Invertebrate borers in the Mediterranean sea grass Posidonia oceanica: biological impact and ecological implications

The effects of polychaete (Lysidice ninetta, L. collaris, Nematonereis unic ornis (Annelida: Polychaeta)) and limnoriid isopod (Limnoria tuberculata (C rustacea: Isopoda)) borers in the Mediterranean sea grass Posidonia oceanic a (Potamogetonaceae) were assessed in the meadows of Olbia and Genoa (weste rn Mediterranean Sea). Borer invertebrates were mainly found into the old l eaf sheaths of the sea grass. At Olbia, a mean density of borers (202.6 +/- 34.8 ind m(-2)) significantly higher than at Genoa (23.2 +/- 7.3 ind m(-2) ) was observed. In 3.3% of samples from Olbia, large polychaetes and isopod s were also observed boring into living plant tissues. Only fractions from 0.8-3.9% of the sheath production was actually removed by borers at the two studied meadows. The fact that sheath weight loss with time was higher at Genoa than at Olbia regardless of the presence of borers suggests that othe r factors than the activity of such organisms are important in the sheath d ecay process.