The genus Olivancillaria is revised and restricted to eight living species: O. deshayesiana, O. carcellesi, O. urceus, O. contortuplicata, O. orbignyi, O. teaguei, O. auricularia and O. vesica. The last two species were previously considered as subspecies. The name O. uretai is a junior synonym of O. orbignyi. The genus Olivancillaria is endemic to the southwestern Atlantic, ranging from Bahia state, Brazil (~12°S) to Chubut province (42°37’S), Argentina, intertidal to 70 m depth. It is a clear representative of the Argentine Malacological province. Redescriptions and re-illustration of types were done for each species. Shell ultrastructure, radulae, penes, siphon papillae and egg capsules are described and illustrated by SEM images. A geometric morphometric analysis was carried out to confirm shell variation on size and shape among species. The geographic distribution of each species is provided based on field observations as well as on museum records.

Beach filling activities were conducted along the Mar del Plata coastline, Argentina, with sediments dredged from the harbour mouth between November 1998 and April 1999. We determined the percentage of imposex of Olivancillaria deshayesiana before, during and immediately after beach filling. The current situation (2005–2007) of imposex incidence was compared between populations from Mar del Plata and Mar Chiquita, a less impacted area. In addition, we evaluated differences in shell shape and body weight between females with and without imposex. The percentage of imposex was significantly higher during (85.3%) and after (73.9%) beach filling than before it (31.0%). No differences in body weight were found in the female population but shell length was significantly lower in females with imposex. As expected, the Mar Chiquita population showed no signs of imposex. This is the first report of imposex in beaches nourished with sand dredged from the port. Our results highlight the importance of analysing more carefully the sediment before beach filling. On the basis of the high level of imposex recorded near the port of Mar del Plata, O. deshayesiana can be used as a biological indicator of marine contamination with butyltin compounds.

Phenotypic variation on shell size and shape of Olivancillaria carcellesi from four representative localities is confirmed using geometric morphometric techniques. This species lives along the entire range of the genus in subtidal soft bottoms from Rio de Janeiro (Brazil) to Punta Pardelas (Chubut, Argentina). Thickness of columellar callus, length and width are the main shell differences from specimens collected at several intermediate localities: Cassino (Brazil), La Paloma (Uruguay) and Mar del Plata (Argentina). All forms showed significant differences in shell size except O. carcellesi from La Paloma compared with those from Mar del Plata. The group with larger specimens was from Cassino. Olivancillaria carcellesi from Punta Pardelas, the southernmost population, was the smaller sized group and showed allometry between size and shape and this is responsible for their relatively higher spires. The possible ecological implications of the observed pattern are discussed. It is confirmed that O. carcellesi although variable, is a clearly defined species.

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