10 páginas, 5 figuras, 2 tablas. ; The global NaGISA (Natural Geography in Shore Areas) initiative focusing on long-term monitoring of coastal biodiversity has recently been expanded to the Mediterranean. On the basis of datasets from four Mediterranean hard substrate sites and by using themost abundant polychaete family (Syllidae), two questions are addressed in this study: (a) Is the biodiversity sampled in the two NaGISA sites, based on the family Syllidae, representative of the regional one of the Mediterranean Sea? (b) Are local syllid taxo-communities assembled at random from the regional species pool? Randomisation tests determining to what extent local species lists and their phylogenetic relationships are assembled at random from a regional one showed that at small observational scales (e.g. replicate units, depths) the phylogenetic diversity is assembled at random from the one known to exist locally. It is assumed, therefore, that local processes here have a stronger influence on community assembly than historical- evolutionary processes. The latter is in accordance with previous results derived from using polychaetes to test the same hypotheses at a pan-European level. Local syllid biodiversity in the western Mediterranean is a randomsubset of theMediterranean one,whereas that of the NaGISA sites in the eastern Mediterranean is only under certain conditions. Therefore, the currently observed biodiversity of the Mediterranean sites can be considered as representative of that of the entire regional sea only under certain assumptions, as far as the syllid taxo-communities are concerned, demonstrating the potential of the taxon as a surrogate of the polychaete biodiversity. ; Peer reviewed