Original scientific article UDC 574.5:551.442(497.5 Istra) Received: 2006-05-29 BENTHIC MACROFAUNA OF A SUBMARINE CAVE ON THE ISTRIAN PENINSULA (CROATIA) Lisa FARESI, Nicola BETTOSO & Ida Floriana ALEFFI Osservatorio Alto Adriático, ARPA FVC, 1-34010 Trieste, Via A. Piccard 54, Italy E-mail: lisa.faresi@lbmtrieste.it Giuliano OREL Dipartimento di Biologia, Universita degli Studi di Trieste, I-34127 Trieste, Via E. Weiss 2, Italy ABSTRACT Karst phenomena are common under the sea level in the submarine zone of Istrian Peninsula: a typical phenomenon of karst hydrography is the presence of submarine fresh water springs locally known as "vrulije". Submarine caves are considered ideal habitats, where selection of species is remarkable due to the reduced light, slow water circulation and low nutrient level. From the entrance to the inward end of the cave there is a progressive reduction in the number of taxa and total biomass. This study examines the macrofauna inhabiting the soft bottom, including a thanatocoenosis analysis, and the sessile fauna living on the hard substrate of the wall and the ceiling in a small submarine cave in the Adriatic Sea. The cave presents, on hard substrata, the circalittoral community of the semi-dark cave and there is a horizontal zonation of the communities inhabiting the wall. The macrofauna inhabiting the soft bottom did not show any clear zonation in terms of species richness or abundance. Key words: macrobenthos, submarine cave, Northern Adriatic, Croatia LA MACROFAUNA BENTONICA Dl UNA GROTTA SOTTOMARINA NELLA PENISOLA ISTRIANA (CROAZIA) SINTESI Lungo la costa Istriana sono frequenti i fenomeni dell'idrografia carsica che danno origine a grotte ed a risorgive subacquee localmente conosciute con il nome di "vrulije". Nelle grotte subacquee parametri come la riduzione dell'intensita luminosa, la diminuzione dei nutrienti e della circolazione delle masse d'acqua provocano una progressiva riduzione del numero di taxa e della biomassa totale procedendo dall'ingresso verso il fondo della grotta. Il presente lavoro esamina le comunita macrobentoniche del substrato molle, l'analisi delle tanatocenosi e la fauna sessile insediata sul substrato roccioso delle pareti e del soffitto di una piccola grotta subacquea del Mare Adriatico. La grotta presenta, sul substrato roccioso, comunita circalitorali caratteristiche delle grotte semioscure ed indica una zonazione orizzontale delle comunita insediate lungo le pareti. La macrofauna presente nel substrato molle non indica una chiara zonazione in termini ricchezza e di abbondanza. Parole chiave: macrobenthos, grotta sottomarina, Alto Adriatico, Croazia INTRODUCTION The Croatian coastal and channel area is mostly a submerged karst relief. Various karst phenomena (sinkholes and caves) are common under the sea level in the submarine zone of the Adriatic Sea, including the islands in the Kvarner region (Bozicevic, 1992; Arko-Pijevac et a/., 2001). A typical phenomenon of karst hydrography is the presence of submarine freshwater springs (locally known as "vrulje") connected to the Adriatic Sea by coastal underground water circulation systems, which flow mostly through marine caves (Al-firevic, 1966). Their number in the Adriatic Sea is relatively large along the eastern shore. They are present from the west coast of Istria to the Albanian waters, and in the Adriatic archipelago (Alfirevic, 1969). Submarine caves are considered ideal habitats to study the influence of many environmental parameters on the settled benthic communities (Riedl, 1978) due to the presence of strong environmental gradients on spatial scales of a few meters (Ott & Svoboda, 1976; Cinelli et a/., 1977). In the interior of a cave, changes in the intensity of light and in the hydrodynamic regime are easily perceived (Benedetti-Cecchi et a/., 1996). Due to