Dictyota (PROSEA)

Plant Resources of South-East Asia Introduction

List of species

Dictyota cervicornis - 1, habit of apical portion of a thallus with truncate apices; 2, section through a young tetrasporangium with an one-celled stalk cell and an involucrum. D. friabilis - 3, habit of apical portion of a thallus with rounded apices. D. dichotoma - 4, habit of apical portion of a thallus with acute apices; 5, cross-section of a branch with one layer of large medulla cells surrounded by one layer of cortex cells. D. crispata - 6, habit of an apical portion of a thallus with apiculate apices; 7, habit. D. bartayresiana - 8, section through a tetrasporangium with 4 tetraspores, an one-celled stalk cell and no involucrum. D. ciliolata - 9, section of an antheridial sorus surrounded by paraphyses; 10, section of an oogonial sorus.

Dictyota J.V. Lamour.

Protologue: J. Bot. (Desvaux) 2: 38 (1809).

Family: Dictyotaceae

Chromosome number: 2n= 18, 24, 32, 48

Major species and synonyms

• Dictyota bartayresiana J.V. Lamour., J. Bot. (Desvaux) 2: 43 (1809), synonyms: Dictyota bartayresii J.V. Lamour. (1809), nom. illeg., D. patens J. Agardh (1882), D. neglecta Hörnig & Schnetter (1992), non Dictyota bartayresiana auct. non J.V. Lamour. sensu Vickers (1908).

• Dictyota cervicornis Kütz., Tab. phycol. 9: 8, pl. 17, fig. 1 (1859), synonyms: Dictyota indica Sond. ex Kütz. (1859), D. pardalis Kütz. (1859). Accepted name is now: Canistrocarpus cervicornis (Kützing) De Paula & De Clerck in De Clerck et al. (2006).

• Dictyota ceylanica Kütz., Tab. phycol. 9: 11, pl. 25, fig. 1 (1859).

• Dictyota ciliolata Sond. ex Kütz., Tab. phycol. 9: 12, pl. 27, fig. 1 (1859), synonyms: Dictyota ciliata J. Agardh (1841), D. beccariana Zanardini (1872).

• Dictyota crispata J.V. Lamour., J. Bot. (Desvaux) 2: 44 (1809), synonyms: Dictyota bartayresiana auct. non J.V. Lamour. sensu Vickers (1908), D. apiculata auct. non J. Agardh sensu Weber Bosse (1913), D. dentata auct. non J.V. Lamour. sensu Vannajan & Trono (1978), D. mertensii auct. non (Mart.) Kütz. sensu Trono (1997). Accepted name is now: Canistrocarpus crispatus (J.V.Lamouroux) De Paula & De Clerck in De Clerck et al. 2006.

• Dictyota dichotoma (Huds.) J.V. Lamour. var. dichotoma, J. Bot. (Desvaux) 2: 38 (1809), synonyms: Dictyota acuta Kütz. (1845), D. volubilis Kütz. (1849).

• Dictyota dichotoma (Huds.) J.V. Lamour. var. intricata (C. Agardh) Grev., Alg. brit.: 58 (1830), synonyms: Dictyota divaricata J.V. Lamour. (1809), D. dichotoma (Huds.) J.V. Lamour. var. implexa (Desf.) Gray (1821), D. cirrhosa Suhr (1839).

• Dictyota friabilis Setch., Univ. Calif. Publ. Bot. 12: 91-92 (1926), synonym: Dictyota ceylanica Kütz. var. rotundata Weber Bosse (1913).

Origin and geographic distribution

Dictyota is represented in most regions of the world, except around the poles. Species diversity is greater in the tropics than in temperate waters. The highest diversity of Dictyota is found in Australia. Approximately 40 different species names are recorded from the Indian Ocean, which however, can be reduced to 23 accepted species. Of those mentioned above, D. dichotoma is most often recorded as occurring on almost all coasts of South-East Asia, often together with its variety intricata. Because of taxonomic confusion, however, these distributional data (and those for D. bartayresiana) are not reliable. Most probably neither D. dichotoma var. dichotoma nor var. intricata do occur in South-East Asia.

