| Summary: | A new species of the genus Tethya (Porifera, Demospongiae, Hadromerida) from the North European Sea is described, T. hibernica n. sp. from Rathlin Island in Northern Ireland. Tethya hibernica has a spherical body with tubercles on the surface. The colour in life is ochreous to yellow and shows in alcohol a white colour with greyish core. The oxyspherasters are evenly and densely scattered throughout the whole cortex. Their diameter is about 31 – 60 µm and the R/C value range between 0.40 and 0.78. Micrasters form a discrete layer allocated to the exopinacoderm surface and support the endopinacoderm of the lacunae. Both the main and the auxiliary megascleres are strongyloxeas. Tethya hibernica possesses in contrast to the closely related T. norvegica auxiliary megascleres. In addition to the morphological description we analysed a fragment of the cytochrome oxidase subunit I (COI) mtDNA gene for T. hibernica and the other European species T. aurantium, T. citrina and T. norvegica to check for differences in nucleotide and amino acid sequence between the species. We found that T. hibernica is more closely related to the North-West Atlantic species T. norvegica and T. citrina than to T. aurantium or other non-European species. In conclusion, it is possible to differentiate the new species T. hibernica clearly from all other European species both by morphological and molecular methods.
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