Composition and diel vertical distribution of euphausiid larvae (calyptopis stage) in the deep southern Adriatic

Summary Diel changes in the vertical distribution and abundance of calyptopes were studied in the deepest area of the southern Adriatic over four seasons (July 2003, February 2004, October 2004, April 2009). Temperature variations were limited to the upper 100 m and salinity variations were small. Of previously known adult euphausiid species – 12 for the Adriatic ( Gangai et al., 2012 ) and 13 for the Mediterranean ( Mavidis et al., 2005 ) – calyptopes of 11 species of euphausiids were recorded. Abundance of calyptopes of all species was the highest in spring. Species were characterized according to their mean depth: surface (0–50 m), sub-surface (50–200 m), mesopelagic (200–800 m), or bathypelagic (800–1200 m) and vertical dispersion (scattered or non-scattered). Four diel patterns emerged: (i) nocturnal ascent to upper layers ( Euphausia brevis , E. hemigibba , E. krohnii , Nematoscelis megalops , N. couchii ), (ii) migration to upper layers at middle of the day and at night, and descent during the morning and evening ( Stylocheiron maximum – only winter), (iii) weakly-migrating or non-migrating ( S. longicorne ), (iv) irregular migration independent of the day/night cycle ( S. abbreviatum , S. maximum – during spring, summer and autumn, T. aequalis ).