An overview of the Mediterranean cave-dwelling horny sponges (Porifera, Demospongiae)

Abstract The present synthesis focuses on the so called ‘horny sponges’ recorded from marine caves of the Mediterranean Sea. The main aim is to provide a list of all recorded species, diagnostic keys to their identification up to family and genus level, and exhaustive, formally uniform descriptions at the species level contributing to sharing of information on the faunistics and taxonomy of Mediterranean cave-dwelling species, including habitat preferences. The majority of species was recorded in 105 Mediterranean marine caves hosting four orders of horny sponges belonging to 9 families, 19 genera and 40 species. Species endemic to the Mediterranean Sea harboured in marine caves are 14 with an endemicity value of 35%. For each species morphological descriptions are supported by illustrations both original and from the literature, including the diagnostic traits of the skeleton by light and scanning electron microscopy giving further characterization at the specific level. A detailed map together with a list of all caves harbouring horny sponges is also provided with geographic coordinates.


Introduction
The Mediterranean area represents a hot spot of biodiversity and needs more and deeper studies together with urgent conservation plans on its marine biocoenosis and ecosystems. Among dominant benthic taxa Mediterranean sponge species number over 600 with a high endemicity value (ca. 40%) (Pansini andLongo 2003, 2008;Pronzato 2003;Pansini et al. 2011). The horny sponge fauna also is characterized by high levels of endemism (18 species=31.6% endemicity) from all Mediterranean biotopes (Pansini 1992;Pansini andLongo 2003, 2008;Pronzato 2003;Voultsiadou 2005;Pronzato and Manconi 2011). Although the last synthesis by Van Soest et al. (2012a) reports 654 species, 203 genera, and 86 families of Porifera, the real species richness of the Mediterranean Sea is, apparently, highly over-or under-estimated.
As far as vulnerable biotopes such as marine caves are concerned, data on sponges are scattered widely in the literature and several records are published in not easily accessible regional journals or books. After the pioneering work of Michele Sarà, who collected cave-dwelling sponges by snorkelling in semi-submerged (mid-littoral) caves (Sarà 1958), sampling methods by SCUBA diving highly improved data on biodiversity also from submerged caves (Riedl 1966;Rützler 1966). Results on cave-dwelling sponges highlighted the fact that the taxon Porifera is dominant in these cryptic Mediterranean biotopes, performing a key role in the benthic community structure of caves.
The present paper reports all known records of the horny sponges (Orders Dendroceratida, Dictyoceratida, Halisarcida,Verongida) from a wide array of marine caves in the entire Mediterranean Sea with a checklist and diagnostic keys to benefit an online open-access supporting global sharing of information on faunistics and taxonomy ( Fig. 1; Tables 1, 2). Exhaustive and formally uniform morphological descriptions of species are provided although some were previously reported in part by Pronzato and Manconi (2011) in a rather regional and not widely accessible data source.   Figure 2. Horny sponge skeleton. All orders to which horny sponges belong share a wide array of growth form supported by skeletal architecture of spongin ranging from dendritic-arborescent to reticulate network, with fibres filled or not by mineral detritus a digitate growth form with conulose surface is a very common trait, but also massive or encrusting habits are displayed by a number of species b the sponge surface is, in several species, armed by granular mineral debris sometimes appearing as ornamentation; c) reticulate fibrose surface of an encrusting horny sponge species with the osculum surrounded by conules d vertical section of a conule supported by an ascending primary fibre, with mineral inclusions, connected with a network of thinner secondary fibres free of inclusions e the dendritic skeleton is sometimes ramified f, g, h differently cored primary and secondary fibres network i skeletal network composed only by secondary fibres free of inclusions j detail of the opaque fibrillar medulla coring the skeleton of some horny sponge species k the absence of an horny skeleton occur only in a few species l triradiate horny spicules free in the skeleton characterize a few sponge species m thin long filaments ending in a rounded button (knob) are an exclusive diagnostic trait of the family Irciniidae. Modified from several historical sources.
