Deep-Dwelling Populations of Mediterranean Corallium rubrum and Eunicella cavolini: Distribution, Demography, and Co-Occurrence

Abstract

Corallium rubrum and Eunicella cavolini are two octocorals, reported as co-occurring species in the deep rocky habitats of the Mediterranean Sea with a high hydrodynamic and moderate eutrophication. Their spatial distribution and demography in the deep sea are mainly affected by temperature and direct and indirect anthropogenic activities; however, knowledge of the factors that potentially influence their co-existence is scarce. This paper provides novel data on the distribution and demography of these two species, at depths between 50 and 290 m in the Western Mediterranean Sea, providing insights on their co-occurrence. Both species exhibited the highest population density at deeper sites (>150 m), showing an inverse size–density relation. Density values ranged from 0.03 colonies m−2 to 32 and 80 col. m−2 for yellow gorgonian and red coral, respectively. The two species co-occurred in 13% of the total frames examined, mostly dwelling between 120 and 160 m depth. Distance-based linear modeling (DistLM) emphasized that when co-occurring the variability of the two species’ densities were significantly driven by the density—rather than the morphology (i.e., height)—of the other species. We stress the need for further studies to elucidate the possible mutual effects of suspension feeders and to test the role of different environmental factors potentially influencing inter-specific relationships.

Structure and status of the Italian red coral forests: What can a large-scale study tell?

“…However, overall, it seems that, among all factors, depth does not influence the size of the colonies. This is in accordance with Carugati et al (2022), reporting no significant variation of colony height with depth in the North-western Sardinian populations (50-290 m), but contrasts with the findings of other authors sustaining the major size (as well as diameter and branching) of deepdwelling colonies, a trend mainly attributed to a lower harvesting pressure (e.g., Tsounis et al, 2007;Rossi et al, 2008;Priori et al, 2013;Angiolillo et al, 2016;Cau et al, 2016). Moreover, the same authors provided evidence of a size/density relationship within C. rubrum populations, with larger, but more scattered colonies in deeper waters, that did not result from the present dataset.…”

“…Differently from height, density is strongly driven by depth, with a linear decrease from shallow to deep sites, confirming the preliminary qualitative results obtained by Toma et al (2022) and in accordance with known literature (e.g., Rossi et al, 2008;Carugati et al, 2022). In addition, on the continental shelf and shelf break, higher variability in terms of density was observed, with dense aggregations and populations made of sparse colonies equally present, while at bathyal depth, populations of scatted colonies were dominant.…”

“…The deep circulation of warm waters in proximity to volcanic areas (Bavestrello et al, 2014) combined with the seafloor movements in seismic areas (Lodolo et al, 2017;Bavestrello et al, 2021) may explain the extensive mortality events observed in the Pontine Archipelago, Campanian Archipelago and the offshore banks of the Sicily Channel. For the Sardinian sites, however, there is no clear explanation of the putative factors triggering the coral death, neither for the population along the North-eastern sector (Carugati et al, 2022) nor for those in the southern one. Most of the mortalities were recorded in submarine canyons, which may support the fact that the particular environmental conditions (e.g., turbidity currents) occurring in these systems very close to the shorelines, may have caused the events (Moccia et al, 2021).…”

“…From a conservation point of view, morphometric data are essential to estimate the health status of the populations, which, in turn, are fundamental to identify the most vulnerable sites and the correct management actions. Although several pieces of evidence are available for shallow-water populations, very little data have been recorded about the morphometric structure of deep populations (Rossi et al, 2008;Priori et al, 2013;Angiolillo et al, 2016;Cau et al, 2016;Carugati et al, 2022). These studies, mainly conducted on a local or regional scale, showed that deepdwelling populations are generally less harvested than the shallow-water ones, even if high percentages of the colonies are still above the commercial size (≥ 7 mm of basal diameter).…”

Environmental Status and Geomorphological Characterisation of Seven Black Coral Forests on the Sardinian Continental Shelf (NW Mediterranean Sea)

“…Since both of these sites showed a high geomorphological complexity, we assume that the low I s found could be related to their proximity to two important ports (<13 km Olbia port and <5 km from Arbatax port, Figure 1) and, in turn, to a potentially higher presence of marine traffic and fishing activities. Although rocky substrata are usually not trawled by fisherman, these activities are generally more intense in the proximity of canyon heads [73] and can produce a strong resuspension of fine sediment that increases water turbidity and could negatively affect benthic assemblages [74,75].…”