Food availability and diet composition of three coexisting Mediterranean limpets (Patella spp.)

The gut contents of three intertidal patellid limpets were analysed by collecting foraging specimens on a breakwater on the Tyrrhenian coast (central Italy) between May 1988 and October 1989. The three species coexist there showing a different, but partially overlapping zonation: Patella aspera dominates the infralittoral fringe; the majority of P. caerulea inhabits the lower midlittoral, while P. rustica is most abundant in the upper midlittoral. The algae present on slivers of substratum over which each limpet collected was moving were identified. Moreover, floristic surveys were made along the shore in order to characterize the algal cover of the different zonal belts. The floristic study revealed that the basic elements of algal communities typical of western Mediterranean rocky shores are present in the study area. The algae found on the slivers under the foraging limpets were generally representative of the algal community typical of the same zone. There was a marked difference between the diets of P. rustica and P. aspera due to the fact that the first species forages on a few low lying epilithic and endolithic Cyanophyceae, while P. aspera feeds on a large number of species belonging to all the main algal classes and life forms considered, including frondose epilithics and epiphytics. The diet of P. caerulea resembles that of P. aspera in algal heterogeneity, but is dominated by Cyanophyceae as in P. rustica. A detailed analysis of the differences between gut contents of each limpet species and the relative slivers showed an obvious general correspondence, but revealed also that the diets of the three species do not completely reflect the availability of algae. These findings suggest that the basic diet segregation mechanism between the three populations is their zonal separation. However, the difference in gut contents of heterospecific limpets foraging in the same zone suggests the existence of supplementary morphy-functional or behavioural mechanisms for diet segregation between the three species.