Biological and ecological characterisation of the crinoid Antedon mediterranea (Lamarck, 1816) in surface waters of La Spezia gulf (Italy)

Antedon mediterranea (Lamarck, 1816) is an endemic crinoid species widely distributed throughout the Mediterranean Sea and is more frequently found between 10-100 m in depth. However, preliminary observations indicated that, in the Gulf of La Spezia (Ligurian Sea, Northern Italy), a large population unexpectedly inhabits the surface waters of a side bay along floating piers. In this study we aimed to understand which factors may influence the distribution and abundance of A. mediterranea within the gulf. We selected our stations, two outside (Lerici and Portovenere) and two inside (Le Grazie and Santa Teresa) the dam built to protect La Spezia's harbour and evaluate the presence/abundance of the species through bimestrial sampling over the course of one year. In parallel, we measured key physico-chemical parameters of the water column as well as biological ones (zooplankton, biocenosis and the reproductive cycle). The results showed that, in Lerici and Portovenere, the species was absent throughout the year, while, in Le Grazie and Santa Teresa, A. mediterranea was present, with Le Grazie having the highest abundance (mean density up to 46 ind/m2). The physico-chemical parameters did not allow us to univocally explain the distribution and abundance of the species; however, the station with the highest abundance was characterized by several indices of high trophy (e.g. zooplankton density). We suppose that the high abundance of A. mediterranea in surface water of Le Grazie could be related to the presence of nearby fish farms. Furthermore, in this station a remarkable seasonal variation of A. mediterranea density was observed (peak in July, drop in September); this may relate to the reproductive cycle or migration phenomena. These data underline the need to increase our knowledge on the biology and ecology of this endemic species, trying to better understand which factors drive its distribution.