This unexpected disappearance of the animal fraction from the diagenesised structure
may be due to the erosion activity of clionid
sponges and their allied species, which might be
considered as the main controlling factor on animal populations settled on shady organogenous
rocks. These sponges develop inside the first centimetre of substrate and attack the calcareous
skeletons of sessile organisms, boring through
their base. This action weakens the substrate as a
whole, leading to the periodical detachment of a
large part of the settled organisms. Since the animal fraction in coralligenous biocenosis generally lives by hanging from the ledges, its fall occurs
by simple gravity.
This is in accordance with the population
dynamics of L. pruvoti studied for a long period
of time (Pronzato et al. 1994), which pointed to a
massive mortality (about 80%) of the young
polyps during the first year since their settlement. The severe impact of clionids on the newly
settled scleractinians is due to the small diameter
of the corallite bases (1-2 mm), compared to the
diameter of the sponge boring chambers.
Practically. one boring chamber inside the corallite is enough to cause its detachment. However,
the larger the base becomes, the greater will be
the polyp stability. Moreover, Harmelin (1990)
suggested that a collar, consisting of an encrusting bryozoan at the base of the corallite. might
preserve the polyp against clionid attack while
the individuals are growing.
The mineral structure of the ledges may
result out of clionids attacking carbonates
deposed by animals and, in particular. by cnidarians, rather than those produced by algae. Among
the 11 boring sponges recorded in the Portofino
waters (Bavestrello et al. 1999). only Cliona
viridis seems to be able to attack the crustose
thalli of coralline algae. However, C. viridis is a
photophilous species living at 10-15 m depth.
Therefore, algae constituting the upper side of
deeper coralligenous ledges cannot be attacked
(Barbieri et a1. 1995; Bavestrello et al. 1996).
Consequently, living Lithophyllum fr0ndo5um,
growing between 30 and 50 m depth, is the calcareous substrate least attacked by boring
species along the diff of the Portofino
Promontory. Probably, when the coralligenous
structure began to form in shallower waters, the
boring activity of the sponges was offset by the
quick growth of algae. After the rise in sea level,
accretion of these bioconstructions, now in deepThe Role of Sponges in Coralligenous Dynamics
239
er waters, certainly decreased. However, at these
depths. no clionid was able to continue its boring
activity, hence these structures were preserved.
In conclusion. boring sponges may be considered as a key-stone group in coralligenous
biocoenosis: eroding the substrate, influencing
reef paleo structure and acting as intermediate
disturbers (Connell 1978), they may represent
one of the strongest forces modelling coralligenous communities, as they do in the coral reefs
(Davies 1983; Sorokin 1993; Becker and ReakaKudla 1997).
Acknowledgements. This work was financially supported by
Italian MURST funds.
References
AdeyWH (1986) Coralline algae as indicators of sea-level. In: Van
de Plaasche 0 (ed), Sea level research: Ii manual for the collection and evaluation of data. Geo Books 9: 229-280
Barbieri M, Bavestrello G, Sar~ M (1995) Morphological and ecological differences in two electrophoretically detected
species of Cliona (Porifera, Demospongiae). Bioi J Linn Soc
54: 193-200
Bavestrello G, Calcinai B, Cerrano C, Pansini M, Sara M (1996)
The taxonomic status of some Mediterranean clionids
(Porifera, Demospongiae) according to morphological and
genetic characters. Bull lnst R Sci Nat Belg, Bruxelles 66:
185-195
Bavestrello G, Calcinai B, Cattaneo-Vietti R, Cerrano C, Pansini M
(1999) Distribuzione e modalita d'erosione di alcune specie
mediterranee eli Clionidi (Porifera, Demospongiae) as80ciati Corallium rubrum. In: Cicogna F, Cattaneo-Vietti R,
Bavestrello G (eds) Mediterranean Red coral and
Gorgonians biology. Ministero per Ie Politiche Agricole
Becker LC. Reaka-Kudla ML (1997) The use of tomography in
assessing bioerosion in corals. In: Proc 8 th Int Coral Reef
Symp Panama 2, pp 1819-1824
Bellan-Santini D (1985) The Mediterranean benthos: reflections
and problems raised by a classification of the benthic assemblages. In: Moraitou-Apostolopoulou M, Kiortsis V (eds)
Mediterranean marine ecosystems. Plenum, New York, pp
19-48
Connell JH (1978) Diversity in tropical rain forests and coral
reefs. Sci NY, 199: 1302-1310
Davies PJ (1983) Reef growth. In Barnes DJ (ed) Perspectives on
coral reefs. Aust Inst Mar Sci, Townsville, pp 69-106
Harme1in IG (1990) Interactions between small sciaphilous
Scleractinians and epizoans in the Northern Mediterranean.
with particular reference to bryozoans. PSZNI Mar Bcol 11:
351-364
Hong JS (1982) Contribution it l'etude des peuplements d'nn fond
de concn!tionnement coralligene dans la region marseillaise
en Mediterran~ Nord-occidentale. Bull KOROl 4: 27-51
Laborel J (1987) Marine biogeniC constructions in the
Mediterranean: a review. Sci Rep Port-Cros Nat! Park Fr
13:97-126
Laubier L (1966) Le coralligene des Alberes: monographie biocenotique. These Doct, Fac Sci Univ Paris
Peres JM, Picard J (1964) Nouveau manuel de bionomie benthique
may be due to the erosion activity of clionid
sponges and their allied species, which might be
considered as the main controlling factor on animal populations settled on shady organogenous
rocks. These sponges develop inside the first centimetre of substrate and attack the calcareous
skeletons of sessile organisms, boring through
their base. This action weakens the substrate as a
whole, leading to the periodical detachment of a
large part of the settled organisms. Since the animal fraction in coralligenous biocenosis generally lives by hanging from the ledges, its fall occurs
by simple gravity.
