Sessile and non-sessile morphs of Geodia cydonium (Jameson) (Porifera, Demospongiae) in two semi-enclosed Mediterranean bays


Mercurio, M.; Corriero, G.; Gaino, E.

Marine Biology 148(3): 489-501

2006


Morphological plasticity and ecological aspects of the demosponge Geodia cydonium (Jameson) were studied from seasonal samples collected over 1 year in two semi-enclosed Mediterranean bays of the Southern Italian coast (Marsala lagoon and Porto Cesareo basin). Sponge specimens present two morphs: sessile and non-sessile, both of which showed constant size distribution and density over the studied year. Sessile specimens were larger in size than non-sessile ones. This feature is particularly evident at Porto Cesareo, where these sponges have a more compact skeletal network than at Marsala (evident both in the cortical spicule size and sponge silica content). Sessile specimens adhere to hard rocky substrates (Porto Cesareo) or phanerogam rhizomes (Marsala); non-sessile ones occur on soft bottom areas. Several morphological and structural features of the non-sessile forms differ in the two environments, but the difference in body shape seems to play the most relevant role in enhancing the colonization of incoherent substrates. Indeed, at Marsala, where the large amount of silt and clay determines the occurrence of a markedly reduced anoxic layer just below the surface of the sediment, non-sessile specimens of G. cydonium are fairly spherical and thus able to roll, dragged by slow circular currents. In addition, the usual association with the red alga Rytyphloea tinctoria, which almost constantly forms a thick and continuous layer around the sponge, allows them to avoid contact with the substrate. The non-sessile specimens from Porto Cesareo inhabit sandy soft bottoms and are flattened. In such an environment, affected by moderate wave turbulence, the flattened shape widens the contact surface between the body and the substrate, thereby reducing the risk of stranding. The evident signs of abrasion, provided by scanning electron microscopy investigations, on both cortical spicules and outermost sponge surface suggest that sponges rub on the bottom. Sediment, epibiontic organisms, and the phanerogam leaves protect this sciaphilous sponge from high solar radiation, allowing the specimens to live in these shallow environments.

Marine
Biology
(2006)
148:
489-501
DOI
10.1007/s00227-005-0092-4
RESEARCH
ARTICLE
M.
Mercurio
G.
Corriero
E.
Gaino
Sessile
and
non-sessile
morphs
of
Geodia
cydonium
(Jameson)
(Porifera,
Demospongiae)
in
two
semi-enclosed
Mediterranean
bays
Received:
12
February
2005
/
Accepted:
7
April
2005
/
Published
online:
28
September
2005
©
Springer-Verlag
2005
Abstract
Morphological
plasticity
and
ecological
aspects
of
the
demosponge
Geodia
cydonium
(Jameson)
were
studied
from
seasonal
samples
collected
over
1
year
in
two
semi-enclosed
Mediterranean
bays
of
the
Southern
Italian
coast
(Marsala
lagoon
and
Porto
Cesareo
basin).
Sponge
specimens
present
two
morphs:
sessile
and
non-
sessile,
both
of
which
showed
constant
size
distribution
and
density
over
the
studied
year.
Sessile
specimens
were
larger
in
size
than
non-sessile
ones.
This
feature
is
par-
ticularly
evident
at
Porto
Cesareo,
where
these
sponges
have
a
more
compact
skeletal
network
than
at
Marsala
(evident
both
in
the
cortical
spicule
size
and
sponge
silica
content).
Sessile
specimens
adhere
to
hard
rocky
sub
strates
(Porto
Cesareo)
or
phanerogam
rhizomes
(Mar-
sala);
non-sessile
ones
occur
on
soft
bottom
areas.
Several
morphological
and
structural
features
of
the
non-sessile
forms
differ
in
the
two
environments,
but
the
difference
in
body
shape
seems
to
play
the
most
relevant
role
in
enhancing
the
colonization
of
incoherent
sub-
strates.
Indeed,
at
Marsala,
where
the
large
amount
of
silt
and
clay
determines
the
occurrence
of
a
markedly
reduced
anoxic
layer
just
below
the
surface
of
the
sedi-
ment,
non-sessile
specimens
of
G.
cydonium
are
fairly
spherical
and
thus
able
to
roll,
dragged
by
slow
circular
currents.
In
addition,
the
usual
association
with
the
red
alga
Rytyphloea
tinctoria,
which
almost
constantly
forms
a
thick
and
continuous
layer
around
the
sponge,
allows
them
to
avoid
contact
with
the
substrate.
The
non-sessile
specimens
from
Porto
Cesareo
inhabit
sandy
soft
bottoms
and
are
flattened.
In
such
an
environment,
Communicated
by
R.
Cattaneo-Vietti,
Genova
M.
Mercurio
G.
Corriero
(21)
Dipartimento
di
Zoologia,
Via
Orabona,
4-70125
Bari,
Italy
E-mail:
g.corriero@biologia.uniba.it
Tel.:
+
39-80-544-3358
Fax:
+
39-80-544-3358
E.
Gaino
Dipartimento
di
Biologia
Cellulare
e
Ambientale,
Via
Elce
di
Sotto,
06123
Perugia,
Italy
affected
by
moderate
wave
turbulence,
the
flattened
shape
widens
the
contact
surface
between
the
body
and
the
substrate,
thereby
reducing
the
risk
of
stranding.
The
evident
signs
of
abrasion,
provided
by
scanning
electron
microscopy
investigations,
on
both
cortical
spicules
and
outermost
sponge
surface
suggest
that
sponges
rub
on
the
bottom.
Sediment,
epibiontic
organisms,
and
the
phanerogam
leaves
protect
this
sciaphilous
sponge
from
high
solar
radiation,
allowing
the
specimens
to
live
in
these
shallow
environments.
Introduction
Sponges
are
organisms
whose
gross
morphology
is
not
static.
Indeed,
remodelling
processes
take
place
throughout
their
life
cycle,
allowing
sponges
to
adapt
to
their
environment
(Palumbi
1984;
Bond
and
Harris
1988;
Gaino
et
al.
1995;
Bell
et
al.
2002).
This
plastic
morphology
is
essential
for
survival,
since
it
gives
better
opportunity
for
feeding
(Vogel
1981),
reproduction
and
dispersal
(Wullf
1985,
1991,
1995;
Corriero
et
al.
1996a),
water
current
adaptation
(Vogel
1981;
Palumbi
1984;
Kaandorp
1991, 1999;
Kaandorp
and
Kluijver
1992;
Bell
and
Barnes
2000;
Bell
et
al.
2002;
McDonald
et
al.
2002),
and
protection
against
predators
(Guide
1976;
Hill
and
Hill
2002)
as
well
as
preventing
sediment
set-
tlement
(Bell
2004).
