Aspects of the ecology of habitat-forming Cnidaria in Maltese deep waters

Abstract

The present study focused on deep-sea cnidarians occurring within the Malta Graben. Four main habitat-forming species – Madrepora oculata, Lophelia pertusa, Leiopathes glaberrima and Callogorgia verticillata – were studied in detail with the aim of updating current information on their spatial and bathymetric distribution. The present study also aimed to determine whether bathymetry and bottom-type have an effect on the structure of the cold-water coral assemblages present in the study area. Data were collected by analysing video imagery recorded during 84 remotely operated vehicle (ROV) dives along the southeastern end of the Malta Graben as part of the “LIFE BaĦAR for N2K” project. The data extracted from the video footage included the location (geographical coordinates) and water depth of each cnidarian individual, together with the habitat type where it was found. The size of colonies of M. oculata and C. verticillata was also determined. These data were used to plot the spatial distribution of the four habitat-forming species, and their bathymetric range. Multivariate analyses (cluster analysis, nMDS, SIMPER and CAP) were carried out on the species-sample data matrix to assess variation in cnidarian assemblage structure in relation to habitat type and depth. All four habitat-formers were found in higher denisities in the southeastern end of the Malta Graben, but abundances of L. pertusa were notably lower than those of the other three species. M. oculata was recorded at depths between 250m and 1050m, L. pertusa was found between 400m and 1050m, C. verticillata was found between 350m and 600m and again between 750m and 1050m and L. glaberrima was found between 200m and 500m and again between 600m and 850m. The multivariate analyses revealed a marked difference between hard and soft substrata, and also differences in the coral assemblages in terms of depth for some bottom-types. SIMPER revealed that M. oculata, L. glaberrima and C. verticillata were the main contributors to these differences. As seen in other studies in the Mediterranean region, L. glaberrima, C. verticillata and M. oculata have similar spatial distributions. Their bathymetric distributions are also similar to those found in other regions in the Mediterranean, except for C. verticillata which occurs in deeper waters in the Malta Graben. The data generated through the present study will inform local management of deep-sea ecosystems and providebaseline data that can be used in conservation efforts.