Unveiling the deep biodiversity of the Janua Seamount (Ligurian Sea): first Mediterranean sighting of the rare Atlantic bamboo coral Chelidonisis aurantiaca Studer, 1890

Highlights

• The Janua Seamount hosts a unique bathyal benthic community.

• The bamboo coral Chelidonisis aurantiaca is described for the Mediterranean Sea.

• Fossil traces testify to the luxurious past of the deep coral community.

• ROV footage reveals a moderate fishing impact by long lines.

• Seamounts hold a paramount ecological role for deep-sea ecosystems.

Abstract

Seamounts represent ecologically important marine environments and undeniably play a crucial role in the functioning of the deep-sea ecosystems, thanks primarily to their contribution in supporting offshore benthic and pelagic biodiversity. In the last decades, these features have received attention in all oceans of the world, but they were relatively overlooked in the Mediterranean Sea. In particular, the Ligurian Sea hosts six major structures, whose summits, ranging from 150 to nearly 2000 m depth, are virtually unknown from the biological point of view. The summit region of the Janua Seamount, located approximately at 40 nautical miles off the harbour of Genoa at 810 m depth, was explored for the first time using Remotely Operated Vehicles. The sea bed topography is characterized by a heterogeneous hard bottom, dominated by sub-fossil thanatocoenoses of Lophelia pertusa (Linnaeus, 1758) covered in a thick Ferrous-Manganese crust, and colonized by a scattered bathyal assemblage dominated by hexactinellids and benthic cnidarians showing a distinct regionalization around the summit. Cnidarians observed in this site include a rare Atlantic bamboo coral (Isididae, Chelidonisis aurantiaca Studer, 1890), described herein for the first time for the Mediterranean Basin. These findings provide new information on the biogeographic patterns of colonization of the bathyal Mediterranean fauna but also highlight the large knowledge gap that still exists for deep-sea biodiversity of this basin.

Introduction

Seamounts are prominent topographic features of the ocean floor and worldwide are recognized as important offshore deep ecosystems (Rowden et al., 2005; Pitcher et al., 2008; Consalvey et al., 2010; Ramirez-Llodra et al., 2010). The often oceanographically turbulent conditions found along their flanks and summits trigger rich benthic communities dominated by filter feeders, particularly corals and sponges, many of which are recognized as fragile, vulnerable, and long-lived faunal components of the deep-sea (Genin et al., 1986; Rogers, 2004; McClain, 2007; Pitcher et al., 2008; Althaus et al., 2009; Consalvey et al., 2010). Seamounts also attract a rich associated fauna including pelagic predators that find the features to be optimal foraging areas. As a result, globally seamounts are seen to support important offshore fishing grounds and hotspots for cetaceans (Worm et al., 2003: Dower and Brodeur, 2004; Johnston et al., 2008; Althaus et al., 2009; Morato et al., 2010; Fiori et al., 2016). Various studies have shown the importance of conducting global-scale research focussing on seamounts to address critical ecological paradigms of these ecosystems, specifically those regarding biodiversity, connectivity and vulnerability, and to use the results to evaluate best management practices (Pitcher et al., 2008; Clark et al., 2010, 2012; Schlacher et al., 2010).

A recent bibliographic review showed that there are 242 seamounts within the Mediterranean Basin, one of the least considered geographic regions in terms of biocoenotic characterization of these ecosystems (Danovaro et al., 2010; Bo et al., 2011; Morato et al., 2013; Würtz and Rovere, 2015). Substantial data on the benthic assemblages have been collected for a small number of these features in the region (Würtz and Rovere, 2015). They and their summit depths and key faunal communities are as follows: i) the Eratosthenes Seamount (780 m), located in the Levantine Basin (Galil and Zibrowius, 1998) and currently managed as a Fishery Restriction Zone; ii) the Vercelli Seamount, with a euphotic summit at 60 m in the Tyrrhenian Sea and supporting an abundant coralligenous community with kelp and gorgonian forests (Bo et al., 2011); iii) the Aceste and Enarete Seamounts along the northern margin of Sicily (120 and 320 m, respectively) and hosting living white corals (Freiwald and Bohrmann, 2011; Aguilar et al., 2013); iv) various seamounts in the Alboran Sea and Balearic region including the Seco de los Olivos Seamount and the “Stony Sponge Seamount” supporting a deep lithistids sponge ground (Hebbeln et al., 2009; Aguilar et al., 2013; Morato et al., 2013; Maldonado et al., 2015; De la Torriente et al., 2018); v) the Santa Lucia Seamount (150 m) located in the Ligurian Sea and hosting mesophotic black coral forests (Bo et al., 2014a).

Mediterranean benthic surveys have revealed the existence of priority Vulnerable Marine Ecosystems (VMEs) over these features and have highlighted a varying amount of impact from fishing activities (Bo et al., 2011, 2014a; Freiwald et al., 2009, 2011). No large-scale scientific exploration effort, however, has ever been carried out within the basin region. It was in this context that the surveys under the project BioMount were conducted in the Ligurian and Tyrrhenian Seas. The main aims of the surveys were to describe the faunal biodiversity patterns, vulnerability and levels of connectivity between the mesophotic, bathyal, and abyssal structures. Hence, a reference base for the biological functioning of Mediterranean underwater mountains could be established (Bo et al., 2018a).

The Ligurian Sea, located in the North-western part of the Mediterranean Basin, is considered, based on historic surveys research data, an iconic geographic region with high biodiversity levels. Deep-sea biocoenotic investigations have been carried out in this region for decades, primarily in the fishing areas where over time significant trawling has occurred (Rossi, 1958; Relini et al., 1986; Fanelli et al., 2017; Enrichetti et al., 2018). Nevertheless, few specific benthic surveys have been carried out on the six seamounts within the basin area (Würtz and Rovere, 2015) (Fig. 1A). Exceptions to this are a preliminary Remotely Operated Vehicle (ROV) survey carried out along the Santa Lucia Seamount (Bo et al., 2014a), a small amount of geological sampling conducted to investigate coral thanatocoenoses in the region (Delibrias and Taviani, 1984; Allouc, 1987), and data collected on species by-catch from fishing activities.

The Janua Seamount (Janua referring to “Genoa” in Latin) is the deepest of the so-called Genoese seamounts (Ulisse, Penelope and Janua) and is located on the eastern flanks of the Bisagno canyon 40 nautical miles from Genoa Harbour (Fig. 1A). Two ROV surveys conducted in 2017 and 2018 enabled, for the first time, an investigation on this deep-sea feature to characterize the benthic megafauna at the seamount peak, to help assess its vulnerability in terms of anthropogenic impacts, and to evaluate the occurrence of rare species.