Estuarine, Coastal and Shelf Science (2000) 51, 627–635
doi:10.1006/ecss.2000.0674, available online at http://www.idealibrary.com on
The Sponge Community of a Subtidal Area with
Hydrothermal Vents: Milos Island, Aegean Sea
M. Pansinia, C. Morria and C. N. Bianchib
a
Dip.Te.Ris., Università di Genova, Corso Europa 26, I16132 Genova, Italy
Marine Environment Research Centre, ENEA Santa Teresa, P.O. Box 316, I19100 La Spezia, Italy
b
Received 16 November 1999 and accepted in revised form 30 May 2000
Sponges were sampled by SCUBA diving at six subtidal rocky sites, three of which were close to hydrothermal vents, a
common feature on the sea-floor off the south-east coast of Milos. Twenty-five species (2 Calcarea and 23 Demospongiae) were found, few compared with the 589 recorded for the Mediterranean, but an important addition to the scant
information on the sponge fauna of the Aegean Sea. The number of species found at vent sites was consistently higher
than that found at non-vent sites, but no vent-obligate species could be identified. However, Geodia cydonium and three
species of Cliona (C. copiosa, C. nigricans and C. rhodensis) showed a tendency to colonize vent areas. The former might
take advantage of increased silica availability, the latter of the enhanced deposition of carbonates near vents. Substratum
cover by sponges (estimated from wire-framed photographs of 0·7 m2), varied greatly both among and within sites, mostly
according to slope. Most sponge species preferred vertical to overhanging, shaded substrata. Proximity to vents seemed
to have little or no influence on sponge cover, notwithstanding a primary effect on species diversity.
2000 Academic Press
Keywords: porifera; sponge communities; substratum cover; hard bottom zoobenthos; hydrothermal vents; eastern
Mediterranean Sea
Introduction
Together with cnidarians, sponges are the most
important components of the sessile macrobiota at
shallow hydrothermal vents (Fricke et al., 1989;
Benedetti Cecchi et al., 1998; Tarasov et al., 1999).
However, sublittoral epifaunal assemblages characterized by sponges are a common occurrence world-wide
(Todd, 1998), so there is no clear evidence of whether
dominance by sponges is favoured by vent activity or
not. In deep-sea cold seep communities, certain
sponge species take advantage by methane emission
(Vacelet et al., 1995, 1996; Maldonado & Young,
1998), but sponges are otherwise uncommon at
deep-sea hydrothermal vents (Grassle, 1986). Early
SCUBA diving observations by Laborel (1960) at
shallow hydrothermal vents in the Aegean Sea suggested that sponges were not negatively affected by
vent activity. In the Aeolian Islands (Tyrrhenian Sea),
Bavestrello et al. (1995) stated that proximity of
hydrothermal springs had no influence on sponge
distribution.
These two studies, however, considered only the
general physiognomy of sponge communities. Here
we analysed the structure of sponge communities in a
subtidal hydrothermal area in the Aegean, taking
0272–7714/00/110627+09 $35.00/0
account of both qualitative (species composition) and
quantitative (percentage cover of the substratum)
aspects.
The results add to the scant information previously
available on the sponge fauna of the Aegean Sea, as
compared with other parts of the Mediterranean Sea
(Voultsiadou-Koukoura & van Soest, 1993).
Material and methods
The study was conducted at Milos, an island in the
Hellenic Volcanic Arc where hydrothermal systems
and biota have been intensively studied in recent years
(Morri et al., 1999; Dando et al., 2000). Fieldwork
was done in June 1996 at six sites off the south-east
coast of Milos (Figure 1). Two sites, SR (3640.20N,
2430.64E) and E (3640.22N, 2431.61), were
located in shallow water (13 m bottom depth) within
Palaeochori Bay. Sites CR (3639.73N, 2431.24E)
and ST (3640.01N, 2432.14E) were on two
rocky reefs rising from a sandy bottom at 32 m
depth, about 1 km offshore. The remaining two sites,
VS (3639.55N, 2431.37E) and S (3638.14N,
2434.50E) were at two and 5 km further offshore,
respectively, at depths of 41 to 44 m. All sites were
located in a hydrothermally active area (Dando et al.,
2000 Academic Press
628 M. Pansini et al.
Milos Is.
