LA
Hydrobiologia 440: 55-64, 2000.
qq M.B. Jones, J.M.N. Azevedo, A.I. Neto, A.C. Costa & A.M. Frias Martins (eds), Island, Ocean and Deep-Sea Biology.
02000 Kluwer Academic Publishers. Printed in the Netherlands.
A sponge diversity centre within a marine 'island'
James J. Bell & David K.A. Barnes
Department of Zoology and Animal Ecology, U n i v e r s i ~College Cork, Cork, Ireland
E-mail: sponge@indigo. ie
Key words: sponge diversity, Lough Hyne, sublittoral, temperate
Abstract
The exposed and gulf-stream warmed south-west coast of Ireland has a Lusitanean fauna composed of elements
of the colder waters to the north and east, and others from the warmer Mediterranean Sea. Lough Hyne, a small
marine body, is unusual on this coast in being very sheltered, but also in being characterised by many different
niches within a small space (1 km2). Sponges are particularly abundant, morphologically varied and more than
100 species have been described. Species diversity was measured at 6 m intervals on vertical and inclined profiles
(to a maximum of 30 m) at six sites, spanning a range of flow rate and sedimentation regimes. Diversity, richness,
evenness and density varied significantly with both flow regime and depth, but was much lower on the surrounding
Atlantic coast. Four different sponge communities were differentiated on the basis of sponge species assemblages
which correlated with different environmental conditions. At sites of turbulent and fast flow conditions, sponge
diversity and richness were lowest, with the highest values being found at the sites of moderate and high sedimentation. Significant differences were observed in all four ecological variables with respect to substratum angle with the
exception of the site experiencing the most turbulent flow conditions. Lough Hyne was found to possess the second
highest sponge species diversity (H=3.626) and richness (77 species) of all available figures from temperate, polar
and tropical areas (of similar sized sampling area). The uniqueness, diversity and species composition of the sponge
community at this location suggests Lough Hyne is, biologically, a marine island within the island of Lreland.
Introduction
Islands may exist in two forms: true islands which are
geographic land masses (e.g. Ireland) or conceptual
'islands' such as FADS (Fish Aggregating Devices),
hydrothermal vent fields and even boulders. Lough
Hyne is one such conceptual 'marine island' due to
its limited exchange with the adjacent Atlantic Ocean,
with currents inside the Lough only being detected
during inflow of water. The conditions within Lough
Hyne are extremely different from the exposed coastline outside the Lough and, as a result, so are the
sublittoral cliff communities (Picton, 1990).
The intertidal and sublittoral sponge communities
at Lough Hyne are known to be extremely rich with
over 100 species being reported (Van Soest & Weinberg, 1980; Van Soest et al., 1981; Picton, 1990).
Several sponge species found within the shallow waters of Lough Hyne have only been found elsewhere
in deep-water environments (e.g. Eurypon sp.). This
is thought to be due both to the sheltered nature of
the site and the large number of niches within such a
small area (1 km2). Quantification of sponge species
diversity, comparable to other marine ecosystems (see
Diaz et al., 1990; Schmahl, 1990), has not been obtained previously at Lough Hyne, even though species
richness is thought to be extremely high (Lilly et al.,
1953; Van Soest & Weinberg, 1980; Van Soest et al.,
1981; Picton, 1990). Many sponge species, including
those from the genera Polymastia and Eurypon, have
not yet been described with up to 8 new species from
the genus Eurypon and 5 from Polymastia awaiting
description (Bernard Picton, pers corn.).
Specific sponge species diversity within tropical
areas is generally thought to be high (Alcolado, 1990)
providing Shannon Wiener diversity indices ranging
from 0.5 (Diaz et al., 1990) in Venezuela to 3.39 in
Southern Florida (Schmahl, 1990). Published sponge
species diversity values from Polar regions have not
been reported, although sponge species richness ap-
pears to be high (SarB et al., 1992), area effects,
however, need to be considered. Sponges are known to
form a conspicuous part of the sessile fauna in many
sublittoral temperate communities such as Lundy Island (Hiscock et al., 1983) and Strangford Lough (Erwin, 1976). However, these sites are much larger than
Lough Hyne. Quantitative estimates of sponge species
diversity within sublittoral temperate regions have not
been produced. The reason for so few studies providing quantitative information of this nature may be due
to the problems of field recognition of sponge species
(Ackers & Moss, 1987; Bell & Barnes, unpublished
data) and the time required for such surveys.
