Octocorals in the Mediterranean Sea constitute a small group of 51 species (Vafidis in Coll et al. 2010: Table S13) but are widespread from shallow to significant depths (Freiwald et al. 2009, Mastrototaro et al. 2010) on both hard and soft substrates (Aguilar 2004) and are important components of the valuable Mediterranean coralligenous communities (Ballesteros 2006). Among them, some long-living species are considered as ‘‘ecosystem engineers’’ (Jones et al. 1994, Ponti et al. 2014) in Mediterranean marine hard-bottom communities, with significant effects on the structure, biomass and biodiversity of coralligenous communities (Ballesteros 2006). Although scientific studies and conservation efforts focus increasingly on octocorals and mainly gorgonians, our knowledge on their presence and distribution in the eastern Mediterranean is yet insufficient (Salomidi et al. 2009).

Octocoral species from the Marmara Sea have been previously reported (Demir 1954, Öztürk and Bourguet 1990, Topçu and Öztürk 2013) but studies on the abundance and distribution of the species in the Marmara Sea have been rarely addressed. Although the Marmara Sea is connected to the Mediterranean via the Çanakkale Strait (Dardanelles), it has peculiar oceanographic, ecological and geomorphologic characteristics (Öztürk and Öztürk 1996). This semi-enclosed sea, connecting the Black Sea to the Aegean Sea via the Turkish Straits System (TSS), is characterized by a two-layer stratification, with the brackish surface layer formed by the Black Sea water mass flowing into the Marmara Sea through the Istanbul Strait with a salinity of 18-24 and a temperature of 20-24°C in summer and 8-9°C in winter. More saline (up to 38.5) Mediterranean Sea water with a constant temperature of about 15°C resides 15-20 m below this layer (Beşiktepe et al. 1994). The TSS serves as an ecological barrier, a biological corridor and an acclimatization zone for the biota of the Mediterranean and the Black Sea (Öztürk and Öztürk 1996). Suspended particulate organic matter and zooplankton, which constitute the bulk of octocorals’ diet, are very abundant in the whole Marmara Sea (Çoban-Yıldız et al. 2000, İşinibilir et al. 2011), although the phytoplankton and microzooplankton biomass and production show a decreasing trend from the Istanbul Strait to the Aegean Sea (Zervoudaki et al. 2011).

The aim of this study was to determine species composition and abundances of octocoral assemblages in the Marmara Sea and to compare the communities in the southern parts (closer to the Çanakkale strait connecting to the Aegean Sea) and northern parts (closer to the Istanbul Strait [Bosphorus] connecting to the Black Sea) with those in the Mediterranean Sea.

A total of 14 octocoral species were collected from the Sea of Marmara (Supplementary Material Table S1), with 11 of them from the Prince Islands coasts and 14 from the Southern Marmara Islands. Abundance values were obtained for only 9 species (Table 2) because rare species were generally not encountered in transects and encrusting corals were not counted. Octocoral abundance in the Marmara Sea was 5.21±5.11 colonies m^–2 (mean±SD) on average calculated from a total of 1390 colonies counted in transects. Total octocoral abundance varied among stations from 0.30 to 17.70 colonies m^–2 and octocoral abundance was higher in the north of the Marmara than in the south, though not significantly. Although more species were found in the south, the north of the Marmara Sea (Simpson mean=1.98±0.88) was significantly more diverse than the south (Simpson mean=1.24±0.38) (p<0.05).

Species composition differed significantly between the north and the south as well as among different substrate types (Table 3). 60.5% of the variation was explained in axis 1 of the PCO plot (Fig. 2), where the discrimination of stations in the horizontal direction was by groups of substrate types.

The stations with soft substrates were not significantly different between north and south in terms of either species composition or abundance but were significantly different between hard and mixed substrates (Table 3). Veretillum cynomorium was responsible alone for 86% of the similarity observed between stations of soft substrates (Table 4). Both species composition and abundances differed significantly on hard substrates and mixed substrates of the north and south (Table 3).

Species composition on hard and mixed bottoms was significantly different from each other in the north and south but abundances of the common taxa were not significantly different from each other in both zones (Table 3). Paramuricea macrospina, Spinimuricea klavereni, Alcyonium palmatum and to a certain extent Paralcyonium spinulosum seem to be the most common taxa on either hard or mixed substrates of the northern Marmara Sea (Fig. 3; Table 4) but P. macrospina and S. klavereni were rarely observed in the south. Alcyonium acaule was a common species on mixed and hard substrates of the south, with abundances of 0.8 to 3.4 colonies m^–2 (Fig. 4A), whereas it was rarely observed in the north.

Abandoned fishing gears were encountered in 9 of the 17 stations (N1; N2; N4; N5; N7; N8; N9; N10; N14) on the Prince Islands coasts. The gears found most were purse seine nets, followed by set nets and fish lines.