PALAEOCHORI
10 m
SR
E
20 m
36° 40' N
GREECE
ST
30 m
40 m
CR
50 m
VS
Milos Is.
100 m
N
Aegean Sea
S
36° 38'
1 km
150 m
24° 32'
24° 34' E
F 1. Study area and location of the six sites (SR, E, ST, CR, VS and S) where sponges were collected. Sites SR, CR
and VS were close to hydrothermal vent systems on the sea floor of the area. In four sites (CR, E, ST and S), sponge
assemblages were also sampled photographically to estimate substratum cover.
2000), but SR, CR and VS were closer to the vents,
and emission of fluid (Table 1) was observed there
during the dives. Therefore, for each pair of sampling
sites, one is a ‘ vent site ’ and one is a ‘ non-vent site ’.
To establish a check-list of the principal sponge
species occurring in the area, samples of sessile
epibenthic assemblages at each site were handcollected and/or scraped off the substratum with
hammer and chisel by SCUBA divers. Quantitative
data on substratum cover by sponges were obtained
using wire-framed photographs (0·7 m2), also taken
by SCUBA divers. Because of logistic and weather
constraints, it was possible to photograph only sites
CR, E, ST and S, so quantitative data are available for
only one of the three vent sites. At each site, photographs were taken on the rock nearest to the actual
vent (or to the boat anchor in non-vent sites), firstly
on horizontal to gently sloping slabs and secondly on
overhangs. Six colour slides were randomly taken in
each situation, leading to a grand total of 48 images
(4 sites2 situations6 slides).
Total sponge cover was analysed by two-way
ANOVA to test for differences among sites and situations. Percentage cover values were arcsine transformed to meet the assumption of homogeneity of
variances (Underwood, 1997). Qualitative and quantitative species composition of sponge assemblages at
the different sites were compared through correspondence analysis (Legendre & Legendre, 1998). For
qualitative aspects, correspondence analysis was
applied to a presence–absence matrix. For quantitative aspects, the arcsine-transformed cover data were
used. In both cases, significance of the axes extracted
was evaluated using the tables of Lebart (1975).
Results
Qualitative aspect
A total of 25 species was found: 2 Calcarea and 23
Demospongiae (Table 2). The number of species per
site ranged from 9 to 15, and plotting the number
of species against sites ordered by their depth
and/or distance from shore seemed to follow a
humped pattern, suggesting species richness may be
greater in intermediate circumstances (Figure 2).
The highest number of species (15) was found
in the vent site CR and, in general, the three vent
sites were more species-rich than their non-vent
counterparts.
The sponge community of a subtidal area with hydrothermal vents 629
T 1. Some characteristics of hydrothermal fluids from subtidal vents at Milos Island
Parameter or substance
Temperature (C)a
pHa
Mean water flow (l h 1)b
Composition of watera
DIC (mM)
Si (mM)
SO4 (mM)
NH3 (mM)
Na (mM)
Ca (mM)
Mg (mM)
Mn (mM)
Mean gas flow (l h 1)b
Composition of gasesa
CO2 (%)
H2S (%)
H2 (%)
CH4 (%)
Precipitates and depositsa
minerals
metals
Particulate fluxes in the areac
total mass (mg m 2 day 1)
organic C (mg m 2 day 1)
organic N (mg m 2 day 1)
Observed value or occurrence
108
5·2–7·1
26·1 (0·7–124·3)
2·6–11·8
0·2–3·2 (seawater: 0·1)
12–25 (seawater: 30·5)
20–452 (seawater: 0·3–3)
512–1254 (seawater: 499)
12–58 (seawater: 10·8)
23–56 (seawater: 56·8)
8–171 (seawater: <0·2)
8·1 (range: 0·1–56·6)
76·6
1·34
2·75
0·27
silica, galena-sphalerite, chalcopyrite
As, Sb, Fe, Mn, Tl, Cu, Zn, Pb
7–5900 (control: 11–1000)
up to 1359 (control: 115)*
up to 238 (control: 21)*
a
Dando et al. (1999).
Dando et al. (2000).
Miquel et al. (1998).