This is in accordance with the population
dynamics of L. pruvoti studied for a long period
of time (Pronzato et al. 1994), which pointed to a
massive mortality (about 80%) of the young
polyps during the first year since their settlement. The severe impact of clionids on the newly
settled scleractinians is due to the small diameter
of the corallite bases (1-2 mm), compared to the
diameter of the sponge boring chambers.
Practically. one boring chamber inside the corallite is enough to cause its detachment. However,
the larger the base becomes, the greater will be
the polyp stability. Moreover, Harmelin (1990)
suggested that a collar, consisting of an encrusting bryozoan at the base of the corallite. might
preserve the polyp against clionid attack while
the individuals are growing.
The mineral structure of the ledges may
result out of clionids attacking carbonates
deposed by animals and, in particular. by cnidarians, rather than those produced by algae. Among
the 11 boring sponges recorded in the Portofino
waters (Bavestrello et al. 1999). only Cliona
viridis seems to be able to attack the crustose
thalli of coralline algae. However, C. viridis is a
photophilous species living at 10-15 m depth.
Therefore, algae constituting the upper side of
deeper coralligenous ledges cannot be attacked
(Barbieri et a1. 1995; Bavestrello et al. 1996).
Consequently, living Lithophyllum fr0ndo5um,
growing between 30 and 50 m depth, is the calcareous substrate least attacked by boring
species along the diff of the Portofino
Promontory. Probably, when the coralligenous
structure began to form in shallower waters, the
boring activity of the sponges was offset by the
quick growth of algae. After the rise in sea level,
accretion of these bioconstructions, now in deepThe Role of Sponges in Coralligenous Dynamics
239
er waters, certainly decreased. However, at these
depths. no clionid was able to continue its boring
activity, hence these structures were preserved.
In conclusion. boring sponges may be considered as a key-stone group in coralligenous
biocoenosis: eroding the substrate, influencing
reef paleo structure and acting as intermediate
disturbers (Connell 1978), they may represent
one of the strongest forces modelling coralligenous communities, as they do in the coral reefs
(Davies 1983; Sorokin 1993; Becker and ReakaKudla 1997).
Acknowledgements. This work was financially supported by
Italian MURST funds.
References
AdeyWH (1986) Coralline algae as indicators of sea-level. In: Van
de Plaasche 0 (ed), Sea level research: Ii manual for the collection and evaluation of data. Geo Books 9: 229-280
Barbieri M, Bavestrello G, Sar~ M (1995) Morphological and ecological differences in two electrophoretically detected
species of Cliona (Porifera, Demospongiae). Bioi J Linn Soc
54: 193-200
Bavestrello G, Calcinai B, Cerrano C, Pansini M, Sara M (1996)
The taxonomic status of some Mediterranean clionids
(Porifera, Demospongiae) according to morphological and
genetic characters. Bull lnst R Sci Nat Belg, Bruxelles 66:
185-195
Bavestrello G, Calcinai B, Cattaneo-Vietti R, Cerrano C, Pansini M
(1999) Distribuzione e modalita d'erosione di alcune specie
mediterranee eli Clionidi (Porifera, Demospongiae) as80ciati Corallium rubrum. In: Cicogna F, Cattaneo-Vietti R,
Bavestrello G (eds) Mediterranean Red coral and
Gorgonians biology. Ministero per Ie Politiche Agricole
Becker LC. Reaka-Kudla ML (1997) The use of tomography in
assessing bioerosion in corals. In: Proc 8 th Int Coral Reef
Symp Panama 2, pp 1819-1824
Bellan-Santini D (1985) The Mediterranean benthos: reflections
and problems raised by a classification of the benthic assemblages. In: Moraitou-Apostolopoulou M, Kiortsis V (eds)
Mediterranean marine ecosystems. Plenum, New York, pp
19-48
Connell JH (1978) Diversity in tropical rain forests and coral
reefs. Sci NY, 199: 1302-1310
Davies PJ (1983) Reef growth. In Barnes DJ (ed) Perspectives on
coral reefs. Aust Inst Mar Sci, Townsville, pp 69-106
Harme1in IG (1990) Interactions between small sciaphilous
Scleractinians and epizoans in the Northern Mediterranean.
with particular reference to bryozoans. PSZNI Mar Bcol 11:
351-364
Hong JS (1982) Contribution it l'etude des peuplements d'nn fond
de concn!tionnement coralligene dans la region marseillaise
en Mediterran~ Nord-occidentale. Bull KOROl 4: 27-51
Laborel J (1987) Marine biogeniC constructions in the
Mediterranean: a review. Sci Rep Port-Cros Nat! Park Fr
13:97-126
Laubier L (1966) Le coralligene des Alberes: monographie biocenotique. These Doct, Fac Sci Univ Paris
Peres JM, Picard J (1964) Nouveau manuel de bionomie benthique