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Electronic Flora of South Australia Family Fact Sheet

ORDER GRACILARIALES Fredericq & Hommersand 1989a: 225

Phylum Rhodophyta – Class Florideophyceae

Thallus usually erect, branches terete to flat, radially or complanately branched; or parasitic and forming hemispherical pustules. Growth usually multiaxial (or uniaxial, but without an apparent central axis), pseudoparenchymatous, with a small-celled cortex grading to a large-celled medulla, with numerous secondary pit-connections. Apical and outer cortical cells dividing by oblique, longitudinal, concavo-convex walls followed by oblique or transverse division of the subterminal cell, with or without an apical cap of smaller cells. Cell walls containing agar; pit-plugs with one cap layer.

Reproduction: Gametangial thalli usually dioecious; auxiliary cells absent. Carpogonial branches 2-celled, outwardly directed from an intercalary mid cortical multinucleate supporting cell which also bears two or more sterile branches; hypogynous cell binucleate to multinucleate, carpogonium uninucleate with a straight trichogyne. Auxiliary cells absent, cells of the sterile branches fusing with the fertilized carpogonium which forms a fusion cell developing laterally and outwardly multiple gonimoblast initials which establish secondary fusions with gametophytic cells in the cystocarp floor and the pericarp, then produce a dense, erect, tuft of filaments surmounted with chains or clusters of subspherical to ovoid carposporangia. Cystocarps protruding, hemispherical, with a thick walled ostiolate pericarp, or as slight bulges in pustular taxa. Spermatangia forming a surface layer or borne in conceptacle-like depressions or cavities.

Tetrasporangia (or bisporangia) scattered in the outer cortex or within a shallow nemathecium, cruciately divided.

Life history triphasic with isomorphic gametophytes and tetrasporophytes.

Taxonomic notes: The Gracilariales were distinguished at ordinal level by Fredericq & Hommersand (1989a), separated from the Gigartinales (where they were previously placed as a family) by lacking an auxiliary cell and in details of apical cell divisions, development of the carposporophyte and the spermatangia. Fredericq & Hommersand (1990a) also placed the family Pterocladiophilaceae, including 3 parasitic genera, in the Gracilariales.

References:

FREDERICQ, S. & HOMMERSAND, M.H. (1989a). Proposal of the Gracilariales ord. nov. (Rhodophyta) based on an analysis of the reproductive development of Gracilaria verrucosa. J. Phycol. 25, 213–227.

FREDERICQ, S. & HOMMERSAND, M.H. (1990a). Morphology and systematics of Holmsella pachyderma (Pterocladiophilaceae, Gracilariales). Br. phycol. J. 25, 39–51.

The Marine Benthic Flora of Southern Australia Part IIIB complete list of references.

Author: H.B.S. Womersley

Publication: Womersley, H.B.S. (28 June, 1996)
The Marine Benthic Flora of Southern Australia
Rhodophyta. Part IIIB. Gracilarialse, Rhodymeniales, Corallinales and Bonnemaisoniales
Reproduced with permission from The Marine Benthic Flora of Southern Australia Part IIIB 1996, by H.B.S. Womersley. Australian Biological Resources Study, Canberra. Copyright Commonwealth of Australia.

KEY TO FAMILIES OF GRACILARIALES

1. Thallus parasitic, forming pale or pigmented pustules on the host branches (Gracilaria spp.)

PTEROCLADIOPHILACEAE

1. Thallus free living, erect, many centimetres high

GRACILARIACEAE


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