A reassessment of the diversification of the genus Cerithiella (Gastropoda: Newtoniellidae) in Southern South America

Newtoniellidae is a family of globally distributed gastropods, with most species occurring in polar, subpolar, and deep waters. Most members of this family in Antarctic waters and adjacent regions belong to the genus Cerithiella (Newtoniellinae). However, this contrasts with that known from Southern South America, a region influenced by sub-Antarctic waters. Only one species has been described from this area under this genus, Cerithiella axicostulata Castellanos, Rolán & Bartolotta, 1987, while two others were assigned without supporting evidence. This study aims to determine which and how many species of Cerithiella occur in Southern South America and to reassess if the genus is as diversified as previously inferred.

We examined 158 Newtoniellidae from 69 sampling events along the Atlantic and Pacific coasts of Southern South America, the Beagle Channel, and the Magellan Strait. Shell morphology (including protoconch and teleoconch), radulae, jaws, and opercula were studied using scanning electron microscopy.

The placement of Cerithiopsis macroura Melvill & Standen, 1912 and Bittium burdwoodianus Melvill & Standen, 1912 in Cerithiella is confirmed. The former is reported for the first time since its description and is abundant and widely distributed along the Atlantic coast of South America. The latter was found along the Atlantic coast, the Beagle Channel, and the Magellan Strait, and shows considerable intraspecific variability in shell morphology, ranging from cancellate to spirally sculptured or nearly smooth. Both shell and radula exclude Cerithiella axicostulata from this genus, and a new species of Cerithiella is recognized based on material collected between Isla de los Estados and Burdwood Bank. Our results show that Newtoniellinae are poorly diversified in Southern South America, where Eumetulinae dominate. Despite low diversity, the species found suggest that Cerithiella, as currently considered, is not monophyletic.