An evaluation of the systematics of the genus Thalia (Thaliacea, Salpidae) based on new material from the EASTROPAC and MORO expeditions

Until 1973, when Van Soest revised Thalia Blumenbach, 1798, this genus comprised only two species: Thalia democratica (Forsskål, 1775) and Thalia longicauda (Quoy & Gaimard, 1824). The former showed great variability, which led Van Soest to distinguish five species: T. rhomboides, T. cicar, T. democratica, T. orientalis, and T. sibogae. Some authors have ignored this revision considering the observed differences of infraspecific level. In the present work new material is studied analysing variables not previously considered, such as the development of blastogenic stolons in the solitary zooids and the location of the tunnel opening through which the bud blocks are liberated. These characters previously permitted a separation of closely related species in the genus Ihlea. The results show that stolon features, even though they permit a separation of solitary zooids of Thalia longicauda, do not discriminate all species of the “T. democratica group” since they are only useful in distinguishing T. democratica from T. sibogae. Other variables, mainly qualitative, do permit differentiation. The sympatric distribution of these taxa supports Van Soest’s hypothesis as well.