In
order
to
meet
the
environmental
needs,
sponges
are
capable
of
modifying
their
body
shape
(Palumbi
1986;
Kaandorp
1999)
but
phenotypic
variations
are
not
unlimited
because
a
sponge
can
arrange
its
body
pattern
into
a
limited
range
of
forms
(Kaandorp
and
Kluijver
1992).
The
skeletal
components
are
also
not
static,
since
seasonal
changes
in
the
number
of
spicules
have
been
observed,
resulting
in
an
increased
inorganic
content
(Stone
1970;
Schonberg
and
Barthel
1997;
Mercurio
et
al.
2000).
Moreover,
differences
in
skeletal
compo-
nents
may
also
be
related
to
differences
in
wave
exposure
(Palumbi
1986;
Bell
et
al.
2002;
McDonald
et
al.
2002).
TYRRHENIAN
SEA
a
l
a2
-
MARSALA
.
. .
'
0
1Km
I:
4
490
The
finding,
in
Mediterranean
lagoons,
of
peculiar
non-sessile
specimens
of
Geodia
cydonium
coexisting
with
the
typical
sessile
forms
(Mercurio
et
al.
1997a,b),
stresses
the
fact
that
acclimatization
can
lead
to
a
free-
existing
morphotype
in
a
phylum
typically
represented
by
sessile
forms.
The
occurrence
of
these
non-sessile
specimens
corroborates
Burton's
(1932)
assumption,
derived
from
observations
of
a
sponge
collection
con-
sisting
of
specimens
belonging
to
different
taxa,
that
even
adult
sponges
may
spend
their
life
floating
just
above
the
sea-floor
and
may
be
lifted
by
currents
of
moderate
force.
Unattached
sponges
have
been
found
in
other
shallow
sub-littoral
environments
(Sara
and
Vac-
elet
1973;
Corriero
1990;
Bell
and
Barnes
2002)
and
in
the
deep
sea,
where
conditions
are
relatively
stable
over
time
(Barthel
and
Tendal
1993).
The
purpose
of
this
study
is
to
analyse
populations
of
G.
cydonium
(Jameson)
(Porifera,
Demospongiae)
in
two
semi-enclosed
bays
located
along
the
Southern
Italian
coast,
namely
Marsala
lagoon
(Sicily)
and
Porto
Cesareo
basin
(Apulia),
in
order
to
investigate
the
morphology
and
skeletal
architecture
of
sessile
and
non-
sessile
specimens
along
with
the
strategy
employed
by
this
sciaphilous
sponge
to
protect
itself
from
high
solar
radiation.
Materials
and
methods
Study
sites
Both
the
studied
sites
are
characterized
by
low
water
movement
and
moderate
input
of
organic
matter
(Magazza
1977;
Genchi
et
al.
1983;
Congedo
1988).
Marsala
lagoon
(Fig.
la)
(NW
Sicily,
37°14'N;
12°40'E)
covers
an
area
of
20
km
2
with
a
maximum
depth
of
2.5
m
(Magazzil
1977).
It
is
characterized
by
a
high
water
exchange
with
the
sea,
mainly
through
the
southern
opening,
which
reduces
thermohaline
oscilla-
tions.
However,
the
presence
of
'reefs'
of
Posidonia
oceanica
(L.)
Defile
(Molinier
and
Picard
1953;
Calvo
and
Frada
Orestano
1984)
greatly
decreases
the
wave
action
even
in
the
outermost
part
of
the
lagoon
(Corri-
ero
et
al.
1989).
Hard
substrates
are
scarce,
mainly
represented
by
algal
or
animal
calcareous
concretions
and
by
the
rhizomes
of
the
phanerogam
P.
oceanica.
Demosponges
are
a
very
important
component
of
the
macrozoobenthos,
being
represented
by
36
species
and
high
biomass
values
(Corriero
1989,
1990).
Porto
Cesareo
basin
(SW
Apulia,
40°15'N;
17°54'E)
(Fig.
lb)
measures
2,500
m
in
length
and
700-800
m
in
width,
with
a
maximum
depth
of
2.5
m
(Passeri
1974).
The
basin
communicates
with
the
sea
through
a
channel
system
that
allows
a
considerable
inflow
of
sea
water
(Corriero
1990).
The
bottom
of
the
basin
consists
pri-
marily
of
mixed
sands
but
rocky
shores,
rocks
and
pebbles
are
also
present.
Thirty-nine
species
of
demo-
sponges
occur
in
the
basin
showing
high
coverage
values
(Corriero
1990;
Corriero
et
al.
1996b;
Mercurio
et
al.
2001).
Species
studied
Geodia
cydonium
(Jameson
1811)
is
a
well-known
Atlantic—Mediterranean
demosponge
that
is
very
com-
mon
in
sciaphilous
environments
(Uriz
1981).
At
the
study
sites,
in
addition
to
rich
populations
of
sessile
%
ON
200
Km
Fig.
1
The
study
sites
with
the
sampling
areas
of
sessile
(black
squares:
al,
bl)
and
non-sessile
(white
squares:
a2,
b2)
specimens.
a
Marsala
lagoon
(NW
Sicily,
37°14';
12°40').
b
Porto
Cesareo
basin
(SW
Apulia,
40°15';
17°54')
b.
:
-
.
.
PORTO
CESAREO
b2
IONIAN
SEA
0
1Km
491
specimens,
many
non-sessile
specimens
also
occur
(Corriero
1990;
Mercurio
et
al.
1997a,b,
2001).
In
Marsala
lagoon,
the
sessile
specimens
settle
pri-
marily
on
the
rhizomes
of
P.
oceanica,
at
depths
ranging
between
0.5
and
2
m
(Corriero
1989)
(Fig.
la,
al).
The
non-sessile
specimens
occur
on
the
soft
bottom
of
the
lagoon
in
wide
storage
areas,
about
2
m
deep
(Mercurio
et
al.
1997a)
(Fig.
la,
a2).
In
Porto
Cesareo
basin,
the
sessile
specimens
of
G.
cydonium
mainly
colonize
the
middle
part
of
the
basin
on
rocky
calcareous
substrates,
at
a
depth
of
1-2
m
(Fig.
lb,
b1).
The
non-sessile
specimens
are
located
in
an
adjacent
area
but
on
soft
bottoms
(Mercurio
et
al.
1997b)
(Fig.
1
b,
b2).
Sampling
protocol
In
each
site,
sampling
of
G.
cydonium
was
carried
out
using
SCUBA
equipment;
the
collection
of
data
was
limited
to
the
two
areas
where
sessile
and
non-sessile
specimens
were
observed
during
previous
research
(Corriero
et
al.
1984;
Corriero
1989,
1990;
Mercurio
et
al.
1997a,b;
Gherardi
et
al.
2001)
(Fig.
la,
b).
