Ecological patterns of polychaete assemblages associated with the Mediterranean stony coral Cladocora caespitosa (Linnaeus, 1767): a comparison of sites in two biogeographic zones (Adriatic and Aegean Sea)

The Mediterranean stony coral Cladocora caespitosa (Linnaeus, 1767) is a well-known habitat builder, and as such hosts a diversified faunal assemblage. Although polychaetes are one of the most abundant and diverse macrobenthic groups associated with C. caespitosa colonies, our knowledge of their ecological features in this association is still limited. The aim of this paper was to gather and compare the most comprehensive data available on polychaetes associated with C. caespitosa in the Adriatic and the Aegean Seas, and to test for differences between these geographic areas. To this end, differences were tested in terms of: (i) richness and structure of polychaete assemblages; (ii) feeding and functional traits of assemblages; (iii) the main factors influencing those aspects, (iv) the relationship between polychaete assemblages richness and Cladocora colony size, and estimate richness. Differences were observed between the Adriatic and the Aegean Seas, in terms of richness, species composition and relative proportion of the dominant feeding guild (filter feeders most abundant in the Aegean and carnivores in the Adriatic) and motility mode (sessile most abundant in the Aegean and motile in the Adriatic). Conversely, cosmopolitan and Atlanto-Mediterranean species dominated the assemblages in both geographic areas, and the same Species-Area Relation model proved to be effective for richness estimation in both geographic areas.

Co-occurrence of a reef-building coral and canopy-forming macroalgae in the Mediterranean Sea

Canopy-forming macroalgae are amongst the main competitors of corals by affecting coral recruitment, growth of recruits and adults, fecundity and in the worst-case scenario causing coral bleaching and necrosis. However, potentially reef-building coral Cladocora caespitosa (Linnaeus, 1767) and canopy-forming macroalgae of the order Fucales (Cystoseira sensu lato) are known to concur in a few places of the Mediterranean Sea. Here we look at the small-scale relationships between Cladocora abundance and Cystoseira s. l. densities at three different places where they coexist. Relationships have turned out to be both species and site-specific even though most relationships are neutral, pointing to a predominant concurrence of corals and macroalgae at the small scale. These findings shed new information on the relationship between corals and fleshy macroalgae in a temperate environment and serve as a starting point for future studies addressing the interactions between C. caespitosa and Cystoseira s. l.

Vulnerability of Northern Adriatic to Warming and Intensification of Marine Heat Waves

<p>Northern Adriatic Shelf (NAS) is a shallow, semi-enclosed northern part of the Adriatic basin, and as such rapidly responds to climate change. Multidecadal satellite and in-situ sea surface temperature (SST) time series on the NAS indicate a warming trend. During 1995-2015, SST in the Gulf of Trieste increased at a rate of 0.08°C ± 0.01°C per year (amounting to 1.6°C in 20 years), a trend indicative of the entire NAS shelf.</p><p>We use a centennial SST time series from Trieste (Raicich and Colucci, 2019) to construct a climatological year as a base for SST day-of-year anomaly estimation. We show that yearly number of discrete periods of extreme warming (Marine Heat Waves - MHW) and extreme cooling (Marine Cold Spells - MCS) exhibit clear seasonality. Both positive and negative anomalies from climatological SST manifest maximum variance in the summer months. The frequency of MHW has increased, while the number of Marine Cold Spells (MCS) is declining.</p><p>Sea warming and MHW intensification are potent agents of disturbance, particularly for sessile taxa and species residing near their warm range edges. In the NAS extreme events may force regression of habitat-forming species such as seagrass Zostera marina and increase bleaching episodes of coral Cladocora caespitosa. Warming events may be associated with the inflow of invasive non-indigenous species and expand the period of occurrence, such as harmful gelatinous invader Mnemiopsis leidyi. In contrast, a reduced number of MCS during winter may enhance survival of Aurelia polyps generating through strobilation more intense jellyfish blooms.</p>

Shifts in marine invertebrate bacterial assemblages associated with tissue necrosis during a heatwave

Marine heatwaves (MHWs) are periods of extremely high seawater temperature that affect marine ecosystems in several ways. Anthozoans (corals and gorgonians) and Porifera (sponges) are usually among the taxa most affected by MHWs. Both are holobiont entities that form complex interactions with a wide range of microbes, which are an essential part of these organisms and play key roles in their health status. Here, we determine microbial community changes suffered in two corals (Cladocora caespitosa and Oculina patagonica), one gorgonian (Leptogorgia sarmentosa), and one sponge (Sarcotragus fasciculatus) during the 2015 MHW. The microbial communities were different among hosts and displayed shifts related to host health status, with a higher abundance in necrosed tissues of Ruegeria species or of potential pathogens like Vibrio. We also carry out a meta-analysis using 93 publicly accessible 16S rRNA gene libraries from O. patagonica, C. caespitosa and L. sarmentosa to establish a Mediterranean core microbiome in these species. We have identified one Ruegeria OTU that maintained a stable and consistent association with these species, which was also related with tissue necrosis in their hosts. Therefore, Ruegeria sp. could play an important and still underexplored role in the health status of its hosts.

Coexistence of the reef-building coral Cladocora caespitosa and the canopy-forming alga Treptacantha ballesterosii: Description of a new Mediterranean habitat

Shallow Mediterranean rocky environments are usually dominated by macroalgae, but the stony colonial zooxan­thellate coral Cladocora caespitosa is able to build extensive banks in some particular areas. Although zooxanthellate corals and benthic macroalgae are expected to compete for light and space when overlapping in the same habitat, there is previous evidence that C. Caespitosa and Mediterranean macroalgae do not suffer from competitive exclusion when living together. Here we characterize a new and unique Mediterranean habitat where the reef-building coral C. Caespitosa and erect seaweeds of the order Fucales (Cystoseira s.l.) coexist. In this new habitat C. Caespitosa reaches 34% cover and densities of Cystoseira s.l. (mainly Treptacantha ballesterosii) are much higher than values reported from other sites. Interestingly, abundances of T. Ballesterosii and C. Caespitosa show a positive relationship, suggesting that some kind of facilitation mechanism is taking place. These findings challenge the theory of competitive exclusion between corals and macroalgae and launch a wide array of possible open discussions on coral-macroalgae interactions.

Response of the temperate scleractinian coral Cladocora caespitosa to high temperature and long-term nutrient enrichment

Anthropogenic nutrient enrichment and increased seawater temperatures are responsible for coral reef decline. In particular, they disrupt the relationship between corals and their dinoflagellate symbionts (bleaching). However, some coral species can afford either high temperatures or nutrient enrichment and their study can bring new insights into how corals acclimate or adapt to stressors. Here, we focused on the role of the nutrient history in influencing the response of the Mediterranean scleractinian coral Cladocora caespitosa to thermal stress. Colonies living naturally in nutrient-poor (<0.5 µM nitrogen, <0.2 µM phosphorus, LN) and nutrient-rich (ca. 10–20 µM nitrogen, 0.4 µM phosphorus, HN) locations were sampled, maintained under the right nutrient conditions, and exposed to a temperature increase from 17 °C to 24 °C and 29 °C. While both HN and LN colonies decreased their concentrations of symbionts and/or photosynthetic pigments, HN colonies were able to maintain significant higher rates of net and gross photosynthesis at 24 °C compared to LN colonies. In addition, while there was no change in protein concentration in HN corals during the experiment, proteins continuously decreased in LN corals with increased temperature. These results are important in that they show that nutrient history can influence the response of scleractinian corals to thermal stress. Further investigations of under-studied coral groups are thus required in the future to understand the processes leading to coral resistance to environmental perturbations.