Stability of the sponge assemblage of the Mediterranean coralligenous along a millennial span of...

ORIGINAL ARTICLE

Stability of the sponge assemblage of Mediterraneancoralligenous concretions along a millennial time spanMarco Bertolino1, Barbara Calcinai2, Riccardo Cattaneo-Vietti2, Carlo Cerrano2, Anna Lafratta2,Maurizio Pansini1, Daniela Pica2 & Giorgio Bavestrello1

1 Dipartimento di Scienze della Terra, dell’Ambiente e della Vita, Universit�a di Genova, Genova, Italy

2 Dipartimento di Scienze della Vita e dell’Ambiente, Universit�a Politecnica delle Marche, Ancona, Italy

Keywords

Coralligenous; ecosystem stability; Ligurian

Sea; Porifera; sponge spicules.

Correspondence

Marco Bertolino, Dipartimento di Scienze

della Terra, dell’Ambiente e della Vita,

Universit�a di Genova, Corso Europa 26,

16132 Genova, Italy.

E-mail: [email protected]

Accepted: 4 March 2013

doi: 10.1111/maec.12063

Abstract

The Mediterranean coralligenous substratum is a hard bottom of biogenic ori-

gin, mainly composed of calcareous algae, growing in dim light conditions.

Sponges are among of the most representative taxa of the coralligenous assem-

blages, with more than 300 recorded species of different habits: massive, erect,

boring and insinuating. When sponges die, their siliceous spicules remain

trapped in the biogenic concretion, offering the opportunity to describe the

coralligenous spongofauna over a very long span of time, virtually dating back

to a large part of the Holocene period. The data reported here were obtained

from core samples collected from four coralligenous concretions. Each block

was collected in a different locality of the Ligurian Sea: Santo Stefano Shoals,

Bogliasco, Punta del Faro (Portofino Promontory) and Punta Manara. Radio-

carbon age determinations indicate for these conglomerates a maximal age

between 1600 and 3100 years. The spicules trapped in the cores show deep dis-

solution marks in the form of circular holes on their surface or present an

enlargement of the axial canal. However, their original shape, generally intact,

suggests the absence of mechanical injuries and allows a tentative identification

at the species level. The analysis of these old spicules reveals an ancient sponge

assemblage composed of 30 recognisable species. This indicates that almost one

half of the sponge community today settled on coralligenous substrata has been

present in the conglomerates for their entire existence.

Introduction

Coralligenous concretions are calcareous formations of

biogenic origin in Mediterranean benthic environments

produced by the accumulation of encrusting algae and

invertebrates with calcareous structures. Coralligenous

communities, together with Posidonia oceanica meadows

(Ballesteros 2006), are the most important ‘hot spots’ of

biodiversity in the Mediterranean. The few existing radio-

carbon datings of the coralligenous assemblages suggest

that these concretions had their maximum development

between 8000 and 5000 years ago. After that period,

appreciable accumulation rates are recorded only for con-

cretions situated in relatively shallow waters (10–

35 m depth), whereas the accumulation rates of corallig-

enous assemblages deeper than 50 m are negligible (Sar-

toretto 1996; Sartoretto et al. 1996; Ballesteros 2006).

Sponges, living with different growing strategies in the

different micro-habitats offered by the concretions, are

one of the most diversified groups of the coralligenous

assemblage, with 304 recorded species (Bertolino et al.

2009; Bertolino 2011). Generally, species living on the

coralligenous surface (massive, branching or encrusting)

are well known, whereas studies of boring and insinuating

species, which penetrate deep into the concretion, are

rare. In spite of this paucity of knowledge, these sponges

play important roles in the stability of the concretions. In

fact, the highly destructive boring species play a major

Marine Ecology (2013) 1–10 ª 2013 Blackwell Verlag GmbH 1

Marine Ecology. ISSN 0173-9565

role in the recycling of the carbonates stored in the

coralligenous concretions (Davies 1983; Sorokin 1993;

Becker & Reaka-Kudla 1997). In contrast, insinuating spe-

cies, filling the crevices of the concretions, enhance the

cohesion of the conglomerate (Cerrano et al. 2001).

