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Electronic Flora of South Australia Genus Fact Sheet
Phylum Rhodophyta – Class Florideophyceae – Order Rhodymeniales – Family Rhodymeniaceae
Thallus usually erect, occasionally prostrate, cartilaginous, foliose (some perforate) and simple or divided, or with flat, subdichotomous to lateral, complanate branches 2–20 mm broad, with or without surface or marginal proliferations (often proliferous from damaged ends of branches), stipes usually present; holdfast crustose or stoloniferous. Structure multiaxial, with a small-celled cortex 2–5 cells thick and a pseudoparenchymatous medulla of larger ovoid cells, elongate lengthwise to the branch, in older or robust thalli often with smaller intermixed cells and in some species with a secondary cortex in the stipe.
Reproduction: Gametangial thalli dioecious. Carpogonial branches 3- or 4-celled, borne on inner cortical (supporting) cells together with a 2- or 3-celled auxiliary cell branch. Carposporophyte developing outwards, usually with several branched basal fusion cells and a dense mass of ovoid to angular carposporangia. Basal nutritive tissue present, erect filaments absent or disintegrating apart from inner stellate cells to the pericarp. Cystocarps scattered (often clustered) on the blades or proliferations, protruding, usually basally constricted, pericarp thick, usually smooth (verrucose in one species). Spermatangia in sori, subapical on the branches or scattered on foliose thalli, cut off from elongate outer cortical initials.
Tetrasporangia in subapical sori or nemathecia, or scattered, terminal on cortical cells or transformed from mid cortical cells, decussately or cruciately divided, with adjacent cortical cells in most species becoming elongate and often dividing.
Life history triphasic with isomorphic gametophytes and tetrasporophytes.
Lectotype species: R. palmetta (Lamouroux) Greville [= R. pseudopalmata (Lamouroux) Silva 1952: 265].
Taxonomic notes: Rhodymenia is a common world-wide genus of 50–60 described species, but the limits of many species are uncertain. The most recent attempt to survey the genus is by Dawson (1941), and critical studies of the genus are needed in most countries.
Rhodymenia is here considered to include Epymenia Kützing (1849, p. 787), a southern hemisphere genus with the lectotype species E. obtusa (Greville) Kützing. Epymenia has been distinguished by the presence of small proliferations on the thallus surface, these bearing the reproductive sori or cystocarps. However, as others (e.g. Sparling 1957, p. 356) have noted, this is not a clear-cut distinction since several species form a few surface proliferations, many species form marginal or apical proliferations (especially following damage) and while such proliferations often bear reproductive structures, some species (e.g. R. prolificans, R. obtusa, R. australis) also have cystocarps and tetrasporangial sori on the branch surfaces. In R. halymenioides tetrasporangia occur on small leaflets but cystocarps occur on the thallus surface.
The origin of the tetrasporangia in the cortex is not easy to establish. The genus is sometimes said to have intercalary tetrasporangia (Irvine & Guiry 1983, p. 93) but the illustrations of Guiry (1974, fig. 1) appear to show them as terminal in the type species. Hawkes & Scagel (1986, p. 1570) refer to them as terminal for the genus while Lee (1978, p. 84) for R. pertusa, states initials intercalary to terminal. Few authors have clearly illustrated pit-connections on the tetrasporangia. Such pit-connections are usually slender and faint, and are usually visible only with juvenile, undivided, tetrasporangia. As the sporangium matures and enlarges greatly, it extends both towards and away from the thallus surface so that the basal pit-connection, as well as ones to small outer cortical cells if present, are often moved to lateral positions on the inner half of the sporangium. In southern Australian species, even within a species, it appears that tetrasporangia may be either terminal on cortical cells or transformed from intercalary mid cortical cells. This needs further study since other genera of the Rhodymeniaceae are separated on whether the tetrasporangia are terminal or intercalary.
While a few species are distinctive in habit, others appear to be very variable depending on their habitat, probably influenced by depth, shading, and water motion in particular. One such species, R. australis, on southern Australian coasts, is interpreted broadly, and several other specimens in AD are not currently placed within a species. Further detailed studies, based on in-situ populations and from ecological and seasonal bases, are needed to clarify most species.
The following key and descriptions do not include all species on southern Australian coasts. One species occurs as loose lying plants in bays (e.g. Marion Bay) on SE Tasmanian coasts, being 20–40 cm long with distant branches 6–10 mm broad, often proliferous from damaged ends. No reproductive structures are known.
References:
DAWSON, E.Y. (1941). A review of the genus Rhodymenia with descriptions of new species. Allan Hancock Pacif. Exped. 3(7), 115–181.
GREVILLE, R.K. (1830). Algae Britannicae. (Maclachlan & Stewart: Edinburgh.)
GUIRY, M.D. (1974). A preliminary consideration of the taxonomic position of Palmaria palmata (Linnaeus) Stackhouse = Rhodymenia palmata (Linnaeus) Greville. J. mar. biol. Ass. U.K. 54, 509–528.
HAWKES, M.W. & SCAGEL, R.F. (1986). The marine algae of British Columbia and northern Washington: division Rhodophyta (red algae), class Rhodophyceae, order Rhodyméniales. Can. J. Bot. 64, 1549–1580.
IRVINE, L.M. & GUIRY, M.D. (1983). Rhodyméniales. In Irvine, L.M. (1983). Seaweeds of the British Isles. Vol. 1 Rhodophyta Part 2A Cryptonemiales (sensu stricto), Palmariales, Rhodyméniales. (British Museum (N.H.): London).
KÜTZING, F.T. (1849). Species Algarum. (Leipzig.)
LEE, I.K. (1978). Studies on Rhodyméniales from Hokkaido. J. Fac. Sci., Hokkaido Univ., Ser. V (Bot.) 11 (1), 1–194, Plates 1–5.
SILVA, P.C. (1952). A review of nomenclatural conservation in the algae from the point of view of the type method. Univ. Calif Pubis Bot. 25, 241–324.
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.
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 SPECIES OF RHODYMENIA
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1. Thallus moderately to much branched, essentially complanately, subdichotomous to irregularly lateral or proliferous, branches | 2 |
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1. Thallus foliose, simple or divided below into cuneate lobes | 7 |
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2. Thallus with tetrasporangial sori or nemathecia and cystocarps on the branches, proliferous only from damaged branch ends | 3 |
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2. Thallus with numerous surface or marginal proliferations bearing the tetrasporangia or cystocarps | 6 |
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3. Thallus with erect, clumped, linear fronds from a crustose base, branched near the apices | R. stenoglossa |
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3. Thallus with one to a few spreading, subdichotomous fronds from a small crustose base, with or without stolons | 4 |
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4. Basal stolons and stipes slender, prominent, | R. leptophylla |
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4. Basal stolons (if present) and stipes relatively robust, usually | 5 |
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5. Cystocarp pericarp smooth, tetrasporangial sori with an irregular and not sharply defined margin | R. australis |
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5. Cystocarp pericarp verrucose, tetrasporangial nemathecia with a sharply defined margin | R. verrucosa |
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6. Thallus with frequent fertile surface proliferations, branches | R. obtusa |
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6. Thallus with numerous marginal proliferations and occasional surface ones, branches | R. prolificans |
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7. Thallus simple or irregularly lacerate, often perforate, surface smooth if cystocarpic or covered with numerous ovate, tetrasporangial, proliferations | R. halymenioides |
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7. Thallus with several cuneate lobes | R. cuneata |
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