Feeding habits of pandalid shrimps in the Alboran Sea (SW Mediterranean): influence of biological and environmental factors

The feeding habits of 4 species of Plesionika inhabiting the Alboran Sea (SW Mediterranean) at depths between 29 and 790 m were analyzed. Included within the trophic guild of nonmigratory macroplankton feeders, Plesionika spp. preferably exploited mesopelagic resources (mainly euphausiids and Myctophidae). Deep-bathyal species (P. martia, P. gigliolii and P. edwardsi) fitted better within this guild, while the shallower living species P. heterocarpus, which inhabits the shelf-slope break, consumed benthic organisms (e.g. polychaetes, bivalves and tanaids). In a multi-species MDS analysis, diets of P. martia and P. heterocarpus were grouped as a function of depth. Latitude, phytoplankton pigment (PP) concentration recorded 4 wk before the sampling date (PP4) and depth were the explanatory variables of stomach fullness of Plesionika (all species: R-2 = 0.674 in a multiple linear regression model, MLR). The abundance of P. martia also showed a positive significant correlation (Spearman r) with PP4. Possible causes of higher shrimp aggregations in P. martia could be related to reproductive processes because peaks of mature females were reported at similar periods in neighboring areas, In both dominant Plesionika species (P. martia and P. heterocarpus), MDS dimensions for diets were also correlated with PP4, although significant correlations with other PP data taken at different periods (from simultaneous PP data to PP6, 6 wk before sampling) were also found. These trends suggest that macroplankton-feeder Pandalidae may couple some aspects of their biology with blooms of plankton and ultimately with peaks of primary production. Prey size selection occurred among Plesionika spp., which suggested a hunting activity. Correlations between Plesionika spp. size (CL, mm) and prey size were positive, both for fish (R-2 = 0.437) and for euphausiids (R-2 = 0.440). Otolith length (mainly of Benthosema glaciale) and the size of euphausiid mandibles (Meganyctiphanes norvegica and Euphausia hemigibba) increased with predator size. Further evidence of active predation by deep pandalids, for example changes in the daily feeding activity of species, were evidenced for P. martia with higher foregut fullness at the beginning of the daytime period (from 06:00 to 08:00 h) related to predation on euphausiids.