The skeletons of most demosponges (excluding horny

sponges and a few other species) are composed of dis-

crete siliceous elements called spicules, which show a

large variety of shapes and sizes; these are commonly

used in sponge identification. Sponge spicules enrich the

fine sediment fraction of biogenic silica when dead ani-

mals decay (R€utzler & Macintyre 1978). In Antarctic

waters, sponge spicules can form mats 1.5 m or more

thick (Koltun 1968; Dayton 1979; Barthel 1992; Cattaneo-

Vietti et al. 1999; Gutt 2007).

The numbers and types of sponge spicules in dated

sediment core samples have been used to reconstruct the

dynamics of freshwater sponge assemblages in lakes or

lagoons (Paduano & Fell 1997; Volkmer-Ribeiro et al.

2007). Bavestrello et al. (1996) evaluated the contribu-

tion of this form of low-soluble silica to the bottom

sediments of different areas of the Ligurian Sea. Micro-

scopical analysis of the fine fraction of the sediments,

coming from several stations, demonstrated that sponge

spicules represent nearly the total BSi (biogenic silica),

other siliceous remains, such as diatom and silicoflagel-

late tests, constituting a negligible fraction. Similar

results were recorded from cores taken from a Posidonia

oceanica meadow in the Mediterranean (Bertolino et al.

2011).

When sponges living in coralligenous crevices die, their

siliceous spicules likely become trapped in the biogenic

concretion, offering the unique possibility to describe

the coralligenous spongofauna over a very long span of

time, virtually dating back to a large part of the Holocene

period.

The aim of this paper is to describe the ancient sponge

assemblages on the base of spicules recorded inside blocks

of coralligenous material, assuming that spicules present

inside the conglomerates may have approximately the

same age of the surrounding bio-deposed carbonates. The

comparison of these ancient sponge assemblages with the

living ones recorded on the surface of the concretions

allowed an estimation of the stability level of the corallig-

enous community along a millennial span of time.

Material and Methods

The coralligenous blocks were collected by SCUBA diving

from four localities of the Ligurian Sea: at the Santo Stef-

ano Shoals (Western Ligurian Riviera), Bogliasco, Punta

del Faro (Portofino Promontory) and Punta Manara

(Eastern Ligurian Riviera) (Fig. 1). In the first three

localities, the conglomerate blocks came from the hori-

zontal rim at the base of the vertical cliffs, at a depth

ranging of 30–40 m. At Bogliasco, the sampled block was

from a vertical pinnacle arising from a coralligenous bank

on a flat bottom at 15 m depth. The portions of con-

glomerate were detached using hammer and chisel. For

each block, with a volume of 30–40 l, all the living

sponge species present on the surface (massive, erected or

encrusting) were collected and identified to species level

(Fig. 2A). Sponges without spicules (horny sponges,

Chondrosia reniformis, Oscarella sp.) were identified but

were not considered in the following analysis because of

the impossibility of checking their presence in the ancient

assemblages.

Each block was cut in 2-cm-thick slices running from

the surface toward the plane of junction with the rocky

substratum, perpendicular to the plane of development of

the coralline algae (Fig. 2A,B). Each side of the slices was

observed under stereomicroscope to detect the living,

endolithic (boring or insinuating) sponges present in the

first layer (1–2 cm thick) of the conglomerates (Fig. 2C).

These specimens were identified to species level.

Finally, from the central slice of each block, one verti-

cal sample 9 cm long extending from the surface (consid-

ered the youngest part of the conglomerate) to the face

Fig. 1. The four studied localities along the

Ligurian Coast: St. Stefano Shoal (1),

Bogliasco (2), Punta del Faro (3) and Punta

Manara (4).

