The genus Antithamnion (Ceramiales, Rhodophyta) in the Mediterranean sea: nomenclatural and taxonomical problematics

The genus Antithamnion Nägeli 1847 (1), as currently defined, is restricted to those filamentous red algae with axes completely lacking rhizoidal cortication, determinate “branchlets” arranged in opposite pairs, presence of a small quadrate cell at the base of each “branchlets”, presence or absence of gland cells, cruciate tetrasporangia, and carpogonial branches borne singly on the basal cells of the “branchlets” (2, 3, 4). According to Guiry’s AlgaeBase (5), the genus includes 41 species widely distributed between the tropics and temperate regions worldwide. Several authors have suggested that some of these species have extended their distribution range from the Pacific to the Atlantic and Mediterranean coasts (6, 7, 8, 9, 10, 11, 12). The branching pattern of the determinate “branchlets”, the origin of new indeterminate lateral axes, and the position of gland cells are useful characters for discriminating between species in Antithamnion (3, 13, 14). No molecular data are available on Mediterranean populations. On this basis, seven species are presently reported in the Mediterranean Sea (5, 6, 8, 15): A. amphigeneum Millar, A. decipiens (J.Agardh) Athanasiadis, A. cruciatum (C.Agardh) Nägeli, A. heterocladum Funk, A. hubbsii Dawson, A. piliferum Cormaci & G. Furnari and A. tenuissimum (Hauck) Schiffner. In this work, we present, for the first time, a molecular and morphological study on an Antithamnion population from the Straits of Messina (Sicily, Italy). Our result highlighted a discrepancy between molecular and morphological data. Specifically, the studied population could be attributed to A. amphigeneum using the morphological characters, while its rbcL sequence is determined as A. nipponicum (sensu 3) in BOLD and GenBank online databases. This discrepancy has led us to a careful analysis of the morphological and molecular data reported in the literature on the Antithaminon species. A number of imprecisions, producing a confusing state on both taxonomic and nomenclature aspects, were detected and are here discussed. In conclusion, we can say that although the genus Antithamnion is one of the most wellknown marine taxon and several articles on its taxonomy and systematics have been published, yet attribution of samples at species level is problematic. This uncertainty is surely due in part to the high variability of the morphology of some species, but also to the lack of care of some authors in comparing the morphological characteristics of their specimens with those reported in the literature. This is further aggravated by the fact that type specimens are hardly used as reference for identifications and names with uncertain application are commonly adopted. It is advisable that more morphological and molecular data on the type material of each species of Antithamnion are done to assess its taxonomy. Therefore, the starting point would be the characterization of A. cruciatum, the generitype, from the type locality (Trieste, Italy).