Colonization patterns of the date mussel Lithophaga lithophaga (L., 1758) on limestone breakwater boulders of a marina

Background and Purpose: The European date mussel (Lithophaga lithophaga) is widespread along the whole Mediterranean rocky coastline where it is frequently but illegally harvested. It is well known that the growth of the date mussel is very slow; however, patterns of recolonization of exploited surfaces have been scarcely investigated. The objective of this study is to assess colonization patterns of the date mussel on limestone boulders which have been in the sea for 19 years. These results could be useful in predicting the reconstitution of natural populations of the date mussel after harvesting. Materials and Methods: Limestone breakwater boulders were placed along the dike of the Marina of Rovinj (northern Adriatic, Istrian peninsula, Croatia) in 1984. Sampling was carried out in summer 2003 by SCUBA diving from six habitats of different inclination and topography: horizontal, inclined, vertical, sheltered, vaults, and whole stones. The abundance and biomass of L. lithophaga at the Marina were compared with those in natural control locations. Results: At the Marina, no date mussels were found on the horizontal and inclined sides of the boulders, and very few date mussels were found on the vertical side. On the contrary, sheltered and vault sides of boulders and whole stones were intensively colonized by L. lithophaga. In vaults, the abundance was similar to that in nature. In these habitats, L. lithophaga biomasswas generally lower than in nature. However, large date mussels, of lengths from 50 to 70 mm, were already present on boulders of theMarina. They amounted to 35% in the sheltered habitat, 24% in vaults, and 3% of the total number in whole stones. Conclusions: An unexpected high colonization rate and growth of date mussels was detected in certain unexposed rocky habitats at the boulders of the Marina. However, on the natural rocky Istrian coast, the date mussel was mostly collected on exposed inclined and vertical rocky surfaces where repopulation after harvesting may require very long periods