2 Marine Ecology (2013) 1–10 ª 2013 Blackwell Verlag GmbH

Stability of Mediterranean coralligenous sponge assemblage Bertolino, Calcinai, Cattaneo-Vietti, Cerrano, Lafratta, Pansini, Pica & Bavestrello

of the block originally attached to the substratum (con-

sidered the oldest part of the conglomerate) was obtained

and divided into nine portions, each 2 9 2 9 1 cm

(4 cm3) (Fig. 2D). Each small portion of the vertical sam-

ple was placed in hydrogen peroxide (240 vol) that was

changed three times at intervals of 24 h. This method

released all of the sediment entrapped in the cavities of

the conglomerate.

The average amounts of sediment obtained from the

different localities were 0.45 � 0.09 g, 0.8 � 0.12 g,

0.33 � 0.06 g and 0.21 � 0.06 g for St. Stefano Shoal,

Bogliasco, Punta del Faro and Punta Manara, respectively.

The conglomerate pieces were removed and dried while

the sediments were treated with boiling nitric acid to

eliminate the carbonatic fraction, rinsed twice in distilled

water and twice in alcohol 95%, and dried. For each sam-

ple of sediment, three replicates of 50 mg were suspended

in 300 ll of Bioclear solvent and mounted on a micro-

scope slide. Microscopic analysis of the spicules contained

in the sediment was done to identify the sponge species

living in ancient times. We have identified only the spe-

cies characterized by peculiar, diagnostic spicules. Finally,

the conglomerate fragments were gold-sputtered (Balzers

Union Evaporator) and observed by scanning electron

microscopy (SEM Philips XL 20).

After the sediment extraction, three subsamples, the most

superficial, the deepest and an intermediate one (Fig. 2D),

were used for dating by 14C using high-resolution

mass spectrometry at the Center of Dating and Diagnostic

(CEDAD) of the University of Salento (http://cedad.unisal-

ento.it).

Statistical analysis

Sponge data were analysed using PAST 2.15, a software

package of computer programs including common sta-

tistical and modelling functions (Hammer et al. 2001).

Data on presence/absence of sponge species were com-

piled prior to analysis. Cluster analysis was used to

check for similarities between the ancient and living

sponge assemblages in the four localities (St. Stefano

Shoal, Bogliasco, Punta del Faro, Punta Manara). This

analysis was run using the average linkage algorithm

(UPGMA method) of Bray–Curtis distances among

samples.

Results

Living sponge assemblages

The living sponge species recorded on and inside the

blocks collected in the four localities are listed in Table 1.

Of a total of 75 determined species (excluding horny

sponge species and other species lacking spicules), 46

(61.3%) were encrusting, 14 (18.6%) insinuating, 10

(13.3%) massive and seven (9.3%) boring.

The assemblages of the different blocks are strongly

diversified: only the insinuating Dercitus (Stoeba) plicatus

was present in the blocks of all the four localities. Com-

paring pairs of localities hosting coralligenous rims (St.

Stefano Shoal, Punta Manara, Punta del Faro) the num-

ber of shared species ranged from 5 to 15. Comparing

Bogliasco, hosting coralligenous banks, with the other

A

D

B

C

Fig. 2. Protocol for processing the

coralligenous blocks. From a newly collected

block (A) all the sponges living on the surface

were recorded. The block was then cut into

2- cm thick slices (A–B) and the insinuating or

boring specimens (arrow) observed in each

slice were collected (C). From the central slice

of the block, a vertical core sample 9 cm high

was obtained and divided (D) into nine

portions, each 2 9 2 9 1 cm. Each portion

was used for the ancient spicules extraction.

The most superficial, the deepest and the

intermediate portions (arrows) were used for

radiocarbon dating.


Bertolino, Calcinai, Cattaneo-Vietti, Cerrano, Lafratta, Pansini, Pica & Bavestrello Stability of Mediterranean coralligenous sponge assemblage

Table 1. List of the living sponge species recorded on the blocks col-

lected in the four sampled localities.