Systematics and phylogenetic relationships of horny sponges have only recently begun to be tested using current biochemical and molecular approaches, partly confirming the classical morphological classification scheme (Borchiellini et al. 2004;Lavrov et al. 2008;Erpenbeck et al. , 2012. Molecular analyses showed that Dictyoceratida, Dendroceratida, Verongida, and Halisarcida are in fact closely related (Borchiellini et al. 2004;Lavrov et al. 2008).
The order Halisarcida was recently suggested to be moved to the order Chondrosida Ereskovsky et al. 2011). The phylogenetic tree based on molecular data (Ereskovsky et al. 2011, Fig. 46, p. 26) shows Halisarca spp. close to Chondrilla nucula although this status is weakly supported by the relationship in the same tree of Halisarca spp. with Ephydatia muelleri (Suborder Spongillina) and Aplysina fulva (Order Verongida). As a consequence the entire phylogenetic tree must be considered with caution (see also Erpenbeck et al. 2012). We have given these results serious consideration but assume a conservative approach until better diagnostic molecular markers are available; therefore, we maintain the traditional taxonomic status of the order Halisarcida.
Basic references on "Keratosa" are few (von Lendenfeld 1889; de Laubenfels 1948;Bergquist 1980aBergquist , b, 1996Cook and Bergquist 2002;Bergquist and Cook 2002a, b, c;Pronzato and Manconi 2011). After the last fundamental worldwide taxonomic revision (Hooper and Van Soest 2002), 56 genera of sponges with fibrous skeletons are considered valid, although the final number of species at the global level is still under discussion.
The discovery of new taxa showed a continuous and constant increase up to the present (see Pronzato 2003). First data on the Mediterranean Sea as the type locality of horny sponges are reported in the 13th edition of Systema Naturae (Linnaeus 1789). Starting from the description of Spongia officinalis L., 1759 a total of 20 authors are involved from 1759 to 2007 in the discovery of new horny sponge species with a maximum of 2-3 new species per decade. Out of that trend is the period 1862-1938, of intense inventory activity resulting in the discovery of a high number of new species and genera by Emile Topsent, Oscar Schmidt and Franz Eilhard Schulze. In recent times only a few studies were published on horny sponge fauna mostly reporting on restricted geographic areas of the Mediterranean Sea (Vacelet 1959;Pronzato 1975;Pulitzer-Finali andPronzato 1976, 1980;Rubió-Lois et al. 1981;Voultsiadou-Koukoura and Koukouras 1993;Uriz and Maldonado 2000;Pronzato et al. 2004;Manconi 2008, 2011).

Materials and methods
Specimens were collected, by the authors and others, using SCUBA diving. Specimens were preserved in 95% ethanol, 4% formaldehyde or dried. For specimens registered in collections we use acronyms published in the Systema Porifera (Hooper and Van Soest 2002).
A detailed study of the external morphology was performed on growth form, surface traits e.g. dimensions and topographic distribution of conules, oscules, and inhalant apertures. For species identification, skeleton preparations for light microscopy (LM) were made by hand dissection under a stereomicroscope, which were dried and mounted in Canada balsam or similar media under a cover slip. Similar preparations for Scanning Electron Microscopy (SEM) were air dried and attached to a stub with drops of silver glue. Preparations were viewed, measured, and photographed to characterize diagnostic micro-traits.
Morphological descriptions of cave dwelling-species refer basically both to recent analyses of specimens in the authors' collections, of type materials, and/or original and historical descriptions, also in those cases in which taxa were first reported from other seas.
The cave-dwelling horny sponges were critically reviewed for synonymies and based on recent trends in taxonomy following, in part, Systema Porifera (Hooper and Van Soest 2002), Fauna d'Italia (Pansini et al. 2011;Pronzato and Manconi 2011), and taxonomic databases such as the World Porifera Database (WPD) and WoRMS (Van Soest et al. 2012b; www.marinespecies.org/porifera). For more detailed synonymies and distribution patterns of some all species see Pronzato and Manconi (2011). Some divergent points of view of the authors on the taxonomic status of a few taxa with respect to the previous papers fonts are discussed in the text.

Study area
All studied caves are submerged or semi-submerged and, in most cases, the entrances are no more than 20 m in depth.
