Taxonomic diversity and distribution of polychaete and isopod borers on the sheaths of the seagrass Posidonia oceanica: analysis at regional scale along the coast off Sardinia (Italy)

Abstract

Polychaete and isopod borers of Posidonia oceanica leaf tissues have been identified as a peculiar guild of detritus consumers, colonizing the seagrass sheaths (remains of the leaf bases which persist along the rhizome). Taxonomic diversity and distribution of borer organisms have been in 89 stations along the coast off Sardinia during summer 2001: 34 in shallow Posidonia stands (5–12 m depth), 50 in deep stands (22–26 m depth), and 5 at the lower limit (36–38 m). Shoot density was measured at each station. Borers and their traces were recorded for each rhizome. In each station relative abundance of species (%), and the Indices of Borer (IB) and of traces were calculated for each borer species. A total of 6643 vertical (orthotropous) P. oceanica shoots were examined (mean 75 shoots per station), 26.6% of which hosted borers, while 42.5% showed only traces of their previous colonization. Four species of polychaete Eunicidae (Lysidice collaris Grube, Lysidice ninetta Audouin & Milne‐Edwards, Nematonereis unicornis Grube, Marphysa fallax Marion & Brobretzky), and a single species of isopod Limnoriidae (Limnoria mazzellae Cook‐son & Lorenti) were identified. Polychaetes were collected in all the 89 stations sampled, while L. mazzellae was restricted to 32 stations, mainly in shallow stands. The mean IB values of L. mazzellae were significantly higher in shallow stands, with respect to deep ones. The species L. collaris and M. fallax showed higher frequencies in deep stands, while L. ninetta and N. unicornis had similar IB values, regardless of depth. The lepidochrono‐logical analysis, performed on most of the shoots, revealed that L. mazzellae selects more recent sheaths (first 2 lepidochronological years), while eunicid polychaetes are more frequent in aged scales (>2 years). Most of the significant differences found in IB indices and relative abundances of borer species, were recorded between normal density or high sub‐normal density meadows, and the “altered” density ones, regardless of depth. The Spearman's analysis revealed various significant correlations of IB indices of borers with shoot density, although with relatively low coefficients. The analysis of some stations selected in potentially human‐impacted and in pristine areas showed that while shoot density was significantly higher in un‐impacted areas with respect to impacted ones, the IB indices of borer groups and individual borer species did not show differences among the two kinds of sites, except for that of polychaetes. The studied data set demonstrates that borer organisms occur in P. oceanica beds under different ecological situations (e.g., depth range, bed typology, shoot density, potential human impact), with different distribution among species that are discussed also in relation to some biotic factors (life habit, habitat partitioning, competition).