Species Habit

Localities

SS BO PF PM

Order Homosclerophorida

Oscarella sp.a Ms X

Plakina trilopha Schulze, 1880 Ec X

Plakinastrella copiosa Schulze, 1880 Ec X

Plakortis simplex Schulze, 1880 Ec X X

Order Spirophorida

Samus anonymus Gray, 1867 In X

Order Astrophorida

Stelletta grubii Schmidt, 1862 Ec X

Stelletta lactea Carter, 1871 In X

Jaspis incrustans Topsent, 1890 In X

Jaspis johnstoni (Schmidt,1862) In X X X

Pachastrissa sp. In X

Penares euastrum (Schmidt, 1868) In X

Erylus discophorus (Schmidt, 1862) In X X

Geodia conchilega Schmidt,1862 In X

Geodia cydonium Schmidt,1862 In X

Pachastrella monilifera Schmidt, 1868 In X

Poecillastra compressa

(Bowerbank, 1866)

In X X

Dercitus (Stoeba) plicatus

(Schmidt,1868)

In X X X X

Triptolemma simplex (Sar�a, 1959) In X X

Delectona ciconiae Bavestrello,

Calcinai & Sar�a, 1996

Br X

Order Hadromerida

Cliona celata Grant, 1826 Br X X

Cliona janitrix Topsent, 1932 Br X X

Cliona schmidti (Ridley, 1881) Br X

Cliona viridis Schmidt, 1862 Br X

Cliona sp. Br X X

Diplastrella bistellata

(Schmidt, 1862)

Ec X

Spirastrella cunctatrix Schmidt, 1868 Ec X

Aaptos aaptos (Schmidt,1864) Ec X

Pseudosuberites sulphureus

(Bowerbank, 1866)

Ec X

Suberites sp. Ec X

Terpios gelatinosa (Duchassaing &

Michelotti, 1864)

Ec X X

Timea stellata (Bowerbank, 1866) Ec X

Timea unistellata (Topsent, 1892) Ec X

Dotona pulchella mediterranea

Rosell & Uriz, 2002

In X

Spiroxya heteroclita Topsent, 1896 Br X

Order Chondrosida

Chondrosia reniformis Nardo,1847a Ms X X X X

Order Poecilosclerida

Acarnus souriei Levi, 1952 In X

Clathria (Microciona) toxivaria

(Sar�a, 1959)

Ec X

Clathria (Microciona) sp. Ec X X

Eurypon clavatum (Bowerbank, 1866) Ec X X

Table 1. Continued

Species Habit

Localities

SS BO PF PM

Eurypon major Sar�a & Siribelli,1960 Ec X X X

Eurypon vesciculare Sar�a &

Siribelli,1960

Ec X X X

Eurypon sp. Ec X X X

Raspaciona aculeata

(Johnston, 1842)

Ec X

Crambe crambe (Schmidt,1862) Ec X X X

Crella (Grayella) pulvinar

(Schmidt, 1868)

Ec X X X

Hemimycale columella

(Bowerbank, 1864)

Ec X

Hymedesmia (Hymedesmia)

baculifera Topsent, 1901

Ec X

Hymedesmia (Hymedesmia) cf.

gracilisigma Topsent, 1928

Ec X

Hymedesmia sp. Ec X X

Phorbas fictitius Bowerbank, 1866 Ec X X

Phorbas tenacior (Topsent,1925) Ec X X

Phorbas sp. Ec X

Plocamionida ambigua

(Bowerbank, 1866)

Ec X

Merlia normani Kirkpatrick, 1908 Ec X

Order Halichondrida

Axinella damicornis (Esper,1794) Ms X X

Axinella polypoides Schmidt, 1862 Ms X

Phakellia sp. Ec X

Bubaris carcisis Vacelet,1969 Ec X

Acanthella acuta Schmidt, 1862 Ms X X X

Dictyonella incisa (Schmidt,1880) Ec X X X

Halichondria (Halichondria)

genitrix Schmidt, 1862

Ec X

Halichondria sp. Ec X X

Order Agelasida

Agelas oroides Schmidt,1864 Ms X X

Order Haplosclerida

Dendroxea lenis (Topsent, 1892) Ec X

Haliclona (Gellius) lacazei

(Topsent, 1893)

