Distribution, Habitats and Behavior of the Non-Indigenous Brachyuran Percnon gibbesi (Percnidae) of Southwestern Mediterranean (Algeria)

Some ecological aspects of the alien crab Percnon gibbesi (H. Milne Edwards, 1853) have been approached by visual surveys in the southwestern Mediterranean waters (East, Algeria). This sampling method, applied on a coastal line of 16.2 km and between 0 and − 3 m depth, allowed us to collect information on abundances and densities by size category, nature of the habitat and behavior of this species. Overall 22 diurnal dives (3 observations/dive) were carried out in 2 different adjacent costal sectors, the Bay of Annaba (Sector 1: Lever de l’Aurore-Amphore), located at the east, and Cap de Garde (Sector 2: Cap de Garde - Djenen El Bey beach), located at the west of the Bay. We recorded an average total of 5325 individuals (± 15.62) with numerical dominance in sector 2 (4472 ± 9 ind.). Medium-sized individuals dominated numerically in sector 1 (412 ± 10 ind.), while large individuals were better represented in sector 2 (1980 ± 12.77 ind.). Average densities in two sectors varied to according on measurement unit (surface, dive time). Whatever the sector, the rock cracks sheltered the greatest number of individuals. No individual was observed on homogeneous bottoms. Overall, crabs are indifferent in the presence of an observer.