Shallow-water sea fans: the exceptional assemblage of Leptogorgia sarmentosa (Anthozoa: Gorgoniidae) in the Genoa harbour (Ligurian Sea)

F Betti,M Bo,S Bava,M Faimali,G Bavestrello

doi:10.1080/24750263.2018.1494219

Abstract

An exceptional assemblage of the sea fan Leptogorgia sarmentosa (Anthozoa: Gorgoniidae) was observed inside the Genoa harbour, which represents one of the major Mediterranean ports. The studied assemblage is confined in a shaded portion of a floating dock thriving in extremely shallow water, with specimens even touching the sea surface. It represents the shallowest population of this species and of this genus known worldwide. A total of 188 specimens were observed and measured: the maximum density of 45 specimens m−2 was reached in the most shaded part of the dock, where the maximum height of colonies (30 cm) was also recorded. Light measurements showed that the illuminance along the dock was comparable to that observed outside the harbour at 20 m depth, where the nearest colonies of L. sarmentosa were recorded for this region. This suggests that high levels of incident light might be putatively interpreted as the limiting factor in the upper bathymetrical distribution of the species. Despite the extremely shallow distribution, however, the population cannot be defined as intertidal as the floating dock avoids exposing the gorgonians to air. The chance to grow far from the silted bottom, but still in a turbid, sciaphilous and nutrient-enriched environment, probably enhanced the settling and growth of the colonies and allowed the formation of a dense and healthy population.

Highlights

Harbours are recognised as extreme marine ecosystems characterised by a wide array of environmental constraints such as high silting levels, low water circulation, sediment and water hypoxia, and high concentration of nutrients and pollutants, as well as a frequent occurrence of artificial substrates and ample variations of temperature and salinity not necessarily related to natural seasonal cycles (Ruggieri et al 2011)
The low water renewal rate, the high sedimentation rate and the high concentrations of pollutants strongly select for soft-bottom benthic communities with low species richness (Guerra-Garcia & GarciaGomez 2004); hard-bottom assemblages are usually confined to the walls of jetties, docks and seawalls
Light and Scanning Electron Microscope (SEM) analysis of the sclerites of the collected samples confirmed that the gorgonians colonising the floating dock belonged to the species Leptogorgia sarmentosa, excluding the possibility of an invasive species

Summary

Introduction

Harbours are recognised as extreme marine ecosystems characterised by a wide array of environmental constraints such as high silting levels, low water circulation, sediment and water hypoxia, and high concentration of nutrients and pollutants, as well as a frequent occurrence of artificial substrates and ample variations of temperature and salinity not necessarily related to natural seasonal cycles (Ruggieri et al 2011). The harbour marine fauna is constituted by many native species, selected by their natural capability to thrive in the difficult conditions characterising these sites. The low water renewal rate, the high sedimentation rate and the high concentrations of pollutants strongly select for soft-bottom benthic communities with low species richness (Guerra-Garcia & GarciaGomez 2004); hard-bottom assemblages are usually confined to the walls of jetties, docks and seawalls. Light penetration represents an important constraint factor for this type of community: Blockley (2007), for example, noticed that in the Sydney Harbour it was possible to distinguish two assemblages, depending on the presence or absence of wharves and their influence

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