Water flow is important to many suspension feeding organisms including sponges (Vogel, 1974, 1977,
1978). It is thought that inhalant water flow through
sponges is enhanced by increased ambient flow (Vogel, 1977). Wave action and current speed is also be_ lieved
__--to be important as has been shown by transplant
experiments, resulting in slower growth of sponges in
weak currents (Wilkinson & Vacelet, 1979), and increased stiffness and lower profiles in wave exposed
areas (Palumbi, 1984). Sponpes have also been found
to, show morphological variation in relation to flow
and depth at Lough Hyne (Bell & Barnes, 2000) with
more delicately branching sponges being found where
disturbance is moderate to low.
This study details sponge species diversity, richness, evenness and density from an area of high
sponge diversity in a temperate region. This information is compared with data from both the literature
and those collected by the present authors for sponge
communities found within tropical, polar and other
temperate ecosystems. This study also investigates
changes in the aforementioned population statistics
within Lough Hyne as a function of site, flow regime
and depth.
the in-flowing water moves across the lough, current
speed decreases, resulting in a sedimentation gradient from east to west (Bassindale et al., 1957). Five
sites within Lough Hyne were sampled: Whirlpool
Cliff, Glannafeen, Goleen, West Cliff and Labhra
Cliff and one, Bullock Island, on the adjacent Atlantic coast (Fig. 1). Bullock Island is subject to severe
wave action especially during strong winds and winter
storms, therefore, encrusting organisms are dominant
at this site. Whirlpool Cliff is subjected to strong but
predictable unidirectional flow conditions,(during inflow), hence the environment is favourable to many
organisms. Glannafeen lies between the extremes of
the sedimentation gradient and, therefore, experiences
moderate current flow ( t 1 5 cms-I) which leads to
considerable sedimentation upon cliff surfaces. Cliff
surfaces at Goleen, Labhra and West Cliff are highly
sedimented as only slight water currents ( t 5 cms-I)
move across these cliff surfaces. The sublittoral communities, including sponges found upon the cliff sites
within Lough Hyne, are described in detail by Picton
(1990).
Materials and methods
Ten 0.25 m2 quadrats were sampled at 6 m depth
intervals on both vertical ( x 90") apd inclined (X
45") surfaces at each of the sites. The number of
each species of sponge in each quadrat was recorded.
Photographs and samples were taken of sponges that
could not be identified in situ for laboratory analysis.
Patterns of species diversity were described kith the
Shannon and Weiner (Krebs, 1989) information function H'= - C pi In pi. Paired t-tests were u s e d t o
examine differences between rock surface types.
Bray-Curtis
hierarch. . using
- __ ,..,,--.i similarity analysis,
- .
..
i~~_a~~!~me~at~v~~~r~~~..ave~~~e_~:~~,s!e_:ir!~.~
performed upon data using the unweighted pair s r.---.
oup
method
using
arithmetic
averages
(UPGMAJ
using
-_=*
..._,-...p..
.--Mari,ne
.., - _ .I-i-.
Lqbarat-).
the PRIMER
=--.
programLp_ly.mouth
.*--- Data
were
,t,ran>foO~edd
t_o~e<zc: the -im----*
-. _Io,g.~,~-~l)
ortance
of
extreme
values.
As
sites
were sampled to
..
L--different depths. All data on each surface type from
each depth sampled were combined and then standardised by dividing by total abundance at each site, to
account for different population sizes. Ordination of
samples was performed using Correspondence Analysis upon untransformed data, but standardisation, as
used for Bray Curtis Analysis, was performed upon
species composition data.
