Acrochaete geniculata (Gardner) comb. nov. (=Pseudodictyon geniculatum Gardner) is a distinctive marine chaetophoracean alga endemic to the Pacific coasts of North America and Asia, where it occurs exclusively as an endophyte in Laminariales. The reticulate habit typical of A. geniculata occurs only when the alga is endophytic, and is not reproducible in culture. The presence of setae, and the formation of pseudoparenchymatous ‘cushion’ stages in nature and in culture, are structural features of A. geniculata reported for the first time. The alga reproduces by means of quadriflagellate zoospores and biflagellate anisogametes, and the life history involves an alternation of isomorphic phases. The gametophytes may be monoecious or dioecious. Sporangia and gametangia are flask-shaped and undergo sequential cleavages to produce motile cells that are released without a vesicle by the explosive rupture of an exit papilla. As P. geniculatum is the type of the genus, Pseudodictyon is reduced to the synonymy of Acrochaete Pringsheim. Several of the features of A. geniculata, especially the sporangial and gametangial development patterns and the life history, are common to Acrochaete, Endophyton, Entocladia, Ochlochaete, Pringsheimiella, and Ulvella. These genera form a distinct assemblage of closely related organisms, whose features suggest a close affinity with the Ulvales.
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Charles J. O’Kelly (1983) studied this question.