SPATIAL AND SEASONAL DIFFERENCES IN THE DIET OF PORTUMNUS LYSIANASSA (HERBST, 1796) (DECAPODA, PORTUNIDAE)

The natural diet of the portunid crab, Portumnus lysianassa was studied. In order to investigate possible spatial differences, 117 individuals were collected from three different sandy sediment stations. In order to determine possible seasonal differences, also 265 individuals were collected from one of the three above stations during the four consecutive seasons of 2003. P. lysianassa seems to be an omnivorous species, mainly feeding on invertebrates while macroalgae also participated in its diet. At all three stations, Crustacea constituted the dominant prey category followed by Polychaeta and macroalgae. However, statistically significant differences in the diet composition of this species were detected among the three stations. Concerning the seasonal samples, in autumn the most dominant prey category was composed of Crustacea followed by Polychaeta and macroalgae. In contrast, in spring and summer the order was Polychaeta followed by macroalgae and Crustacea, and in winter, macroalgae followed by Crustacea and Polychaeta. The differences detected in the diet composition of this species among the four seasons, appeared to be statistically significant. The observed spatial and seasonal diet differences seem to be directly related to the corresponding food composition in the community this crab inhabits. Stomach fullness percentages indicated that this species has a rather low feeding activity, both seasonally and spatially. Furthermore, it was ascertained that there is a negative, statistically significant correlation between carapace size and the values of the vacuity coefficient. It seems that smaller individuals more often have empty stomachs. This should be attributed to the pause of feeding during the period of ecdysis, which occurs more often in smaller individuals.