Uses

The thalli of several Dictyota are edible and used in Indonesia (Sulawesi), Malaysia and Thailand. In the Hawaiian Archipelago, D. acutiloba J. Agardh is cultivated in "algal gardens" and sold on local markets. Dictyota is either eaten raw or cooked with coconut milk, pickled or preserved by smoke-drying and is very nutritious. Some Dictyota are known to have a somewhat bitter taste. Acetone extracts of "Dictyota dichotoma" in India showed synergism with a number of insecticides used to kill mosquito larvae. This synergism may be due to inhibition of some detoxifying enzymes in mosquito larvae by the algal extract. If this acetone extract can be combined with synthetic insecticides, it will result in reduced consumption of the latter, thus reducing aquatic pollution.

Properties

Like most brown algae Dictyota contains alginic acid, an acid membrane mucilage with emulsifying, gelling and thickening properties which is very important on a world scale in a wide variety of industries. D. bartayresiana is known to contain a very high percentage of alginic acid: 17-18%. Unfortunately a high alginic acid content is not the only prerequisite for industrial utilization; the size and density of the crops are extremely important as well. Because of its relatively small size Dictyota is less attractive for commercial exploitation. The chemical composition of a Dictyota sp. identified as "D. divaricata" in the Arabian Gulf showed a very low protein content compared to similar studies of Dictyota spp. in different Indian Ocean localities: 1.7% of dry weight, compared to 19.8% in India. This may be due to the high salinity of the Arabian Gulf waters. D. dichotoma possesses a high folic acid content. Different experiments have proven that lipid-soluble extracts of several Dictyota possess antibiotic and antiviral substances (terpenes). The amount of these substances may vary from place to place and may depend on the season. D. crenulata J. Agardh was found to be active against a form of lymphocytic leukaemia and tumours. Other Dictyota, among which D. dichotoma, inhibit the growth of several human pathogenic and phytopathogenic fungi.

Description

• Thalli composed of dichotomously to irregularly branched straps, each with single lens-shaped apical cell; attachment by rhizoids or stolonoidal fibres.
• Branching by equal to unequal division of apical cell.
• One to several layers of large, thick-walled, medullary cells surrounded by single layer of small, pigmented cortical cells, iridescent or not - in dried specimens iridescence lost.
• Sori of hairs present or absent.
• Life cycle diplo-haplontic and isomorphic.
• Sporophyte forming unilocular tetrasporangia each containing 4 non-motile tetraspores; tetrasporangia grouped in sori or scattered over whole or parts of fronds.
• Gametophytes dioecious; oogonia and antheridia surrounded by one to several rows of paraphyses, grouped in sori.

D. bartayresiana.

• Thalli erect, forming hemispherical tufts, 8-20 cm tall, attached by numerous patches of rhizoids, straw-coloured, never iridescent, texture somewhat crisp; individual straps up to 10-15 cm long, 5.7-7.3 mm wide, regularly dichotomously branched, branching angle 70-80°, interdichotomies short; margins smooth; surface proliferations absent; hair tufts common.
• Tetrasporangia scattered on both surfaces of straps, single, not surrounded by a ring of enlarged, sterile cells (involucrum).
• Gametangia unknown.

D. cervicornis.

• Growth form very variable; thalli 10-15(-30) cm long, with or without well-developed base, attached by rhizoids, medium brown; straps of whole thallus somewhat coarse, often with strongly elongated, linear interdichotomies in erect parts, average width 2.3-3.1 mm; apices truncate to rounded; branching regular dichotomous in lower and middle parts of thalli (branching angle 60-70°), apical parts typically cervicorn (branching angle 70-100°) or with recurved branches; margins smooth, sinuously curved; surface proliferations absent; hair tufts common.
• Tetrasporangia scattered over both surfaces of straps, not divided, mostly single, never in apical segments, surrounded by involucrum.
• Gametangia unknown.

