Distribution and population size structures of the megabenthic cnidarians Isidella elongata and Funiculina quadrangularis in Maltese waters

Abstract

Cold water communities play an important role as benthic habitat formers in the deepsea environments, providing a variety of ecosystem services to many fish and invertebrate communities, including food, shelter and breeding grounds. Information and data about these deep-sea communities, including diversity, abundance and distribution is to the present day very limited. This study analysed two important megabenthic cnidarians occurring within the Maltese, namely Funiculina quadrangularis and Isidella elongata, in the aid of bridging knowledge gaps regarding spatial distribution, bathymetric distribution and colony size structures. These two critically endangered octocorals were analyzed using underwater video footage recorded in years 2015 and 2016 by a remotely operated vehicle within the Malta 25- nautical mile fisheries management zone at depths ranging between 100 and 950m. Results show distinctive preferences with regards to bathymetry and terrain slope, with similar preferences with respect to sediment type. From all the stations analyzed, Funiculina quadrangularis was found to occur in wider bathymetric ranges (135- 930m) with a total of 686 individuals. Whilst the occurrence of I. elongata was present between depth ranges of 320-790m, with a total of 629 individuals recorded. The highest density of F. quadrangularis was recorded in depths ranging between 100- 150m, with a total of 40 individuals per 1000m2 and a general trend of a decrease in density with an increasing depth. Whilst the highest density of I. elongata was present in depths ranging between 600-650m, with a total of 21 individuals per 1000m2 and a general trend of an increase in density with an increasing depth. A large proportion of F. quadrangularis analyzed at the colony level was of intermediate size (class C), with larger size classes being much less common (in terms of mean standard abundance) than smaller size classes. Similarly, a large proportion of F. quadrangularis analyzed at the colony level was of intermediate size (class C), however the populations seem to be shifting to one with larger colonies. In addition to this, a change in colony size was indicted with a variation in depth. Distribution and population size structures are also affected by anthropogenic impacts such as bottom trawling, varying also based on the morphological structure of the species. Ultimately this knowledge can serve as an important contribution for ecological and biological studies, with the main aim being that of safeguarding the species and their ecosystems given that Mediterranean CWC habitats have been recognized and classified as Essential Fish Habitats.