Bennettella ceticola(Nelson ex Bennett) Holmes on the skin of Franciscana dolphin (Pontoporia blainvillei) of the Argentinean Sea: an emendation of the generic description

The presence of epizoic diatoms on the skin of a variety of large marine vertebrates, such as whales and dolphins, has been often reported in the scientific literature. The first observations of an algal growth over on the skin of cetaceans were probably made around 1920. Later studies showed the presence of a new species, Cocconeis ceticola,on skin samples. Morphological features of this species led to the transference of C. ceticola,together with related forms, to the new genus BennettellaHolmes.Here, we report the occurrence of a diatom film of Bennettella ceticolaon the skin of the Franciscana dolphin (Pontoporia blainvillei), obtained from bycatch in fishermen’s gillnets at Samborombon Bay, Argentina, in April 2005. For comparison the holotype of B. ceticola,deposited in the Natural History Museum (London, U.K.) dated 1920 and labelled as Cocconeis ceticolawas examined. Detailed analysis of the fine structure of B. ceticolashows that it mostly matches the original description. However, the external raphe ending is not bifurcated (as originally described), and has only one simple, parallel sided slit and polar fissures hidden by a triangular flange. The triangular flange is the main diagnostic feature distinguishing Bennettellafrom EpipellisHolmes and CocconeisEhrenberg, allowing us to propose an emended description of Bennettella.The taxonomic affinities of B. ceticolawith related genera, such as Cocconeisand Epipellis, are discussed. The present study constitutes the first report of B. ceticolaas an epizoic diatom of a cetacean species in waters of the Argentinean Sea.