D. ceylanica.

• Growth form ascending, with very intricate appearance; thalli small, 4(-6) cm long, without conspicuous base, attached by rhizoids, medium to pale brown, blue to yellowish iridescent; straps procumbent, repent or erect, mostly filiform but extremely variable in width, (0.2-)0.8-1.3(-5.8) mm broad; apices acute or rounded; branching mainly regularly dichotomous, branching angle broad divaricate (70-100°); margins smooth; surface proliferations absent, marginal proliferations common and obscuring the original branching pattern; hair tufts common.
• Tetrasporangia scattered over both surfaces of fronds, often forming a narrow, longitudinal line in the middle of the straps; sporangia rarely divided and not surrounded by involucrum.
• Only oogonial gametangia sori known, scattered on both surfaces of the straps, with a variable number of oogonia (12-58) per sorus.

D. ciliolata.

• Growth form completely erect, 8-15(-25) cm long, with conspicuous stupose holdfast giving rise to single frond of somewhat crisp texture, brown, often with broad transverse banding pattern of slightly iridescent zones alternating with non-iridescent zones; straps in whole thallus of same width, (1.5-)3.7-9.1(-19.5) mm, width and height being highly variable between different specimens; apices regularly dichotomous or slightly irregularly dichotomous, never alternate, branching angle 35-50°; margins usually dentate, teeth slightly to prominently directed towards the apices, giving rise to marginal proliferations obscuring original branching pattern; surface proliferations absent; hair tufts common.
• Tetrasporangia scattered over both surfaces of fronds, sporangia without involucrum, divided sporangia frequently observed.
• Gametangia occasionally observed, evenly distributed over whole frond, also present in the apical segments; female gametangial sori with about 20 oogonia per sorus; male gametangial sori with about 25 oogonia per sorus, surrounded by 3-4 rows of unicellular paraphyses.

D. crispata.

• Growth form ascending, 9-20(-30) cm long, with small prostrate base giving rise to several stiff, crisp, erect straps, harsh to the touch, brown to pale brown, not iridescent; average width between different specimens extremely variable, (1.0-)4.9-6.4(-22) mm, with anisotomous dichotomous branching, branching angle 30-70°; margins smooth; surface proliferations common to abundant, marginal proliferations absent.
• Tetrasporangia scattered on both surfaces of straps, single or grouped in small, longitudinal sori often placed in apical segments, involucrum present, sporangia not divided.
• Both oogonia and antheridia (surrounded by 2-3 rows of pigmented paraphyses) in sori evenly distributed over whole surface of straps.

D. dichotoma var. dichotoma.

• Extremely polymorphic, very difficult to characterize in a single description.
• Thalli erect, 10-35 cm tall, sometimes spirally twisted, medium brown; main straps 4-10 mm broad, usually with rounded apices; branching regularly dichotomous; margins smooth; proliferations on surface absent; hair tufts common.
• Tetrasporangia scattered over whole surface, except for a narrow, sterile marginal strip.
• Sporangia not surrounded by involucrum. Oogonia and antheridia common in Europe.

D. dichotoma var. intricata.

• Similar to var. dichotoma but with characteristic long and narrow (1.1-1.8 mm broad) interdichotomies, with acute apices, proliferations on surface very common, often growing into new straps of similar shape.
• Tetrasporangia always single, dispersed over both surfaces of fronds, absent from apical segments.

D. friabilis.

• Growth form completely procumbent, forming dense imbricate mats (up to 20-30 cm in diameter), composed of several layers of somewhat brittle (friable) straps, resulting in typical jigsaw aspect; thalli attached by marginal bundles of rhizoids arising from all parts, medium brown, pale bluish iridescent, often with conspicuous non-iridescent longitudinal stripes.
• Individual thalli small (3-5 cm long), straps with typical short and broad interdichotomies, 2.5-6.4(-11.0) mm broad, branching angle 60-110°; apices rounded to obtuse; margins smooth; marginal proliferations rare, surface proliferations absent; hair tufts common.
• Tetrasporangia scattered on upper surface of fronds, single, absent from apical segments, not surrounded by involucrum.
• Gametangia unknown.

