236
C. Cerrano et al.
tion, inducing a strong microhabitat differentiation. Laubier (1966) defmed these structures like
eco-ethological cross-roads rather than a simple
biocoenosis. According to Picard (1985), this is a
common feature in what he called "complex climatic meso ecosystems", formed by a polybiocenotie species assemblage.
Radiocarbon dating has allowed to determine the age of these bioconstructions: the deeper ones date back to the early Holocene or to the
late Pleistocene, during the last great transgression, caused by the general increase in temperature at the end of the Wurm period (Adey 1986).
Sartoretto et al. (1996), dated the oldest
Mediterranean reefs at around 8,500 B.C., suggesting they had formed at a depth not greater
. than 10-15 m. The coralligenous buildings growing on soft bottoms along the Apulian coasts are
constituted by mounds of calcareous subfossil
algae (Neogoniolithon mamillosum), developed
probably 10,000 years ago in the littoral zone
(Sara 1973).
Laubier (1966) and Sara (1969) observed the
fast sessile fauna turnover in different coralligenous communities, and, in general, the balance
existing between growth and erosion processes.
Owing to this fast turnover, the calcareous skeleton contributes to the increase in detritic sediments lying below the cliff (Peres and Picard
1964). However, no data are so far available to
quantify the real· impact of boring sponges on
the dynamics of calcareous organisms involved
in coralligenous accretion.
This paper is a first attempt to verify the
impact of sponge boring on sessile fauna
turnover, in order to provide an evaluation of its
role in coralligenous building dynamics.
Materials and Methods
The study has been carried out on the Portofino
Promontory cliff (Ligurian Sea) as follows:
1. Three blocks, from horizontal ledges between
35 and 45 m depth, have been cut by diamond
saw in 1 cm thick sections, showing both lit
and shady sides. Each section has been examined by stereo microscope to detect the
organisms involved in its formation.
2. The boring sponge activity on the corallites
of the scleractinian Leptopsammia pruvoti
has been evaluated. Corallites have been collected in standard surfaces of 20 x 20 cm at
20,25,30,35 m depth (three replicas per each
depth). In bored corallites, the sponge tissue
was collected directly from the boring chambers to prepare spicule slides in order to
identify the different species.
3. The animal fraction present in the detritic
bottoms below the cliff has been considered.
The analysis has been conducted on the
coarse fraction (> 2 mm) collected at 45 m
depth. In this fraction, carbonatic remains of
organisms have been sorted by taxa and
weighed.
Results
On the Portofino Promontory cliff, the coralligenous ledges show an upper, lit surface occupied
by coralline algae (Mesophyllum lichenoides and
Lithophyllum frondosum) and a lower shady side
characterised by the growth of an assemblage
mainly composed of animal organisms with a
calcified skeleton. The main organisms were
anthozoans
(Corallium
rubrum
and
Leptopsammia pruvoti) and massive and erect
bryozoans (Smittina cervicornis, Myriapora
truncata and Pentapora fascialis). Among these
elements, a cryptic fauna of serpulids, encrusting
bryozoans, brachiopods, and bivalves, could be
detected, all likely to add their calcareous
remains to the bioconstruction.
The sections of coralligenous rocks (Fig. la)
show that, although the animal component is
very abundant on their lower surface, the inner
rock structure is mainly constituted of coralline
algae. The lower, shady side of the rocks shows a
continuous layer of 1 cm thick perforations,
caused by the action of boring sponges (Fig.
Ib,e). This etching action is practically absent in
the upper surface (Fig. lc). Perforations by
sponges, consisting of boring spherical chambers - 1-2 mm in diameter - penetrate from the
substratum into the calcareous skeleton of benthic organisms (Fig. Id,e). Insinuating sponges
are often present in the large crevices produced
by the coalescence of boring chambers inside the
coral rock (Fig. If).
Quali-quantitative analysis of L. pruvoti
corallites bored by sponges at different depths
showed that the number of bored corallites
ranges between 60 and 75% of the total (Fig. 2).
The specific composition of the boring species
inside the corallites varies according to depth.
