Gracilaria viridis sp. nov. (Gracilariales, Rhodophyta): a new red algal species from the Mediterranean Sea

We characterized a new Gracilaria species from the Venice Lagoon, Italy, using molecular analyses based on the plastid large subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase gene (rbcL) and the intergenic RuBisCO spacer (rbcL-rbcS), combined with morphology data. This new entity was recorded on the artificial substrata of the Venice Gulf from March to July, adding to 12 Gracilaria taxa already recorded in the Mediterranean and Adriatic seas. Thalli exhibited a green-yellowish pigmentation with pink shades, and there was dense branching in the distal portions. Tetrasporangia were scattered on thallus cortex and distributed mostly on short, stipitate branchlets. The inner pericarp was connected to the gonimoblast by tubular nutritive cells. The male gametophytic plants formed round- elliptical spermatangial verrucosa-type conceptacles. This species grew attached on artificial rocky substrata of the low midlittoral and upper sublittoral zone in spring and early summer. Molecular analyses based on the plastid-encoded rbcL gene showed a 99.66% nucleotide identity with another Gracilaria sp. from southern Sicily. We compared our rbcL-rbcS spacer sequences with those of two cryptic species, and the phylogenetic analyses confirmed that the Venice populations were a new species. We suggested that the discovery of this new species was not due to an extra- Mediterranean introduction but the consequence of its misidentification as Gracilaria gracilis, which has a similar gross morphology.