also be explained by enhanced larval settling rates caused by odors of conspecifics. The results suggest that the invasion of H. sanguineus has not yet reached its equilibrium, and population abundances in the North Sea are expected to further increase in the future.