Ontogenetic changes in vertical migratory rhythms of benthopelagic shrimps Pasiphaea multidentata and P. sivado

8 pages, 3 figures, 1 tableVertical 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 bottomThe authors thank Dr. F. Sardà, Chief Investigator of the project NERIT (MAR98-0935) funded by the Spanish CICYT and the crew of the RV ‘García del Cid’ (CSIC) for their support during sampling. Special thanks also to B. Molí and A. Castellón for their valuable help during sampling operations. J. Aguzzi is a Postdoctoral Fellow on the Juan de la Cierva ProgramPeer reviewe