Syringodermatales ord. nov. and Syringoderma floridana sp. nov. (Phaeophyceae)

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Abstract
S. floridana sp. nov. (Phaeophyceae), a monostromatic species from Florida [USA], has a heteromorphic life history in which zoospores from unilocular sporangia are scarcely motile and the gametophyte is reduced to 2 cells, both of which differentiate as biflagellate, morphologically similar gametes. S. abyssicola is interpreted as having similar 4-celled gametophytes which develop within the unilocular sporangium and release 5 biflagellate isogametes. Evidence for heteromorphic sporophytes and gametophytes in all 4 known species of Syringoderma, and the presence of several vegetative characters of Syringoderma differing from those of both the Dictyotales and Sphacelariales, support the placement of Syringoderma in a separate order SYRINGODERMATALES ord. nov., containing the single monogeneric family SYRINGODERMATACEAE fam. nov.