According to the areas investigated in the past by cave sponge workers and following previous biogeographical analyses the Mediterranean Sea was divided into 14 areas (Table 1), namely the Alboran Sea, Balearic Sea, Sardinian Sea, Gulf of Lions, Ligurian Sea, Northern Tyrrhenian Sea, Central Tyrrhenian Sea, Southern Tyrrhenian Sea, Sicily Channel, Ionian Sea, Northern Adriatic Sea, Southern Adriatic Sea, Aegean Sea, and the Levantine Basin (Van Soest 1994;Pansini andLongo 2003, 2008;Xavier and Van Soest 2012;Cadeddu 2012;Gerovasileou and Voultsiadou 2012). Not a single record of cave-dwelling horny sponges is reported for the Alboran Sea or the Northern Tyrrhenian Sea.
Additional data on new records ( Fig. 1; Tables 1, 2) have been included in the historical dataset after recent investigations in some Italian Marine Protected Areas (MPA) of seven submerged caves of the Capo Caccia-Isola Piana MPA (n=3), the Plemmirio MPA (n=3), and the Pelagie MPA (n=1) Cadeddu 2012). These new records are indicated by asterisks in the text.

Taxonomic accounts
We use the obsolete designation "horny sponges" sensu von Lendenfeld (1889) not acting as greenhorn taxonomists but for convenience, to avoid listing all four orders that once were included in one, Keratosa sensu de Laubenfels (1948) whenever referring to the group. Because of the trait "absence of mineral spicules in the skeleton", the taxonomy of "horny sponges" is based on fewer characters than the other demosponges. In general, some valuable diagnostic traits for a correct identification are the spatial organization of spongin fibres and collagenous filaments in the skeleton, the homogeneous or laminate architecture of fibres, and the presence/absence of exogenous mineral inclusions within spongin (Fig. 2). In other cases supplementary characters include the shape and dimension of flagellate chambers, the richness of cellular types, and larval architecture. The morphological plasticity of sponges (see Gaino et al. 1995) is one of the key problems for a correct identification of taxa bearing exclusively a fibrous skeleton ) with a few morphological traits sometimes constrained by the influence of environmental parameters. In any case, first-hand experience of many species, including live material, is important for the difficult task of horny sponge identification at the species level.
The following keys are useful aids for understanding cave-dwelling horny sponge diversity, even if they are necessarily imperfect due to the incongruence and uncertainties still present in the field. The diagnostic keys reach the family or genus level, whereas identification at the species level is based on detailed descriptions and illustrations provided here. In a few cases the species are known only from the original description and there are no subsequent findings, and so no images support the diagnoses. Moreover the validity of some taxa is strongly under debate, in-depth revisions are needed and the possibility of synonymies is real. The present overview is systematically conservative and aims at facilitating the identification of Mediterranean cavedwelling horny sponges.
Order Dendroceratida Minchin, 1900 Diagnosis (emended after Bergquist and Cook 2002a). Demospongiae with skeleton exclusively composed by horny fibres arising from a spongin basal plate. In one genus free fibrous spicules in the choanosome. No endogenous mineral elements in the skeleton. Fibres dendritically arranged as small, adjacent, ascending fibres, sometimes anastomosing. In a few genera a fibrous network characterises the skeleton (this diverging trait is problematic for the homogeneity of the taxon). Choanocyte chambers either diplodal (small, spherical) or eurypylous. Mode of reproduction viviparous. Larvae large, brooded parenchymellae with a posterior clump of long cilia.

Order Halisarcida
Order Verongida Bergquist, 1978 Diagnosis (emended after Bergquist and Cook 2002c). Skeletal network, absent in some genera, without inclusions and with no distinction between primary and secondary fibres. The fibre structure is concentrically laminar surrounding a pith of thin fibrillar material. Taxa lacking skeleton show "peculiar verongid characters" such as the presence of complex brominated tyrosine derivates. Choanocyte chambers either diplodal or eurypylous. Mode of reproduction oviparous, larvae unknown.