Due
to
the
remarkable
population
dispersal
at
Porto
Cesareo,
with
respect
to
Marsala
(Corriero
et
al.
1984;
Corriero
1989,
1990;
Mercurio
et
al.
1997a,b),
sponges
were
collected
inside
sampling
squares
differing
in
dimensions:
a
square
of
25
m
2
at
Porto
Cesareo
and
a
square
of
5
m
2
at
Marsala.
Sampling
was
carried
out
with
seasonal
frequency
(January,
April,
June,
and
October
2002)
in
the
areas
al,
a2,
bl,
and
b2
(Fig.
la,
b).
Sessile
specimens
were
collected
by
removing
the
rhi-
zomes
of
P.
oceanica
(Marsala)
or
by
scraping
the
cal-
careous
substrate
(Porto
Cesareo).
Non-sessile
specimens
were
collected
using
a
dredge
with
a
5
mm
mesh
size
(knot-to-knot).
Immediately
after
collection
specimens
were
fixed
in
4%
formaldehyde
in
sea
water.
The
density
values
of
both
the
sessile
and
non-sessile
specimens
were
evaluated.
For
each
specimen,
measurements
of
volume
and
shape
were
taken.
Volumes
(ml)
were
measured
by
enveloping
each
specimen
with
a
plastic
film
and
dipping
it
into
a
graduated
beaker
containing
water;
the
size
distri-
bution
is
expressed
as
the
logarithm
of
volume
+
1.
The
shape
of
sessile
and
non-sessile
specimens
is
expressed
as
the
ratio
between
the
maximum
base
diameter
and
the
maximum
height
(shape
coefficient).
Epibiotic
and
sediment
covering
on
the
sponge
surface
Before
collection,
a
semi-quantitative
analysis
of
epibi-
otic
organisms
was
carried
out
covering
the
external
surface
of
each
specimen
(expressed
as
a
percentage)
using
a
15x15
cm
2
,
divided
by
a
nylon
thread
into
25
smaller
squares
of
side
3
cm.
A
semi-quantitative
abundance
index
(according
to
Relini
2003)
was
assigned
to
the
epibiotic
organisms
in
each
smaller
squares
to
indicate
their
coverage
(
+
=
<
1%;
1=
<
5%;
2
=
5-25%;
3
=
25-50%;
4
=
50-75%;
5
=
75-100%).
The
coverage
reported
for
each
sponge
specimen
is
the
mean
value
of
the
25
smaller
squares.
The
most
common
epibiotic
organisms
were
identified.
Moreover,
a
quan-
titative
estimation
of
the
sediment
layer
covering
the
sponge
surface
was
made
using
a
small
corer
(2
cm
in
diameter).
Two
samples
were
collected
for
each
speci-
men
by
introducing
the
corer
vertically
into
the
sponge
to
a
depth
of
2
cm.
The
sediment
layer
was
removed
from
the
sponge
fragment
and
then
the
dry
weight
sed-
iment
was
measured.
In
order
to
investigate
the
cortical
skeletal
ultra-
structure
of
sessile
and
non-sessile
sponges,
fragments
of
sponge
tissue,
dissected
from
some
specimens
before
their
fixation
in
toto,
were
fixed
for
scanning
electron
microscopy
(SEM)
in
2.5%
glutaraldehyde
in
a
buffer
of
cacodylate
and
filtered
sea
water
to
a
final
pH
7.4.
After
2
h,
the
fixed
material
was
then
dehydrated
in
a
graded
ethanol
series
and
critical
point
dried
using
a
CO
2
Bal-
Tec
CPD
030
apparatus,
mounted
on
stubs
with
silver-
conducting
paint,
and
coated
with
gold
palladium
(20
nm)
in
an
Emitech
K
550X
Union
Evaporator.
Specimens
were
observed
under
an
SEM
Philips
XL30
at
an
accelerating
voltage
of
10
kV.
Spicule
size
and
silica
content
For
morphometric
analysis
of
the
spicule
size,
sponge
samples
(0.5
cm
3
)
were
cleaned
by
boiling
in
nitric
acid
65%,
washed
repeatedly
in
distilled
water
and
alcohol,
and
gently
agitated
to
suspend
the
spicules.
Thereafter,
the
spicule
suspension
was
spread
over
a
microscope
slide
and
evaporated
to
dryness.
The
average
seasonal
diame-
ters
of
the
cortical
sterraster
and
choanosomal
oxyaster
were
calculated
by
measuring
50
randomly
selected
spic-
ules
for
each
slide.
The
silica
concentration
of
the
cortex
and
choano-
some
was
determined
for
each
specimen
by
the
following
procedure:
a
piece
of
sponge,
measuring
about
1
cm
i
,
was
weighed
after
dehydration
at
100°C
for
24
h,
and
the
silica
was
subsequently
eliminated
by
digestion
with
5%
hydrofluoric
acid
for
12
h.
After
drying,
the
sample
was
again
weighed
and
the
silica
content
estimated
by
calculating
the
difference
between
the
two
dry
weights.
Environmental
parameters
Samples
of
soft
bottoms
were
collected
from
areas
inhabited
by
non-sessile
sponges
(a2
and
b2)
in
order
to
obtain
the
particle-size
analysis.
Sampling
was
performed
using
a
corer
(3.2
cm
in
diameter)
introduced
vertically
into
the
sediment
for
20
cm.
The
sediment
was
divided
into
two
fractions
by
wet
sieving
(63
µm).
The
larger
fraction
(gravel
and
sand)
was
then
analysed
dry
using
a
mechanical
sieve
shaker,
and
the
fraction
less
than
63
gm
(silt
and
clay)
by
determination
of
sedimentation
rate.
Moreover,
redox
measurements
were
made
in
situ
according
to
Bagander
(1976).
NUMBER
OF
SPECIMENS
b
12
11
10
9
8
7
6
5
4
3
2
1
0
r
Fr
492
Results
Population
structure
Marsala
A
total
of
50
sessile
specimens
of
G.
cydonium
and
19
non-
sessile
ones
were
collected
over
the
sampling
period.
The
mean
density
of
sessile
specimens
was
2.5
±
0.48/m
2
,
ranging
from
2.2/m
2
recorded
in
October
to
3.0/m
2
recorded
in
January;
the
mean
density
of
non-sessile
specimens
was
0.95
±
0.1/m
2
ranging
from
0.8/m
2
recorded
in
July
to
1.0/m
2
recorded
in
January,
April,
and
October.
Seasonal
monitoring
did
not
show
significant
dif-
ferences
in
the
volume
distribution
of
the
sessile
and
non-sessile
specimens
(P>
0.05;
Kruskal—Wallis
test).
The
sessile
specimens
had
larger
dimensions
than
the
non-sessile
ones,
their
mean
volume
being
525.74
±
1012.4
vs
102.89
±
97.53
ml
(P<
0.05
Mann—
Whitney
U-test).
