Different settlement strategies explain intertidal zonation of barnacles in the Eastern Mediterranean

The Mediterranean mid-littoral zone is inhabited by two sympatric chthamalid barnacles: Chthamalus stellatus and Euraphia depressa, C. stellatus extends from the high midtidal zone, above the algal belt, to the supra-littoral fringe, E. depressa is restricted to the uppermost intertidal levels in wave-beaten places and to cryptic habitats lower on the shore within the belt of C. stellatus. Previous studies have suggested that the reason for the fragmented distribution pattern of E. depressa is competitive displacement by the sympatric C. stellatus, following random settlement. This hypothesis is in agreement with the common model of zonation suggested by Connell that lower distribution limits are determined by biotic factors (competition and predation), while upper limits are set by physical factors. It is hard to test the validity of this model for this barnacle pair since the early ontogenetic stages of the species are morphologically indistinguishable, hindering our ability to understand distribution processes. Using 16S mtDNA as a genetic marker in a multiplex PCR system, cyprids and spats were individually identified. Settlement and recruitment rates were assessed using settlement plates, and the effect of post-settlement processes was tested with transplantation of settlers between zones. Results showed different strategies in each species: settlement of E. depressa was habitat-specific, while settlement of C. stellatus was random. Shifting individuals of C. stellatus to the high and cryptic zones resulted in high mortality; however, exposing juveniles of E. depressa that settled in artificially cryptic low shore habitat to C. stellatus presence had no effect on their survival. These finding do not agree with the formerly suggested hypothesis that zonation is mainly determined by post-settlement factors, and that the interspecies boundary is determined by interspecific competition, implying that competition model cannot be adapted to Mediterranean intertidal zonation and that other models, dominated by physical enforcement and pre-settlement recruitment-limiting factors, may prevail in this ecosystem.