1a. Rhizome monopodial, herbaceous, with a short erect stem at each node. Leaf-sheath persisting longer than the leaf-blade. Anthers stalked. . . 2 1b. Rhizome sympodial, ligneous, with elongate, more or less branched, erect stems at certain nodes. Leaf-blade shed with its sheath. Anthers subsessile 4
2a. Leaves flat. Flowers solitary 3
2b. Leaves subulate. 'Flowers' arranged into a cy-
mose inflorescence Syringodium
3 a. Nerves 3. Anthers not attached at the same height on the peduncle. Ovary with one undivided style Halodule
3b. Nerves 7-17. Anthers attached at the same height on the peduncle. Style divided into two stigmata Cymodocea
4a. Rhizome with two unbranched or little branched stems at every fourth rhizome node; roots 15 on the node preceding the stem-bearing one. Leaves with parallel nerves and denticulate apex. 'Flowers' enclosed by four leafy bracts. Anthers connate over their entire length, each crowned with one appendage. Style with two stigmata. False fruit composed of 1 or 2 fertilized ovaries surrounded by the enlarged inner bract Thalassodendron
4b. Rhizome with 1-2 branching roots at each node and at a distance of (1-) 4-8 nodes one profusely branched stem. Leaves entire, with pseudoparallel nerves, and a bidentate apex. 'Flowers' enclosed by normal leaves. Anthers connate with their lower parts, each crowned with 2-3 appendages. Style with three stigmata. 'Fruit' consisting of one fertilized ovary with four pectinate, spreading lobes arising just above its base; viviparous Amphibolis
Type species: Diplanthera tridentata Steinheil (= H. uninervis (Forssk.) Ascherson).
The typification of Halodule has been complicate. Du Petit Thouars (1806) was the first to describe the genus from Madagascar under the name Diplan-thera, but he unfortunately did not add a species name to his material. Steinheil (1838) described the material as D. tridentata. Steudel (1840) referred to the same material and named it, without a description, D. madagascariensis. He recognized a second species, D. indica that turned out to be Halophila ovalis. From Steudel's work it becomes also obvious, that the genus name Diplanthera has been used also for other genera in very different families. Endlicher (1841) referred to the material of Du Petit Thouars (1806) and the work of Steinheil (1838) as he founded the genus Halodule, but regrettably he failed also to transfer the species name to the new genus. Miquel (1855) was the first to describe a species in the genus as H. australis, for material from Indonesia, similar to Steinheil's species; therefore, the new epithet was superfluous. It took till 1882, before this material was properly named.
The genus consists of seven species. The main characters used for the identification of the species are the shape of the leaf tip and the width of the leaves (den Hartog, 1970). There are too few data of the generative structures, and the degree ofvariation of these is not yet clear; at present they cannot be used to identify the taxa. Moreover, they are not known for several taxa.
H. uninervis commonly occurs in the tropical Indo-West Pacific with a narrow- and a wide-leaved form. It is possible that these two forms represent different taxa; in that case the name H. unin-ervis is linked to the wide-leaved form, while the narrow-leaved form should be named H. tridentata (Steinheil) F. von Mueller. In the West Pacific a second species, H. pinifolia, occurs as well; in the
Indian Ocean it is less common and restricted to India. In the Caribbean H. wrightii is widely distributed from Cuba and the smaller Antilles; along the coast of South America it crosses the mouth of the Amazon River, and extends southward along the coast of Brazil (De Oliveira et al., 1983). Other localities of this species are on the Atlantic coast of Africa, e.g. in Mauritania, Senegal, and Angola; it is expected that in future more locations will be discovered. It is remarkable that populations satisfying the diagnosis of this species have been found in Kenya. In southern Brazil the species is replaced by H. emarginata. Finally H. beaudettei occurs in the Gulf of Mexico, and less frequently in the Caribbean. It extends along the Atlantic coast of the USA northward up to North Carolina. This species has been found also along the Pacific coast, from Panama up to Mexico, where it reaches its northernmost locations in the Sea of Cortes. In the USA H. beaudettei is traditionally referred to as H. wrightii, but it relates in no way with the true H. wrightii which has been described after material from Cuba. The remaining two species are only known from one collection, H. ciliata from Pacific Panama, and H. bermudensis from the Bermuda Islands.