Description. Growth form encrusting (less than 2 mm in height). Surface conulose, ornamented by a network of rounded meshes (200-300 µm in diameter) loaded of inclusions; inside the meshes surface is smooth and perforated by small apertures (15-40 µm in diameter). Colour from grey to violet (Vacelet 1959(Vacelet , 1969. Dendritic modules (tree-shaped) of the skeleton with fibres apically branched (80 µm in diameter at their base, 20 µm at the apical branch level).
Mediterranean caves. Blava, Calamars, Misidacis caves (Balearic Sea); Endoume, Figuier, Trèmies caves (Gulf of Lions) (Pouliquen 1972;Uriz et al. 1992;Martì et al. 2004;Pronzato and Manconi 2011). Description. Growth form encrusting. Surface conulose bearing a reticulate dermal membrane with fibre tips supporting conules. Colour in vivo "rouge carmin" as reported by the author, bright red. Dendritic skeleton arising from a basal spongin plate with the main fibres (up to 4 mm in height, 60-160 µm in diameter) evidently laminated and free of foreign material, with variably dense granular axial pith. Fibres. Horny spicules triactines free or connected to the main skeleton (rarely each to one another), with actins ca. 1.1-1.25 mm in length and 45-50 µm in diameter, gradually tapering towards the sharp tips. Rays linear, usually 3, rarely 2 or 4. Spicules sometimes with pith.
Remarks. Pronzato (1975) considered the Mediterranean species Darwinella simplex Topsent, 1892 as junior synonym of the Pacific species D. australiensis Carter, 1885 (senior synonym) sharing diagnostic morphological traits as also focused by Topsent (1892). A re-evaluation of original descriptions vs. old and new materials allow us to consider Darwinella simplex Topsent, 1892 a valid species. The validity of D. simplex solves the extremely disjunct Australian-Mediterranean geographic pattern and matches the hypothesis of a species complex. (Vosmaer, 1883) http://species-id.net/wiki/Spongionella_gracilis Fig. 6 Velinea gracilis Vosmaer, 1883: 439.

Spongionella gracilis
Description. Tubular habit with ten to fifteen slightly clavate hollow cylinders (up to 2 cm high, with a diameter of 5-8 mm) partly coalescing and arising from a common basal spongin plate (ca. 4.5 × 3 cm in diameter). Consistency soft and elastic, as the rule in all Spongionella species. Oscules apical (2-3 mm in diameter). Surface finely conulose with conules supported by tips of ascending fibres (conules ca. 100 µm high, 300 µm apart). Skeleton reticulate with a more or less regular network of generally quadrangular meshes (100-300 µm in diameter). Primary fibres (25-30 µm in diameter) connected by rare and irregular tracts (5-10 µm in diameter). Fibres laminated, clear, and uncored, with a transparent axis.
Remarks. The reticulate fibrous skeleton is atypical for Dendroceratida.
Remarks. The Mediterranean specimens ascribed to this species, are very different from the Atlantic ones.
Remarks. As for diagnostic traits the genus Pleraplysilla is anomalous among the Dictyoceratida, for the trait 'dendritic not anastomosing skeleton'. As for the taxonomic status Pleraplysilla minchini is regarded by Vacelet (1959) as a synonym of P. spinifera. Later authors, as Cabioch (1968) and Borojevic et al. (1968), considered both species as valid. The material available for our study seems to confirm a specific divergence between the two. P. spinifera is generally recognizable at sight by the very pronounced, spaced conules. Its fibres reach a length of 12 mm, with a thickness of  450 µm near the base; they are generally branched; sometimes more than one fibre starts from a common basal plate; the inclusions are mostly closely-packed sand grains. In P. minchini the fibres are less widely spaced, they reach not more than 3 mm in length and a diameter of 160 µm near the base; they are generally not branched and there is a prevalence of sponge spicules in their inclusions.
Description. Growth form encrusting, up to 2 cm thick, as irregular patches (several cm in diameter) characterized by a smooth and conulose mucous surface. Conules very evident, up to 8-10 mm in height. Colour from whitish to very light brown. Consistency very soft. Exhalant and inhalant apertures (up to 1 mm in diameter) irregularly scattered on the surface. Skeleton of dendritic fibres generally arborescent with 2-5 branches. Each fibre with a basal plate strictly adhering to the substrate. Spongin laminated and cored by sand grains and spicule fragments. These stout fibres (1.5-2.0 mm in height) can reach 400 µm in diameter at their base, with a sandy core of 80 µm. Fibres usually light yellow and transparent show, in many cases, a red-brown colour due to microscopic algae.
