Isidella elongata (Cnidaria: Alcyonacea) facies in the western Mediterranean Sea: visual surveys and descriptions of its ecological role

Abstract

Isidella elongata is a candelabrum-shaped alcyonacean forming important facies on the bathyal muddy bottoms of the Mediterranean Sea, currently considered a sensitive habitat and heavily impacted by deep-sea fisheries. Until a few decades ago, this facies was a widespread habitat of the deep Mediterranean seabed and I. elongata was a common species in the trawling fishery's bycatch. Despite its current persistence in dense aggregations being very scarce, a dense facies of I. elongata was revealed during several ROV (Remotely Operated Vehicle) surveys carried out from 2010 to 2014 on the muddy bottoms between two seamounts east of Ibiza (Balearic Sea). The facies developed in an area between 480 and 615 m in depth where trawling is forbidden, with an extraordinary density of about 2300-2683 colonies/ha, representing one of the biggest facies of I. elongata currently known for the Mediterranean Sea considering the surface covered and the colonies' density. The associated community was surveyed, with 50 taxa identified. Moreover, a canyon southwest of Formentera characterised by the presence of I. elongata together with a high trawling impact was investigated. The density of the colonies was 53-62 colonies/ha and only 19 taxa of associated fauna were observed. The results of the two areas are compared and discussed in the framework of the protection of such an important habitat.

Mesophotic and Deep-Sea Vulnerable Coral Habitats of the Mediterranean Sea: Overview and Conservation Perspectives

“…are among the most common species that form large aggregations on mesophotic coralligenous habitats, as well as on other coherent substrata, often on the continental shelf [59,[62][63][64][65]. Acanthogorgia hirsuta Gray, 1857, Callogorgia verticillata (Pallas, 1766) (Figure 1d On soft bottoms, the bamboo coral Isidella elongata (Esper, 1788) can form extensive populations on compact mud between 110 and 1600 m depth, on relatively flat or gently inclined seabed [69] (Figure 1e). This typology of substratum is also suitable for bottom trawling, which has a strong mechanic impact on I. elongata colonies as well as on other sessile species.…”

“…For this reason, the Mediterranean populations of I. elongata are in strong decline and the species has been categorized as "critically endangered" by IUCN [16]. Fortunately, some populations of the species are currently surviving in places where trawling is not carried out [39,69].…”

“…In fact, some recent new records of deep-sea invertebrates in the Mediterranean Sea occurred within I. elongata populations [70][71][72]. Among the suite of ecosystem goods and services that these habitats provide ( Table 1), they are known to act as refuge, feeding, spawning and nursery areas for many associated species, including species of commercial value [69,[73][74][75].…”

“…Black coral forests can take long time to be formed and are highly sensitive to artisanal fishery, as well as to indirect effects from trawl fishing; thus, protected areas should be created to protect the last surviving forests of the Mediterranean Sea [104]. Soft-bottom habitats such as I. elongata gardens are the most sensitive to fishing pressures, particularly to trawling; they are critically endangered, representing a priority for conservation measures [39,69]. In the same way, sea pen fields are sensitive to trawl fishing and are very important to be protected [88,105].…”

Finding of the macrophagous deep-sea ascidianDicopia antirrhinumMonniot, 1972 (Chordata: Tunicata) in the Tyrrhenian Sea and updating of its distribution

“…This is particularly true for fragile species that are destroyed by trawling or that live in irregular areas where trawls cannot be deployed, to update the knowledge about their distribution (e.g. Mastrototaro et al 2016Mastrototaro et al , 2017Chimienti et al 2018c). Moreover, visual techniques can allow the study of population structure and dynamics in non-invasive ways (e.g.…”