*The difference between vent and control areas may be interpreted as due to microbial production at vents.
b
c
Correspondence analysis on the presence–absence
matrix (25 species6 sites) gave two significant axes
(P<0·05), together explaining 78·2% of the total
variance. In the plane formed by the first two axes
(Figure 3), the points representing the three vent sites
(SR, CR and VS) were well separated from the three
non-vent sites (E, ST and S). In particular, the
distinction between the vent sites group and the
non-vent sites group appeared obvious along the first
axis. The second axis opposed the two offshore sites
(VS and S) to the remaining sites.
Species points were spread more or less regularly
over most of the plane formed by the first two axes
from correspondence analysis (Figure 3). The points
for species occurring in most sites (Agelas oroides,
Hymeniacidon sanguinea, Phorbas tenacior and Sycon
raphanus) came to occupy a central position in the
graph, whereas the points of the species found exclusively in a single site lay close to the relevant site
points. Excluding the five species recorded only once
(Axinella cannabina, A. damicornis, Corticium candelabrum, Erylus euastrum and Mycale retifera), four species
(Geodia cydonium, Leuconia solida, Cliona rhodensis and
Mycale lingua) were exclusive to vent sites. No species
was exclusive to non-vent sites.
Quantitative aspect
Sponges often covered large portions of the substratum, reaching high values at sites ST and S, and
an absolute maximum of 60% at ST. Highest cover
was seen on vertical walls and under overhangs (Figure 4), where sponges were usually the dominant
taxon. Two-way ANOVA indicated that differences in
total sponge cover among sites and situations (substratum slope) were both highly significant, as was the
interaction between the two variables (Table 3). At
the vent site CR, total sponge cover on vertical substrata was lower, and that on (sub)horizontal substrata
was higher than at the remaining sites (Figure 4), but
differences between the two situations at CR were still
significant (Tukey’s test, T8,40 =0·122, P<0·05).
Eighteen sponge species were recognized on the
images, close to the total of 25 species obtained in the
reference collection. Most sponges were large and
630 M. Pansini et al.
T 2. Taxonomic list of the sponge species found in the six studied sites at Milos, Aegean Sea.
Species codes are those used in Figures 3, 5–7. Asterisks mark the species also recognized on the
in situ photographs
Vent sites
Code
Aaa
Aca
Ada
Aor
Ave
Cca
Cco
Ccr
Cni
Cre
Crh
Dav
Eeu
Gcy
Hym
Ior
Iva
Lso
Mli
Mre
Pfi
Pte
Sfo
Sof
Sra
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
Non-vent sites
Species
SR
CR
VS
E
ST
S
Aaptos aaptos (Schmidt)
Axinella cannabina (Esper)
Axinella damicornis (Esper)
Agelas oroides (Schmidt)
Axinella verrrucosa (Esper)
Corticium candelabrum Schmidt
Cliona copiosa Sarà
Crambe crambe (Schmidt)
Cliona nigricans (Schmidt)
Chondrosia reniformis Nardo
Cliona rhodensis Rützler et Bromley
Dysidea avara (Schmidt)
Erylus euastrum (Schmidt)
Geodia cydonium (Jameson)
Hymeniacidon sanguinea (Grant)
Ircinia oros (Schmidt)
Ircinia variabilis (Schmidt)
Leuconia solida (Schmidt)
Mycale lingua (Bowerbank)
Mycale retifera Topsent
Petrosia ficiformis (Poiret)
Phorbas tenacior (Topsent)
Sarcotragus foetidus Schmidt
Spongia officinalis L.
Sycon raphanus (Schmidt)
—
—
—
+
—
—
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—
—
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—
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colourful species, with an erect (Axinella damicornis,
A. verrucosa), massive (Agelas oroides, Chondrosia reniformis, Cliona nigricans, Ircinia oros, I. variabilis, Petrosia ficiformis, Sarcotragus foetidus) or encrusting
(Phorbas tenacior, Crambe crambe) habit, and were
easily recognized on photographs. Clearly, however,
cryptic and inconspicuous species may well have
escaped detection on the images. Moreover, only four
of the six sites, were sampled also photographically; so
the four species found exclusively at SR (Mycale
retifera) or VS (Axinella cannabina, Corticium candelabrum and Erylus euastrum) were not expected to be
observed on the images.