Habitat. Cave, muddy and rocky bottom. Here we report a new record from a submerged cave in the NW-Sardinian karst. Bathymetric range shallow water up to 80 m.
Remarks. The present description is based on the holotype LMJG 15499 (Museum Joanneum of Graz, Austria), O. Schmidt collection, from Lesina (Adriatic Sea), and other specimens belonging to the Schmidt's collection preserved in the same museum. The study in depth of this dry holotype material resulted in the evidence that it does not belong to the genus Ircinia but perfectly matches the genus Sarcotragus. The holotype is, probably, a fragment of a bigger specimen and does not exceed 15 cm in diameter; no traces of dermal membrane or choanosomal architecture are visible, suggesting that it can be a beached specimen. The type material of Pallas Spongia fasciculata is missing and the single specimen of Ircinia fasciculata belonging to the Schmidt's collection (NHMG 15499) must be ascribed to the genus Sarcotragus. Pronzato et al. (2004) investigated the species formerly named Ircinia fasciculata (Pallas, 1766); the result was that Ircinia variabilis (Schmidt, 1862) became the type species of the genus Ircinia Nardo, 1833 and the specimen LMJG 15499, of I. fasciculata, was moved under the genus Sarcotragus Schmidt, 1862 affirming that: "a further study will decide if this species is a good one or a synonym". Pronzato et al. (2004) focused the problematic status of the taxon but did not describe the species. Here a new combination for S. fasciculatus is proposed. Sarcotragus fasciculatus is clearly different from the other species ascribed in the genus, also when compared with extra-Mediterranean species (Pronzato et al. 2004) because all its fibres are free of inclusions and primary ones are formed by "fascicules of secondaries".
Description. Growth form irregularly massive to globular (up to 1 m in diameter, 50 cm in height); oscules large (0.5-1 cm in diameter) with a short collar, often grouped in a central depression at the top of the body. Consistency soft and strong. Colour is medium grey, but brown or black varieties have been also recorded (Vacelet 1959). Surface is smooth or covered by several epizoans. Conules are 2-3 mm high and 10-15 mm apart. Dry specimens become very hard and smaller (1/5) than living ones, also colour changes regularly into black. The skeleton does not differ from the other Mediterranean species belonging to the genus; the main skeleton composed by a reticulate network of primary (ca. 100-200 µm in diameter) and secondary (ca. 50-100 µm in diameter) fibres. Filaments abundant (1-3 µm in diameter).

Coscinoderma sporadense Voultsiadou-Koukoura, Van Soest and Koukouras, 1991
http://species-id.net/wiki/Coscinoderma_sporadense Fig. 24 Coscinoderma sporadense Voultsiadou-Koukoura, Van Soest and Koukouras, 1991: 195. Description. Growth form massive, cushion shaped, lobose (6 to 30 cm 2 surface area, ca. 5 mm avg thickness). Colour light brown, lighter in formalin. Consistency soft, spongy and compressible. Surface conulose with conules ca. 1 mm in height and 2-4 mm apart. Oscules few (2-4 mm in diameter). Ostia visible in some areas with a diameter of 50-200 µm. Ectosome (100-350 µm in thickness) detachable and armoured with sand grains and foreign spicules.  Ascending primary fibres (50-80 µm in diameter) cored with foreign material to such a degree that sometimes spongin is hardly visible. Foreign material usually sand grains mixed with low amounts of spicules, although some fibres cored exclusively with spicules. Primary fibres connected to a dense, irregular, network of secondary fibres which, in the vicinity of the primary fibres, has the form of a perforated plate. Secondary fibres (10-40 µm in diameter) often with rounded or broadly acute free tips, thin and hardly anastomosing. The secondary network, in its greater part, resembles an unwound clew.
