Distribution and species composition of mass occurrences of large-sized sponges in the northeast Atlantic

The geographic and bathymetric distribution of ostur, that is mass occurrences of large-sized astrophorid demosponges, first recognized at the Faroe Islands during the internordic BIOFAR programme (Marine Benthic Fauna of the Faroe Islands), are mapped for the northeast Atlantic. This is done on the basis of information obtained during the sampling of the BIOICE programme (Benthic Invertebrates in Icelandic Waters) as well as during cruises at Karmoy (southwest Norway), the Trondheim Fjord (middle Norway), the Koster area (southwest Sweden) and the Denmark Strait (southeast Greenland). In addition, information has been acquired from Nordic and German biologists and fishermen regarding the occurrence of ostur. These data together with the sparse information in the literature show that the geographic distribution of the ostur areas follows two band-shaped arcs, defined by the Norwegian Atlantic Current and the Irminger Current. The local occurrence of ostur is, however, to a great extent dependent on areas of variable topography where a hard bottom is present. The results show that two main types of ostur can be recognized in the northeast Atlantic. Firstly a boreal ostur which is dominated by Geodia barretti, Geodia macandrewi, Geodia atlantica, Isopsphlegraei, Stryphnusponderosus and Stelletta normani, and occurs around the Faroe Islands, Norway, Sweden, parts of the western Barents Sea and south of Iceland. Secondly a cold water ostur characterized by the same genera but represented by different species, viz. Geodia mesotriaena, Isops phlegraei pyriformis and Stelletta rhaphidiophora, which is found north of Iceland, in most of the Denmark Strait, off East Greenland and north of Spitzbergen. A number of hexactinellid species are also represented in the cold water ostur, the most frequently occurring being Schaudinnia rosea. Suggestions are given regarding the possible causes for observed changes in the distribution of ostur as well as to the biological importance of these areas. (C) 2004 Elsevier Ltd. All rights reserved.