the reduced light, slow water circulation and low nutrient level, submarine caves are considered to be a habitat where selection of species is remarkable (Riedl, 1966; Harmelin et a/., 1985; Zabala et a/., 1989; Bian-chi & Morri, 1994). Dark caves show some similarities with the bathyal zone (Arko-Pijevac et a/., 2001) with regard to hydrodynamics, nutrient level (Fichez, 1990, 1991a, b) and fauna composition including sponges, anthozoans, serpulids and bryozoans (Harmelin, 1985; Harmelin et a/., 1985). All studies on Mediterranean caves have revealed a remarkable horizontal zonation of the animal communities inhabiting the walls (Laborel & Vacelet, 1958; La-borel, 1960; Pérès & Picard, 1964; Gili et a/., 1982; Bibiloni et a/., 1984). The number of taxa from the entrance to the inward end of the caves is progressively reduced; sponges, scleractinians and polychaetes become dominant at the expense of other taxa, and total biomass decreases (True, 1970; Gili et a/., 1986). This study examines the macrofauna inhabiting the soft bottom, including a thanatocoenosis analysis, and the sessile fauna living on the hard substrate of the wall and the ceiling in a small submarine cave in the Adriatic Sea (Columbera cave) characterized inside by the presence of freshwater springs. MATERIAL AND METHODS Columbera cave (45"10'18"N, 14"14'07"E) is located near Brsec, on the eastern coast of the Istrian Peninsula (Fig. 1). The cave entrance is about 3 m high at a depth of 6 to 9 m at the bottom of a cliff. The cave has a linear shape and is approximately 11 m long. The length-width ratio is about 3:1 and this ratio is characteristic of a "Sackhohle" cave, as described by Riedl (1966) (Fig. 2). Muddy-sand and rough organic detritus, mostly shells fragments, cover the cave floor. Two freshwater springs are located in the cave at 10.6 and 2 m from the entrance: the former in muddy sediment, the latter in de-tritic sand. Fig. 1: Location of Columbera cave. SI. 1: Lokacija jame Columbere. Direct sampling was performed in April 2001 using scuba dive. The sessile macrofauna on the wall and ceiling of the cave was analyzed by visual-census in each meter from the entrance to the bottom of the cave (1 to 9 meters) and organisms were photographed with a Nikon F90 in an underwater housing; species of uncertain determination were collected for identification in the laboratory. Hierarchical classification and MDS based on the Bray-Curtis similarity coefficient was calculated, using complete linking (PRIMER software package developed at the Plymouth Marine Laboratory). Three sites were chosen to study soft bottom communities: site A located in the inner area of the cave, site B situated among the springs, site C close to the outer spring. A set of four squares (0.1 m2 x 0.20 m) were positioned, next to each other, on site A and B to collect the sediment using a small shovel (Di Geronimo & Robba, 1976). In site C it was impossible to place the square into the sediment and thus only 9 l of sediment were collected directly using a shovel. Samples were sieved on 1 mm mesh and preserved in 4% formalin. In the laboratory, living organisms were sorted and determined at the lowest possible taxonomic level. The thanatocoenoses were analyzed by sorting intact shells and skeletal structures; each empty bivalve shell was counted as a separate individual (Peharda et a/., 2002). 