Description. Growth form vase-or fan-shaped, large (up to over 1 m). Surface finely conulose, inhalant and exhalant openings of the aquiferous system on the outer and inner sides, respectively, of the vase, or on the opposite sides of the fan. Wall 5-10 mm thick. Inhalant apertures large and irregular. Oscules small with a diameter ca. 1.5 mm and grouped in clubs regularly scattered. Colour in vivo from grey to brown. Surface conulose. Ectosomal skeleton covered by a dermal membrane rich of sand, as a network of secondary fibres (15-20 µm in diameter) connected to the apices of primaries. Choanosomal skeleton as an irregular network of secondaries (20-40 µm in diameter) with evident tracts of primary fibres (50-80 µm in diameter) extended between inner and outer surfaces. Primary fibres cored by mineral inclusions.
Description. Growth form massive-lobate, surface finely conulose, single oscules scattered or at the apex of lobes, pre-oscular cavities well evident. Colour in vivo from light grey to black. Ectosomal skeleton as apices of primary fibres joining secondary fibres to form the conical reticulum which supports the conules. Choanosomal skeleton: network dense with irregular polygonal meshes of secondaries joining to form ascending primaries. Primary fibres (50-100 µm in diameter) typically twisted with ornamentations as parallel ridges along the main fibre axis mainly developed and evident towards the surface, cored with sand grains and spicules. Secondaries (20-35 µm in diameter) with ornamentations as parallel ridges along the main fibre axis, twisted and characterised by concentric layers of compact spongin surrounding the compact axial core without inclusions.

Spongia zimocca Schmidt, 1862
http://species-id.net/wiki/Spongia_zimocca Fig. 30 Spongia zimocca Schmidt, 1862: 23. Description. Massive to globular growth form, small size, usually not over 15 cm in diameter. Surface softly hairy, densely conulose with very long conules (2-3 mm high and less than 1 mm apart) sometimes a single conule supported by 2-3 converging primary fibres. Oscules not evident and located in small deep superficial depressions. Colour in vivo never reported. Consistency very soft, elastic and strong. Skeleton as a network of regular meshes (100-200 µm) with primary fibres bearing very rare inclusions (particularly fragments of spicules) and secondaries completely free of inclusions; primary fibres typically formed by anastomosing secondaries in fascicules (50-80 µm in diameter).
Habitat. Cave, rocky bottom, coralligenous community. Bathymetric range 1-40 m. Here we report a new record from the Bisbe Cave in the NW-Sardinian karst.
Remarks. It is a problematic species, indeed the Schmidt's type specimen (naked skeleton, Cyprus, no further data), preserved in the Graz Museum (LMJG 15470/0) is clearly a S. officinalis. Moreover many authors, in various papers, described this species differently, contributing to determine its problematic taxonomic status. In contrast with that, the commercial "Zimoccas" really belong to a species distinctly different from the other species hitherto ascribed to the genus Spongia as reported also by Schmidt (1862), Schulze (1879a) and de Laubenfels (1948). As a consequence the Graz Museum type needs to be carefully studied. The present description is based on the specimens TRG Ker 346, DTRG Ker 347, Jerba-El-Jem (Tunisia), 3-4 m, soft bottom, August 2006. Many traders consider "Zimocca" as the best commercial Mediterranean sponge.
Remarks. We do not accept that Cacospongia scalaris and C. proficens belong to the genus Scalarispongia on the basis of the genus diagnosis by Cook and Bergquist (2002). Indeed the comparative analysis of diagnostic traits of Scalarispongia vs. Cacospongia Schmidt, 1862 clearly indicates that no diverging morphological characters exist among them except for the ladder-like arrangement of skeletal polygonal meshes that in some species, i.e. C. scalaris, are mostly but not always rectangular. Rectangular meshes are displayed less frequently also in other species of Mediterranean cacospongias. We consider the trait 'skeleton ladder-like with rectangular meshes' not diagnostic at the genus level in agreement with Schmidt (1862), Vacelet (1959), Pulitzer-Finali andPronzato (1976) and Pronzato and Manconi (2011). Moreover molecular data (see Borchiellini et al. 2004) indicate that C. scalaris belongs to the genus Cacospongia. Cacospongia proficens and C. scalaris belong therefore to the genus Cacospongia.
