Petar Kružić

Banks of the coral Cladocora caespitosa (Anthozoa, Scleractinia) in the Adriatic Sea

Cladocora caespitosa (Linnaeus, 1767) is a colonial scleractinian coral belonging to the family Faviidae and occurs in the Mediterranean Sea and the IberoMaroccan Gulf. The coral branches build big irregular colonies, up to 1 m in diameter, which fuse into banks and produce reef-like structures (Fig. 1C). Large banks of C. caespitosa, both living and fossil, have been found at several sites in the Mediterranean Sea: near the Tunisian coast, and in the Aegean and Ligurian Seas (Laborel 1961; Zibrowius 1980; Peirano et al. 1998). In the Adriatic Sea, Abel (1959) also describes extensive colonies of C. caespitosa in the channel of Lim near Rovinj whilst Pax and Müller (1962) mention banks of C. caespitosa near Mljet Island (Fig. 1A). Our recent surveys are focused on four locations where banks occur in the Adriatic Sea: in the Lim channel and near Prvić Island in the northern part of the Adriatic; near Pag Island in the central part of the Adriatic; and in Veliko Jezero (National Park ‘‘Mljet’’) in the southern part of the Adriatic. In the area where the banks occur, the annual sea temperature range—from 6 C in winter (the Lim channel) to 29 C in summer (Mljet Island)—is rather wide. The C. caespitosa bank in Veliko Jezero occurs at depths from 4–18 m and covers an area of 650 m and is thus one of the largest bank of C. caespitosa found in the Mediterranean Sea. The strong sea currents, which occur as a result of tidal exchange in the channel, appear to favor the growth of the bank. The colonies of the C. caespitosa from Mljet have similar growth rates to those in tropical reefs (growth rates range from 2.8 to 6.2 mm/year) measured with x-radiography and ‘‘Alizarin’’ staining method (Schiller 1993a; Kružić and Požar-Domac 2002) and slightly higher growth rates than those measured in colonies of the C. caespitosa in the Ligurian Sea (1.3–4.3 mm/year) by Peirano et al. (1999). The bank harbors a high diversity of associated species, especially sponges, polychaetes, and molluscs. Coral bleaching during summer was observed only in shallow waters (4–10 m depth) and in only a few colonies.

The highly invasive alga Caulerpa racemosa var. cylindracea poses a new threat to the banks of the coral Cladocora caespitosa in the Adriatic Sea

The invasive green alga Caulerpa racemosa var. cylindracea was observed in the Mediterranean for the first time in 1991 in Libya. Within 20 years it has expanded to almost all regions of the Mediterranean. Like Caulerpa taxifolia, C. racemosa var. cylindracea is crowding out the algal flora, diminishing biodiversity, and also overgrowing corals (Piazzi et al. 2001). This has previously been observed on coral reefs in the Caribbean (Littler and Littler 2000, 2003). Predominantly spreading by sea currents, by the end of 2006 it was found at more than 50 locations along the eastern Adriatic coast (Žuljević and Antolić 2004). In the summer of 2004, it was found in the National Park Mljet in the Channel Soline at 0.5–4 m depth, along 200 m of coastline (Fig. 1d). An additional large colony of the alga was found just outside the channel in Gonoturska Cove. In September 2004, the alga was found in Veliko jezero at 8–14 m depth, on 150 m of coastline and the marginal part of the coral bank of Cladocora caespitosa (Fig. 1a–c). The C. caespitosa bank in Veliko jezero occurs at depths between 6 and 18 m and covers an area of 650 m, and is one of the largest banks in the Mediterranean (Kružić and Požar-Domac 2003). The alga smothered live coral colonies, causing the coral to completely retract, leaving the calyx rim deprived of tissue coverage, and then produced large quantities of mucus. The alga’s ability to propagate from fragments and the strong sea currents, a result of tidal exchange in the channel, appear to favor the spread of algal fragments in the Veliko jezero.

Impact of tuna farming on the banks of the coral Cladocora caespitosa in the Adriatic Sea

Anthropogenic activities such as industrial and urban sewage discharges, trawl fishing and coastal works, and introduction of the tropical algae Caulerpa taxifolia (Vahl) C. Agardh and Caulerpa racemosa (Forsskal) J. Agardh have caused a major decline of the colonial coral Cladocora caespitosa (Linnaeus, 1767) in the eastern part of the Adriatic Sea (Kružić 2005). There is now evidence that the recent increase in fish farming in the Adriatic Sea is imposing further anthropogenic pressure. C. caespitosa is the only reef-building coral in the Mediterranean (Kružić and Požar-Domac 2003). Near Iz Island (Central Adriatic Sea) a C. caespitosa bank occurs at depths of 12–16 m and covers an area of 150 m. This bank is located 300 m away from tuna fish farm cages. Fish farming induces high organic and nutrient loading in the surrounding water. These nutrients enhance phytoplankton and macroalgal blooms, which reduce light reaching the benthos. During an algal bloom, the coral bank was covered with algae (Fig. 1a), and underneath the coral had started to bleach (Fig. 1b). Subsequently, there was a heavy bloom of pluricellular filamentous algae, principally Acinetospora crinata (Carmichael ex Harvey) Kornmann. In recent years the occurrence of benthic mucilaginous aggregates has become an increasing problem along the Adriatic coastline and in many other areas of the Mediterranean. These aggregates appear as small, yellowish tufts in early spring and continue until the end of summer, forming, under favorable environmental conditions, extensive patches on the seabed. A heterogeneous algal community of diatoms, blue–green algae and fragments of macroalgae grows inside the benthic aggregates (Lorenti et al. 2005). The macroscopic development is caused by few filamentous and associated species: Nematochrysopsis marina (Feldmann) Billard and Chrysonephos lewisii (Taylor), two fast-growing multicellular benthic chrysophytes, together with a free-living form of the brown alga Acinetospora crinita (Fig. 1d). Between 2001 and 2005, as a result of these impacts, more than 90% of C. caespitosa colonies on the bank died (Fig. 1c).

