INSHORE AND OFFSHORE DIEL MIGRATIONS IN EUROPEAN BENTHOPELAGIC MYSIDS, GENERA GASTROSACCUS, ANCHIALINA AND HAPLOSTYLUS (CRUSTACEA, MYSIDACEA)

The nocturnal migratory activity of the six most abundant Gastrosaccin ae living off the European coast, namely Gastrosaccus sanctus, Gastros accus spinifer, Anchialina agilis, Haplostylus lobatus, Haplostylus lo batus var. armata and Haplostylus normani, was studied. Both Gastrosac cus species perform horizontal and vertical migrations near sandy beac hes. These migrations involve the whole population, which exhibits a s uperficial or hyponeustonic distribution in the water column. Anchiali na and Haplostylus perform vertical migrations from inshore to slope w aters. The timing of pelagic phases, as well as the distribution in th e water column, depend on age, sex, depth and clarity. Male A. agilis and H. lobatus show intense migratory activity leading to a hyponeusto nic distribution in mid-shelf and slope waters a few hours after dusk. Females and juveniles show continuous pelagic phases and a uniform di stribution from dusk to dawn. Haplostylus lobatus var. armata and H. n ormani populations exhibit the same behaviour, consisting of overnight migrations and a uniform to deep nocturnal distribution. In shallow w aters, male Anchialina and Haplostylus no longer exhibit hyponeustonic behaviour and their nocturnal distribution becomes more and more deep as clarity increases. Above the slope, animals that migrate from the continental shelf during the night sometimes adopt permanent pelagic b ehaviour above more than 200 or 300 m in depth, but upward nocturnal m igration persists and leads to superficial stratification, albeit dela yed. Slow morning descents to the bottom can occur even further from t he 500 m isobath.