Electronic Flora of South Australia Family Fact Sheet

Tribe SPONGOCLONIEAE Schmitz 1889: 450

Phylum Rhodophyta – Order Ceramiales – Family Ceramiaceae

Thallus erect, 5–30 cm high, much branched usually with long axes and lateral branches, apices conspicuous or not, usually overtopped by branchlets from below, axes and branches loosely corticated below with rhizoids from basal cells of laterals; ecorticate lateral branchlets more or less determinate, some becoming indeterminate; gland cells absent. Cells multinucleate.

Reproduction: Gametophytes dioecious. Carpogonial branches borne on the subterminal cell of short, 3 (–5)-celled, branchlets, with 2 periaxial cells on the third cell; the 3 sterile cells (terminal and 2 periaxial cells) enlarging and becoming rounded after fertilization. The subterminal cell cuts off an auxiliary cell which produces successive, rounded, gonimolobes of carposporangia, and a loose involucre of filaments develops from axial cells below the carposporophyte which terminates a lateral branch. Spermatangial heads are borne on branchlet cells, sessile or pedicellate, ovoid to elongate, with a row of axial cells bearing whorls of cells producing outer spermatangia.

Tetrasporangia or polysporangia occur on cells of the lateral branchlets, sessile or pedicellate, subspherical to ovoid, with 4 or 32–40 spores.

Life history triphasic, with isomorphic gametophytes and tetrasporophytes.

Taxonomic notes: The tribe Spongoclonieae includes probably three genera, Spongoclonium, Pleonosporium and Lophothamnion. It is characterised by the 3-celled female axis with carpogonial branches on the subterminal cell, a single auxiliary cell, and the 3 sterile cells enlarging, becoming rounded, but not dividing. This distinctive cell arrangement has been interpreted by Kylin (1925, p. 58), Ardré et al. (1982, p. 13), Norris (1985, p. 60) and Wollaston (1990, p. 21) as having the apical cell pushed aside to become one of the periaxial cells, and the subapical cell cutting off a supporting cell which forms a sterile cell and a 4-celled carpogonial branch; a second sterile periaxial cell is cut off opposite the first periaxial (originally apical) cell. If this development occurs, it is not easily seen, and in many cases it appears that the two periaxial cells (which occur almost simultaneously and are of similar size and shape) are both cut off the third cell of a fairly straight 3-celled axis, and the carpogonial branch directly from the second cell. Carpogonial branches are borne directly on cells of the whorl-branchlets in the Antithamnieae and Heterothamnieae, and it would not be surprising for them to be borne directly on cells of a female axis which then act as supporting cells. While this suggested displacement is not convincingly evident in all southern Australian species, the situation of 3 rounded and enlarged sterile cells is a conspicuous feature of Spongoclonium and the tribe Spongoclonieae.

Mesothamnion shows essentially the same reproduction as Spongoclonium and cannot be satisfactorily separated generically, as having a not or only slightly corticated thallus (as does S. australicum, described below). Mesothamnion is here considered a synonym of the older genus Spongoclonium.

The genus Lophothamnion is here kept separate from Pleonosporium, both bearing polysporangia instead of tetrasporangia. Some species of Spongoclonium (e.g. S. australicum, S. brownianum) which typically bear tetrasporangia do very occasionally show further divisions to octosporangia but are here considered distinct from species which uniformally bear multi-spored (32 or more) polysporangia. In contrast to the view of Norris (1985, p. 61), the distinction between tetra (rarely octo) sporangia and multispored polysporangia is considered an adequate generic separation. Lophothamnion is separated from Pleonosporium by its dense, radially and spirally branched thallus, with overtopped apices, in contrast to the distichously branched thallus with exserted apices of the type species (P. borreri) and most other species of Pleonosporium. Norris (1985, p. 60) claims species of Pleonosporium are either spirally or distichously branched, but this does not occur, apparently, within any one species.

References:

KYLIN, H. (1925). The marine red algae in the vicinity of the biological station at Friday Harbour, Washington. Lunds Univ. Årsskr. NF Avd. 2, 21(9), 1–87.

NORRIS, R.E. (1985). Studies on Pleonosporium and Mesothamnion (Ceramiaceae, Rhodophyta) with a description of a new species from Natal. Br phycol. J. 20, 59–68.

SCHMITZ, F. (1889). Systematische Ubersicht der bisher bekannten Gattungen der Florideen. Flora, Jena 72, 435–456, Plate 21.

WOLLASTON, E.M. (1990). Recognition of the genera Spongoclonium and Lasiothalia Harvey (Ceramiaceae, Rhodophyta) in southern Australia. Bot. Mar. 33, 19–30.

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

Author: H.B.S. Womersley

Publication: Womersley, H.B.S. (24 December, 1998)
The Marine Benthic Flora of Southern Australia
Rhodophyta. Part IIIC. Ceramiales – Ceramiaceae, Dasyaceae
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