-
I,,II_<_
Study site
-
- -
_.ll
_.._.-_~X
Lough Hyne (Fig. 1) is a small ( x l km2) semienclosed sea lough on the south-west coast of Ireland
(5 l o 29' N, 9" 18' W) attached to the adjacent Atlantic
coast by a narrow ( ~ 2 m
5 wide) tidal rapids. Water
flow into the lough is essentially unidirectional due to
a sill in the rapids, meaning the incoming tide must
reach the level of the sill before inflow of water can
begin. This results in inflow lasting only four hours
with currents reaching >35 cms-I and outflow lasilng
eight hours when currents are slight ( > 5 cmspl). As
"
z.s...s..,.q*L
-=.A
.--
-,->
%*.>?-A>-.
".l..--l-r
-
'-
LOUGH
HYNE
0
100
LOO
300
Metres
CLANNAFEEN
BULLOCK
Laugh Hyne
IRELAND
Rgure I. The location of Lough Hyne Marine Nature Reserve In Ireland (Long 51' 29'N, Lat 9' 18' W), with detals of study sites where
sponge species diversity and richness was sampled.
Results
Seventy seven species of sponge were identified at
Lough Hyne and along the adjacent Atlantic coast
(Table l ) , of which only one species (Plakortis simplex) has not been identified previously from Lough
Hyne. Twenty-eight of the 77 species were found on
the----Atlantic
- .--- -----coast~Bullock
-- - - - Island)
--- - -of-..which
--.."-"--only
- _ ,-three
---species were exclusive to t h i ~ ~ ~ t _ ~ ~ ~ Z o p hnlgrzconopsis
ans, Leuconia nivea and Haliclona sp 5). Species
diversity, richness, evenness and sponge density varied
_______I__-
with both depth and flow regime at Lough Hyne MNR
(Figs 2a-d).
At Bullock Island (Fig. 2a), sponge species diversity and evenness were moderate compared to the
other five sites sampled. However, sponge richness
and density were the lowest of the six sites sampled.
All four variables remained relatively constant with
depth. Sponge species diversity, evenness, richness
and density showed no significant variation between
surface type ( p 0 . 0 5 for each t-value). Sponge density at this site was low, with only 20 sponges m-2
Table I. Sponges found at Lough Hyne Manne Nature Reserve
and upon the adjacent Atlantic coast. (* species found within
Lough Hyne and on the surrounding Atlantic coast. ** species
found exclusively on the adjacent Atlantic coast)
inclined surfaces. Density of sponges at Whirlpool
Cliff remained constant with depth, with a mean of
50 sponges rnP2 being found upon vertical surfaces
and 100 sponges mP2 on inclined surfaces. Species
lophon ingalli*
Antho involvens*
diversity was low at this site, but species richness was
Iophonopsis nigricans**
Aplysilla rosea
moderate.
Aplysilla sulfurea*
Lnxosuberites incrustans
At Glannafeen and Goleen (Fig. 2b), the relationLeucosolenia complicata**
Axinella damicornis
ships between species diversity, richness, evenness
Leuconia nivea**
Axinella dissimilis
and density with depth were very similar at both
Mycale contarenii
Biemna variantia
sites: higher values occurred on vertical than on inMycale macilenta
Clathrina coriacea
clined surfaces and all increased with depth reaching
Cliona celata*
Mycale rotalis
a
maximum at 18 rn. However, only sponge species
Myxilla rosacea*
Dysidea fragilis*
d
d
density are significantly higher
Myxilla incrustans
Dysidea pallescens
(pt0.05
for
each
t-value)
on .vertical than inclined
Myxilla jimbriata
E~~eriopsisfucorum*
surfaces. Evenness increased from 0 to 6 m and then
Euiypon sp 1
Pachymatisma johnstonia*
remained relatively constant. Sponge ecology at West
Eurypon sp 2
Paratimea constellata*
Cliff (Fig. 2c) showed similar trends to those found at
Eurypon sp 3
Phakellia sp
Eurypon sp 4
Phorbas jictitius*
Goleen and Glannafeen, except that this cliff extended
Plakortis simplex
Eurypon sp 5
to 24 m, rather than 18 rn at Goleen and Glannafeen.