Ec X

Haliclona (Halichoclona) fulva

(Topsent, 1893)

Ec X X X

Haliclona (Halichoclona) parietalis

(Topsent, 1893)

Ec X

Haliclona (Haliclona) sp. Ec X

Haliclona (Reniera) griessingeri van

Lent & De Weerdt, 1987

Ec X

Haliclona (Reniera) sp. Ec X

Haliclona (Soestella) mucosa

(Griessinger, 1971)

Ec X

Haliclona (Soestella) valliculata

(Griessinger, 1971)

Ec X

Haliclona sp. Ec X

Aka insidiosa Johnson, 1899 Br X X

Petrosia (Petrosia) clavata

(Esper,1794)

Ms X



three localities, the number of shared species ranged from

six to seven.

The sponge fauna recorded on the block from the St.

Stefano Shoal is the most diverse (44 species). In the

other three localities the number of species is very similar

(25, 24 and 21 species for Punta del Faro, Punta Manara

and Bogliasco, respectively). Massive and erect species are

the less common forms (two at Punta del Faro to four at

St. Stefano Shoal), whereas the encrusting species are very

abundant (from 13 at Bogliasco to 24 at St. Stefano

Shoal). Insinuating sponges are numerous at the St. Stef-

ano Shoal (11 species) but show lower values at Punta

del Faro (seven), Bogliasco (four) and Punta Manara

(two). Few boring species are recorded in the studied

blocks; in this case as well, the highest number is

observed at the St. Stefano Shoal (five).

Although the values of species richness are different in

the blocks studied, the encrusting species represent the

most abundant category, ranging from 53% to 66% of

the total. The insinuating species vary from 8% to 27%,

the boring species from 4% to 12.5% and the massive

ones from 7% to 14% (Fig. 3A).

Ancient sponge assemblages

The radiocarbon dating of the different portions of the

conglomerates from different localities are summarised in

Table 2. From these data, it is possible to estimate aver-

age growth rates of 0.03 mm year�1 for St. Stefano Shoal,

0.04 mm year�1 for Punta del Faro and Bogliasco and

0.05 mm year�1 for Punta Manara.

Analysis of the sections of the conglomerates shows

that the crevices between the layers of coralline algae are

filled with extremely compacted fine sediments. SEM

observations indicate that several sponge spicules are

embedded in these sediments (Fig. 4A–D). The embedded

spicules show deep dissolution marks in the form of cir-

cular holes on their surface or present an enlargement of

the axial canal. However, their original shape, generally

intact, suggests the absence of mechanical injuries and

allowed the identification of several sponge species

(Fig. 5E–H), enabling the reconstruction of an ancient

sponge assemblage comprised of at least 30 species. Ele-

ven of these (36.7%) were insinuating, 8 (26.0%) encrust-

ing, seven (23.3%) massive and four (13.3%) boring. In

the examined blocks the most abundant sponge category

was insinuating species, except for the block from Bogli-

asco, where massive sponges were the most represented

(Fig. 3B).

Similar to the present species, the highest biodiversity

of the ancient species was recorded at the St. Stefano

Shoal (20 species); the other localities showed almost the

same number of species (16, 15, 15 for Punta del Faro,

Punta Manara and Bogliasco, respectively).

Similar to the present sponge assemblages, the ancient

ones differ strongly from each other at species level. Only

five species are shared by all the sites: one encrusting

(Diplastrella bistellata), two insinuating [Dercitus (Stoeba)

A

B

Fig. 3. (A) percent composition of the living and (B) ancient sponge

assemblage at St. Stefano Shoal (SS), Bogliasco (BO), Punta del Faro

(PF) and Punta Manara (PM). Ec = encrusting sponges; In = insinuating

sponges; Br = boring sponges; Ms = massive sponges.