The
analysis
of
the
volume
frequency
histograms
(Fig.
2a)
indicated
a
concentration
of
the
non-sessile
specimens
in
the
middle
classes,
with
two
peaks
in
the
distribution
between
3.5
and
4.5
(corre-
sponding
to
values
ranging
between
33
and
90
ml).
The
sessile
specimens
showed
a
wider
distribution
up
to
8.5
(4,913
ml).
The
non-sessile
specimens
lacked
small
size
classes
(
<
3
=19
ml).
Porto
Cesareo
A
total
of
47
sessile
specimens
of
G.
cydonium
and
21
non-sessile
ones
were
collected.
The
mean
density
of
sessile
specimens
was
0.47
±
0.075/m
2
,
ranging
from
0.36/m
2
recorded
in
January
to
0.52/m
2
recorded
in
April
and
July;
the
mean
density
of
the
non-sessile
Fig.
2
Geodia
cydonium:
volume—class
distribution
(expressed
as
In
volume
+
1)
of
sessile
and
non-sessile
specimens
collected
throughout
the
sampling
period
in
Marsala
lagoon
(a)
and
in
Porto
Cesareo
basin
(b)
12
-
11
-
10-
9-
8-
7-
6-
5-
4-
3-
2-
1-
0
SESSILE
SPECIMENS
MEAN
VALUE
=
525.74+1012.4
ml
NON-SESSILE
SPECIMENS
MEAN
VALUE
=
102.89197.53
ml
AA
A
I
11
tr•
4,
4) 4)
VZ
4)
CO
4)
C%
41
VS
.7:
tri
47i
6
1,
)
1
4
IA
4
6
VS
4
1
)
C'e
CP
.
CD
''"4
mi
4,
se
4
ce
LN
(VOLUME+1)
SESSILE
SPECIMENS
MEAN
VALUE
=
2050.2112850.72
ml
D
NON-SESSILE
SPECIMENS
MEAN
VALUE
=
515.71+599.65
ml
rV
43
t."P
4,f)
r;,i
Re)
ei
in
7
'?
r.5
17:
LN
(VOLUME+1)
cip
te)
te)
Cid
/1
1
7
CM1
.
46
IN.
sn:
0
6
oc
r
4:4
I I I
I F
i
ia..1
i
t
T
le7
e
yle7
+n7
0-,
lc,
wr
'CI
v,
r•-
r2
r
n
tr
rn
4
,
1
vS
tip
rry
fr)
•I:
VS
LN
(VOLUME-I-I)
op
oz;
od
493
SHAP
E
COEFFICIENT
(
DIA
METER
I
HIGHT)
a)
Fig.
3
Geodia
cydonium:
shape
coefficient
distribution
in
the
volume
classes
(mean
value
±
SD)
of
sessile
and
non-
sessile
specimens
in
Marsala
lagoon
(a)
and
in
Porto
Cesareo
basin
(b)
5
-
SESSILE
SPECIMENS
MEAN
VALUE
=
1.660.28
4.5
-
ONON-SESSI
LE
SPECIMENS
MEAN
VALUE
=
1.540.42
4
-
3.5
3
2.5
2--
1.5
1
0.5
0
I
v
.
)
1
1
1
'1
4e)
r',4
rp
in
7
LN
(VOLUME+I)
r
,
or
h
c:P1
tz
rn
*6
4i
kz;
r.
r:
ap
cci
5
4.5
4-
3.5
3-
2.5
2--
1.5
1
0.5
0
specimens
was
0.21
±
0.02/m
2
,
ranging
from
0.2/m
2
recorded
in
January,
April,
and
July
to
0.24/m
2
recorded
in
October.
The
mean
volume
of
the
sessile
specimens
was
about
four
times
higher
than
that
of
the
non-sessile
ones
(2,050.21
±
2,850.72
vs
515.71
±
599.65
ml),
even
if
the
statistical
analysis
with
the
U-test
(P
=
0.47)
did
not
re-
veal
significant
differences.
The
analysis
of
the
volume
frequency
histogram
(Fig.
2b)
indicated
a
fairly
wide
distribution.
The
sessile
specimens
showed
a
wider
dis-
tribution
up
to
9.5
(10,000
ml).
The
non-sessile
speci-
mens
lacked
small
size
classes
(
<
3
=19
ml)
and
showed
one
peak
between
6
and
6.5
(corresponding
to
values
ranging
between
33
and
90
ml).
Seasonal
monitoring
did
not
show
significant
differences
between
the
volume
distribution
of
the
sessile
and
non-sessile
specimens
(P>
0.05;
Kruskal-Wallis
test).
Sponge
shape
and
external
covering
Marsala
The
mean
values
of
the
shape
coefficient
of
sessile
and
non-sessile
specimens
(1.66
±
0.28
and
1.54
±
0.42,
respectively)
revealed
a
more
spherical
shape
in
the
latter
specimens
(P
<
0.05
Mann-Whitney
U-test)
(Fig.
3a).
Figure
4a
shows
that
the
sessile
specimens
are
glo-
bous
and
have
mamelons
protruding
from
the
sponge
surface,
where
epibionts
and
sediment
form
irregular
SESSILE
SPECIMENS
MEAN
VALUE
•-=
2.410.77
El
NON-SESSILE
SPECIMENS
MEAN
VALUE
=
2.93±0.57
494
Fig.
4
Sessile
specimens
of
Geodia
cydonium
in
Marsala
lagoon.
a
Detail
of
the
external
surface
of
the
sponge
showing
protruding
mamelons
(M).
b
SEM
view
of
spicule
bundles
(SB)
emerging
from
the
sponge
surface.
c
SEM
view
of
sediment
(S)
entrapped
by
sponge
spicule
bundles
(SB).
d
SEM
view
of
cribrous
regions
without
spicule
bundles
1r"
t.
A
2
4
1
5
cm
ft
d
7
77
s
4
.
4
I
74
patches.
Ultrastructural
investigation
allowed
us
to
ob-
serve
that,
while
in
the
regions
covered
with
sediment
the
spicule
bundles
are
very
frequent
(Fig.
4b)
and
retain
the
sediment
(Fig.
4c),
and
in
the
cribrous
regions
the
sur-
face
is
very
clean
and
spicules
are
almost
absent
(Fig.
4d).
In
these
specimens
the
sediment
layer,
ex-
pressed
as
g/cm
2
(dry
weight),
varied
from
0.02
to
0.11
g/cm
2
(mean
value
0.06
±
0.03
g/cm
2
);
the
covering
of
the
sponge
surface
by
epibionts
varied
from
5
to
35%
(mean
value
19.3
±
9.3%).
