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

Alessandro Cau,Rita Cannas,Michela Angiolillo,Lorenzo Bramanti,Katell Guizien,Danila Cuccu,Marzia Bo,Simonepietro Canese,Maria Cristina Follesa

doi:10.1038/srep23322

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.

Highlights

Taking advantage of new technological tools, which provide data on population density and mean colony size in deep-dwelling populations of C. rubrum, this study aims to test three null hypotheses: (1) population density is not related to depth (2) population structure in the deep environment does not depend on habitat quality, namely, slope and sediment accumulation and (3) population density is not related to mean colony size
Water depth explains less than 1% of the observed variance in red coral population density in the present study (R2 = 0.0062, P< 0.001, water depth ranging from 80 to 170 m)
It is noteworthy that while in deep dwelling populations, both high densities (> 10 colonies) and low densities (

Summary

Introduction

Harvesting practices with non-selective gears (i.e., ingenium or St. Andrew cross) exist since ancient times (historical records of fishing practice with these gears date back to the fourth century B.C.)[3,7,8], and since the late 1950s, the development of scuba diving technology has enabled additional selective harvesting in shallow-water populations (shallower than 50 m depth), allowing fishermen to (ideally) target only large and fecund colonies (i.e., producing a high quantity of larvae)[6,9,10]. The few ROV-based investigations performed on deep-dwelling populations across the Mediterranean basin[4,6,10,17,18,19] recorded mostly large and highly branched colonies, distributed sparsely or forming small patches on rocky surfaces[20]. From these observations, the hypothesis that C. rubrum population structure follows a depth-related distribution, with small and densely packed colonies in shallow environments and large and sparsely distributed colonies in deep ones was formulated[6]. In shallow environments it should be expected to find larger red coral colonies than in deep environments; the opposite pattern observed has been explained by more frequent disturbances in the shallow environment, either of anthropogenic[7,22] or environmental origin[23,24,25,26]

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Conclusion