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FAMILY RHODYMENIACEAE Harvey 1849b: 75,120

Phylum Rhodophyta – Class Florideophyceae – Order Rhodymeniales

Thallus erect, encrusting or minute and parasitic, branches terete to flat or segmented, solid or hollow to varying degrees but lacking regular longitudinal filaments in the medulla, branching alternate, di- to polychotomous, or irregular, radial or complanate, often proliferous. Structure multiaxial, cortex 1–3 (–5) cells thick, outer cells often in anticlinal rows, older parts often with a secondary cortex; medulla usually 2–8 (–20) cells across, cells ovoid, with or without small accessory cells or secondary filaments, with secretory cells in some genera; longitudinal filaments bordering the inner medulla absent.

Reproduction: Gametangial thalli usually dioecious; procarpic. Carpogonial branches 3- or 4-celled, auxiliary cell branches 2- or 3-celled, borne on inner cortical cells. Carposporophyte developing outwards, with basal fusion cell(s) and nearly all gonimoblast cells becoming carposporangia. Basal nutritive tissue present, with numerous pit-connections to the supporting cell and auxiliary initial cell, erect filaments slender ("tela arachnoidea") or coarse or non persistent. Cystocarps prominent, protuberant or immersed, with a thick pericarp, ostiolate. Spermatangia cut off from outer cortical cells via initials.

Tetrasporangia scattered in the cortex or in nemathecia or sori and lying between sterile cortical filaments, terminal on inner cortical cells or transformed from inner cells, cruciately or decussately divided (tetrahedrally in Hymenocladia).

Life history triphasic with isomorphic gametophytes and tetrasporophytes.

Taxonomic notes: The Rhodymeniaceae is the largest family of the order, with some 30 genera. The habit varies considerably between the genera but reproductive features are similar, with virtually all gonimoblast cells forming carposporangia. The "tela arachnoidea" (J. Agardh 1876, p. 290), used by Kylin (1931) and other authors is, as Sparling (1957, p. 373) found, a variable feature though usually consistent for the genus. All genera have a basal nutritive tissue of branched chains of darkly staining cells, connected to the supporting cell and auxiliary initial cell by numerous pit-connections, and above connected to slender, erect, elongate (often stretched) cells (the "tela arachnoidea") around the developing carposporophyte and thence to pericarp cells around the ostiole; the elongate cells may be relatively coarse or may consist of stellate inner pericarp cells which may disintegrate to varying extents as the cystocarp develops. This variation makes use of the term "tela arachnoidea" of little value, and this account refers instead to the basal nutritive tissue, with erect filaments which may be persistent or may disintegrate and may be slender or relatively prominent.


AGARDH, J.G. (1876). Species Genera et Ordines Algarum. Vol. 3, Part 1 — Epicrisis systematis Floridearum, pp. i-vii, 1–724. (Weigel: Leipzig.)

HARVEY, W.H. (1849b). A manual of the British marine algae. Edn 2. (Van Voorst: London.)

KYLIN, H. (1931). Die Florideenordnung Rhodyméniales. Lunds Univ. Årsskr. N.F. Avd. 2, 27 (11), 1–48, Plates 1–20.

SPARLING, S.R. (1957). The structure and reproduction of some members of the Rhodymeniaceae. Univ. Calif Pubis Bot. 29, 319–396.

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.


1. Thallus minute (1–4 mm high), parasitic on Rhodymenia


1. Thallus erect, usually 2–40 cm high, not parasitic


2. Thallus terete or saccate, segmented or not, laterals at least saccate


2. Thallus not saccate, foliose or branched, branches terete to flat


3. Axes terete, solid, with clavate to ovoid saccate laterals, firm and not mucilaginous


3. Axes absent or very short, branches hollow, simple or constricted at intervals; soft and mucilaginous


4. Laterals ovoid to obovoid, 3–10 (–14) mm in diameter, borne irregularly on the axes


4. Laterals 1–2 mm in diameter, produced only near apices when fertile, bearing reproductive structures


5. Thallus simple or occasionally basally branched, clavate, 5–30 mm in diameter; tetrasporangia transformed from intercalary cortical cells


5. Thallus simple or much branched usually polychotomously, with ovoid to elongate segments and regular constrictions; tetrasporangia transformed or lateral on inner cortical cells


6. Cortex with a single layer of outer cells; cystocarps protuberant, erect filaments absent around the carposporophyte; tetrasporangia scattered, transformed from inner cortical cells


6. Cortex with outer, short, anticlinal filaments of 2–3 cells; cystocarps immersed, erect filaments around the carposporophyte present or absent; tetrasporangia in nemathecia, cut off from inner cortical cells or lower cells of paraphyses


7. Thallus firm, cartilaginous, simple to complanately and furcately to irregularly branched; tetrasporangia cut off from cortical cells, cruciately or decussately divided


7. Thallus soft to moderately firm, foliose to complanately branched; tetrasporangia cut off from cortical cells and cruciately or decussately divided, or transformed from inner cells and tetrahedrally divided


8. Thallus simple, ovate, relatively thick with a cortex of anticlinal rows of cells and medulla with small cells intermixed with larger cells; tetrasporangia in scattered sori over the blade surface, transformed from intercalary cells


8. Thallus simple and ovate (with proliferations) or much branched largely complanately, furcately or laterally or proliferously, cortex 2–5 cells thick, with rows only in the secondary cortex, medulla of large cells only; tetrasporangia scattered or in sori or nemathecia below branch apices, cut off laterally or terminally from cortical cells or intercalary


9. Cortex with anticlinal rows of small cells


9. Cortex one to a few cells thick, not in anticlinal rows


10. Cortex compact, of dense anticlinal, mostly unbranched, filaments; medullary cells thick-walled, without secondary pit-connections, with layers (1–4 cells thick) of rhizoidal filaments between the large cells. Cystocarps smooth, becoming horned when mature


10. Cortex of lax, dichotomous filaments, medulla with occasional slender rhizoidal filaments and frequent secondary pit-connections. Cystocarps horned


11. Thallus foliose, simple or branched, medulla 2–4 cells thick, of large ovoid cells bordering a central space within which rhizoids develop, at first sparsely then later filling the space. Cystocarps largely immersed, erect filaments non-persistent; tetrasporangia cut off from cortical cells, cruciately divided, small (20–45 µm in diameter)


11. Thallus much branched, pinnately to irregularly, medulla 3–20 cells thick, of large and small cells intermixed. Cystocarps protruding; tetrasporangia transformed from inner cells, tetrahedrally divided, large (60–140 µm in diameter)


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