Invertebrate borers in Posidonia oceanica scales: relationship between their distribution and lepidochronological parameters

The occurrence and pattern of distribution of invertebrate borers in the scales (remains of leaf bases still connected to the rhizome) of the seagrass Posidonia oceanica was studied in two meadows of the Mediterranean Sea. At Noli (Ligurian Sea), Posidonia shoots were sampled at 1, 5, 10 and 15 m depths, while at cape Coda Cavallo (Northern Sardinia) samples were collected only at 6 m depth. The lepidochronology technique (based on the annual cycle variation in the scale thickness) was employed in order to study the relationship between plant and borers and to characterize their microhabitat in the stem. Borers belong to Polychaeta and Crustacea Isopoda. Three species of polychaetes belonging to the family Eunicidae (Lysidice ninetta Audouin and M. Edwards, Lvsidice collaris (Grube) and Nematonereis unicornis (Grube)) and one species of isopod of the family Limnoriidae (Limnoria tuberculata (Sowinski)) were responsible for burrowing within Posidonia scales. Estimated density of borers reached in some instances values as high as 278 individuals m 2. Isopods occurred in younger scales, whereas polychaetes inhabited older scales. A positive trend was observed between polychaete width (3rd body segment) and scale thickness; however, both Spearman rank correlation and linear regression coefficients showed quite low significant values. In some instances polychaetes burrowed also into the living tissues of the shoots. Borers have been recorded also in several other Posidonia beds along the Italian