Table 1. Continued

Species Habit

Localities

SS BO PF PM

Petrosia (Petrosia) ficiformis

(Poiret,1798)

Ms X X

Oceanapia sp. Ms X

Order Dictyoceratida

Ircinia variabilis (Schmidt, 1862)a In X

Spongia (Spongia) virgultosa

(Schmidt, 1868)aIn X X X

Dysidea sp.a Ec X

Pleraplysilla spinifera (Schulze, 1879)a Ec X

SS = St. Stefano Shoal; BO = Bogliasco; PF = Punta del Faro;

PM = Punta Manara; Ec, encrusting; In = insinuating; Br = boring;

Ms = massive.aSpecies without spicules are not considered in the analysis.



plicatus and Triptolemma simplex] and two boring species

(Cliona janitrix and Spiroxya sp.).

Cluster analysis conducted on both living and

ancient assemblages at the different levels of the con-

glomerate (Fig. 5) shows a first cluster including the

living assemblage of Punta del Faro, Punta Manara and

St. Stefano Shoal. A second cluster includes all the

ancient assemblages and the living assemblage of Bogli-

asco. The other groups of ancient assemblages are well

separated, with Punta del Faro and St. Stefano Shoal

closer to each other than to Punta Manara. Within the

ancient assemblages obtained from different levels at

Table 2. Radiocarbon age estimation of three portions of conglomer-

ate per each locality obtained at different levels of the core samples.

Depth in

the core

sample (cm)

Localities

SS BO PF PM

Age estimation (year BP)

1 2014 � 60 1135 � 45 446 � 45 976 � 45

5 2142 � 40 1672 � 45 1828 � 50 1006 � 50

9 3083 � 50 2498 � 45 2259 � 50 1615 � 50

SS = St. Stefano Shoal; BO = Bogliasco; PF = Punta del Faro;

PM = Punta Manara.

A B

C D

E F

G H

Fig. 4. Spicules of ancient sponges. (A–B) old

spicules embedded into the compact

sediments filling the crevices inside the

conglomerate. Pachastrellid calthrops (C) and

amphitriaenes of Samus anonymus (D)

partially embedded into the sediment. (E) Old

mesotriaenes of Triptolemma simplex

showing the circular erosions due to silica

dissolution. (F) Old calthrop of Dercitus

(Stoeba) plicatus with numerous erosion

marks. (G–H) close up of the same spicules

showing the enlargement and the erosions

on the surface.



each locality it is not possible to observe a pattern of

similarity related to the hypothesised age of the con-

glomerate.

Discussion

This study shows that spicules trapped inside coralligenous

conglomerates can be used to describe the diversity of

ancient sponge communities. Data on the dating of the

Mediterranean coralligenous and its growth rates are very

scarce (Sartoretto 1996; Sartoretto et al. 1996). The known

accumulation rate of the constructions is considered very

low (0.006–0.83 mm year�1) and oscillates greatly depend-

ing on depth and time period. The higher accumulation

rates (0.20–0.83 mm year�1) were recorded for deep coral-

ligenous concretions and correspond to a period between

8000 and 5000 years BP. More recently, the only apprecia-

ble accumulation rates (0.11–0.42 mm year�1) have been

recorded for coralligenous concretions situated in relatively

shallow waters (10–35 m depth�1) (Sartoretto 1996; Sarto-

retto et al. 1996). The findings of our study strongly agree

with evaluations obtained from constructions recorded in

the same depth range.

The recorded differences in the number of sponge spe-

cies between living and ancient assemblages (Tables 1–3)are due to the obvious difficulty of identifying species

relying only on disaggregated spicules. Moreover, it is evi-

dent, in all the studied localities, that spicules from

encrusting species are rarely preserved inside the con-

glomerate. This is due to the low biomass of these kinds

of sponges and to their position on the conglomerate sur-

face inducing spicule dispersion when sponges decay.

