MEPS 355:219-233 (2008)  -  DOI: https://doi.org/10.3354/meps07260

ABSTRACT: The feeding habits of the 2 Plesionika species occurring around the Balearic Islands (western Mediterranean) were examined. Samples were taken with a bottom trawl from depths between 247 and 752 m from August 2003 to June 2004 at 2 sites (Cabrera and Sóller) off the island of Mallorca. Both species preferentially exploited mesopelagic resources but the shallower living P. heterocarpus, which inhabits the shelf-slope break, also consumed benthic organisms (e.g. polychaetes, bivalves, tanaids). Spatio-temporal changes were evident in the diet of the 2 pandalids. In P. heterocarpus, 2 seasonal groups were revealed by nMDS analysis, one summer–autumn, the other winter–spring. Differences in prey consumption were observed between Cabrera and Sóller samples, with a temporal shift in the dominance of some prey between the 2 areas. The diet of P. heterocarpus was significantly correlated with geographical (e.g. latitude, depth), chemico-physical variables (mainly those related to sediment characteristics) and biological features (e.g. maturity stages, sex) of the species. Correlation between percentage of mature females and caloric content of prey in the stomach did not show any clear seasonal pattern; also trophic level did not change throughout the year (δ¹⁵N = 7.87‰ ± 0.17). Temporal changes in diet composition were stronger in the deeper pandalid P. martia and strictly related to its reproductive pattern. Three groups were revealed by multivariate analysis: (1) a summer group (August and September samples), (2) an autumn–winter group (November and February), characterized by high predation on gelatinous plankton, salps, hyperiids and pelagic molluscs and (3) a spring group, in which predation was directed to higher energy prey such as euphausiids and decapods. The higher percentage of mature females observed in spring was correlated with the increase in energy requirements in the pre-spawning period. Stable isotope analysis revealed a stronger seasonality for P. martia than for P. heterocarpus. An abrupt decrease in trophic level (δ¹⁵N) and an analogous enrichment in δ¹³C values were observed in February. This trend may be related to various factors, among others a higher consumption of refractory material (e.g. marine snow) in winter, when mesopelagic resources are scarcer, changes in the whole zooplanktonic and suprabenthic communities due to phytodetritus input of low δ¹⁵N /high δ¹³C values, and possible reproductive migration to deeper waters.

KEY WORDS: Pandalid shrimps · Diet · Reproduction · Seasonality · Environmental variables · Benthopelagic fauna · Stable isotope analysis