Soft coral (Octocorallia, Alcyonacea) diversity and distribution along a latitudinal environmental gradient and the role of their chemical defense against predatory fish in the Red Sea
The Red Sea located between 30°N and 12°30’N separates Africa and Asia. It has a length of 1,840 km, an average width of 280 km and a total area of approximate 4,600,000 km2. The Red Sea harbors complex ecosystems such as coral reefs, sea grass beds and mangrove forests. Soft corals are an important component of the reef communities and contribute substantially to the biological diversity in coral reefs of tropical Indo - Pacific region, and indeed globally. This study not only assessed the soft coral distribution along the Saudi Arabian Red Sea including diversity, abundance and coverage but also valuated their relation with environmental parameters along the large scale latitudinal gradient and at the local scale. Moreover, this study asks whether the conspicuous dominance of xeniid soft corals in the Red Sea reef systems may be due to their chemical defenses against predator reef fishes. Rapid ecological assessments (REA) and line intercept transect (LIT) methods were used in the field along the Saudi Arabian coast to record the cover and abundance of soft coral species. For a comprehensive diversity assessment, around 1,000 soft coral samples were collected at 24 sites along the Saudi Arabian coast from shallow (1 m) to deep reefs (38 m) during three subsequent field trips. Further, the environmental parameters such as nutrients, temperature, sedimentation, turbidity and reef types were also recorded during these expeditions. The field surveys were carried out in February and September 2011, and February/March 2012 and the laboratory experiments were conducted from September 2013 to March 2014 at GEOMAR in Kiel, Germany. Seventeen genera of alcyonacean soft corals belonging to five families were found along the Saudi Arabian Red Sea coast by REA: Tubipora, Rhytisma, Klyxum, Cladiella, Sarcophyton, Lobophytum, Sinularia, Anthelia, Xenia, Ovabunda, Heteroxenia, Paralemnalia, Litophyton, Sterenonephtya, Nepthea, Dendronephthya and Siphonogorgia. The highest numbers of genera (fifteen genera) were found in the northern reefs. The southern reefs featured the lowest number of soft corals with eight genera. The most abundant genera throughout the Red Sea included, Sinularia, Xenia/Ovabunda, Sarcophyton and Tubipora. These were found at all reef sites. In contrast, the genera Cladiella, Stereonephtya, Heteroxenia and Siphonogorgia were found in few areas only. Overall, the genera Xenia/Ovabunda and Sinularia featured highest abundances contributing most to the coverage of soft corals throughout the Red Sea. The LIT determined the average soft coral areal cover was 11% (± 3.8 SE), relative cover was lowest at southern reefs (Farasan: 0.6% ± 0.9) and highest in the northern reefs (Al-Wajh: 27% ± 2.1). Eightytwo soft coral species were identified belonging to Alcyoniidae (six genera, 40 species), Xeniidae (five genera, 24 species), Nephtheidae (six genera, 15 species), Nidaliidae, Briareidae and Tubiporidae (one species each). This study reported new distribution of soft coral species records for the Red Sea. Bray-Curtis clustering of soft coral species composition and abundance grouped the sites into three main clusters: representing northern (Maqna and Al-Wajh), central (Yanbu, Jeddah, Rabigh, Mastura and Al-Lith) and southern (Doga and Farasan) reef areas respectively. The factors affecting the pattern of soft coral communities along coastal reefs of Saudi Arabia are substrate, depth, slope morphology, temperature, nutrients, sedimentation and turbidity. These factors, in combination, explained 65% of the total variation in soft coral community structure. The northern section had highest soft coral coverage (27% ± 4.1 SE) and diversity (44 species) and was characterized by lowest temperatures, low nutrient concentrations, steep reef slopes and low sedimentation. The southern section had lowest soft coral coverage (0.6% ± 0.9) and diversity (26 species), and was characterized by high temperature, high nutrient concentration, mostly shallow reef slopes and high sedimentation. The central section was intermediate in cover, diversity and the key environmental factors. Xeniids, notably Xenia/Ovabunda species, were important components of soft coral communities in the Saudi Arabian Red Sea. Xeniids occupied 80% of soft coral cover in some areas. The relative coverage of xeniids ranged from 7.5% (± 2.1 SE) to 14.4% (± 1.9) in the off-shore reefs, and from 0.6% (±1.1) to 8.5% (±3.3) in the near-shore reefs, in response to major differences in water quality parameters. Eighteen species were recorded at the off-shore sites and 13 species in near-shore locations at Al-Wajh, Yanbu, Mastura/Rabigh and Jeddah. Multivariate analyses showed that xeniid communities differed between the eight reef sites surveyed. The xeniid communities were significantly different between inshore and offshore at Yanbu, Mastura/Rabigh and Jeddah reefs. They not only differ in coverage but also in the predominating genera and species diversity varies under different habitat conditions. Community composition partly varied according to anthropogenic impacts at some locations. The crude extract of two xeniid species deterred reef fishes in the field at the Red Sea to 86% (Ovabunda crenata) and 92% (Heteroxenia ghardaqensis. In the laboratory, natural concentration of crude extract deterred the reef fish Thalassoma lunare (moon wrasse) to 83% and 85%, respectively. Crude extract still showed unpalatable for moon wrasse even when reduced to 12.5% of the natural concentration in both species. While Heteroxenia ghardaqensis lacking sclerites, the sclerites of Ovabunda crenata species did not deter moon wrasses in the laboratory even under the increasing double natural concentration suggesting that sclerites provide structural support rather than antifeeding defenses. We conclude from that, the role of chemical defense against predation contributes to the conspicuous abundance of these soft coral species in the Red Sea.
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