Spatio-temporal variability of borer polychaetes in Posidonia oceanica beds and its relation to meadow structure

Posidonia oceanica forms an extremely productive and complex ecosystem in the coastal Mediterranean Sea, providing a suitable habitat for hundreds of plant and animal species. Among motile invertebrates associated to Posidonia meadows, polychaete borers of the plant sheaths represent a unique group that exploits a peculiar microhabitat. They belong to the family Eunicidae with three dominant species, Lysidice collaris, L. ninetta and L. unicornis. Due to their strong association with Posidonia shoots, these animals are particularly suitable to study the plant and animal spatial relationships and their pattern of variability. The aim of this work was to evaluate the frequency of occurrence of these animals (Index of Borer, IB) at different spatial and temporal (summer vs winter) scales in two Posidonia meadows off Ischia Island (Gulf of Naples, Italy), which are exposed to different degrees of human impact and hydrological conditions. Results showed significantly different IB values of L. collaris (the most abundant species) between meadows and at scales from 10s to 100s of meters, as well as between summer and winter. The IB of L. ninetta showed significant differences only at scales of 10s of meters while, on the contrary, the IB of L. unicornis (the less abundant species) did not show variability at any spatial or temporal scale. Most of the variance was in the most impacted meadow and less exposed to water movement, suggesting a higher level of small and intermediate scales of patchiness of borers in this meadow. These variation patterns are discussed in relation to local environmental differences between the studied Posidonia beds.