The
frondose
bryozoans
Margaretta
cereoides
(Ellis
and
Solander)
together
with
the
macroalga
Caulerpa
prolifera
(Forskal)
Lamouroux
were
the
most
common
epibionts.
Figure
5a
shows
that
the
non-sessile
specimens
are
sub-spherical,
which
is
more
evident
in
cross-sectioned
specimens
(Fig.
5b).
The
slow
circular
water
currents
make
sponges
roll
on
the
soft
substrate.
Most
of
them
(about
90%)
are
covered
with
a
thick
layer
of
the
red
alga
RytyphRiea
tinctoria
(Fig.
5a),
which
is
firmly
at-
tached
to
the
sponge
cortical
region
(Fig.
5c).
SEM
images
show
that
even
though
the
thallus
deepens
into
the
cortex,
algal
tissues
do
not
penetrate
into
the
cho-
anosomal
region
(Fig.
5d).
The
alga
tightly
adheres
to
the
sponge
surface
(Fig.
5e).
Only
on
occasion,
does
the
thallus
penetrate
inwards
throughout
the
opening
out-
side
of
the
aquiferous
canal
system.
The
covering
of
the
sponge
surface
with
R.
tinctoria
varied
from
70
to
85%
(mean
value
78.3
±
7.64%);
the
sediment
coat
ranged
from
0.08
to
0.16
g/cm
2
(mean
value
0.12
±
0.04
g/cm
2
).
Porto
Cesareo
The
comparison
between
the
mean
value
of
the
shape
coefficient
calculated
for
sessile
and
non-sessile
sponges
.
4-
'S O
.
411111
r
--
1
Cm
I
6
4
L
AT
r.
t
10
...
.1
.
CL
CL
A
CL
tritn-
-6-).
0.1
1
1.•
1•1
1
4
4,
3
cm
495
NIA
'
W
.
Fig.
5
Non-sessile
specimens
of
Geodia
cydonium
in
Marsala
lagoon.
a
A
subspherical
specimen
covered
with
a
thick
layer
of
the
red
alga
Rytyphloea
tinctoria.
b
Cross
section
of
the
specimen
in
(a).
c
Algal
thallus
(T)
tightly
adherent
to
the
thin
cortical layer
(CL).
d
SEM
view
of
the
algal
thallus
(AT)
deepening
into
the
cortical
layer
(CL).
e
SEM
view
detail
of
the
relationship
between
the
algal
thallus
(AT)
and
the
cortical
layer
(CL).
Note
the
tight
adhesion
between
sponge
and
algal
tissues
(arrows)
0.5
mm
(2.41
±
0.77
and
2.99
±
0.57,
respectively)
indicates
that
the
latter
individuals
show
a
more
evident
flattened
shape
(P
<
0.05,
Mann-Whitney
U-test)
(Fig.
3b).
Sessile
(Fig.
6a)
and
non-sessile
specimens
(Fig.
7a)
show
a
massive
shape,
even
though
the
sessile
sponges
are
more
massive
and
have
a
brain-shaped
pattern.
The
sessile
individuals
are
covered
with
a
continuous
layer
of
macrobenthic
organisms
and
sediment
(Fig.
6a).
Cross
sections
of
sessile
individuals
examined
under
a
stereomicroscope
reveal
the
particular
thickness
of
the
cortical
layer
(Fig.
6b),
a
feature
confirmed
by
SEM
images
that
show
numerous
and
densely
packed
sterr-
asters
filling
this
layer
in
its
extension
(Fig.
6c).
Spicule
bundles
emerge
from
the
cortex
and
form
a
dense
pali-
sade
that
entraps
sediment
and
algal
filaments
(Fig.
6d).
Sediment
on
the
sponge
cortical
surface
varied
from
0.13
to
0.48
g/cm
2
(mean
value
0.30
±
0.11
g/cm
2
),
while
the
covering
by
epibionts
varied
from
10
to
70%
(mean
value
32.3
±
18.01%);
epibiotic
organisms
consisted
of
algae
(mainly
Padina
pavonia
(Linneus)
and
various
species
belonging
to
Cystoseira),
but
many
species
of
demosponges
(mainly
Tedania
anhelans
Lieberkiihn),
hydroids,
bryozoans,
and
worms
were
also
present.
The
non-sessile
sponges
are
characterized
by
an
upper
surface
similar
to
that
of
the
sessile
individuals
(Fig.
7a),
whereas
the
lower
part,
in
contact
with
the
substrate,
lacks
both
epibionts
and
sediment
(Fig.
7b).
The
smooth
appearance
of
this
lower
part
is
due
to
the
scraping
of
the
sponge
on
the
soft
bottom.
Owing
to
the
gradual
loss
of
the
integrity
of
the
cortical
region,
spicule
bundles
break
and
their
main
axes
adhere
to
the
sponge
surface
where
they
are
amidst
the
sterrasters
(Fig.
7c).
The
scraping
of
the
sponge
on
the
substrate
causes
a
gradual
smoothing
of
the
cortical
sterrasters
that
lose
their
typical
pattern
(Fig.
7d).
Sediment
on
the
sponge
cortical
surface
varied
from
0.11
to
0.20
g/cm
2
(mean
value
0.16
±
0.03
g/cm
2
),
while
the
covering
by
epibionts
varied
from
5
to
20%
(mean
value
10.71
±
4.46%).
Epibiotic
organisms
were
the
same
as
the
sessile
speci-
mens.
Skeletal
features
Marsala
The
mean
values
of
choanosomal
oxyaster
diame-
ter,
calculated
for
sessile
and
non-sessile
specimens,
were
19.83
±
3.41
and
21.65
±
2.66
gm,
respectively
(Fig.
8a,
b).
The
mean
value
of
cortical
sterraster
diameter
was
40.05
±
3.13
gm
in
the
sessile
specimens
and
37.43
±
4.35
gm
in
the
non-sessile
ones.
A
seasonal
I
.
ar-
11
,
1
4
,
P
'
04
‘,
••••••••
CL
-;
20
CM
417
.
7
.7
(
d
APT
'
/4.•%
„.„
•%•
1
.
•1.1k
-
..4••
*
1.1
,
7••
-
•'e......
-4..
a
I
—Is
te•
-
f
e
_
SB
••
1.
Fit
-...
#
.:44-
Ah„
-
•:1
...t•-,.
..•
-
,41
1,-.'''''
-..
4
'.•••
'-
iS•
Q
I I
f*
-••••
.
7r
••••
J
i
J
i
t
,
,
A!*,.ge
,i:
-.-00
-
1/4
-7•m-7
.
--4
496
Fig.