The most extensive study concerning corals/gorgonians in the Sea of Marmara dates back to the 1950s (Demir 1954) and there are very few recent studies focusing on corals (Öztürk and Bourguet 1990, Özalp 2012, Topçu and Öztürk 2013). In this regard, this study greatly enhances the knowledge on the composition and abundance of octocorals in the Marmara Sea.

Information available for the neighbouring areas of the Marmara Sea indicate that octocorals are present and diverse in the Aegean Sea (Vafidis in Coll et al. 2010: Table S13), whereas only one species is present in the Black Sea. Virgularia mirabilis is found only in the small section of the southern Black Sea shelf close to the Istanbul Strait that creates a zone of high salinity and provides living conditions for many Mediterranean species (Zaitsev and Mamaev 1997). In the Marmara Sea, connecting the Aegean and the Black Seas, 23 species were found in various studies (Table 5). The high number of species in the Marmara Sea in the Turkish literature is mainly due to the lack of research effort in other areas. The number of species is higher than in the Ionian Basin (15) and similar to that in the Adriatic Sea (20 species) and in the Aegean Sea (28) (species numbers/region were obtained from Vafidis in Coll et al. 2010: Table S13). The Prince Islands form a boundary of the spatial distributions of octocorals in the Mediterranean Sea, except for V. mirabilis.

The soft bottoms of the Marmara Sea in the diving limits of depth seem to be colonized mainly by Pennatulaceans, most commonly V. cynomorium, followed by Pteroeides griseum, the two shallow-water species. These sea pens form large beds of a single species on the soft bottoms of the Marmara Sea, occasionally disrupted by a few alcyonaceans, as is frequently observed in the Mediterranean (Gili and Pagès 1987) and globally (Pérès 1982).

The main rocky habitats of the Marmara Sea covered by coralligenous outcrops were composed of (1) vertical walls or large rocks on steep bottom colonized mainly by Eunicella cavolini with high abundances (10.7-13.9 colonies m^–2) (Fig. 4B) and (2) large-medium size boulders on a slightly steep or flat bottom colonized by P. macrospina, S. klavereni, and P. spinulosum (3-13.3 colonies m^–2) (Fig. 4C). The first type corresponds to one of the common coralligenous assemblages for the Mediterranean Sea, dominated by E. cavolini, as is often the case in the (rarely) gorgonian-dominated eastern Mediterranean coralligenous assemblages (Kružić 2007, Salomidi et al. 2009, Gerovasileiou et al. 2009). E. verrucosa and E. singularis were rare observations in the southern Marmara Sea, as is also generally observed in the eastern Mediterranean.

The Sea of Marmara is highly impacted by various anthropogenic activities, such as wastewater discharge, agricultural run-off, illegal fishing and overfishing, marine litter and shipping (Öztürk et al. 2000, Öztürk 2010a). The semi-enclosed sea recently suffered from red-tides (Türkoğlu 2013) and mucilage events (Aktan et al. 2008, Balkıs et al. 2011). In spite of such severe anthropogenic disturbances, dense assemblages of endemic gorgonians in the Marmara Sea were observed during this study, although gorgonian densities were lower than those of the western Mediterranean (e.g. in Linares et al. 2008). E. cavolini populations can reach densities as high as 180 colonies m^–2 in the bay of Calvi (Corsica, France) (Weinbauer and Velimirov 1996), whereas the highest density in the Marmara Sea was approximately 14 colonies m^–2.

P. macrospina is a Mediterranean endemic known to occur on rocks, detritic or sandy/muddy bottoms, mainly at depths of 40 to 100 m but also deeper (Carpine and Grasshoff 1975). This species is mentioned in the literature dealing mainly with deep sea fauna of the Mediterranean Sea (Watling et al. 2005, Aguilar et al. 2009, Mastrototaro et al. 2010, Bo et al. 2012, Angeletti et al. 2014) except in the Aegean Sea, where it was collected between 20 and 90 m (Vafidis et al. 1994); P. macrospina–dominated assemblages were abundant, especially in the north of the Marmara Sea, and this species was suggested to have greater adaptability than other gorgonians in relation to a relatively fast growing rate (Bo et al. 2012).

On the other hand, S. klavereni, another common species in the northern Marmara Sea, is an endemic Mediterranean species on which very limited information is available. It occurs on hard substrates of the circalittoral zone between 50-80 m deep (Carpine and Grasshoff 1975, Grasshoff 1992). It was recently reported from the Tyrrhenian Sea as a rare occurrence, on muddy substrates with patches of organogenic detritus at depths deeper than 70 m (Bo et al. 2012). In the northern Marmara Sea, it occurs on hard substrates like rocks or small pebbles/shells on sandy/muddy bottom starting from 25 m until our limit of observation (42 m), with abundances varying from 1 to 3.1 colonies m^–2.

