Sandy bottoms, with local patches of rocks, dominate the southern Baltic Sea coast. These rock patches create three-dimensional habitats with additional niches that can support diverse assemblages of organisms. In this study we investigated macrofaunal assemblages associated with the boulder field in the brackish Gulf of Gdansk. Of the 30 recorded taxa three animal species (Mytilus trossulus, Balanus improvisus and Electra crustulenta) together with five species of algae were directly attached to rocks. These engineering organisms provided habitats for a further 22 taxa. Among the fauna directly associated with rocks, barnacles (76%) were the most abundant while among indirectly associated biota, oligochaetes were the dominant group (45%). Rock size and algal biomass explained most variance in macrofaunal structure of the assemblages investigated. There were statistical differences in assemblage structure between two separate localities within the rocky patch, despite environmental homogeneity (salinity, water temperature and structure of sea bottom). These differences in assemblage structure were mostly due to differences in dominance of particular species rather than in species composition. Our results show that rocky patches in an otherwise soft sediment habitat provide additional living space for macrofauna leading to an increase in local biodiversity and organismal abundance.

The genus Gerbillus (Mammalia: Rodentia) is characterized by an important morphological homogeneity associated with an extensive karyotypic diversity. Here we describe a new species of Gerbillus from the Inner Delta of the Niger River in Mali, on the basis of morphological and chromosomal data. This new species seems to be morphologically very similar to G. campestris, another bare-footed species known from northern Africa. Nevertheless, it can be distinguished from the latter species by a different molar pattern and a specific karyotype. The three specimens on which this description is based were caught in a rocky outcrop, and behavioural observations in captivity suggest an adaptation to life in rocky habitats. This finding is unusual as rodents of the genus Gerbillus are generally sand-dwellers. Moreover, this new species enriches the gerbil community of this Sahelian region, in which at least nine species of the genus can be found. Finally, this description highlights the importance of chromosomal studies for the characterization of biological diversity in tropical rodents, where morphological sibling species are numerous.