6
Sessile
specimens
of
Geodia
cydonium
in
Porto
Cesareo
basin.
a
A
specimen
covered
with
a
layer
of
macrobenthic
organisms.
b
Cross
section
showing
the
thick
cortical
layer
(CL).
c
Detail
under
SEM
showing
the
thick
cortical
layer
(CL)
filled
up
by
densely
packed
sterrasters.
d
Detail
under
SEM
of
spicule
bundles
(SB)
that
formed
a
palisade
emerging
from
the
sponge
cortex.
Note
that
sediment
(S)
and
algal
filaments
(AF)
are
intermingled
with
the
spicules
comparison
of
spicule
diameter
both
in
sessile
and
non-
sessile
specimens,
showed
no
significant
differences
(P>
0.05;
Kruskal-Wallis
test).
In
contrast,
cortical
sterraster
diameters
were
significantly
higher
in
the
sessile
specimens
than
in
the
non-sessile
ones
(P
<
0.05,
Mann-Whitney
U-test)
while
no
significant
differences
were
noted
for
the
choanosomal
oxyaster
diameters
between
the
sessile
and
non-sessile
specimens
(P>
0.05,
Mann-Whitney
U-test).
As
for
the
choanosomal
silica
content,
the
mean
values
observed
in
the
sessile
and
non-sessile
specimens
were
58.78
±
7.26%
and
53.99
±
10.57%,
respectively.
The
cortical
silica
content
was
68.03
±
10.51%
in
the
sessile
specimens
and
56.19
±
11.84%
in
the
non-sessile
ones
(Fig.
8c,
d).
A
seasonal
comparison
of
choanosomal
and
cortical
silica
content
both
in
sessile
and
non-sessile
specimens
showed
no
significant
differences
(P>
0.05;
Kruskal-Wallis
test).
The
choanosomal
silica
content
was
not
significantly
different
between
the
sessile
and
non-
sessile
specimens
(P>
0.05,
Mann-Whitney
U-test).
In
contrast,
the
cortical
silica
content
was
significantly
higher
in
the
sessile
specimens
than
in
the
non-sessile
ones
(P
<
0.05,
Mann-Whitney
U-test).
Porto
Cesareo
The
mean
value
of
choanosomal
oxyaster
diameters
observed
in
the
sessile
and
non-sessile
specimens
were
23.96
±
1.85
and
20.53
±
3.82
gm,
respectively
(Fig.
9a,
b).
The
mean
value
of
cortical
sterraster
diameter
was
70.05
±
2.35
gm
in
the
sessile
specimens
and
71.18
±
3.34
gm
in
the
non-sessile
ones
(Fig.
9a,
b).
A
seasonal
comparison
of
spicule
diameter
in
sessile
and
non-sessile
specimens,
showed
no
significant
differences
(P
>
0.05;
Kruskal-Wallis
test).
In
addition,
the
cho-
anosomal
oxyaster
and
cortical
sterraster
diameters
were
not
significantly
different
between
the
sessile
and
non-sessile
specimens
(P>
0.05,
Mann-Whitney
U-test).
1
4"•
-;,,v
4
6
41-
'sea",
e
.
0.3
im
A
,
0.03
mm
10
cm
11 11
,
r
.
.
4'•-
*
kt
b
10
cm
497
Fig.
7
Non-sessile
specimens
of
Geodia
cydonium
in
Porto
Cesareo
basin.
a
The
upper
surface
of
a
non-sessile
specimen
covered
with
sediment
and
macrobenthic
organisms.
b
The
lower
part
of
the
same
specimen
of
(a),
lacking
epibionts
and
sediment.
c
SEM
view
of
broken
spicule
bundles
(SB)
adherent
to
the
outermost
sponge
surface.
d
SEM
view
of
some
cortical
sterrasters
(asterisks)
made
smooth
by
scraping
on
the
substrate
As
for
the
choanosomal
silica
content,
the
mean
values
observed
in
the
sessile
and
non-sessile
specimens
were
69.06
±
4.17%
and
72.56
±
5.27%,
respectively.
The
cortical
silica
content
was
82.92
±
2.9%
in
the
sessile
specimens
and
83.91
±
3.93%
in
the
non-sessile
ones
(Fig.
9c,
d).
The
seasonal
comparison
of
the
silica
con-
tent
of
sessile
and
non-sessile
specimens
showed
no
sig-
nificant
differences
(P>
0.05,
Kruskal—Wallis
test).
Both
the
cortical
and
choanosomal
silica
contents
were
not
significantly
different
between
the
sessile
and
non-sessile
specimens
(P>
0.05,
Mann—Whitney
U-test).
Environmental
parameters
Marsala
Particle
size
analysis
of
the
sediment
collected
in
the
area
a2,
showed
high
contents
of
the
smallest
fractions
(silt
=
22.44%
and
clay
=
24.25%)
(Table
1).
Moreover,
redox
measurement
showed
the
presence
of
a
negative
electric
potential
(-110
mV)
starting
from
1
cm
deep;
this
negative
electric
potential
gradually
increased
with
depth
(Table
1).
Porto
Cesareo
Particle
size
analysis
of
the
sediment
collected
in
the
area
b2,
showed
a
low
content
of
the
smallest
fractions
(silt
=
0.28%
and
clay
=
0.76%),
while
the
sand
fraction
prevailed
(92.55%)
(Table
1).
In
the
same
area,
redox
measurement
showed
the
presence
of
a
weak
negative
electric
potential
(-26
mV)
starting
from
3
cm
under
the
bottom
surface
(Table
1).
Discussion
and
conclusions
Present
data
and
literature
records
(Labate
1968;
Par-
enzan
1976;
Pulitzer-Finali
1983;
Corriero
et
al.
1984,
1996b;
Corriero
1990;
Mercurio
et
al.
2001)
show
that,
in
the
studied
sites
of
Marsala
and
Porto
Cesareo,
G.
cydonium
is
a
very
persistent
species.
It
occurs
with
sessile
and
non-sessile
specimens,
the
former
constituting
the
largest
portion
of
the
populations
in
both
environ-
ments,
and
characterized
by
high
abundance
values
and
large
size.
Rich
populations
and
cases
of
gigantism
have
also
been
previously
reported
for
other
demosponges
(Cliona
copiosa;
Phorbas
paupertas)
from
these
Medi-
terranean
environments
(Corriero
1989;
Mercurio
et
al.
2001).
As
regards
G.
cydonium,
several
massive
speci-
mens,
up
to
1
m
in
diameter,
have
been
described
by
Parenzan
(1976)
for
the
basin
of
Porto
Cesareo.
Santucci
(1922)
mentioned
the
finding
of
a
specimen
of
29
kg
(dripped
weight),
from
a
bay
on
the
east
Adriatic
coast.
Such
large
sizes,
probably
the
largest
among
Mediter-
ranean
shallow
demosponges,
exclusively
occur
in
specimens
inhabiting
sheltered,
shallow
environments
and,
according
to
Corriero
(1990),
may
be
related
to
favourable
trophic
and
hydrodynamic conditions.