Coral tissue mortality of the coral Cladocora caespitosa caused by gastropod Coralliophila meyendorffi in the Mljet National Park (eastern Adriatic Sea)

The corallivorous gastropod Coralliophila meyendorffi (family Coralliophilidae) is a well-known predator of the coral Cladocora caespitosa, particularly in the eastern Adriatic Sea, where population outbreaks can drastically reduce coral cover. Coralliophila meyendorffi ranges in size from 5 to 40 mm in shell total length, and smaller specimens are often found living with the coral C. caespitosa. Specimens of C. meyendorffi feed exclusively on live coral tissue, stripping it from the calcium carbonate skeleton. Recent outbreaks of the gastropod C. meyendorffi have caused considerable damage on a C. caespitosa bank in Veliko Jezero (the Mljet National Park, eastern Adriatic Sea). The bank occurs at depths between 6 and 18 m, covering a 65 m 2 area. During summer of 2010, a substantial outbreak affected about 5%t of the coral colonies. The gastropods were highly aggregated in sheltered areas of the coral bank (up to 41 specimens/dm 2 ) with smaller clusters (from 8 to 12 specimens/dm 2 ) on coral colonies that were influenced by strong bottom currents (up to 1.23 ms -1 ).

Bioconstructions in the Mediterranean: Present and Future

In the Mediterranean Sea, most important habitat formers are bioconstructors. Bioconstructors provide habitats for a large variety of organisms and these organisms rely on bioconstructors as a source of food and shelter. Marine bioconstructors in temperate seas have been recognized to have a structural and functional role of marine biodiversity (as a habitat formers and ecosystem engineers), the same as coral reefs in tropical regions. Bioconstructors are ranging from coralligenous formations (formed usually by coralline algae, sponges, cnidarians, and bryozoans) to vermetid reefs, deep-sea white corals and oyster banks. Some habitats like coral banks formed by shallow-water coral Cladocora caespitosa od deep-water coral Lophelia pertusa, together with coralligenous buildups and maerl beds are of special interest for scientists and people involving with nature protection. Habitat degradation, destruction, fragmentation and loss are the most dramatic consequences of anthropogenic pressures on natural ecosystems and marine bioconstructors as a part of that. Under the present climate warming trend, together with marine acidification, new mass mortality events may occur in the near future, possibly driving a major biodiversity crisis in the Mediterranean Sea, especially in Mediterranean bioconstructors.

The ecology of the Mediterranean stony coral Cladocora caespitosa (Linnaeus, 1767) in the Gulf of Trieste (northern Adriatic Sea): a 30-year long story

ABSTRACT Cladocora caespitosa is an endemic coral of the Mediterranean Sea and an important carbonate bioconstructor that adds 3D complexity to the habitat, thus increasing marine biodiversity. Despite its important role in the ecosystem, the real status of the population along most of the Mediterranean coastline is still poorly investigated and very little is known about the resilience of the species. Using non-destructive visual surveys, colonies of C. caespitosa were investigated by SCUBA diving in 2013 and 2015 at seven sites of the northern Adriatic Sea (southern part of the Gulf of Trieste). Data about colony size, index of sphericity and corallite diameter were collected. Almost all biometrical parameters differed significantly among sampling sites, showing low occurrence of the larger size classes compared to the abundance of small-sized colonies. This pattern of distribution is typical of long-lived organisms. The positively skewed colony size distribution could be due to both a high mortality rate of small colonies unable to reach larger size classes, and to a high fragmentation rate of colonies, related to a strong hydrodynamic forces. The northern Adriatic population of C. caespitosa has previously been investigated by Schiller, who reported size and abundance data of colonies from one site, at a depth range of 2–5 m. We compared these data with our findings from the same sampling site, adding new information about the ecology of C. caespitosa. After a 30-year period, the comparison shows a change in the size distribution of colonies, with a decrease of the small class and an increase of the medium class of colonies. In view of these conclusions, further assessments are required in order to evaluate the trend of the northernmost C. caespitosa population in the Mediterranean Sea.

Inshore capture-based tuna aquaculture impact on Posidonia oceanica meadows in the eastern part of the Adriatic Sea

Mapping and monitoring of the seagrass Posidonia oceanica in the eastern (Croatian) part of the Adriatic Sea since 2004 indicates a significant decline in meadow density in an area impacted by inshore capture-based tuna aquaculture. The density and overall condition of P. oceanica meadows impacted by tuna farms near Fulija Islet was compared to two reference sites (Iž Island and Mrtovnjak Islet). The factors with the most significant influence on P. oceanica meadows were found to be the input of organic matter originating from the cages, as well as high epiphyte biomass caused by nutrient enrichment. Significant differences in nutrient concentrations were found between the sites impacted by tuna farms (Fulija Islet) and the control stations. Shoot density of the P. oceanica meadows decreased at the stations in close vicinity to the tuna farm, which suggests that the tuna farm activity strongly affected the surrounding meadows.