136 specimens), whereas site C was the poorest (8 taxa and 14 specimens), probably due to the small amount of sediment collected. The main taxonomic groups were (Tab. 2): molluscs (8%), polychaetes (37%), sipunculids (31%), crustaceans (2%) and echinoderms (22%). The most representative groups in terms of taxa and abundance were polychaetes and molluscs, respectively, whereas the most abundant species were the sipunculid Aspidosiphon mue//eri and the echinoderm Amphiura chiajei. These two species were present at all sampling sites. Tab. 1: Hard bottom community (sessile fauna). Tab. 1: Zduzba trdega dna (prirasli organizmi). c-c Species group 1 group II group III 0-3 m 3-6 m 6-9 m Tunicata Diplosoma listerianum * Tunicata Halocynthia papillosa * Porifera Anchinoe tenacior * * Porifera Aplysina aerophoba * * Porifera Cacospongia scalaris * * Porifera Chondrosia reniformis * * Porifera Clathrina clathrus * * Porifera Dysidea avara * * Porifera Hemymicale columella * * Porifera Ircinia variabilis * * Bryozoa Myriapora truncata * * Bryozoa Hornera frondiculata * Porifera Agelas oroides * * * Porifera Axinella verrucosa * * * Porifera Crambe crambe * * * Porifera Oscarella lobularis * * * Cnidaria Parazoanthus axinellae * * * Porifera Petrosia ficiformis * * * Porifera Spirastrella cunctatrix * * * Porifera Aplysina cavernícola * * Cnidaria Leptopsammia pruvoti * Fig. 2: Cave longitudinal plane and cross section. SI. 2: Podolžna ploskev in prerez jame. RESULTS A total of 21 sessile species living on the hard substrate were recognized. The more representative taxa were Porifera (15 species), Cnidaria (2 species), Briozoa (2 species) and Tunicata (2 species). Hierarchical classification and MDS performed on sessile species yielded three groups: group I included 18 species from the entrance to 3 meters (1-3), group II with 18 species represented the middle portion to 6 meters (4-6) and group III with 9 species comprised the inner area to 9 meters (7-9) (Fig. 3). Table 1 shows the species present in the cluster groups: seven species were found in each group I-II-III, from the entrance to the bottom of the cave: nine species were found only in groups I and II, to 6 meters, and 1 species in the groups II and III. Two species were found only in group I, one species in group II, and one species in group III. A total of 197 living organisms belonging to 26 taxa were collected in the soft bottom. The richest site in terms of abundance and taxa was site A (23 taxa and Regarding the thanatocoenosis, a total of 6555 specimens belonging to 83 species were collected in the detritus. The richest station in term of abundance and taxa was site B, whereas site C was the poorest. The main taxonomic groups were: molluscs (90%), echino-derms (9%) and brachiopods (1%); of the molluscs, 56% were gastropods, 43.7% bivalves and 0.3% scaphopods (Tab. 3). Of the 83 species, only 5 species were detected as living organisms, accounting for 28% of the total specimens collected as the thanatocoenosis. Twelve species were found at all sampling stations and represented 36% of total abundance (Tab. 3). DISCUSSION AND CONCLUSIONS Submarine caves are mostly found around the islands and islets in the area of the open Adriatic and along the eastern rocky shore; they are fairly scarce in the other parts of the Adriatic Sea. The hard substrate community inside caves is characterized by the presence of the cir-calittoral biocoenosis of semi-dark caves (Gamulin-Brida, 1967) or GSO (Grottes Semi-Obscures) (Pérès & Picard, 1964). This biocoenosis comprises only animals, and sponges represent the dominant fauna (Pérès & Picard, 1964). In the Columbera cave, more than 70% of sessile fauna were sponges. The following species characterized the GSO biocoenosis: Aplysina cavernícola, Petrosia ficiformis (Fig. 4), Oscarella lobularis, Agelas oroides (Porifera); and Parazoanthus axinellae (Fig. 5) and Leptposammia pruvoti (Pérès & Picard, 1964) (Cni-daria). The sponge Aplysina aerophoba, characteristic of the infralittoral biocenosis of photophilic algae, was found nearby the entrance and in the middle zone of the cave. Some species common in the sciaphilic niche of this biocenosis or present in the circalittoral biocenosis of rocks in the open sea are also present in the GSO biocoenosis: Axinella verrucosa (Fig. 5) (Porifera), Echi-naster sepositus (Echinodermata), Bonellia viridis (Echi-urida), Diplosoma listerianum and Halocynthia papillosa (Tunicata), Hornera frondiculata (Bryozoa), as well as Myriapora truncata (Bryozoa), a species characteristic in the circalittoral biocenosis of coralligenous (Pérès & Picard, 1964; Gamulin-Brida, 1967, 1974). 