Fasciospongia cavernosa (Schmidt, 1862) http://species-id.net/wiki/Fasciospongia_cavernosa Fig. 34 Cacospongia cavernosa Schmidt, 1862: 28. Description. Growth form tubular, massive, rounded, usually not larger than 10 cm, sometimes up to 25 cm in diameter. Colour dark brown at the surface, light yellow- ish at the choanosome. Large and abundant irregular cavities and canals scattered in the mesohyl (etymology of the specific name). Consistency strong and cartilaginous; sponge surface covered by very abundant conules (3-4 mm in height) giving a spiny aspect. External membrane smooth, translucent and resistant; flagellate chambers round (25-30 µm in diameter). Skeleton network very strong with large (50-250 µm) rugose or granulated fibres; some of the largest ones cored by foreign debris can be considered as primary fibres.
Description. Growth form sub-spherical or cake shaped, usually less than 10 cm in diameter. Colour black at the surface, greyish-yellow in the choanosome. Consistency very spongy in vivo, quite brittle in dry conditions. Surface conulose (conules 1-2 mm high, 1-2 mm apart). Oscules small, scattered and inconspicuous. Ectosome leathery, densely packed with highly heterogeneous detritus in nature, shape and size. Choanosome moderately cavernous and fleshy, with a ground-work of fibro-reticulations. Flagellate chambers rounded, 25-40 µm in diameter. Skeleton composed by very rare fibres completely filled by foreign materials, ascending primaries (100-350 µm in diameter), secondaries 50-100 µm, meshes very irregular in size, shape and outline; a large amount of variously composed and sized detritus is scattered in disorder in the mesohyl.

Halisarca dujardini Johnston, 1842
http://species-id.net/wiki/Halisarca_dujardini Fig. 36 Halisarca dujardini Johnston, 1842: 192. Description. Growth form encrusting, few mm thick and few cm in diameter. Consistency jelly-like or softly colloidal. Surface smooth with small oscular tubes and not evident inhalant apertures. Colour in vivo pale yellow to dark yellowish, sometimes with more or less dark blue tonalities. Absence of horny skeleton. Flagellate chambers radially arranged around the aquiferous system canals, elongated and typical of the genus (25 µm in diameter, 60-150 µm in length).
Description. Body irregularly massive to digitate (up to 20-30 cm in diameter and height). Colour bright yellow in vivo and dramatically changing in a few minutes after collection or preservation (both alcohol and formalin, but also in dry conditions) into a very dark violet or most frequently pure black. Evident oscules on the top of sponge body or digitations. Sponge body surfaces seasonally covered by thin outgrowths (asexual propagules) up to 5 cm in length and 1 cm in diameter; outgrowths are lost by the mother-sponge as propagules at the end of summer. Consistency firm and fleshy. Surface smooth to slightly conulose, showing a fine (but evident) superficial fibrous network. Skeleton fragile, with fibres of a single dimensional class (80-150 µm) arranged in a regular three-dimensional scaffold. Fibre structure laminar with a large axial core (30-70 µm) inconspicuous in dry condition.
Description. Growth form thinly encrusting and lobate, in large patches. Colour bright yellow in vivo, dark purple in alcohol after releasing a yellowish fluid. Surface finely conulose, entirely wrinkled by small evident collagenous reinforcements irregularly crossing and converging towards small conules, with inconspicuous inhalant apertures surrounding the tiny conules armed by debris. Large oscules in vivo, not visible after fixation in ethanol. Ectosome with bundles of collagen fibrils. Choanosome fragile with large clusters of spherulous cells with large inclusions of heterogeneous size, containing microgranules and microgranular cells. Choanocyte chambers (ca. 40 × 20 µm in diameter) eurypylous, densely packed with 40-60 choanocytes. Bacteria in the mesohyl. Aerophobins 1 and 2 compounds with medium-high natural toxicity.