Eurypon sp 6
Polymastia sp 1
At West Cliff, sponge species diversity, richness and
Eurypon sp 7
Polymastia sp 2
density increased to 18 rn then became constant, but
Polymastia sp 3*
Eurypon sp 8
evenness declined slightly at 12 rn. Species richness
Halichondria bowerbanki
Polymastia sp 4*
at West Cliff was the highest ( ~ 4 species)
0
of all the
Halichondria panicea*
Polymastia sp 5
sites sampled (at a depth of 24 m). Of the four variHaliclona cinerea
Polymastia sp 6
ables, only sponge density was significantly ( p t 0 . 0 5
Polymastia sp 7
Haliclona jistulosa*
for each t-value) higher on vertical surfaces. At Labhra
Polymastia sp 8
Haliclona simulans
Cliff, which extended to 30 m, the same increases with
Haliclona sp l*
Pseudosuberites sulphureus*
depth
occurred with sponge species diversity, even'Rhaphidosiylu kitchingi
Haliclona sp 2*
ness,
richness
and density initially. However, a sharp
Raspailia hispida
Haliclona sp 3*
decrease
in
all
measured variables occurred between
Haliclona sp 4
Raspailia ramosa
24-30
m
(e.g.
from
x H = 3 to H=1.5). Higher variScypha cilatum*
Hnliclona sp 5**
ables
also
occurred
on
vertical surfaces [although only
Stelligera rigida*
Haliclona ~rrceolus
sponge
density
was
significantly
(p>0.05, t =3.55)
Haliclona viscosa
Stelligera stuposa*
higher], which was less pronounced with depth. The
Halicnemia patera
Suberites carnosus
highest sponge diversity ( H = z 3 ) , richness (% 30 speHalisarca sp
S~rberitessp 1
Suberites sp 2*
cies) and density ( ~ 1 5 sponges
Hemimycale columella*
0
rnP2) at this site was
Hymedesmia paupertus*
S~rberitessp 3*
observed at a depth of 24 m.
Hyme~tiacidonperleve*
Suberites sp 4*
Sponge. species diversity at Bullock Island was
Hymernphin stellifern
Tethyn aurantium*
moderate but ri'chness was the lowest of the six
Iophon hyndmnni
sites sampled (both of which remained constant with
depth). The pattern at Whirlpool Cliff was the converse with moderate species richness and low diversity. At Whirlpool Cliff, variation in diversity and
richness
with depth was evident. There was little difbeing found at each depth. Whirlpool Cliff (Fig. 2a)
ference
in
species diversity and richness between the
showed variation in sponge diversity, evenness and
remaining
four sites (Goleen, Glannafeen, West Cliff
richness with depth. All three variables remained reand
Labhra),
with the exception of West Cliff which
latively constant with depth until approximately 12 m,
had
a
higher
species
richness than the other three sites.
after which they increased. Sponge species diversity
Diversity
andrichness
values at 12 rn depth at the
and richness (Fig. 2a) was found to be significantly
six
sites
(Table
2)
showed
the highest diversity ochigher on vertical surfaces (p>0.05 for each t-value),
curring
at
Glannafeen
where
sedimentation and water
however, sponge density was significantly higher upon
.
5
0
10
I5
20
25
Depth (m)
0
5
10
15
20
25
30
Depth (m)
Figure 2. (a-d) Sponge species diversity (Shannon-Weiner), evenness, richness and density with depth at Bullock Island & Whirlpool Cliff (a),
Goleen & Glamafeen (b), West Cliff (c) and Labhra CLiff (d). Filled circles represent data from vertical surfaces and open circles from inclined
surfaces. Significant differences (pc0.05) between vertical and inclined surfaces are indicated by an asterisk (*). Numbers on species diversity
graphs indicate species richness at each depth, with bracketed numbers representing communities from inclined surfaces and non-bracket
numbers from vertical surfaces.
movement is moderate (authors, unpublished data).
Thelowest diversity occurredat Whirlpoolcliff where
flow was strong and unidirectional, -but sedimentat h n was extremely slight (authors, unpublished data).