20 40 CO 80 Group I Group II Group III Stress: 0,02 Fig. 3: (Top) Hierarchical classification and (bottom) multidimensional scaling (MDS) of the hard bottom community. Sl. 3: (Zgoraj) Hierarhična klasifikacija in (spodaj) večdimenzionalno skaliranje (MDS) združbe, živeče na trdem dnu. Cluster analysis and MDS clearly showed a separation among the inner area of the cave and the entrance up to the middle portion. Group III showed a reduction in the number of species (True, 1970; Gili et a/., 1986), and the populations constituted of species capable of living in the inner area of semi-dark caves (Bianchi & Morri, 1994, 1999): in particular, L. pruvoti was present only at this site, and small serpulids, characteristics of dark caves, (Bianchi & Morri, 1994) were noted. At the entrance, all species present at the other stations inside the cave were found, with the exception of H. frondicu/ata, $. cavernico/a and L. pruvoti. D. /isteri-anum and H. papi//osa disappeared in the middle portion, where H. frondicu/ata was recorded and $. cavernico/a appeared. Among the motile fauna we observed, at the entrance, the opistobranch F/abe//ina affinis and Cratena peregrina, the echiurid B. viridis and the echi-noderm E. sepositus, whereas an individual of the cni-darian Cerianthus membranaceus was found in the soft bottom of the middle area. Finally, many individuals of the opisthobranch Discodoris atromacu/ata feeding on the sponge P. ficiformis (Cattaneo-Vietti et a/., 1990; Jaklin, 1998; Turk, 2000) were found between the entrance and the bottom. Tab. 2: Soft bottom community. Tab. 2: Združba mehkega dna. Station A 1 B 1 C Mollusca Cerithium vuljatum i Bittium latreillii 3 2 Polinices nitida i Marshallora adversa i Muricopsis cristata 2 i Nassarius incrassatus i i Myrtea spinifera i Plajiocardium papillosum i i Polychaeta Paraonidae non ¡dent. i i Pseudoleiocapitella fauveli 5 7 Capitellidae non ident. 8 Glycera rouxii i Glycera unicornis 2 i Nereidae non ident. i Nephtys homberji i i Sthenelais sp. i Aponuphis bilineata ii i5 Eunice vittata 2 Marphysa bellii i i Sabellidae non ident. ii 2 Sipunculida Aspidosiphon mulleri* 54 1 2 1 6 Crustacea Athanas nitescens i Anisopoda i Amphipoda i Echinodermata Amphiura chiajei* 25 i6 i Schizaster canaliferus i Total abundance 136 47 14 Total No. of species 23 10 8 + Species present in each sampling station. In the inner area of the cave, 9-11 m, no organisms were found and the visibility was limited owing to the inflowing spring freshwater. The ceiling of this area and of another portion of the cave, directly influenced by two freshwater springs, was completely defaunated with evident erosion phenomena. The macrofauna inhabiting the soft bottom did not show any clear zonation, either in terms of species richness or abundance, from the entrance to the bottom of the cave, as opposed to the case for solid substrata. Am-phiura chiajei (Echinodermata) and Aspidosiphon mul-leri (Sipunculida) were the most abundant species, the former being common in coastal detritic bottoms more or less mixed with mud (Gamulin-Brida, 1967), the latter inhabiting Turritella communis shells, the latter being characteristic for the circalittoral biocenosis of the coastal terrigenous muds (Gamulin-Brida, 1967) or VTC (Vases Terrigènes Cotieres) (Pérès & Picard, 1964; Gamulin-Brida, 1974). Most of polychaetes found are common in the soft bottoms of the northern Adriatic Sea (Aleffi et al., 2003) and the species recorded are