However,
the
large
size
could
also
be
dependent
on
other
ecological
factors,
such
us
the
availability
of
silica
in
the
water.
It
is
well
known
that
the
increase
of
silica
in
the
water
may
enhance
the
silica
content
and
the
spicule
size
in
many
species
of
demosponges
(Jorgensen
1944,
1947;
Hartman
1958;
Stone
1970;
Pe
1973;
Frohlich
and
Barthel
1997;
Schonberg
and
Barthel
1997).
Our
data
show
that
at
Porto
Cesareo,
where
significantly
higher
DIAMETER
(R
m)
b
100
90
80
70
60
50
40
30
20
10
0
DIAMETER
(R
M)
C
SILICA
CONTENT
(
%)
SILI
CA
CONTENT
(
%)
SESSILE
SPECIMENS
100
90
80
70
60
50
CHOANOSOMAL
OXYASTERS
CORTICAL
STERRASTERS
40
i•..i
fif
30
20
p..of
10
0
(•
-
(h
e;‘)
,
0)
en
0)
'a
0)
0)
h
b
h
/*,
h
co
h
q
h
ON
Vj
(7:
VS
'7
7
vi
0)*
vi
b
VJ
n
vi
0
6
(A
c")
O
cz*
ti ti
N
N
M
•••,
O
er:
‘o•
o
z
e,
LN
(VOLUME+1)
SESSILE
SPECIMENS
100
oCHOANOSOME
90
CORTEX
80
70
60
50
40
30
20
10
0
NON-SESSILE
SPECIMENS
o
CHOANOSOMAL
OXYASTERS
CORTICAL
STERRASTERS
est
h
M
h
7
hhhbh
h
co
h
c<5
Vi
q
ev
es/
en
en"
ep
7
V7
Vi
b
IN
(N
.
OD
O6
Di
LN
(VOLUME+1)
d
NON-SESSILE
SPECIMENS
100
-
90
-
80
-
70
-
60
-
50
-
40
-
30
-
20
-
10
-
0
498
a
0
CHOANOSOME
CORTEX
est
h
M
h
o
hhhbh
h
co
h
esi
le)
en
.
<4
h
h
/N
.
h
cc
.
Vj
41
4:4
,
cc
.
ti
(4
(Ni
(
4
)
(n*
@
.
0:
(<
(N
.
0
0
0
6
ci
LN
(VOLUME+1)
,
e)
ti
0)
N
h
0)
0)
"0.
0) 0)
h b h
IN
IzS
V
.
)
h
(•i
v
ei
vS
o
vi
v)*
ReS
b
y
01
,
0s
(
Nt
(•1
M
(vi
ti•
LN
(VOLUME+1)
0)
co
0)
*)
y
06
vS
0)*
I,'
06
06
04
Fig.
8
Geodia
cydonium
in
Marsala
lagoon.
Volume—class
distribution
of
the
spicule
size
(mean
value
±
SD)
in
sessile
(a)
and
non-sessile
(b)
specimens.
Volume—class
distribution
of
the
silica
content
(mean
value
±
SD)
in
sessile
(c)
and
non-sessile
(d)
specimens
silica
water
concentrations
occur
(Mercurio
et
al.
2000),
the
specimens
of
G.
cydonium
are
characterized
by
a
larger
amount
of
silica,
larger
cortical
spicules,
and
larger
body
size
than
those
of
Marsala.
Therefore,
a
strong
skeletal
support,
related
to
a
high
availability
of
water
silica
content,
could
play
a
role
in
the
increase
in
the
sponge
size.
From
the
bulk
of
data
reported
in
the
current
liter-
ature,
G.
cydonium
is
a
sciaphilous
species
with
a
wide
bathymetric range.
In
shallow
waters,
it
is
common
in
caves,
crevices
or
under
large
stones (Uriz
1981;
Pulitzer-
Finali
1983).
In
the
high
light
exposed
bottoms
of
Marsala
and
Porto
Cesareo,
G.
cydonium
protects
itself
against
high
solar
radiation
with
a
sediment
layer
and
an
epibiotic
covering,
both
set
on
the
external
surface
of
the
sponge.
The
deposition
of
sediment
and
the
settlement
of
epi-
biotic
organisms
may
be
favoured
by
the
occurrence
of
a
palisade
structure
consisting
of
monoaxon
spicules
protruding
from
the
cortical
surface
of
the
sponge.
The
occurrence
of
such
a
protective
screen
is
also
observed
in
other
sponges
living
in
these
two
studied
sites,
for
example,
Tethya
aurantium
and
T.
anhelans
(both
sites),
Stelletta
stellata
(Porto
Cesareo)
(Corriero
et
al.
unpublished
data).
In
particular,
in
the
non-sessile
specimens
from
Marsala
lagoon,
the
protection
from
solar
radiation
is
largely
due
to
the
thick
coat
formed
by
the
red
alga
R.
tinctoria,
which
is
almost
constantly
associated
with
the
non-sessile
sponges.
The
algal
thallus
seems
to
contribute
to
strengthening
the
cortical
layer
of
the
sponge
body,
thereby
partially
substituting
the
skeletal
components,
as
indicated
by
the
lower
values
of
cortical
silica
content
and
cortical
spicule
size
than
those
of
the
sessile
ones.
The
sessile
specimens,
which
grow
on
the
rhizome
of
P.
oceanica,
show
a
lower
level
of
sedi-
ment
covering
the
sponge
surface.
In
this
habitat,
how-
ever,
phanerogam
leaves
screen
the
sponge
surface
against
the
solar
radiation.
The
most
relevant
feature
of
G.
cydonium
in
the
studied
environments
consists
of
the
development
of
the
free-living
habitus.
In
such
sink
lagoons,
the
partially
sediment-buried
habitus
is
quite
common
(Corriero
1989;
Ilan
and
Abelson
1995;
Rutzler
1997;
Calcinai
et
al.
2001),
whereas
unattached,
free-living
forms
are
oo
<e)
q
<e)
4
4
°C
.
4
e>
n 0
06
0
d
100
90
80
70
60
50
40
30
20
10
0
SILI
CA
CONTENT
(
/0
)
NON-SESSILE
SPECIMENS
CHOANOSOMAL
OXYASTERS
CORTICAL
STERRASTERS
...fif
i
.f
MX_
I
0
DI
AMETER
(R
m
)
b
100
-
90
-
80
-
70
-
60
-
50
-
40
-
30
-
20
-
10
-
499
a
DIAMETER
(r
i
m)
SILI
CA
CONTENT
(
%)
4e)
,
•44,
h
N
h
et
4e)
7 h
4e.)
h b h
N
4e.)
oo
4e.)
0
,
)
ce
4'
44
v
V
47)
.