Moderate diversity and richness was found at Bulibck Island w h e r e t u i b u l e n t d i t i o n s
and
C
I
_
-
secl&entation
was not observed. An overall sponge
diversity of 3.626 and richness of 77 species (at a
depth of 12 m) was calculated within ~ o u Hyne
~ h to
allow comparison with other global sites where these
variables have been measured.
Tohlr 2. Sponse hpecies diversity ( H ' ) and 1.ichness ; I I \ I \ s l r e \:llnplcd oli h o ~ hvcl-tical ( " J aliil inclined (I)
sul-faces within Lous11 l-lyne n l 13 111
S~te
Di\,e~.<i~y
Rlclilics\
Curl-ent flow
Setltmcntstio~i
Disturbance
Bullock
Island
3.02 1 ' .
7.002
10'
II
Tu1-hule11
t
Lo\\'
Whirlpool C l i f
l.SO10".
II
'
10'
7'
Fast
Llnidil-ectional
Lo\\,
Higli
Glannafee~i
3.21 1-91
2'
34"
3 1.'
Modelxte
Unidirectional
h4ode1-ate
Motlel.ate
Very H i ~ h
Golzen
3.056"
2.823'
34"
-3 l '
Slight
High
Low
West Cliff
2.789'*
-.
7 3747
27'
29t
Slight
High
Low
Lahhsa Cliff
2.70Y"
2.895t
21"
18j
Slight
Hi:h
Low
Four different sponge communities were identified
sing Bray-Curtis similarity analysis (Fig. 3a). ( 1 )
Sponges on vertical and inclined surfi~cesat Bullock
Island, ( 2 ) sponges on vertical and inclined surfaces
at Whirlpool Cliff, ( 3 ) sponges on vertical surfaces
and (4) inclined surfaces at Glannafeen, Goleen. West
Cliff and Labhra Cliff. Clustering of inclined and
vertical surfaces at Whirlpool Cliff and Bullock Island showed a lowei- siinilarity ( 2 6 0 % ) than (lid the
sites and surfaces within other sponge communities
identified ( 1 7 0 % ) . Correspondence Analysis (Fig. 3b)
showed a similar prrtier~i~to
that of Bi-ay Curtis Similarity Analysis, with the sanie four communities being
identified. Species compositions ant1 their corresponding abundances were significantly depe~identupon site
(Chi-square 13 192, p<0.05> d.f. 825). The first three
E i ~ e nValues explain 3 6 7 ~ 29%
.
ancl 7 % of the total
iliertia respectively, antl tl~e~.efcjre
tlie initial t~-i]~lot
(Fig. 3b) explains 72% of the tori11 inertia.
Discussion
Sponges are linon.n to be i m p o r t n ~ ~contributors
t
to
mcist suhticiul marine benthic c o l n l n ~ ~ ~ ~ si tpiaens n i n ~
rrop~calI-cefs iAl\!arer et al.. 1090: Diar c'r 31.. 1990:
Scli~iialil.IC)90).f'olar regions (Sa1.3et al.. 1992). the
cleep sea (B;l~-thelr.t al.. I99 I : Bartllel cCr Tenclnl. 1993)
ancl hanl hotrom temperate co~liiii~~nities
(Lill!] et ;)I..
1933: El.\\:i~i.1976: HisCocli et al.. 19s.: 1. .l-lie reas6is
for SLICII \\~iilespseacloccurl.ence of sponzes is due to I:
number of factors including: morphological plasticity
(Palunibi, 1984, 1986). elficient filter feeding (Pile et
a].. l996), symbiotic associations (Vacelet & Donadey,
1977; Burlando et a].. 1988), toxicity to other marine organisms (Porter & Targett, 1988; Huysecom et
a]., 1990) and fast regenerative capabilities (Aylii~g.
1983).
The density of sponges found in the present study
(LIPto 200 sponges 111-~) has shown sponges to be important contributors to the sublittoral cliffs at Lougli
Hyiie. However. variation between sites experiencing
different flow regimes and sedimentation was evident.
At tlie site experiencing the inost turbulent conditions
(Bullock Island). diversity was not the lowest of tlie
sites sampled (Table 2) as might have been expected.
with tlie lowest density occurring at Whirlpool Cliff.