The vent site CR, with 12 species recorded, also
gave the highest species richness value from the images. Partitioning of substratum cover among sponge
species at CR was more even than at the remaining
sites (Figure 5), with the result that diversity and
equitability were maximal. Overall, the most important species (i.e. those covering more than 10% of the
substratum in at least one image) were Agelas oroides,
Crambe crambe, Petrosia ficiformis and Cliona nigricans.
Agelas oroides was the dominant species in all sites
except E, where it was replaced by Crambe crambe
(Figure 5).
Correspondence analysis on the matrix of arcsinetransformed cover data (18 species48 images) gave
four significant axes (P<0·05), together explaining
65·0% of the total variance. Along the first axis, image
points were arranged in such a way that horizontal
situations separated from vertical situations, irrespective of the site (Figure 6). On the plane formed by the
first two axes, the image points cloud was parabolic in
shape. This shape is derived from the quadratic interdependence of the first and second axes (Guttman
effect), and is frequently considered the result of
the very strong intensity of the ecological gradient
expressed by the first axis (slope).
Because most sponges reached higher values of
substratum cover on vertical walls and under overhangs, their species points were close to vertical image
points in the plane formed by the first two axes. The
only species showing some tendency to colonize (sub)
horizontal substrata were Sarcotragus foetidus, Ircinia
variabilis, Cliona rhodensis, C. nigricans and Petrosia
ficiformis. The third axis opposed the offshore site S,
The sponge community of a subtidal area with hydrothermal vents 631
60
16
CR
Percentage cover
50
14
Number of species
ST
VS
12
SR
S
40
30
20
10
10
0
H
V
CR
E
H
V
H
E
V
H
ST
V
S
F 4. Total percentage cover of sponges on differently
sloping substrata (H=horizontal to gently sloping substrata;
V=vertical to overhanging substrata) at the four sites where
photographs were taken. Values are means (+1 SE) of data
from six photographs.
8
6
Inshore/shallow
sites
Intermediate
sites
Offshore/deep
sites
F 2. Sponge species richness at the six sites, ranked
according to their distance from the shore. Sites SR, CR and
VS were close to hydrothermal vents.
Crh
Sfo
Cre
Mre
Gcy
Cco
SR
Aaa
Dav
E
CR
Aor
Mli
d.f.
MS
F
P
Site
Situation (substratum slope)
Sitesituation
Error
3
1
3
40
0·121
2·170
0·200
0·004
27·7
495·5
45·8
<0·01
<0·01
<0·01
ST
Ior
Ccr Pfi
Ave
Sra
Aca
Source of variation
Iva
Lso
Cca
T 3. Two-way ANOVA on arcsine transformed total
sponge cover data obtained from the photographs
Pte
Sof
Hym
VS
Eeu
Cni
S
Ada
F 3. Ordination plot on the plane formed by the first
and second axes extracted by correspondence analysis on
presence–absence data. First axis (abscissa) explained
48·4% of the total variance, second axis (ordinate), 29·8%.
Site points are indicated by the site names, species points by
the codes reported in Table 2. Sites SR, CR and VS were
close to hydrothermal vents.
(with Axinella damicornis), to the horizontal situation
of the inshore site (with Sarcotragus foetidus), (Figure
7). Finally, the fourth axis mainly opposed images
taken on horizontal substrata at site CR to those
taken on vertical substrata at site E. The former
images were characterized by the high cover of Cliona
nigricans, the latter by the dominance of Crambe
crambe. The strong polarization of some image points
of the vent site CR might suggest that this axis was,
at least in part, an expression of the influence of
vent proximity. Lack of data from other vent sites,
however, precludes firm conclusions on this point.
Discussion
The 25 species reported here seem few if compared
with the 589 sponge species occurring in the Mediterranean (Pansini, 1995, 1996). Nevertheless, they add
significantly to our poor knowledge of the sponge
fauna of the Aegean, which is estimated to comprise
about 120 species (Bogdanos & Zenetos, 1988).
Unfortunately a check-list of the species hitherto
recorded from the Eastern Mediterranean is lacking.
