400
M.e. Gambi and G. Cafiero
the scales (remains of former leaf bases persisting along the rhizome) represent an important
part of the detritic compartment of the
Posidonia system (Romero et al. 1992), and a
unique microhabitat, mainly colonized by microorganisms and fungi. Recently we have described
some boring polychaetes and crustacean
isopods, which burrow inside of P. oceanica
scales (Gambi et al. 1997; Guidetti et al. 1997),
and that represent the only Metazoans known to
exploit this peculiar microhabitat. While isopod
borers were previously observed in Australian
seagrass systems (Brearley and Walker 1993,
1995, 1996). for polychaetes the boring habit has
been reported so far only for the P. oceanica seagrass system (Guidetti et aI. 1997; Gambi 2000).
Polychaete and isopod borers may occur in the
same bed and on some occasions in the same
shoot, but they prefer scales of different age and
structure. Polychaetes colonise older and aged
scales, while isopods prefer younger scales
(Guidetti et al. 1997). Finally, polychaetes
occurred in most of the investigated P. oceanica
beds, while isopod borers showed a more limited
distribution (Gambi et aI. 1997). However, our
previous observations were spatially scattered
along the Italian coasts (Gambi et al. 1997) and
limited to short time of observation (Guidetti et
al. 1997; Guidetti 2000).
The aims of this study were: verify species
composition and distribution of scale boring
polychaetes in Posidonia meadows with different
typology and at different depths; verify the
occurrence of these organisms in time; verify the
occurrence of the boring species also in dead rhizomes, and confrrm their preference for aged
scale.
A preliminary evaluation of the feeding
activity of the borer species, as well as of their
role in scale fragmentation and of their ecological implications for the seagrass system has been
reported in a different study (Gambi et aI.2000).
Study Sites - Material and Methods
The study has been conducted in two P. oceanica
beds located on the northern side of the island of
Ischia (Gulf of Naples): Castello Aragonese and
Lacco Ameno (Cafiero 1998). The two meadows,
where a previous survey revealed the occurrence
of borer polychaetes (Gambi et al. 1997), were
selected according to their different typology
and shoot densities. The Castello is a relatively
limited and sheltered meadow, ranging from 0.5
m to 6 m depth in front of the rocky promontory
of Castello Aragonese. The P. oceanica meadow at
Lacco is quite a large and continuous bed ranging from 1 m to about 30 m depth and has been
intensively studied in the past for the meadow
dynamics (Buia .et al. 1992) as well as for the
associated animal communities (Gambi et al.
1992).
Sampling of orthotropous rhizomes) from
both living and dead shoots, was carried out by
SCUBA diving in four stations at different
depths: 1 m and between 3 and 5 m at the
Castello meadow; between 3 and 5 m, and 22 m
at the Lacco meadow. Stations were established
on the basis of bottom topography, meadow features and shoot density. Samples were collected
monthly, from October 1996 to October 1997,
except in the deep station off Lacco (22 m) where
rhizomes were collected on a seasonal basis. A
total of 40 living rhizomes were collected in each
station and at each month/season of sampling,
except at Castello 1 m depth, where only 20 rhizomes were collected in order to limit the impact
on this spatially limited area. However, due to the
high shoot density of the bed in this zone (mean
= 919 shootlm 2 ), the number of analysed rhizomes is representative of the borer colonisation. To estimate the frequency and abundance
of borers, various "indices" have been calculated:
index of borers (IB= percentage of the number
of rhizomes hosting borers over the total rhizomes analysed); index of traces (IT= % of the
number of rhizomes with only empty traces of
previous colonisation of borers - sinuous galleries on the scale mesophyll, holes and piercing
grooves on the scale epidermis - over the total
rhizomes analysed); index of colonisation (IC=
IB + IT); index of herbivory (IH= % of the number of rhizomes with living tissues bored over
the total rhizomes analysed) (Cafiero 1998;
Gambi 2000).
Abundance of borers was calculated to the
square metre multiplying the number of individuals found in each sample by the shoot density at
that given station, and dividing by the number of
rhizomes analysed in that sample.