Description. Growth form encrusting, thin, with lobes in large patches. Colour faded to pale pink in vivo, brownish in ethanol after releasing of a yellow fluid. Surface highly wrinkled by small evident collagenous reinforcements irregularly crossing and converging towards small conules; well developed (when compared to H. pruvoti and H. crypta) star-shaped network of subdermal canals converging towards oscula; inhalant apertures inconspicuous. Oscules wide, at the apices of short chimneys. Ectosome notably thick. Choanosome soft, fleshy and fragile, difficult to cut. Large clusters of spherulous cells, common at the body surface, with large inclusions containing micro-granules and microgranular cells; choanocyte chambers eurypylous (30±6.3 × 19±2 µm on average) in dense clusters. High natural toxicity.
Remarks. See the original description for more details and figures (Reveillaud et al. 2012).

Conclusive remarks
Mediterranean marine caves host one of the least investigated biocoenosis. Despite the difficulties of accessing these biotopes, their horny sponge fauna was recorded in 51 papers published between 1958 and 2012, that focused on marine submerged and semi-submerged caves, mostly along the Italian coasts ( Fig. 1; Table 1). Several papers refer each to a single or very few sponge records. Caves of the Levant Basin and the northern African coasts are scarcely or absolutely not investigated. Moreover, each pa-per reports a species list which is spot data series with no replicas to indicate the real taxonomic richness and/or population dynamics.
The present faunistic assessment, based on literature and new data, results in high values of taxonomic richness of Mediterranean cave-dwelling horny sponges with 4 orders, 9 families, 19 genera and 40 species (Table 2) recorded in 105 out of ca. 150 investigated caves. The new data refer to the first record of 18 species in recently investigated karstic caves ( Fig. 1; Tables 1, 2) namely, 14 species from the Capo Caccia-Isola Piana MPA (Galatea, Falco, Bisbe caves), six species from the Plemmirio MPA (Mazzere, Gamberi, Gymnasium caves), and nine species from the Pelagie MPA (Taccio Vecchio I Cave, Lampedusa) Cadeddu 2012). The present synthesis demonstrates how cave-dwelling horny sponges are representatives of the taxon Porifera in the whole Mediterranean basin thus confirming the high affinity of this pool of species for marine caves; indeed 70% of Mediterranean species (40 out of 57) were recorded to date in marine caves. Species endemic to the Mediterranean Sea harboured in marine caves number 14 with an endemicity value of 35%.
A few species such as Coscinoderma sporadense, Euryspongia raouchensis, Hexadella crypta and Hexadella topsenti are, however, recorded only once, exclusively from their type locality. Although some few species are reported only from caves, the present overview cannot assert the existence of horny sponge species exclusively restricted to cave habitats. The topographic distribution of horny sponges in each investigated cave is restricted to the cave entrance until the semi-dark zone, while no record is reported for confined zones of the caves matching those reported by Pouliquen (1972).
The census of marine caves sponge fauna is characterized by non-homogeneity of sampling methods and efforts, limiting the possibilities of exhaustive comparative analysis of this biocoenosis in the whole of the Mediterranean Sea. Results highlight also that Mediterranean marine caves host seven horny sponges species listed in the appendices II and III of the Barcelona Convention as "protected species of the protocol SPA/BIO", namely Aplysina aerophoba, Aplysina cavernicola, Sarcotragus foetidus, Sarcotragus pipetta, Spongia lamella, Spongia officinalis and Spongia zimocca. They belong to protected biocoenosis of marine caves registered as Habitat II.4.3, Habitat IV.3.2, and Habitat V.3.2 matching the category of mid-littoral caves, semi-dark caves, and dark caves (Relini and Giaccone 2009;Relini and Tunesi 2009). These horny sponge species have a high economic value and are reported as endangered (see Pronzato et al. 2003). The entire data set highlights how marine caves represent a hotspot of biodiversity needing further scientific investigation and appropriate conservation measures that can exert a key role in supporting survival and random genetic reassortment of populations belonging to these species (i.e. caves as reserves of genetic biodiversity) in all Mediterranean biotopes. This matches perfectly both the UE Habitat 8330 strategy of conservation and the biodiversity assessment of Mediterranean species at risk in the progressive environmental/climatic change of the entire basin.