MEPS 335:167-174 (2007)  -  doi:10.3354/meps335167

ABSTRACT: Vertical migratory rhythms of the western Mediterranean congeneric benthopelagic glass shrimps Pasiphaea sivado and P. multidentata were studied at depths of 400 to 430 m in relation to light intensity cycles (i.e. light regimes) in 2 different seasons: close to the autumn equinox (October) and to the summer solstice (June). Catches were considered a proxy of animal density close to the bottom and sampling was performed with consecutive epibenthic trawl hauls. These were continuously repeated over 4 d in order to determine the frequency and timing of species presence on the bottom. The density of animals present on the seabed at the time of the haul was estimated. In both P. sivado and P. multidentata marked diel differences in density were found which were correlated to variability in diel light intensity. While P. sivado was mostly captured at daytime in both seasons, regardless of size, P. multidentata only showed this pattern in the case of juveniles, i.e. individuals with carapace length <30 mm. Adult P. multidentata were mainly present on the bottom during the night, thus showing an apparent phase inversion in their movement patterns. This modification of diel behaviour can be interpreted as the result of differential habitat use with size/age. The comparison of present results with previous field studies illustrates that while P. sivado and juvenile P. multidentata perform actual vertical migrations, adults of the latter species undertake epibenthic movements along the bottom.

KEY WORDS: Pasiphaea sivado · Pasiphaea multidentata · Ontogeny · Vertical migratory rhythms · Continental slope · Light intensity · Photoperiod length · Behavioural activity