This work analyses the morphology and the microstructural characteristics of some specimens of Crassostrea
gigas collected in diferents points of the Biscay coast. Oysters recollected in yachting harbours, where they
are in contact with tributyl-tin (TBT), have morphologycal and microstructural shell anomalies. The presence
of TBT in the sea water and the associated shell anomalies in oyster are more obvious and detectables in
spring because of the maintenance of the ships. In other sampling areas, although the bivalves have not
shown any malformations, other morphologycal variations associated to the abundance of nutrients have
been determinated