Page 3 - gambi_lorenti_alii_1997

386 M.C. GAYHI, M. LOREVTI. S. BUSSOTTI, P. GUID~TTI

Ustica, Table 1). In the Noli bed (Liguria), a number of 209 borers per square
meter was estimated at 1 m depth, whereas at 15 m minimum abundance was
calculated (76 individuals) (Guidetti et al., in press).
Polychaetes were represented by three species of Eunicidae: Lysidice ninettu
(Audouin & M. Edwards), Lysidice collaris Grube and Nematonereis unicornis
(Grube). Both species of Lysidice are cryptic forms already known as borers
into calcareous algae and therefore commonly found in both shallow and deep
coralligenous-dwelling assemblages. Both species are also known from Posidonia
beds. N. unicornis shows a wider ecological distribution, occurring also in soft
bottom assemblages. Herein, L. collaris and N. unicornis are first recorded as
borers into Posidonia scales, whereas such behaviour was already documented
for L. ninetta by Kerneis (1960).
On the whole, polychaetes appeared.to have a wider distribution in comparison
to isopods. They were present at almost al1 sites surveyed; the most widespread
species was L. ninettu, followed by L. collaris. Besides the sites considered in
this survey, the above polychaete species were observed in Posidonici scales also
along the coasts of Latium (Somaschini, pers. comm.) and northern Sicily
(Badalamenti, pers. comm.).
Examination of scales showed that al1 the three polychaete species dig typical
sinuous burrows into the middle tissues. Such burrows are visible from outside
the scale surface and, when empty, are often filled with sediment, which favoiirs
colonization by various organisms (mainly other polychaetes).
In many instances, the removal of the scale cuticle has been observed. Damage
to the living shoot base, caused by polychaete burrowing, sometimes occurred,
leading to the detachment of the leaves from the stem. The number of individuals
was generally 1 per scale and per stem; occasionally up to 3 individuals per
stem were recorded.
The taxonomical analysis of isopods is still in progress, although the bulk
of specimens found seems to belong to the species Limnoria tuherculata Sowinsky,
a representative of the family Limnoriidae which comprehends wood-, seaweed-
and seagrass-boring forms.
During the present study, limnoriids were found to occur at approx. 60 %
of the investigated sites at depths varying between 1 m and 20 m. They burrowed
branching tunnels inside scales close to the leaf base and still showing a ligula.
The number of individuals per scale varied between 1 and 8 (juveniles and adults).
Within the complex habitat built up by overlapped scales, a separation of
spatial niches between polychaetes and isopods appears to be realized, as revealed
by a lepidochronological analysis carried out for some of the studied sites.
Limnoriids seem to select younger scales closer to the leaf base and to the living
shoot, whereas polychaetes are most abundant in older scales whose thickness
is comprised between 0.7 and 0.9 mm. (Guidetti et al., in press). Niche differentiation
within the scales among the three species of polychaetes are not detectable on
the basis of these data.
As highlighted by the present survey, the occurrence of borers in Posidonia
systems appears to be a widespread phenomenon, having possibly far-reaching
implications for the ecological processes of the systems themselves.