Serpula aggregates and their role in deep-sea coral communities in the southern Adriatic Sea

We document a remarkable abundance of the tubeworm Serpula vermicularis Linnaeus, 1767, in bathyal coral habitats from the Bari Canyon System in the southern Adriatic Sea. Here, the specimens of S. vermicularis grow from muddy substrate either as isolated individuals or as localized clusters of multiple individuals. Peculiar tube aggregations are also associated with Madrepora oculata build-ups and other stony corals including Desmophyllum dianthus. Three types of coral-serpulid (C–S) frameworks have been recognized based upon size and shape. The abundance of S. vermicularis increases with the size of C–S frameworks, which results from superimposition, overgrowth, and/or intergrowth of scleractinians and serpulids. Several generations of S. vermicularis contribute to the C–S frameworks, each most probably accounting for more than 8 years and presumably existing in the area for the last hundreds of years. At a meso-scale, the distribution pattern of serpulids seems to be mainly governed by currents and siltation. A further constraint is the co-occurring solitary coral D. dianthus within frameworks. The successful sea-bottom colonization by S. vermicularis and associated C–S frameworks is possibly related to a high oxygen content and food supply derived from the North Adriatic Dense Water (NAdDW). The flourishing populations of S. vermicularis as well as the peculiarity of C–S frameworks suggest that deep-sea canyons provide important benthic habitats in bathyal environments.