Electronic Flora of South Australia Genus Fact Sheet

Genus SARGASSUM C. Agardh 1820: 1, nom. cons.

Phylum Phaeophyta – Order Fucales – Family Sargassaceae

Thallus usually 10 cm-2 m long, with one to a few simple, terete to compressed, stipes 1–20 cm long from a discoid-conical holdfast; a few species free-floating (in the Sargasso Sea). Stipes bearing radially or distichously long primary branches, produced seasonally from the stipe apex and later deciduous, leaving scars or residues on the stipe. Primary branches usually 10 cm-2 m long, distichously, tristichously or radially branched with terete, angular, compressed or three-sided axes; basal laterals simple or branched, compressed and relatively narrow to (in most species) leaf-like, (1–) 3–15 (–25) mm broad, entire or with dentate margins; upper laterals usually branched, with slender, compressed to terete, ramuli. Vesicles normally present, subspherical to ovoid, petiolate, mutic or apiculate, replacing ramuli or axillary to the laterals. Growth from a three-sided apical cell in an apical depression. Structure of a central medulla of elongate cells in the stipe and branch axes, with a cortex of isodiametric cells and a surface phaeoplastic meristoderm, active in larger branches.

Reproduction: Thalli monoecious or dioecious. Receptacles unisexual or bisexual, developed in axils of laterals or ramuli, simple or usually in branched clusters, terete or compressed, smooth, verrucose or spinous, with scattered conceptacles and ostioles, growing apically with conceptacles maturing below; conceptacles bisexual or unisexual.

Type species: S. bacciferum (Turner) C. Agardh, type cons.

Taxonomic notes: A genus of 150–200 species, most prolific in tropical-subtropical regions where the subgenus Sargassum ("Eusargassum") is most common and often the dominant brown alga. In southern Australia this subgenus has only few species, but the subgenera Phyllotrichia and Arthrophycus have their centre of distribution here and are often common or dominant in the uppermost sublittoral region.

Several subgenera were recognised by J. Agardh (1889), of which three occur on southern Australian coasts (Womersley 1954b). Subgenus Phyllotrichia has branched ("pinnatifid") lower laterals; subgenera Arthrophycus and Sargassum have essentially simple, usually leaflike lower laterals, with Arthrophycus having robust, dark brown, lower laterals and a three-sided primary branch axis, while Sargassum usually has thinner, light to medium brown, lower laterals and terete to compressed or angular, but not three-sided, branch axes. However, not all species clearly fit one of these subgenera: S. linearifolium is placed below in subgenus Sargassum even though the lower laterals have occasional branches and they are dark brown and linear rather than leaf-like.

The seasonal development of the primary branches was described for Phyllotrichia by Womersley (1954b) and applies (with variations in time of year) to species of the other subgenera. In late summer and early winter, only young and short primary branches usually occur, with only the larger basal leaves in Arthrophycus and Sargassum and the branched (usually pinnate) ones in Phyllotrichia. By mid winter, these primary branches extend, with narrower, often filiform," laterals, and by spring vesicles and receptacles are being produced. The receptacles of many species elongate considerably as they develop, and may be much larger later in the year than when first formed and fertile; spines on the upper receptacle may be a later development in some species. The fully fertile primary branches are usually lost by mid to late summer, leaving only a scar or blunt residue on the stipe; by then, young primary branches which will mature in the following spring are present. In some species of Phyllotrichia (e.g. S. decipiens and S. sonderi) the relatively long stipes with numerous, bilateral, branch residues, with production of 2–3 new primary branches each year, permit an approximation of the age of a plant; between 5 and 15 years is not uncommon.

Only mature fertile plants, with basal parts, are adequate for determination; thus collections should only be made in spring and early summer for most species.

This account recognises only 5 species of Arthrophycus and 5 of subgenus Sargassum on southern Australian coasts. There are several further species on the west coast of Western Australia and on the N.S.W. coast, especially of subgenus Sargassum, and detailed accounts of these subgenera on western and eastern coasts of Australia are urgently needed.

One of the few marine algae that is a "weed" is S. muticum (Yendo) Fensholt, which has been introduced from Japan to the Pacific coast of North America via oysters, and later to the south coast of England, where it has spread extensively (Critchley et al. 1983).

References:

AGARDH, C.A. (1820). Species Algarum. Vol. 1, Part 1, Fucoideae, pp. 1–168. (Lund.)

AGARDH, J.G. (1889). Species Sargassorum Australiae. K. Svenska Vetenskapsakad. Handl. 23(3), 1–133, Plates 1–31.

