Morphological and taxonomic studies on Antarctic Ceramiaceae (Rhodophyceae). I.Antarcticothamnion polysporumgen. et sp. nov.

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Abstract
Antarcticothamnion polysporum gen. et sp. nov. (Rhodophyceae: Ceramiaceae) is described from the South Shetland Is. and the Antarctic Peninsula. It differs from all previously described Ceramiaceae in vegetative structure: an indeterminate apex, which divides by alternating oblique septa, produces simple alternate-distichous determinate branches, while indeterminate branches are initiated on main axes in a verticillate arrangement. Reproductive structures are borne on modified indeterminate branch systems. Sporangia are polyhedrally divided. Spermatangia are formed in loose heads. Procarps are borne near the apex, but not always on the subapical cell, of a fertile branch. Two auxiliary cells may participate in the formation of a carposporophyte. A fusion cell is lacking and most cells of the gonimoblast develop into carposporangia, which are binucleate. The suite of morphological characters exhibited by Antarcticothamnion sets this genus sufficiently apart from all previously described Ceramiaceae to warrant placement in its own tribe, Antarcticothamnieae trib. nov. It shares important features, however, with Callithamnieae, Compsothamnieae, and Ptiloteae. Apical division is reviewed in the various tribes of Ceramiaceae. It is concluded that a pattern of alternating oblique septa is correlated with transitory or chronic structural imbalance resulting from a particular combination of timing and spatial sequence in the initiation and development of branches. The distribution of oblique apical division within Ceramiaceae is strongly correlated with features generally considered to be important at the level of tribe. Phylogenetic relationships within Ptiloteae, a tribe characterized inter alia by oblique apical division, are suggested from an analysis of branching patterns. The validity of Falklandiella as a genus distinct from Dasyptilon is emphasized. Gymnothamnion and Tokidaea are removed from Ptiloteae, but without being assigned to another tribe. Tanakaella is removed from Sphondylothamnieae to Compsothamnieae, while Mazoyerella is removed from Compsothamnieae to Spermothamnieae. Spongoclonium orthocladum A. et E. S. Gepp is discussed as a possible species of Antarcticothamnion, while an undescribed alga from the South Orkney Is. is definitely indicated as a second species of the genus.