Electronic Flora of South Australia Family Fact Sheet

ORDER DICTYOTALES Kjellman

Phylum Phaeophyta

Thallus erect or partly prostrate, simple or flabellate to dichotomously (or becoming laterally) branched with more or less ligulate branches; branching usually complanate (spiral and much divided in Lobospira) with at least lesser branches and usually all branches strongly flattened; attachment by basal rhizoids which usually form a matted holdfast. Growth by either a single, usually prominent, slightly protruding or sunken apical cell dividing equally to give dichotomous branches, or by a terminal and marginal row of numerous apical cells which divides or splits irregularly to initiate lateral branches; in each case followed by regular transverse and then longitudinal divisions. Structure in cross section usually of regularly arranged cells which in some genera become rounded and laterally displaced, differentiated into a small-celled densely phaeoplastic cortex 1–5 cells thick formed by further transverse and longitudinal divisions than in the medullary cells, and an inner medulla 1–8 cells thick, usually of distinctly larger cells and with few phaeoplasts; phaeoplasts discoid, without pyrenoids. Phaeophycean hairs present in scattered tufts or concentric zones, developed from patches of surface cortical cells.

Reproduction: The sporophyte by scattered or clustered unilocular sporangia with one, four or eight non-motile spores (meiospores or apomeiotic mitospores), usually borne superficially on the cortex. The gametophytes oogamous, with sori of closely adjacent ovoid oogonia on female plants, and sori of antheridia usually surrounded by sterile paraphyses (or occasionally sunken in the cortex) and with each locule producing a uniflagellate sperm. Reproductive organs borne on both sides of the thallus in most taxa.

Life history diplohaplontic and isomorphic, usually dioecious with distinct sporophytic. female and male thalli; in many taxa sexual plants are much rarer than the sporophytes and in some species sexual plants are unknown; such taxa may be apomeiotic.

Taxonomic notes: The Dictyotales is a distinctive order and members can usually be readily recognised by their apical growth, flattened thallus, regularity of the cells in cross section, the tufts of surface hairs, and the characteristic sporangia. In many taxa sexual plants are rare and it is likely that meiosis in the sporangium does not always occur; this has been shown in Padina by Gaillard (1972) and Liddle (1972), and in Taonia by Mathieson (1966). In many species, however, the sporangia divide only shortly before release of the spores. In some species, the apparent preponderance of sporangial plants may be due in part to sexual plants being commoner in deeper water and less frequently collected.

The Dictyotales have usually all been placed in one family, the Dictyotaceae, but with recognition of two tribes, the Dictyoteae and Zonarieae, based on the presence of a single apical cell or a terminal row of apical cells. J. Agardh (1894a) recognised four families, including the Zonariaceae, and the families Dictyotaceae and Zonariaceae may seem warranted on the basis of apical development of mature thalli. However, juvenile stages of some Zonarieae, such as the "Vaughaniella" stage of Padina, have a single apical cell, making the recognition of two families less satisfactory.

The distinctive tropical genus Dictyotopsis Troll has been shown to be a member of the Dictyotales by Allender (1980), who places it in a separate family, Dictyotopsidaceae, on the basis of the single apical cell segmenting laterally and the monostromatic thallus; its reproduction is still unknown.

A further genus and family (the Scoresbyellaceae) is here assigned to the Dictyotales. The habit (very similar to a broad Dictyota or Dilophus), fairly regular transverse section structure with a relatively thin thallus (4–6 cells thick), the tufts of surface hairs, the typical male sori, and the single apical cell all point to the Dictyotales. However, the apical cell is orientated lengthwise to the thallus and segments on both sides, in contrast to the transverse orientation with posterior segmentation in the Dictyoteae, and this provides a distinctive difference from the latter group.

The Dictyotales are particularly well represented in the temperate southern Australian region, with 15 genera and 40 species of a world total of about 18 genera and about 100 species. Although often regarded (e.g. Bold & Wynne 1985, p. 336) as best developed in tropical and subtropical seas, the order is richer in southern Australia than any warmer region.

References:

AGARDH, J.G. (1894a). Analecta Algologica. Cont. I. Acta Univ. lund. 29, 1–144, Plates 1, 2.

ALLENDER, B.M. (1980). Dictyopsis propagulifera (Phaeophyta)-an algal enigma. Phycologia 19, 234–236.

BOLD, H.C. & WYNNE, M.J. (1985). Introduction to the Algae: Structure and reproduction. 2nd Edn. (Prentice-Hall: New Jersey.)

GAILLARD, J. (1972). Quelques remarques sur le cycle reproducteur des Dictyotales et sur ses variations. Soc. Bot. Fr. Mém. 1972, 145–150.

LIDDLE, L.B. (1972). Development of gametophyte and sporophyte populations of Padina sanctae-crucis Borg. in the field and laboratory. Proc. Int. Seaweed Symp. 7, 80–82.

MATHIESON, A.C. (1966). Morphological studies on the marine brown alga Taonia lennebackerae Farlow ex J. Agardh. I. Sporophytes, abnormal gametophytes and vegetative reproduction. Nova Hedwigia 12, 65–79, Plates 11–14.

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

Author: H.B.S. Womersley

Publication: Womersley, H.B.S. (14 December, 1987)
The Marine Benthic Flora of Southern Australia
Part II
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