Vertical and seasonal distribution of pelagic decapod crustaceans on the shelf-break and slope in two contrasting areas around Mallorca (western Mediterranean Sea)

Abstract

The pelagic decapod crustacean fauna of two different zones (Sóller and Cabrera) characterized by different hydrographic dynamics and oligotrophy levels was studied around the island of Mallorca, in the western Mediterranean. Samples were taken with a pelagic nekton trawl net (PT) and an IKMT net. A total of 14 species were present in the samples taken: five species of dendrobranchiate shrimps, eight caridean shrimps and one scyllarid lobster (postlarval stage). Some species were restricted to the continental shelf-break: Chlorotocus crassicornis and Plesionika heterocarpus. Others were exclusive of deeper waters over the continental slope, such as Gennadas elegans, Pasiphaea multidentata, and Sergia robusta. Pasiphaea sivado was widely occurring in all pelagic strata. Cluster and MDS analyses showed the occurrence of several distinct assemblages related to the bathymetry of the area sampled (shelf-break and slope), as well as to the sampling depth (mainly near-surface epipelagic waters and deep scattering layers, DSL). The two studied zones showed a similar faunistic diversity, with the slope DSL showing the highest species diversity. No significant differences concerning sampling area or seasonality were found. No decapod crustacean species occurred in epipelagic daytime samples. The present study showed that the decapod pelagic community sampled in the Balearic Sea was mainly structured by both the geomorphology (and associated hydrographic characteristics of the shelf/slope transition) and the influence of light in the water column. Size analysis showed the occurrence of species-specific patterns concerning the size/age movements into the water column throughout the day-night cycle. Moreover, the present work reveals for the fist time the presence of adult of P. multidentata in the water column evidencing that the adult fraction of the population also has the ability to perform vertical migrations