Growth and development

Most Dictyota seem to be common all the year round in the tropical seas. Because of the frequent occurrence of tetrasporangia one would expect an equal number of sporo- and gametophytes in a population. However, gametophytes are rarely found. This leads to the assumption that in many cases no meiosis occurs in the tetrasporangia and that the diploid spores develop to new sporophytes.

Other botanical information

A high morphological variability makes identification of species of Dictyota very difficult. There has been persistent taxonomic confusion in both D. bartayresiana and D. dichotoma. Study of the type collections of species described by J.V. Lamouroux has revealed the genuine characters of D. bartayresiana. It is often confused with another pantropical species, which proved to be described by Lamouroux as D. crispata. Both species are common in South-East Asia.

D. dichotoma, the type species of the genus, is without doubt the most frequently reported species in the genus. It has been recorded in nearly every locality where Dictyota spp. occur. A combination of high morphological variability and a lack of distinctive characters makes it difficult to give an accurate and concise description. It has been separated from similar species in the Atlantic Ocean by means of chromosome counts and hybridization experiments. D. dichotona probably does not occur at all in South-East Asia. Specimens named as D. dichotoma var. dichotoma often belong to D. crispata, while specimens named as D. dichotoma var. intricata often belong to D. ceylanica or D. ciliolata.

In this notoriously variable genus misidentifications are very common. Specimens from the Philippines (and possibly also records from Peninsular Malaysia), identified as D. dentata J.V. Lamour. and D. mertensii (Mart.) Kütz., most probably belong to D. crispata.

Ecology

Dictyota is very common in the Indo-Malaysian region. Each species is generally found in its own specific biotope; they are most abundant in shallow lagoons, separating the fringing reef from the coast. Healthy free-floating thalli of D. bartayresiana often thrive well among large seagrasses.

Propagation and planting

There is no phycoculture of Dictyota.

Harvesting

Dictyota is harvested by hand; attached as well as drifting specimens are collected.

Handling after harvest

Dictyota is mainly used fresh or sun-dried.

Prospects

Dictyota is only likely to be used in a restricted way for human food. It might become important as a component of insecticides against mosquito larvae and as a provider of fine chemicals and antibiotics.

Literature

• Abbott, I.A., 1984. Limu. An ethnobotanical study of some Hawaiian seaweeds. 3rd Edition. Pacific Tropical Botanical Gardens, Lawai, Hawaii. 35 pp.
• De Clerck, O., 1999. A revision of the genus Dictyota Lamouroux (Phaeophyta) in the Indian Ocean. Thesis Universiteit Gent, Belgium. 356 pp. + 3 append.
• De Clerck, O. & Coppejans, E., 1997. The genus Dictyota (Dictyotaceae, Phaeophyta) from Indonesia in the Herbarium Weber-van Bosse, including the description of Dictyota canaliculata spec. nov. Blumea 42: 407-420.
• Hörnig, I., Schnetter, R. & Prud'homme van Reine, W.F., 1992. The genus Dictyota (Phaeophyceae) in the North Atlantic. I. A new generic concept and new species. Nova Hedwigia 54: 45-62.
• Subramonia Thangam, T. & Kathiresan, K., 1991. Mosquito larvicidal activity of marine plant extracts with synthetic insecticides. Botanica Marina 34: 537-539.

Sources of illustration

de Clerck, O., 1999. A revision of the genus Dictyota Lamouroux (Phaeophyta) in the Indian Ocean. University of Gent, Department of Biology, Laboratory of Botany, Gent, Belgium. Title page (habit D. crispata); fig. 3.2, p. 41 (shape of apical segments); fig. 3.6, p. 44 (medulla and cortex cells of D. dichotoma var. intricata); fig. 3.7, p. 46 (young tetrasporangium with stalk cell and involucrum of D. cervicornis; tetrasporangium with 4 tetraspores, stalk cell and no involucrum of D. bartayresiana); fig. 3.8, p. 48 (antheridial sorus and oogonial sorus of D. ciliolata). Redrawn and adapted by P. Verheij-Hayes.

Authors

• O. De Clerck, E. Coppejans & W.F. Prud'homme van Reine