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POPULATION STRUCTURE AND DIEL VERTICAL MIGRATION (DVM) OF EUPHAUSIIDS LARVAE IN THE OPEN SOUTHERN ADRIATIC SEA OVER A SHORT TIME PERIOD (JULY 2003)

The present work reports the first data of euphausiids larvae species composition, abundance and diel vertical migration (DVM) in the Adriatic Sea. The composition, bathymetric distribution, and diel vertical migration of larval euphausiids in the oligotrophic southern Adriatic Sea was studied during summer 2003. The investigation was carried out at a fixed station in the oligotrophic southern Adriatic Sea (1200 m depth). We used short-term repetitive sampling over 96 h at morning, midday, evening and night. Eleven euphausiids larvae were identified. The findings showed first time appearance of Thysanoessa gregaria in the Adriatic. Higher frequency of occurrence and abundance were noted for: Euphausia krohnii, Stylocheiron abbreviatum, Stylocheiron longicorne, Stylocheiron maximum and Nematoscelis megalops larvae. S. longicorne furciliae predominated with the maximum of 400 ind. 100 m-³. Several larvae were observed over a wide depth range: N. megalops and S. longicorne (0 - 1200 m), S. abbreviatum (200 - 1200 m), and E. krohnii (200 - 1000 m). Three migration patterns were noted: (i) nocturnal ascent to upper layers (E. krohnii, N. megalops, S. abbreviatum) ; (ii) scattered population through the water column (S. maximum) ; (iii) migration to upper layers at midday and night, and descent during the morning and evening (S. longicorne). Different stages of the same species had different preferences for light intensity.

euphausiids; larvae; diel vertical migration; Adriatic; Mediterranean.

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