Tlie most liliely re~rson this apparent cliscsepancy
is that fast flowing co~~diric~lls
at Whirlpool Clifl. with
:in almost total absence of wave a c t i o ~ ~
means
.
conclitio~~s
for gro\vtli ~ I Pfavourable for many dii'fei-ent
types ol'oryanisms (includin: anthozoans ;\nil bryozoa ~ ~ sO\;eralI
).
c o r n ~ l l ~ ~ ili\lersity
l ~ i t y is l i i ~ l at
i Whirlpool
l i e d Lrncl so
Cliff (Ma~iglianct Rarlles. ~ ~ n p ~ ~ b l i sclata)
is tlie total co\,esaFe of i ) ~ . y n i h m s(Hanimer. 1996).
Tlic n1ocle1-;ttele\lel of dist~lrh;rncrprobabl!/ maintailis
tlie high c o m m ~ ~ n i ilivel-siry
ty
at this site. Spc~nsesare
yenel-ally efl'rcti\ie conll7etitol-s i l l be~ithicco~lirn~lnities. Iio\\,ever. [hey ;II-ci ~ t ' t r o\:ergro\vli
'~~
by ;~sciciia~is.
soft coi-31s ant1 some .;l)ecies of a l y e (Russ. 1982:
Quilln. 1982: Na~iil;ll;~~mirr.
le)C)3). all of \\:liicIi irre
Bullock Island (Vertical)
1
Bullock Island (Inclined)
2
I
-----Whirlpool Cliff (Vertical)
Whirlpool Cliff (Inclined)
------
-
Labhra Cliff (Vertical)
West Cliff (Vertical)
3
Goleen (Vertical)
I
1
Glannafeen (Vertical)
7
West Cliff (Inclined)
4
-
Labhra Cliff (Inclined)
Goleen (Inclined)
,
Glamafeen (Inclined)
7
Bullock Island (Vertical)
0 Bullock Island (Inclined)
A
Whirlpool Cliff (Vertical)
Whirlpool Cliff (Inclined)
Labhra Cliff (Vertical)
Labhra Cliff (Inclined)
West Cliff (Vertical)
0 West Cliff (Inclined)
Goleen (Vertical)
0
Goleen (Inclined)
Glannafeen (Vertical)
Glannafeen (Inclined)
(a)
Figure 3. Bray-Curtis ~ i m i l a r i j and Correspondence Analysis (b) for sublittoral sponge populations on vertical and inclined surfaces at
six sites of Lough Hyne.
very common at Whirlpool Cliff (authors' pers. obs.).
We suggest space acquisition by superior competitors and lack of available space for colonisation causes
sponge diversity to be low compared with other sites
sampled. Although sponges do not appear most diverse in conditions which seem most favourable, this
becomes less of a paradox when the entire sublittoral
community is considered. At Whirlpool Cliff, sponge
species dkcx.wy. richness and evenness w e n d a m l
to increase between 12 and 18 m. Current flow rate
decreases between these depths (authors' pers. obs.),
resulting in a re'duction in disturbance, enabling more
delicate species (including branching species), with
smaller basal attachment areas (less space required)
to become more abundant (Bell & Barnes, 2000).
At Bullock Island, sponge s t ~ M - richnesg
evenness and density are all relatively constant with
depth. The reason for this is that disturbance is high
at all depths sampled and, despite some reduction in
disturbance with depth, it is still high at all depths.
Wave action is known to affect sponge morphological
types (Palumbi, 1984, 1986), where many of the more
delicately branching species are destroyed by drag and
tissue fatigue in high energy environments (Hiscock,
1983). Therefore, limits of morphological adaptation
for any particular sponge species may reduce species
richness at sites of high wave exposure (e.g. Bullock
Island), with encrusting and robust species being more
suited to this environment. Evenness remained constant with depth at Bullock Island, showing no one
species dominated at any of the depths sampled. At
Whirlpool Cliff, evenness remains low ( t 0 . 6 ) until
12 m, suggesting that only a few species dominate
(richness values of 10 species on vertical and t 5 on inclined surfaces). Below 12 m, evenness then increases
showing no single species to dominate.