Owing to this, the diversity of encrusting species was

probably higher in ancient times than the present study

suggests. The relatively high number of spicules belonging

to massive sponges is due to the high quantity of spicules

present in these species. When a massive specimen

decays, a large amount of spicules is probably dispersed

and there is a higher probability that a fraction of them

will penetrate the crevices of the coralligenous. About half

of the ancient species recognised from the spicules

trapped inside the conglomerate belong to insinuating

Fig. 5. Dendogram of the four living sponge

assemblages (SS, BO, PF, PM) and the ancient

sponge assemblages at the different levels of

the core samples from St. Stefano Shoal (SS

1–9), Bogliasco (BO 1–9), Punta del Faro (PF

1–9) and Punta Manara (PM 1–9).



and boring species. Since these species live inside the con-

glomerate, when they die their spicules are not dispersed.

In this paper, we assume that the spicules present at a

specific level inside the conglomerate may have approxi-

mately the same age as the surrounding bio-deposed

carbonates. The evidence that the crevices inside the

conglomerate are completely filled by a very compact

sediment including spicules supports this assumption.

Mixing of the trapped sediments may be due to the

action of boring sponges, as the spicules of Spiroxya sp.

and Cliona janitrix are well represented at all levels of the

conglomerate. These organisms penetrate some centime-

tres into the carbonate, potentially mixing the compacted

sediments (Cerrano et al. 2001). We expect that this kind

of bioturbation is not as important as that operated by

the excavating organisms in soft sediments (Gerino 1990).

Although old spicules trapped in the sediment maintain

their original shape, they are characterised by deep circu-

lar marks of dissolution, similar to those observed in pre-

vious studies (e.g. R€utzler & Macintyre 1978). It is

possible to hypothesise that the pH of the porewater

inside the crevices of coralligenous concretions may reach

unusually high values due to CO2 uptake from photosyn-

thetic activity of the coralline algae, as happens in the cre-

vices of coral reefs (Epstein & Friedman, 1982). This

situation may enhance silica dissolution.

The comparison of the ancient sponge assemblages

reconstructed through the spicules trapped in the con-

glomerate blocks with those recorded alive on their sur-

faces, indicates that the sponge community of the

Mediterranean coralligenous, particularly insinuating and

boring species is very old and probably stable. Almost

half the species of the present sponge community have

been present continuously in the coralligenous conglom-

erates for their entire lifespan.

There are some differences among the living sponge

communities. The block from St. Stefano Shoal hosts a

higher number of species than those of the other locali-

ties. The three assemblages of coralligenous coming from

rocky cliffs (Punta del Faro, St. Stefano Shoal, Punta

Manara) cluster together, whereas the assemblage from

the flat bottom (Bogliasco) is significantly different. These

differences may be related to a higher sedimentation rate

at Punta Manara than in Punta del Faro and St. Stefano

Shoal, whereas the lower depth of the bioconstructions

from Bogliasco induces a proliferation of frondose algae

that compete with the sponges settled on the surface of

the conglomerate. Exactly the same kind of difference

arises from the comparison of ancient assemblages,

strongly suggesting that environmental factors have

remained stable for a very long time in different localities.

The fact that the spatial variability of sponge

communities is influenced by a combination of environ-

mental, biological and stochastic processes, resulting in

strong differences in abundance and diversity, is already

known (Pansini & Pronzato 1985; Pansini & Musso 1991;

Duckworth et al. 2008; Berman & Bell 2010).

The necessity of reconstructing past diversity to under-

stand ecosystem changes has been stressed by Boero &

Bonsdorff (2007). In the last few decades, Mediterranean

marine communities have shown significant changes in

Table 3. Ancient species determined from the disaggregated spicules

obtained from different blocks

Species Habit

Localities

SS BO PF PM

Order Homosclerophorida

Plakina dilopha Schulze, 1880 Ec X X

Plakina trilopha Schulze, 1880 Ec X

Plakortis simplex Schulze, 1880 Ec X

Order Spirophorida

Samus anonymus Gray, 1867 In X X

Order Astrophorida

Stelletta sp. In X

Jaspis johnstoni (Schmidt,1862) In X X X

Penares euastrum (Schmidt, 1868) In X X X

Erylus discophorus (Schmidt, 1862) In X X

Geodia cydonium Schmidt,1862 In X X

Pachastrella monilifera Schmidt, 1868 In X X

Poecillastra compressa

(Bowerbank, 1866)

