Inshore and offshore diel migrations in European benthopelagic mysids, genera <i>Gastrosaccus</i>, <i>Anchialina</i> and <i>Haplostylus</i> (Crustacea, Mysidacea)

Abstract

The nocturnal migratory activity of the six most abundant Gastrosaccinae living off the European coast, namely Gastrosaccus sanctus, Gastrosaccus spinifrr, Anchialina agilis, Haplostylus lobatus, Haplostylus lobatus var. armata and Haplostylus norrnani. was studied. Both Gastrosaccus perform horizontal and vertical migrations near sandy beaches. These migrations involve the whole population, which exhibits a superficial or hyponeustonic distribution in the water column. Anchialina and Haplostylus perform vertical migrations from inshore to slope waters. The timing of pelagic phases, as well as the distribution in the water column, depend on age. sex, depth and clarity. Male A.agilis and H.lobatus show intense migratory activity leading to a hyponeustonic distribution in mid-shelf and slope waters a few hours after dusk. Females and juveniles show continuous pelagic phases and a uniform distribution from dusk to dawn. Haplosiylus lobatus var. armata and H.normani populations exhibit the same behaviour, consisting of overnight migrations and a uniform to deep nocturnal distnbution. In shallow waters, 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 behaviour above more than 200 or 300 m in depth. but upward nocturnal migration persists and leads to superficial stratification, albeit delayed. Slow morning descents to the bottom can occur even further from the 500 m isobath.