44
•te'
4
Vi
<4
b .
44
4,
ifS
co*
ifS
ci
,
)*
O
",
,
s1
esi
"1
"5
7
0:
V1
4n
.
b
‘e
,
c.6
LN
(VOLUME+1)
SESSILE
SPECIMENS
100
CHOANOSOMAL
OXYASTERS
90
CORTICAL
STERRASTERS
80
70
60
50
40
30
20
10
0
rp
4
e)
•••
4
e)
4e)
<e)
b
4e)
N
h
‘z
6
44
7%
<e)
e;i
<e)
rp"
t
n
.
QJ
0
4
4
N
.
0 0
N
M
CI:
Vj
Vj
0
0
g
LN
(VOLUME+1)
SESSILE
SPECIMENS
100
-
CHOANOSOME
90
°
CORTEX
80
cl
orT
t
'
W
70
60
50
40
30
20
10
0
4
e,
,
7
N
h
M
,
n
7
,
n
h
<e)
b
<e)
I
,
en
oo
<e)
0
V)
';:=
7
VS
'7
VS
'7;
VS
'7)
.
7,
t'
VS
V:
VS
'd
N
tei
c)*
N
en
7.
R
•Ti
44
44
)4
b
LN
(VOLUME+1)
NON-SESSILE
SPECIMENS
CHOANOSOME
CORTEX
V
0
N
h 7 ,
e)
a h
te)
4t
b
4t
N
,
e)
0
0
,
n
q
,
n
O
4
e)
7:
4
eS
N
4
n
7.
4
e)
0
.
4
4
N
.
VS
°C.
4
eS
0
)
.
0
CS
""+
e":
rst
fq
M
'Y
.
7
tir
<
4
06
0
6
ci
LN
(VOLUME+1)
Fig.
9
Geodia
cydonium
in
Porto
Cesareo
basin.
Volume—class
distribution
of
the
spicule
size
(mean
value
±
SD)
in
sessile
(a)
and
non-
sessile
(b)
specimens.
Volume—class
distribution
of
the
silica
content
(mean
value
±
SD)
in
sessile
(c)
and
non-sessile
(d)
specimens
Table
1
Particle-size
analysis
and
redox
measurement
in
Marsala
lagoon
and
Porto
Cesareo
basin
Marsala—a2
area
Particle-size
analysis
Redox
Gravel
Sand
Silt
Clay
0.92%
52.39%
22.44%
24.25%
Under
water
surface
Sediment
surface
1
cm
(Under
bottom
surface)
2
cm
(Under
bottom
surface)
3
cm
(Under
bottom
surface)
155
mV
110
mV
110
mV
185
mV
223
mV
Porto
Cesareo—b2
area
Particle-size
analysis
Redox
Gravel
Sand
Silt
Clay
6.41%
92.55%
0.28%
0.76%
Under
water
surface
Sediment
surface
1
cm
(Under
bottom
surface)
2
cm
(Under
bottom
surface)
3
cm
(Under
bottom
surface)
144
mV
123
mV
97
mV
50
mV
—26
mV
rarer.
Temporary
non-sessile
stages
have
been
described
for
sponges
from
soft
bottoms
of
shallow
tropical
environments
(Ayling
1980;
Battershill
and
Bergquist
1990;
Wulff'
1985,
1991).
More
recently,
Bell
and
Barnes
(2002),
monitoring
the
shallow
demosponges
Hymeni-
acidon
perlevis
and
Suberites
ficus
over
a
1-year
period,
noticed
the
presence
of
unattached
forms
showing
fluc-
tuations
in
their
density
in
relation
to
local
current
flow.
500
In
the
studied
lagoons,
the
non-sessile
forms
of
G.
cydonium
are
persistent,
being
repeatedly
reported
over
the
last
15
years
(Mercurio
et
al.
1997a,b,
2001;
Corriero
1989,
1990).
Constant
density
and
size
distribution
val-
ues
have
been
observed
throughout
the
year
of
study,
even
though
they
were
lower
than
those
recorded
for
the
coexisting
sessile
specimens.
The
non-sessile
forms
differ
in
the
two
environments
in
several
morphological
and
structural
features,
but
the
differences
in
body
shape
seem
to
play
the
most
relevant
role
for
enhancing
the
colonization
of
such
incoherent
substrates.
At
Marsala,
the
non-sessile
specimens
of
G.
cydonium
are
fairly
spherical.
As
observed
by
Riggio
and
Sparla
(1985),
owing
to
its
spherical
shape,
the
sponge
is
able
to
roll
on
the
soft
bottom
dragged
by
slow
circular
currents.
This
feature
protects
sponges
from
sinking
into
the
sedi-
ment.
Here,
indeed,
the
large
amount
of
silt
and
clay
on
the
bottom,
determines
the
occurrence
of
a
markedly
re-
duced
anoxic
layer
just
below
the
surface
of
the
sediment.
In
this
environment,
the
possibility
of
exposure
to
a
non-
oxygenated
condition
is
the
major
problem
for
the
unat-
tached
sponges.
In
addition,
the
usual
association
with
the
red
alga
R.
tinctoria,
which
forms
a
thick
and
continuous
layer
around
the
sponge,
allows
non-sessile
specimens
of
G.
cydonium
to
avoid
contact
with
the
substrate.
At
Porto
Cesareo,
the
non-sessile
sponges
inhabit
sandy
bottoms
affected
by
moderate
wave
turbulence
(Corriero
1990)
and
are
flattened.
Their
flattened
shape
widens
the
contact
surface
between
the
body
and
the
substrate,
thereby
reducing
the
risk
of
stranding.
The
signs
of
abrasion,
evident
on
both
cortical
spicules
and
outermost
sponge
surface,
suggest
that
these
sponges
rub
on
the
bottom.
The
free-living
habitus
of
G.
cydonium
may
have
a
particular
relevance
in
affecting
the
local
and
temporal
distribution
of
this
species
and
may
play
a
pivotal
role
in
enhancing
the
colonization
process
of
environments,
often
lacking
hard
substrates.
In
conclusion,
the
documented
persistence
of
unat-
tached
specimens
of
G.
cydonium
in
semi-enclosed
bays
suggests
that
these
free-moving
sponges
can
be
regarded
as
the
expression
of
a
peculiar
strategy
for
survival.
Further
investigations
on
the
reproductive
features
and
relationships
between
sessile
and
non-sessile
forms
may
prove
if
this
phenomenon
is
actually
adaptive
and
how
it
can
contribute
to
population
maintenance.
Acknowledgements
This
work
was
financially
supported
by
the
Italian
Ministero
dell'Universita
e
della
Ricerca
Scientifica
e
Tecnologica
funds
(ex
MURST
40
and
60%).
All
the
experiments
complied
with
the
current
Italian
laws.
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