Drai (1985), reported 593 sponge species from the
entire Mediterranean and observed that 543 (87·3%)
were present in the western Mediterranean, 253
(40·7%) in the Adriatic Sea and only 130 (20·9%)
in the eastern part of the basin. Subsequently 29
632 M. Pansini et al.
20
15
10
CR
Aor
Cni
5
Hym
Pfi
Iva
Cco
Ior
Crh
Ccr
Pte
Aaa
Mli
0
20
15
E
Ccr
Percentage cover
10
5
Pfi
Aor
Iva
Sfo
Hym
Cre
Ave
0
20
Aor
15
ST
10
Pfi
Ior
5
Hym
Ccr
Sof
Aaa
Cre
Dav
Pte
0
20
Aor
15
S
10
Ior
5
Ccr
Pfi
0
1
2
3
4
Pte
Cni
5
Sof
6
7
Species rank
Ada
8
9
10
11
12
F 5. Mean (+1 SE) percentage cover of the individual sponge species, ranked in order of decreasing quantitative
importance, at the four sites where photographs were taken. Species codes as in Table 2.
species of horny sponges (Voultsiadou-Koukoura &
Koukouras, 1993) and nine species of Suberitidae
(Voultsiadou-Koukoura & van Soest, 1993) have also
been recorded in the North Aegean. In addition, five
new species have been described from the Island of
Crete and the Aegean Sea between 1983 and 1996
(Voultsiadou-Koukoura & van Soest 1991a, b;
Voultsiadou-Koukoura et al., 1991; Pansini, 1996).
This clearly indicates that further investigation will
increase the total number of sponge species of the
Aegean Sea, which is certainly underestimated at
present.
The sponge community of a subtidal area with hydrothermal vents 633
+
+
Cni
+
+
Crh
Cco
SH
+
+
Ada
Dav
SV
+
Aaa
+
+
+
+
Aor
Ior
+
+
+
Aaa
Pte
+
+
Sof
+
+
ST V
+
+
Cco
+
+
Hym
Mli Pte
+ CR V
+
+
+
+
+ +SV+ +
Ada + + +
EV
Cre + +
+
+
Crh
Cni
Pfi
Ccr
+
+
Ave
+
CR H
+ +
+
+
+
+
+
ST H
+
+
Iva
+
+
+
+
+
+
Aor
Mli
+
+
Ave
+
+
+ ST H +
Pfi
Hym
+
Cre
+ EH +
+
+
Sfo
+
+
+
EH
+
Iva
+
+
+
Dav
CR V
+ ST V
+ Ior
+
+
+
+
+
Sfo
+
Sof
+
+
SH
+
+
+
+
+
CR H
+
+
+
+ EV
+
+ Ccr
+
F 6. Ordination plot on the plane formed by the first
and second axes extracted by correspondence analysis on
arcsine-transformed cover data. The first axis (abscissa)
explained 22·9% of the total variance, second axis (ordinate), 16·4%. Image points (small crosses) were grouped by
site and by slope (H=horizontal to gently sloping substrata;
V=vertical to overhanging substrata); species points are
labelled with the species codes reported in Table 2.
F 7. Ordination plot on the plane formed by the third
and fourth axes extracted by correspondence analysis on
arcsine-transformed cover data. The third axis (abscissa)
explained 14·4% of the total variance, fourth axis (ordinate),
11·3%. Image points (small crosses) are grouped by site and
by slope (H=horizontal to gently sloping substrata;
V=vertical to overhanging substrata); species points are
labelled with the species codes reported in Table 2.
Lack of comparable inventories in the whole
Aegean Sea does not allow for zoogeographic comments upon our findings at Milos. Our species list
corresponds well to the sponge fauna typically found
in most subtidal rocky bottoms of the Mediterranean
Sea. The occurrence of Cliona copiosa, C. rhodensis and
Axinella cannabina, together with the relative abundance of horny sponges (Dysidea, Ircinia, Sarcotragus
and Spongia), suggest a warm-water affinity of the
sponge fauna at Milos and its similarity to that of
other southern sectors of the Mediterranean basin
(Pansini, 1992). This seems consistent with the
geographic location of Milos.
