Abstract

A filamentous species of Desmarestia (Desmarestiaceae, Fucophyceae) is recorded for the first time in the algal flora of the lagoon of Venice. Its vegetative habitat and reproductive structure is described, and some data on its ecology are given. The determination of this new alga highlights the problem of the conspecificity of Desmarestia viridis (O. F. Müller) Lamouroux, which is distributed in the Northern Hemisphere, and Desmarestia confervoides (Bory) Ramirez et Peters (Desmarestia willii Reinsch), which is found in the Southern Hemisphere. The two species are difficult to differentiate on the basis of anatomical characteristics (size, medulla organization around a small central cell, branching, and degrees of cortication) due to the wide range of variations. In the present work, the conspecificity of both species and the probable origin of the present material are discussed, considering that there are several aquaculture sites in the Lagoon of Venice and that several exotic species have already been introduced since the 1990s.