not specific to particular biocoenoses or sediment texture. The same considerations are valid for other taxa, except for Plagiocardium papillosum and Schizaster canaliferus, which are considered to prefer the circalittoral biocenosis of sand-detritus more or less mixed with mud (DC-E) (Gamulin-Brida, 1974). Gastropods shells constituted about 50% of shell detritus and most of them are common on the solid substrata in the infralittoral and me-diolittoral zones (Vio & De Min, 1999), such as Bittium reticulata and Diodora sp. Some species are restricted to or prefer the infralittoral biocenosis of photophilic algae, such as: Columbella rustica, Patella caerulea, Cerithium vulgatum and Rissoa variabilis (Gamulin-Brida, 1967, 1974). Many of bivalve shells present are commonly found in detritic bottoms and some species are considered characteristics or preferential of the circalittoral biocenosis of coastal detritic bottoms like: Pitar rudis, Venus casina, Tellina balaustina, P. papillosum, Lima hians, Chlamys varia and C. flexuosa (Gamulin-Brida, 1967, 1974). Among endolitic bivalve burrowers, Litho-phaga lithophaga lives also abundantly in the boring holes on the rock round Rijeka Bay from the lower me-diolittoral zone down to 13 m depth (Hrs-Brenko et al., 1998). The faunal composition and zonation on solid substrata in Columbera cave were similar to those described in a submarine cave near Vrbnik on the Island of Krk (Arko-Pijevac et al., 2001): but the number of taxa in Columbera cave (Porifera 15, Cnidaria 3 and Echino-dermata 2) was always lower than in the latter cave (Porifera 22, Cnidaria 7 and Echinodermata 5). Although the length of the cave and its entrance location are not as deep as at Vrbnik cave, this study revealed a marked horizontal zonation within the animal communities inhabiting the walls (Laborel & Vacelet, 1958; Laborel, 1960; Pérès & Picard, 1964; Gili et al., 1982; Bibiloni et al., 1984), even if not considering the abundance and the cover of sessile fauna. Species that are characteristic or common in the biocenosis of rocks in the open sea were found. This biocenosis occurs at the boundary between the circalittoral and bathyal zone, near the break in the slope of the continental plateau (Gamulin-Brida, 1967). Reduced light penetration and slow water circulation are probably the main environmental parameters to permit the settlement of these species in few meters depth, thus showing some similarities with the bathyal zone (Arko-Pijevac et al., 2001). Below Fig. 4: Discodoris atromaculata (Foto: B. Furlan). SI. 4: Discodoris atromaculata (Foto: B. Furlan). 1% of superficial light intensity, benthic populations are markedly sciaphylic (Bianchi & Morri, 1999), and sciaphylic organisms, mostly sponges and cnidarians, were observed just several meters under the sea surface in the crevices of breakwater dams in the Gulf of Trieste (Bettoso ef a/., 1999). In this area, Orel & Specchi (1967) already pointed out the role of light conditions and substratum morphology as the main features determining the zonation of benthic organisms in a cavity of the tidal zone. Water circulation determines the structure of benthic populations in caves (Bianchi & Morri, 1999). Gili ef a/. (1986) found no gradient for temperature, salinity, oxygen, chlorophyll a or suspended particles, suggesting a constant circulation within the caves which guarantees water-exchange. In Columbera cave, the Tab. 3: Thanatocoenosis. Tab. 3: Tanatocenoza. Station ABC Gastropoda A/van/a aspera 8 6 A/van/a cance//afa 49 45 A/van/a c/mex 313 258 A/van/a ger\on/a 38 27 ß/ff/um ref/cu/afa 243 157 ßo/ma rugosa 1 2 Ca///osfoma /aug/er/ 9 13 Cer/fh/ops/s minima 5 7 Cer/fh/ops/s fuberco/ar/s 9 6 C/ancu/us cora///nus 9 24 C/ancu/us cruc/afus 3 2 D/odora graeca 10 D/odora /fa//ca+ 45 61 1 Fmarg/nu/a ocfav/ana 41 75 