The rocky intertidal zone of sheltered shores in Breiðafjörður, north-west Iceland is dominated by monospecific stands of canopy-forming brown algae Ascophyllum nodosum, which provide habitat for mobile organisms and has been subjected to long-standing rotational harvesting. We investigated the assemblage composition of little-studied mobile brachyuran crabs in this area, to track distributional shifts in a native species responding to climate change and extent of occupancy of the intertidal by a primarily subtidal invasive non-native species. Potential interactive effects of seaweed harvesting were explored. Breiðafjörður was compared with two reference sites in Faxaflói, south-west Iceland. The study revealed clear poleward expansion of the native European green crab Carcinus maenas in the region, displacing the native spider crab Hyas araneus particularly at mid-shore levels. The invasive non-native Atlantic rock crab Cancer irroratus had negligible occupancy in the intertidal zone, indicating limited effects on the intertidal crab assemblage, composition, and abundance. The current harvesting regime of A. nodosum in Breiðafjörður did not affect the composition and abundance of the brachyuran crab assemblage in the rocky intertidal zone. H. araneus is likely being squeezed by displacement subtidally by C. irroratus, and intertidally by C. maenas. Overall, we provide insights into the potential interactions between climate change, invasive species, and human activities in the rocky intertidal zone.

Landmark-based geometric morphometric methods were used to test which chela is most suitable to use in population differentiation of brachyuran crabs. Two species were analysed: Pachygrapsus marmoratus, a homochelous species, and Carcinus maenas, a heterochelous species. Variance analysis of shape variables and of each chela was calculated to determine which claw presents less variation within populations. The results showed that the right chela of P. marmoratus is more appropriate to use when the goal is to differentiate populations. In C. maenas, the males major chela and females right chela are more suitable to use, probably due to its heterochely. To test these results, specimens from different geographical locations were compared. Pachygrapsus marmoratus populations presented significant differences in manus shape, and C. maenas populations had differences in polex size and shape. Differences in carapace shape were also encountered between populations of both species. This phenotypic variation was not corroborated with genetic data, which were obtained using the mitochondrial DNA gene cytochrome oxidase I as a marker. These opposite results suggest that environmental factors, such as diet and parasitism, can be determining shape differentiation.

Baseline ecological studies of pristine mangroves are important for monitoring, management and conservation of mangrove ecosystems. Diversity, density, biomass and community structure of crab and molluscan macrofauna were studied in a near-pristine mangrove forest in Sematan, Sarawak. In each of the nine 100-m2 plots, six 15-min catching periods were used to collect crabs and three 1-m2 quadrats were sampled for molluscan epifauna. Environmental variables and vegetation were also recorded in each 100-m2 plot. Thirty-one crab species and 44 molluscan species were recorded, including a few species not previously reported from Sarawak. Crab community structure was correlated with topographical height and surface water pH and salinity. Gastropod community structure was correlated with redox potential of water at depth, topographical height, surface water pH and leaf litter. Crab community structure and species number were positively correlated with tree and seedling community structure and diversity, suggesting that the mangrove vegetation is important to the crab fauna as a habitat and food supply. Molluscan abundance was positively correlated with sapling diversity and negatively correlated with the numbers of mangrove associates and tree species. The young leaves on mangrove saplings may provide a better food source than those on mature trees or mangrove associates. The data from this study provide a valuable baseline for future use at this site and for comparison with more degraded mangrove habitats elsewhere in South-East Asia.