Habitat constraints and self-thinning shape Mediterranean red coral deep population structure: implications for conservation practice

Abstract

The Mediterranean red coral, Corallium rubrum, is one of the most precious corals worldwide. Below 50 m depth, C. rubrum populations are generally characterised by large and sparse colonies, whereas shallow populations (above 50 m depth) show high densities of small colonies. We show here instead that populations dwelling between 80 and 170 m depth exhibited a continuous range of population density (from 2 to 75 colonies per 0.25 m2), with less than 1% of variance explained by water depth. An inverse relationship between maximum population density and mean colony height was found, suggesting that self-thinning processes may shape population structure. Moreover, demographically young populations composed of small and dense colonies dominated along rocky vertical walls, whereas mature populations characterised by large and sparsely distributed colonies were found only in horizontal beds not covered by sediment. We hypothesise that, in the long term, shallow protected populations should resemble to present deep populations, with sparsely distributed large colonies. Since the density of red coral colonies can decay as a result of self-thinning mechanisms, we advise that future protection strategies should be based also on a measure of red coral spatial coverage instead of population density.

Does historical harvesting affect colony size distribution and genetic diversity in Corallium rubrum (Linnaeus, 1758)? Evidences from the Southern Mediterranean commercial banks

“…For decades, this impediment prevented gathering information about commercial stocks. In the last years, studies on deep populations of C. rubrum in the northwest of the Mediterranean Sea have been conducted (Priori et al, 2013;Costantini et al, 2013;Angiolillo et al, 2016;Benedetti, et al, 2016;Cau et al, 2016) mainly in relation to the rising concern about the conservation of the species.…”

“…Shallow water populations, in a depth range between 15 and about 60 m, have been heavily overexploited for centuries and now are made by small and shortlived colonies (Santangelo & Abbiati, 2001). Populations at a depth range of about 60-300 m are mainly made by larger, sparse, and long-lived colonies (Rossi et al, 2008; but see also Cau et al, 2016). Nowadays, they provide most of the Mediterranean red coral commercial harvest.…”

Marine Heat Waves Hazard 3D Maps and the Risk for Low Motility Organisms in a Warming Mediterranean Sea

“…Even though these effects are confined to the upper portion of the water column, they still would affect C. rubrum populations and coralligenous communities living above ∼30 m depth. In the case of C. rubrum these populations are already severely depleted because of overharvesting Santangelo and Bramanti, 2010;Cau et al, 2016) and are thus less resilient. The most likely outcome of the increase in heat waves therefore is a deepening of the survival horizon for C. rubrum and, possibly, other coralligenous macroinvertebrates.…”

“…C. rubrum is exploited by the jewelry industry since ancient times for its glossy red skeleton and is currently considered over-harvested (Tsounis et al, 2010(Tsounis et al, , 2013. In fact, populations down to 50 m are usually characterized by dense patches of small individuals as a consequence of selective harvesting of the large specimen Santangelo and Bramanti, 2010;Cau et al, 2016). Besides fishing, C. rubrum populations above 30 m depth were severely affected by the 1999 and 2003 mass mortality events with percentage of affected colonies as high as 80% in some sites (Garrabou et al, 2001(Garrabou et al, , 2009.…”

Reproductive patterns in deep versus shallow populations of the precious Mediterranean gorgonian Corallium rubrum (Linnaeus, 1758) (Sardinia, central-western Mediterranean)

“…Because of the wide bathymetric range of this species, which leads to huge sampling limitations for deeper populations (Genin et al, 1992), little attention is given to the deeper populations (50-200 m ) (Rossi et al, 2008;Cau et al, 2015bCau et al, , 2016Angiolillo et al, 2016;Benedetti et al, 2016).…”