Fmarg/nu/a s/cu/a 3 19 Fp/fon/um acu/eafum 12 7 Fp/fon/um commune 48 22 Fu//m/a b///neafa 1 2 Fus/nus rosfrafus 31 41 Ha//of/s /ame//osa+ 12 70 1 +oma/opoma sangu/neum 1 -u/ub/nus exasperafus 22 35 /e/osfraca g/abra 3 1 Mange//a mu/f///neo/afa 5 3 Mange//a sfoss/c/ana 50 52 Mange//a un/Tasc/afa 39 16 Marsha//ora adversa* 152 171 M/fra n/gra 2 4 Mur/cops/s cr/sfafa* 273 192 Nassar/us /ncrassafus* 172 105 Pafe//a caeru/ea 6 Pafe//a u/\ss/ponens/s+ 8 2 12 Ph///berfh/a bo/v7//ana 4 Po//n/ces n/f/da* 30 16 Raph/foma //near/s 18 24 Station A B C R/ssoa guerr/n// 10 18 R/ssoa sp/end/da 16 7 R/ssoa var/ab/V/s 17 13 R/sso/na brugu/er/ 13 8 Tr/co//a pu//us 2 Turr/fe//a communis 33 11 Vex///um fr/co/or 3 3 Bivalvia Nucu/a n/f/dosa 9 16 Abra fenu/s 15 1 AcanfKocard/a ecK/neafa 22 11 AnodonfK/a Irag///s 11 52 Anom/a epK/pp/um 6 4 Arca noae+ 2 11 1 Arca fefragona 7 19 Azor/nus cKamaso/en 4 5 ßarbaf/a barbafa+ 110 158 23 CK/am\s flexuosa 1 CK/am\s mu/f/sfr/afa 28 41 CK/am\s var/a+ 312 364 73 Cora///opKaga //fopKage//a 2 6 Corbu/a g/bba 2 2 Cfena decussafa 1 Cusp/dar/a cusp/dafa 1 Cou/d/a m/n/ma 14 4 H/afe//a arcf/ca 75 101 /rus /rus 5 2 //ma K/ans+ 4 36 4 //fKopKaga //fKopKaga+ 6 8 4 Myi/'/asfer m/n/mus+ 1 1 12 Nucu/ana pe//a 1 1 Pa///o/um /ncomparab//e 2 Parv/card/um ex/guum 51 21 P/far rud/s 11 3 P/ag/ocard/um pap///osum*+ 240 279 28 PseudocKama gr\pK/na+ 19 48 17 Scrob/cu/ar/a coffard/ 27 29 Spond\/us gaederopus 1 1 Te///na ba/ausf/na 87 49 TKyas/ra //exuosa 1 T/moc/ea ovafa 6 6 Venus cas/na+ 9 39 4 Venus verrucosa 7 2 Scaphopoda Denfa//um vu/gare 1 Denfa//um /naequ/cosfafum 10 9 Fusf/ar/a rubescens 1 Brachiopoda Arg\rofeca cordafa 1 Arg\rofeca cuneafa 25 12 Echinodermata FcK/nocyamus pus///us 151 451 Total abundance 3034 3340 181 Total No. of species 74 78 13 + Species present in each sampling station. * Species detected as living organisms. output of freshwater flowing along the ceiling draws the external sea water inside the cave. This fact might promote the water-exchange, although defaunation was observed at sites directly influenced by spring activity. The lack of zonation of the soft bottom community could be related to spring activity, which disturbs sedimentary patterns and consequently community structure. Thus no indicative soft bottom benthic community was present in the cave. The cave probably constitutes a deposit zone for shell detritus coming from outer area because the entrance is open to southern and northern winds (mainly the wind known as "bora"). BENTOŠKA MAKROFAVNA PODMORSKE JAME V ISTRI (HRVAŠKA) Lisa FARESI, Nicola BETTOSO & Ida Floriana ALEFFI Osservatorio Alto Adriatico, ARPA FVC, 1-34010 Trieste, Via A. Piccard 54, Italy E-mail: lisa.faresi@lbmtrieste.it Giuliano OREL Dipartimento di Biologia, Universitä degli Studi di Trieste, I-34127 Trieste, Via E. Weiss 2, Italy POVZETEK Podmorske jame najdemo predvsem ob otokih v odprtem delu Jadranskega morja in vzdolž njegove vzhodne skalnate obale. V istrski jami, imenovani Columbera, obstajata dva izvira sladke vode, kar je značilen pojav kraške hidrografije. Podmorske jame so habitati posebne vrste: parametri, kot so svetloba, vsebnost hranil in kroženje vode, se občutno zmanjšajo že po nekaj metrih in tako vplivajo na izjemen izbor vrst. Trda podlaga v jami je dom cirkali-toralne združbe slabo presvetljenih vodnih okolij, medtem ko med združbami, živečimi na steni jame, obstaja očitna horizontalna conacija. V makrofavni, ki poseljuje mehko dno, ni bilo zaslediti jasne conacije, kar zadeva gostoto in številčnost vrst. Vrste, živeče na mehkem dnu, so značilne za obalno detritično dno, bolj ali manj pomešano z gle-nom. Dejstvo, da tu ni conacije, bi lahko pripisali sladkovodnima izviroma, ki moteče delujeta na sedimentne vzorce in zatorej tudi na strukturo tam živeče združbe. 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