Changes in vertical location in the water column of meso- and bathypelagic species through development: Behaviour vs hydrography

Abstract

The species composition, distribution and abundance of meso- and bathypelagic species from oceanic waters of the NE Atlantic have been analysed to investigate the modifications in vertical distribution patterns from larvae to adult stages. Plankton and micronekton samples from surface to 1800 m depth, were taken in May 2018 in 8 stations placed in a transect from 20°N to 55°N. The transect crossed intense productivity, thermal and salinity gradients, from the North Atlantic Subtropical Gyral Province (NASGP) to the north Atlantic Artic Province (NAAP). Hydrographical boundaries were evident in the species composition and abundance, with lower diversity in the northernmost station in the NAAP. Larvae and transforming stages of myctophiforms and stomiiforms dominated all over the region. Although the Brunt-Väisälä frequency indicates higher water column stability in the NASGP stations than in the north, vertical distributions were similar along the region. Myctophid larvae were always concentrated in the first 100 m, irrespective of the time of the day. Their transforming stages occurred from surface to 1200 m at night (with no clear migration), and from 400-700 during the day. Stomiiform larvae appeared in the upper 200 m, while transforming showed a wide distribution from surface to 1000 m both day and night. The distribution of the gonostomatid Cyctothone (7 species) is an interesting case of study with larvae always in the upper 100 m, and non-migrating juvenile and adults located in the mesopelagic zone (even reaching the bathypelagic layers in the case of C. microdon). As in previous investigations, it was globally observed that the deepening in the water column is an ontogenetic process related with development that starts when ventral groups of photophores become formed. The wider vertical distribution of this transition stages compared to both larva and adult indicates that they have not developed yet the vertical displacement characteristic of adult stages