In order to evaluate the microdistribution of
the borers along the rhizomes, the scales of the
rhizomes collected at all stations in July were
separated into different groups according to
their increasing age: the frrst scale; group 1 = the
M.e. Gambi and G. Cafiero
the scales (remains of former leaf bases persisting along the rhizome) represent an important
part of the detritic compartment of the
Posidonia system (Romero et al. 1992), and a
unique microhabitat, mainly colonized by microorganisms and fungi. Recently we have described
some boring polychaetes and crustacean
isopods, which burrow inside of P. oceanica
scales (Gambi et al. 1997; Guidetti et al. 1997),
and that represent the only Metazoans known to
exploit this peculiar microhabitat. While isopod
borers were previously observed in Australian
seagrass systems (Brearley and Walker 1993,
1995, 1996). for polychaetes the boring habit has
been reported so far only for the P. oceanica seagrass system (Guidetti et aI. 1997; Gambi 2000).
Polychaete and isopod borers may occur in the
same bed and on some occasions in the same
shoot, but they prefer scales of different age and
structure. Polychaetes colonise older and aged
scales, while isopods prefer younger scales
(Guidetti et al. 1997). Finally, polychaetes
occurred in most of the investigated P. oceanica
beds, while isopod borers showed a more limited
distribution (Gambi et aI. 1997). However, our
previous observations were spatially scattered
along the Italian coasts (Gambi et al. 1997) and
limited to short time of observation (Guidetti et
al. 1997; Guidetti 2000).
The aims of this study were: verify species
composition and distribution of scale boring
polychaetes in Posidonia meadows with different
typology and at different depths; verify the
occurrence of these organisms in time; verify the
occurrence of the boring species also in dead rhizomes, and confrrm their preference for aged
scale.
A preliminary evaluation of the feeding
activity of the borer species, as well as of their
role in scale fragmentation and of their ecological implications for the seagrass system has been
reported in a different study (Gambi et aI.2000).
Study Sites - Material and Methods
The study has been conducted in two P. oceanica
beds located on the northern side of the island of
Ischia (Gulf of Naples): Castello Aragonese and
Lacco Ameno (Cafiero 1998). The two meadows,
where a previous survey revealed the occurrence
of borer polychaetes (Gambi et al. 1997), were
selected according to their different typology
and shoot densities. The Castello is a relatively
limited and sheltered meadow, ranging from 0.5
m to 6 m depth in front of the rocky promontory
of Castello Aragonese. The P. oceanica meadow at
Lacco is quite a large and continuous bed ranging from 1 m to about 30 m depth and has been
intensively studied in the past for the meadow
dynamics (Buia .et al. 1992) as well as for the
associated animal communities (Gambi et al.
1992).
Sampling of orthotropous rhizomes) from
both living and dead shoots, was carried out by
SCUBA diving in four stations at different
depths: 1 m and between 3 and 5 m at the
Castello meadow; between 3 and 5 m, and 22 m
at the Lacco meadow. Stations were established
on the basis of bottom topography, meadow features and shoot density. Samples were collected
monthly, from October 1996 to October 1997,
except in the deep station off Lacco (22 m) where
rhizomes were collected on a seasonal basis. A
total of 40 living rhizomes were collected in each
station and at each month/season of sampling,
except at Castello 1 m depth, where only 20 rhizomes were collected in order to limit the impact
on this spatially limited area. However, due to the
high shoot density of the bed in this zone (mean
= 919 shootlm 2 ), the number of analysed rhizomes is representative of the borer colonisation. To estimate the frequency and abundance
of borers, various "indices" have been calculated:
index of borers (IB= percentage of the number
of rhizomes hosting borers over the total rhizomes analysed); index of traces (IT= % of the
number of rhizomes with only empty traces of
previous colonisation of borers - sinuous galleries on the scale mesophyll, holes and piercing
grooves on the scale epidermis - over the total
rhizomes analysed); index of colonisation (IC=
IB + IT); index of herbivory (IH= % of the number of rhizomes with living tissues bored over
the total rhizomes analysed) (Cafiero 1998;
Gambi 2000).
Abundance of borers was calculated to the
square metre multiplying the number of individuals found in each sample by the shoot density at
that given station, and dividing by the number of
rhizomes analysed in that sample.
In order to evaluate the microdistribution of
the borers along the rhizomes, the scales of the
rhizomes collected at all stations in July were
separated into different groups according to
their increasing age: the frrst scale; group 1 = the