Vent activity in the study area did not seem to have
selected any sponge species. All the species found at
Milos are already known from ‘ normal ’ sites in the
Mediterranean Sea and, therefore, no vent-obligate
species (as defined by Barry et al., 1996) could be
recognized. However, there were species collected
exclusively at vent sites (Geodia cydonium, Leuconia
solida, Cliona rhodensis and Mycale lingua) or exhibiting
higher cover at the vent site CR (Cliona nigricans and
C. copiosa). Geodia cydonium, in particular, was found
in all of the three vent sites and in none of the others,
and this appears consistent with the fact that this
species has been shown to grow better in areas influenced by hydrothermal waters (Morri et al., 1994).
Two species of Geodia were perhaps endemic at
deep-sea cold seeps of the southern Barbados prism
(Olu et al., 1997).
Carney (1994) introduced the term ‘ colonist ’ for
the opportunistic species that were more common
around vents than in the surrounding environment.
For example, it has been suggested that Geodia cydonium takes advantage from enrichment in silica by
vent emissions (Bianchi et al., 1998). The three
species of Cliona may take advantage by the enhanced
deposition of carbonate substrates at vents (Cocito
et al., 2000): clionids bore into calcareous substrates
(Bavestrello et al., 2000), which are virtually absent
from the volcanic island of Milos. Moreover, the
bio-eroding activity of clionids is known to be enhanced by increasing nutrients and organic matter in
the water (Holmes, 1997), as is common situation
around shallow hydrothermal vents (Dando et al.,
1999; Miquel et al., 1998). In the case of symbiotic
clionids, their zooxanthellae, which in turn affect
sponge fitness (Rosell & Uriz, 1992), may also benefit
from enrichment in CO2 at vents.
As observed by Morri et al. (1999) for the whole
sessile epifauna, sponge species richness was consistently higher at vent sites than at the corresponding
non-vent sites. Quantitative data at the vent site CR
also showed a more even partitioning of the substratum among the different species as compared to
non-vent sites, where one or two species dominated
634 M. Pansini et al.
the sponge assemblages. Higher species diversity at
vent sites may be a consequence of episodic mortality
caused by the emission of toxic fluids (Bianchi &
Morri, 2000). Commoner occurrences at vents of the
two species of Calcarea, which are early colonists
(Vacelet, 1980), is consistent with this hypothesis.
Vent activity could therefore act as an intermediate
disturbance of the sponge community, preventing
monopolization of the substratum by dominant
species and allowing for the coexistence of a larger
number of species.
Correspondence analysis on presence–absence data
suggested that proximity to vents might be the major
cause of variation in species composition of the sponge
assemblages at Milos. However, quantitative data
showed great differences in sponge cover both among
and within sites. Considerable spatial variation in
sponge distribution and abundance is a common
pattern in subtidal rocky reefs (Pansini & Pronzato,
1990; Roberts & Davis, 1996). The major source of
variation in sponge cover at Milos was substratum
slope, which in turn determines the amount of light
reaching the substratum. Most sponge species exhibited higher cover on shaded substrata, thus conforming to a known pattern (Sarà, 1968; Glasby, 1999).
With a few exceptions, sponges tend to avoid well-lit
substrata, probably being adversely affected by solar
radiation and algal competition (Sarà & Vacelet,
1973; Pansini, 1997). Among the handful of sponges
that at Milos preferentially colonized (sub)horizontal
substrata, were species having phototrophic endosymbionts, zooxanthellae in Cliona nigricans or zoocyanellae in Petrosia ficiformis (Sarà et al., 1998), so
that they may behave as photophilic organisms
(Wilkinson & Trott, 1983).
Cover and species composition reflected different
responses to the putative effect of vent proximity on
sponge assemblages at Milos. The former indicated
little or no effect, in agreement with previous observations in other Mediterranean sites (Laborel, 1960;
Bavestrello et al., 1995; Benedetti-Cecchi et al.,
1998); the latter suggested a primary influence,
mostly through increased species diversity. Further
studies are necessary to establish significance and
extent of the contrasting responses to vents given
by the qualitative or quantitative aspects of sponge
assemblages.
Acknowledgements
This study is part of the project AG-HY-FL (Hydrothermal fluxes and biological production in the
Aegean), co-ordinated by P. R. Dando (Bangor) and
funded by the EC under contract MAS3-CT95-0021.
S. Varnavas (Patras) organized logistics in Milos, S.
Cocito and A. Peirano (La Spezia) took the samples
and the photographs, A. M. De Biasi (Leghorn)
analysed the slides for cover data.
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