C. Cerrano et al.
tion, inducing a strong microhabitat differentiation. Laubier (1966) defmed these structures like
eco-ethological cross-roads rather than a simple
biocoenosis. According to Picard (1985), this is a
common feature in what he called "complex climatic meso ecosystems", formed by a polybiocenotie species assemblage.
Radiocarbon dating has allowed to determine the age of these bioconstructions: the deeper ones date back to the early Holocene or to the
late Pleistocene, during the last great transgression, caused by the general increase in temperature at the end of the Wurm period (Adey 1986).
Sartoretto et al. (1996), dated the oldest
Mediterranean reefs at around 8,500 B.C., suggesting they had formed at a depth not greater
. than 10-15 m. The coralligenous buildings growing on soft bottoms along the Apulian coasts are
constituted by mounds of calcareous subfossil
algae (Neogoniolithon mamillosum), developed
probably 10,000 years ago in the littoral zone
(Sara 1973).
Laubier (1966) and Sara (1969) observed the
fast sessile fauna turnover in different coralligenous communities, and, in general, the balance
existing between growth and erosion processes.
Owing to this fast turnover, the calcareous skeleton contributes to the increase in detritic sediments lying below the cliff (Peres and Picard
1964). However, no data are so far available to
quantify the real· impact of boring sponges on
the dynamics of calcareous organisms involved
in coralligenous accretion.
This paper is a first attempt to verify the
impact of sponge boring on sessile fauna
turnover, in order to provide an evaluation of its
role in coralligenous building dynamics.
Materials and Methods
The study has been carried out on the Portofino
Promontory cliff (Ligurian Sea) as follows:
1. Three blocks, from horizontal ledges between
35 and 45 m depth, have been cut by diamond
saw in 1 cm thick sections, showing both lit
and shady sides. Each section has been examined by stereo microscope to detect the
organisms involved in its formation.
2. The boring sponge activity on the corallites
of the scleractinian Leptopsammia pruvoti
has been evaluated. Corallites have been collected in standard surfaces of 20 x 20 cm at
20,25,30,35 m depth (three replicas per each
depth). In bored corallites, the sponge tissue
was collected directly from the boring chambers to prepare spicule slides in order to
identify the different species.
3. The animal fraction present in the detritic
bottoms below the cliff has been considered.
The analysis has been conducted on the
coarse fraction (> 2 mm) collected at 45 m
depth. In this fraction, carbonatic remains of
organisms have been sorted by taxa and
weighed.
Results
On the Portofino Promontory cliff, the coralligenous ledges show an upper, lit surface occupied
by coralline algae (Mesophyllum lichenoides and
Lithophyllum frondosum) and a lower shady side
characterised by the growth of an assemblage
mainly composed of animal organisms with a
calcified skeleton. The main organisms were
anthozoans
(Corallium
rubrum
and
Leptopsammia pruvoti) and massive and erect
bryozoans (Smittina cervicornis, Myriapora
truncata and Pentapora fascialis). Among these
elements, a cryptic fauna of serpulids, encrusting
bryozoans, brachiopods, and bivalves, could be
detected, all likely to add their calcareous
remains to the bioconstruction.
The sections of coralligenous rocks (Fig. la)
show that, although the animal component is
very abundant on their lower surface, the inner
rock structure is mainly constituted of coralline
algae. The lower, shady side of the rocks shows a
continuous layer of 1 cm thick perforations,
caused by the action of boring sponges (Fig.
Ib,e). This etching action is practically absent in
the upper surface (Fig. lc). Perforations by
sponges, consisting of boring spherical chambers - 1-2 mm in diameter - penetrate from the
substratum into the calcareous skeleton of benthic organisms (Fig. Id,e). Insinuating sponges
are often present in the large crevices produced
by the coalescence of boring chambers inside the
coral rock (Fig. If).
Quali-quantitative analysis of L. pruvoti
corallites bored by sponges at different depths
showed that the number of bored corallites
ranges between 60 and 75% of the total (Fig. 2).
The specific composition of the boring species
inside the corallites varies according to depth.