CRITCHLEY, A.T., FARNHAM, W.F. & MORRELL, S.L. (1983). A chronology of new European sites of attachment for the invasive brown alga, Sargassum muticum, 1973–1981. J. mar. biol. Ass. U.K. 63, 799–811.

WOMERSLEY, H.B.S. (1954b). Australian species of Sargassum, subgenus Phyllotrichia. Aust. J. Bot. 2, 337–354, Plates 1–5.

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
©Board of the Botanic Gardens and State Herbarium, Government of South Australia

KEY TO SPECIES OF SARGASSUM

1. Lower laterals on primary branches branched, usually pinnately but in some species only once or twice branched, with branchlets linear to tapering, usually not broad and leaf-like; axes of primary branches compressed or angular to terete; receptacles usually simple but in racemose clusters

3 (Subgen. PHYLLOTRICHIA)

1. Lower laterals on primary branches simple (rarely once branched), leaf-like, usually broadest centrally and tapering to base and apex; axes of primary branches 3-sided or angular to terete; receptacles simple, or branched and clustered

2

2. Lower laterals leaf-like, usually dark brown, markedly larger than upper laterals of fertile fronds; branch axes 3-sided (triquetrous)

8 (Subgen. ARTHROPHYCUS)

2. Lower laterals usually leaf-like but slender in some species (rarely linear with a single branch), usually light to medium brown, markedly to only slightly larger than upper laterals; branch axes angular to terete, not 3-sided

13 (Subgen. SARGASSUM)

3. Mature primary branches distichous, branch axes flat and winged, 5–10 mm broad, tapering evenly to narrow apices

S. decurrens

3. Mature primary branches basally distichous but not above, branch axes terete or compressed but usually less than 5 mm broad (lower segments of S. heteromorphum up to 1 cm broad)

4

4. Lower laterals not retroflex from primary branches; vesicles 1–3 mm in diameter

5

4. Lower laterals retroflex from primary branches; vesicles 4–10 mm in diameter

7

5. Lower parts of primary branches 5–10 mm broad, distinct from the upper narrow branchlets; stipe terete, with radial primary branches

S. heteromorphum

5. Lower parts of primary branches 1–5 mm broad, grading to the upper narrow branchlets; stipe compressed, with distichous primary branches

6

6. Lower laterals of primary branches 1–3 cm long and 1–2 mm broad, simple or branched, not or only slightly distinct from the upper laterals

S. decipiens

6. Lower laterals of primary branches 3–6 cm long and 2–5 mm broad, deeply pinnate

S. sonderi

7. Stipe compressed with distichous primary branches, mature lower laterals markedly retroflex from the compressed axes of the main branches

S. varians

7. Stipe terete with radial primary branches, lower laterals retroflex, old branch axes usually flexuose

S. verruculosum

8. Receptacles terete (or becoming slightly compressed), without spines

9

8. Receptacles compressed (at least above) or triquetrous, with spines

10

9. Lower laterals undulate, usually spinous on margins, upper laterals mostly 5–10 mm broad; receptacles (3–) 5–10 (–15) mm long and 700–1500 µm in diameter, in open racemes

S. paradoxum (male)

9. Lower laterals more or less flat and smooth, usually with entire margins; upper laterals terete to slightly compressed, 0.3–1 mm broad; receptacles 1–2 (–4) mm long and 300–900 µm in diameter, terete or with sterile awns, in (usually) dense clusters in axils of upper branchlets

S. fallax

10. Receptacles compressed (often terete when young) or slightly triquetrous above, with irregular spines especially on their upper part

11

10. Receptacles distinctly triquetrous, with prominent and fairly regular spines along the edges

12

11. Lower laterals undulate, usually spinous on margins; receptacles in open racemes, (3–) 5–10 (–15) mm long and 700–1500 µm in diameter, terete below, compressed to slightly triquetrous above with occasional spines

S. paradoxum (female)

11. Lower laterals smooth, margins entire; receptacles (1–) 2–3 (–5) mm long, in dense clusters, compressed with spines on their upper parts

S. vestitum

12. Laterals usually entire and smooth

S. tristichum

12. Laterals deeply incised, especially the lower ones

S. lacerifolium

13. Lower laterals of main branches linear, simple or furcate, dark brown, usually entire; receptacles in dense, much branched, axillary clusters

S. linearifolium

13. Lower laterals of main branches simple, lanceolate, basally attenuate, light-brown, with a serrate margin; receptacles simple or (usually) in branched axillary clusters

14

14. Receptacles terete, verrucose, without spines

S. spinuligerum

14. Receptacles terete or compressed, with spines or a dentate wing

15

15. Receptacles terete, without wings

S. podacanthum

15. Receptacles compressed, with a dentate wing around the swollen, central region

S. distichum


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