The Marmara Sea faced severe disturbances, particularly from the 1980s onward, due to rapid population growth and industrial revolution in the surrounding region (Burak 2008) and in parallel to the catastrophic degradation period in the Black Sea (Bakan and Büyükgüngör 2000). In order to compare the present status of corals in the Sea of Marmara to that of the post-disturbances period (1960 to the 1970s), local divers and fishermen were questioned. Although we could not obtain quantitative data, they all agreed that they used to see more abundant dense gorgonian assemblages in the past. We also obtained a short video (Supplementary Material Video S1) taken in 1975 at Balıkçı Island by a local diver. In the video, a very dense assemblage of Savalia savaglia/P. clavata is clearly seen, revealing large colonies. Today, though both species are still present in the Marmara Sea, such dense assemblages are not encountered. Abandoned purse seine nets were pointed out by divers and small scale fishermen as the main reason behind the decrease of gorgonians and corals. In fact, this problem is continuous in the whole Marmara Sea (Yıldız and Karakulak 2010), causing serious damage to the ecosystem despite fishing net cleaning operations by local and environmental associations. Abandoned fishing gears highly impact corals and gorgonians via entanglement and overgrowth by epibionts (Bavestrello et al. 1997, Bo et al. 2014). Abandoned nets were encountered at 53% of the stations in the north of the Marmara Sea during this study (Fig. 4D, E, F), causing harm to octocoral species and other sessile fauna.

The continuous disturbances by fishing gears, together with the peculiar oceanographic conditions in the Marmara Sea, could be responsible for the abundance of the opportunistic gorgonians. The Marmara Sea has received a significant pollution load in the last 30 years from both the Black Sea inflow and increased anthropogenic input (Tuğrul and Polat 1995). The semi-enclosed Marmara Sea is highly turbid, preventing daylight at very shallow depths (Çoban-Yıldız et al. 2000) and the temperature below 20 m is lower (approximately 14.5°C) than at depths of the same range in the Mediterranean, so the conditions are similar to those of higher depths in the Mediterranean Sea. This could be the reason for high abundances at 25-40 m of P. macrospina and S. klavereni, which are found at greater depths in the Mediterranean. These relatively fast-growing gorgonians might be colonizing more easily the habitats emptied by other species due to the disturbances by fishing gears under the highly turbid conditions of the Marmara Sea since the 1970s.

A series of thermal anomalies recently affected Mediterranean benthic assemblages, causing mass mortalities at some locations and gorgonians were among the organisms most affected (Perez et al. 2000, Garrabou et al. 2009). In the Marmara Sea, temperature variances below 20 m are very low and the temperature is generally about 15°C. Therefore, gorgonian assemblages do not risk mortality events related to thermal stress as in the Mediterranean Sea, but an important threat arises from mucilage events. Mucilage events resulting from single-cell organisms are periodically observed in the Marmara Sea (Aktan et al. 2008, Balkıs et al. 2011) and the sedimentation of the aggregates causes negative effects by covering benthic organisms (Aktan et al. 2008). The impact of mucilage on gorgonians has been previously reported, though it was caused by a different type of mucilage formed by the aggregation of filamentous algae; mucilage gets trapped on gorgonian branches positioned perpendicularly to currents and causes damage (sometimes irreparable) to gorgonians (Giuliani et al. 2005). We observed in 2011 mucilage fragments on S. klavereni colonies and local divers informed us that several S. klavereni colonies died after the severe mucilage events of 2007-2008. The dead branches can still be observed at station N9, which is covered by epibionts (Fig. 4G).

Most of the octocoral species found in the Marmara Sea, such as A. coralloides, A. acaule, P. clavata and E. cavolini, are considered typical components of Mediterranean coralligenous communities, which are stated as the most important biocoenosis in “Guidelines for the Establishment and Management of Mediterranean Marine and Coastal Protected Areas” (Lopez Ornat 2006). According to Council Regulation 1967/2006 of the European Union (EU), fishing with trawl nets, dredges, shore seines or similar nets above coralligenous habitats shall be prohibited. Recently, purse seine fisheries were prohibited around the Prince Islands Area; however that ban does not cover all of the vulnerable assemblages. In the Marmara Sea, there is no regional or national legislation for the protection of corals and gorgonians except the complete prohibition of fisheries of Corallium rubrum and S. savaglia, according to the Statements 2012/66 and 2012/65. In order to ensure the conservation of coral assemblages in the Marmara Sea, we emphasize the need for specific measures such as the removal of abandoned fishing nets, enlargement of the prohibited area for purse seining, prohibition of anchoring, placement of mooring buoys and better prosecution of offenders. This study provides scientific support to complete some tasks required by the EU Marine Strategy Framework Directive in order to achieve and maintain Good Environmental Status (GES) by 2020 in Turkish waters.