At West Cliff, Labhra Cliff and Goleen, where
currents are slight and sedimentation is high, sponge
species diversity, richness, evenness and density all
increased with depth. The low sponge community
composition in shallow water is most likely d u e to
fast growing calcareous algae, which smother sponges
and other sessile organisms. As depth increases, so
does sedimentation (Bell & Barnes, 2000) and hence
light penetration is reduced. Sponges then become a
more conspicuous part of the sessile fauna. Sponges
become more branched with depth to prevent sediment accumulation upon their surfaces as is seen with
other sessile organisms (Chappell, 1980). Water currents at the Glannafeen site are moderate ( t 1 5 cm
s-') as is sedimentation, and sponge communities are
very similar to the highly sedimented sites of West
Cliff and Labhra Cliff. The Bray Curtis and Correspondence analyses performed in the present study
indicated that only three different sponge communities existed within Lough Hyne and one outside on the
adjacent Atlantic coast. The sudden decrease (between
24 and 30 m at Labhra Cliff) which occurred in all four
variables may be due to a seasonal thermocline that
develops within the deep basin on the western side of
the Lough (Kitching et al., 1976; ktching, 1987). The
thermocline develops at approximately 24 m during
summer months, below which an anoxic layer forms
which kills off most of the organisms below this depth.
Many subtidal sponges are slow growing (Ayling,
1983; Fowler & Laffoley, 1993) and so cannot'recolonise cliff surfaces between the annual formation
Table 3. Global values for sponge species divers~ty(H') and richness.
All values shown are taken from a depth of
12 m over similar sized
sampling areas on hard substrata
Location
Spccics Species Source
diversity Richness
Mozambique (Quirimba) 4.07
Ireland (Lough Hyne)
3.63
Southern Florida
3.23
Cuba (Havana)
2.86
Panama (San Blas)
2.45
2.22
Britain (Sussex)
Antarctica (Signy)
2.29
Ireland (Cork)
2.12
Cape Verde (Sal)
1.94
Britain (Cornwall)
1.40
Venezuela
~1.00
73
77
43
80
17
12
19
13
9
6
15
Bell & Bames (unpub.)
Bell & Barnes (unpub.)
Schmahl (1990)
Alcollado (1990)
Bell & Barnes (unpub.)
Bell & Barnes (unpub.)
Bell & Bames (unpub.)
Bell & Bames (unpub.)
Bell & Bames (unpub.)
Bell & Bames (unpub.)
Diaz et a1 (1990)
of the thermocline. Slight currents at this site may
also be responsible for slow growth rates (Wilkinson
& Vacelet, 1979). Some sponges are present below the
thermocline, namely those from the genus Eurypon,
which appear to be able to survive seasonal immersion
in this anoxic layer. Sponges which may survive heat
stress, by means of a smaller resistant stage (Gaino
et al., 1996), may employ similar strategic solutions
to survive anoxic,stress. ~ t ' ~ a b h Cliff,
r a West Cliff,
Goleen and Glannafeen, evenness is low at the surface
( ~ 0 . 6 )suggesting
,
that several'fast growing sponges
dominate, which can compete with-the fast growing
calcareous algae. Evenness increases quickly between
0 and 6 m, meaning no single species dominates as
depth increases.
At Goleen, Glannafeen and Whirlpool Cliff, species diversity was higher on vertical than inclined surfaces. At Goleen and Glannafeen, this probably relates
to the differences in sedimentation falling upon the different surface types, with larger amounts settling upon
inclined surfaces. Increased sediment is thought to reduce water pumping rates in sponges which is likely
to be detrimental to the animal (Gerrodette & Flechsig, 1979). Other effects of sediment upon sponges
are currently unknown. Sponge morphology is-know
to be related to the degree of sedimentation and flow
regime, but whether these variables affect other aspects of sponge ecology still remains undiscovered,
At Whirlpool Cliff, we suggest the higher diversity on
vertical surfaces results from decreased algal populations (not quantified) on vertical surfaces, relating to
reduction in light intensity. No significant differences
were found between species diversity upon vertical
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