Diversity and abundance of sponges in bathyal coral reefs of Rockall Bank, NE Atlantic, from boxcore samples

Abstract

Recent (2004) boxcoring on the SW Rockall Bank, west of Ireland (approx. 55°N 15°W, depths between 557 and 1407 m) yielded 95 species of sponges in 20 boxcore attempts. There are no published reports of Rockall Bank coral reef sponge fauna, but comparison with trawl and dredge efforts in neighbouring parts of the NE Atlantic (east of Rockall Trench, off the Scottish coasts) made in the late 19th and early 20th century leads to the conclusion that our boxcoring efforts yielded similar numbers of species as these more elaborate collecting programs. Numbers of specimens for all boxcore samples combined was 466, together occupying a total volume of approx. 3.38 l, based on length x width x height measurements of all individuals. These results indicate a generally low biomass of sponges contributing to the deep water coral reef fauna. Species composition in this bathyal habitat shows a high heterogeneity (Eveness J’: 0.81–0.97) and the majority of sponge individuals does not exceed 1 cm³ in volume. Nevertheless, high densities of a large hexactinellid species, Asconema aff. setubalense (up to 30 cm in height), were encountered locally. Sponge community analysis showed that there was no clear correlation between coral cover and sponge diversity, and only a weak correlation between coral cover and abundance and volume of sponges. This, combined with the overall very small size of the sponges, suggests that substratum is not a limiting factor for their occurrence.

Keywords:

• Rockall Bank
• bathyal
• sponges
• boxcores

Henk de Haas (NIOZ) graciously allowed us to join the ‘Moundforce’ 2004 cruise. His team and the captain and crew of RV ‘Pelagia’ are thanked for a smooth trip. Artur Palacz (International University Bremen) helped analyze densities of sponges from underwater videos. Mario de Kluijver (Zoological Museum of the University of Amsterdam) did the statistics and Jaap Kaandorp (Institute for Informatics, Faculty of Science, University of Amsterdam) performed the MINAR analysis. The material of this research was collected during cruise 64PE229 with the RV ‘Pelagia’. This cruise forms part of the Moundforce Project (813.03.006/855.01.040) that falls within EUROMARGIN Programme of the European Science Foundation. The research itself forms part of the BIOSYS project funded by the Dutch Science Foundation (NWO/ALW) and was carried out as a preliminary study to the HERMES Project. EC contrast on GOCE-CT-2005-511234, funded by the European Commission's Sixth Framework Programme under the priority Sustainable Development, Global Change and Ecosystems.

Notes

Published in collaboration with the University of Bergen and the Institute of Marine Research, Norway, and the Marine Biological Laboratory, University of Copenhagen, Denmark