Reproductive morphology ofItonoa marginifera(J. Agardh) gen. et comb. nov. (Nemastomataceae, Rhodophyta)

Abstract

The marine red alga Platoma marginiferum (J. Agardh) Batters (Nemastomataceae, Gigartinales) is known only from relatively few localities in the north-eastern Atlantic. A study of fertile gametophytes from south-western Ireland has shown that after fertilisation, connecting filaments arise directly from the carpogonia in contrast to those of the type species, P. cyclocolpum (Montagne) Schmitz, which are formed from nutritive auxiliary cells fused or connected to the fertilised carpogonia. Itonoa gen. nov. is therefore proposed [type species: Itonoa marginifera (J. Agardh) comb. nov.; basionym: Nemastoma marginiferum J. Agardh] and the new genus is referred to the Nemastomataceae. Itonoa is characterised by the following combination of vegetative and reproductive features: connecting filaments arising directly from fertilised carpogonia; X-shaped cells consistently present in the medulla; gland cells absent; supporting cells and auxiliary cells forming in an intercalary position on cortical fascicles; sterile or fertile lateral branchlets occurring on the carpogonial branches; connecting filaments branching once immediately prior to fusion with an auxiliary cell, with one of the branches terminating at the auxiliary cell; special nutritive cells adjacent to the auxiliary cells absent. The life history of Itonoa marginifera is probably of the Bonnemaisonia type with a loosely filamentous tetrasporophyte, but tetrasporangia are presently unknown. The family Schizymeniaceae (Schmitz et Hauptfleisch) stat. nov. [basionym: Schizymenieae Schmitz et Hauptfleisch, lectotypified with Schizymenia J. Agardh] is proposed to accommodate the genera Schizymenia, Platoma and, perhaps, Titanophora.