In X

Poecillastra sp. In X

Dercitus (Stoeba) plicatus

(Schmidt, 1868)

In X X X X

Triptolemma simplex (Sar�a, 1959) In X X X X

Order Hadromerida

Cliona sp. Br X X X

Cliona janitrix Topsent, 1932 Br X X X X

Diplastrella bistellata (Schmidt, 1862) Ec X X X X

Spirastrella cunctatrix Schmidt, 1868 Ec X

Timea unistellata (Topsent, 1892) Ec X

Timea bifidostellata Pulitzer-Finali, 1983 Ec X

Timea sp. Ec X X

Spiroxya heteroclita Topsent, 1896 Br X X X

Spiroxya sp. Br X X X X

Order Poecilosclerida

Acarnus sp. Ms X X X

Eurypon sp. Ms X X

Rhabderemia sp. Ms X

Crella sp. Ms X X

Merlia sp. Ms X X

Order Agelasida

Agelas oroides Schmidt, 1864 Ms X X X

Order Haplosclerida

Petrosia (Petrosia) ficiformis

(Poiret, 1798)

Ms X

Ec = encrusting; In = insinuating; Br = boring; Ms = massive; SS = St.

Stefano Shoal; BO = Bogliasco; PF = Punta del Faro; PM = Punta

Manara.



composition and distribution. These modifications are

considered, directly or indirectly, to be related to the cur-

rent climatic changes, which may influence the spreading

capacity of some species, modifying their range (Astraldi

et al. 1995; Morri & Bianchi 2001; Bianchi & Morri 2004;

Cerrano & Bavestrello 2009). Moreover, a simplification of

the superficial benthic communities has occurred, mainly

in consequence of a sharp reduction of structuring organ-

isms (above all sponges, bryozoans and gorgonians) after a

number of mass mortality events that took place from 1999

to 2006 (Coma et al. 2009; Garrabou et al. 2009; Maldonado

et al. 2010; Cebrian et al. 2011; Di Camillo et al. 2012).

Sponge spicules represent a powerful tool for the tenta-

tive reconstruction of ancient assemblages both from a

quantitative (Bertolino et al. 2011) and qualitative (this

paper) point of view. Data from this study, obtained

from the most diversified assemblage of the Mediterra-

nean Sea, indicate that the diversity of coralligenous

sponges has remained stable over a millennial span of

time. This extended stability may be related to the pecu-

liarly protected inner habitat of the coralligenous crevices.

Conclusion

During the few last decades, Mediterranean marine com-

munities have shown significant changes in taxa composi-

tion and distribution. These modifications can be related,

directly or indirectly, to current climatic changes that, in

turn, have influenced the diffusive capacity of certain spe-

cies modifying their range. It is difficult to interpret the

changes, especially in the coastal environment, where dif-

ferent anthropic disturbances may merge with warming

effects, and where there is an almost complete absence of

historical quantitative data on the composition of benthic

communities. This problem was addressed through quan-

titative studies on the remains (e.g. mineral skeletal struc-

tures) that benthic organisms leave in the sediment after

death. Sponge spicules represent a good potential proxy

for the evaluation of long-term sponge community

dynamics. In this study the sponge spicules trapped inside

the crevices of coralligenous concretions collected in four

localities of the Ligurian Sea were extracted and studied.

In this way, the ancient sponge assemblage of the

coralligenous habitat was described over a millennial span

of time. This assemblage was composed of 30 recognisable

species representing almost half of the sponge community

present today on the same coralligenous concretions.

Acknowledgements

We would like to thank Dr Mario Mori for the helpful

suggestions. This study was conducted within the Ritmare

Project and Prin Cofin 2010–2011 Project.

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