Although the identification characters show some variability at present no other means for identification are available. Studies of chromosome numbers, isozymes, and molecular analyses may be helpful to establish definitively the validity of the present species. The only chromosome count available, as far is known, suggests that possibly polyploidy is involved (den Hartog et al., 1979). McMillan (1980, 1982) found differences in the isozyme composition of East African and Texan Halodule populations, but provided no morphological descriptions of the used material.
Cymodocea Konig in Konig & Sims, Ann. Bot. 2 (1805) 96; nom. cons.
Type species: Cymodocea aequorea Konig (= C. nodosa (Ucria) Ascherson)
The genus, consisting of four species, has a mainly tropical distribution. C. rotundata and C. serrulata have a more or less similar pattern of distribution along the coasts of the tropical Indo-West Pacific. C. nodosa occurs in the subtropical Mediterranean and extends its area in the Atlantic northward to Portugal and southward to Senegal; further it is common on the Canary Islands. The fourth species, C. angustata, is endemic to the north-western part of Australia.
The distribution of this genus has been much wider in the past. In the Avon Park formation, a late Middle Eocene deposit in Florida, well preserved remains of seagrasses were found; among them were two species of Cymodocea (Lumbert et al., 1984). C. floridana differs from the present-day species, but is close to C. angustata. The second species appears to be preserved less completely; its leaf-blade is rather similar to that of C. nodosa. Fossil fruits have been found often in various deposits along the Mediterranean (Ruggieri, 1952). Another species, C. mich-eloti, has been recorded from the Miocene of Sulawesi (Celebes), Indonesia (Laurent and Laurent, 1926). This species is identical with the present-day C. serrulata.
Syringodium Kiitzing in Hohenacker, "Meeralgen" (Algae Marinae Exsiccatae) 9 (1860) no. 426 Type species: Syringodium filiforme Kutz.
This genus contains two species. S. isoetifolium is widely distributed in the coastal waters of the Indian Ocean and the western Pacific; along the west coast of Australia it penetrates far into the temperate zone and occurs even south of Perth. S. filiforme is restricted to the Gulf of Mexico and the Caribbean, and it has been recorded from Bermuda.
Thalassodendron den Hartog, Sea-grasses of the World (1970) 186
Type species: Zostera ciliata Forsskal (= Thalasso-dendron ciliatum (Forssk.) den Hartog)
This genus contains two species. T. ciliatum is widely distributed in the tropical Indo-West Pacific. In the Indian Ocean it is dominant on the reefs of the coasts of East Africa, many of the oceanic islands, and along the Red Sea. It does hardly occur along the northern coasts, as it absent from Pakistan, India, Sri Lanka, and BanglaDesh. In the western Pacific its occurrence is scattered, the northernmost locality being in the South Chinese Sea, the southernmost in Queensland. The other species, T. pachyrhizum, has a small area of distribution in the temperate south-western part of Western Australia, where it occurs on exposed reefs in the open ocean.
A fossil species, T. auricula-leporis has been described from the Middle Eocene Avon Park formation, Florida (Lumbert et al., 1984); its rhizome, however, does not show the regular features of the still living representatives of the genus; probably does it belong to an extinct genus of the Cymod-oceaceae.
Amphibolis C. Agardh, Spec. Alg. 1 (1823) 474 Type species: Amphibolis zosteraefolia C. Agardh (= A. antarctica (Labill.) Sonder et Ascherson.
According to Ducker et al. (1977, p. 68) the genus Amphibolis was erected by C. Agardh (1823, p. 474) as a green alga with two species, A. bicornis (1823, p 474), and A. zosteraefolia, (1823, p. 475) from New Holland. Later, Agardh (1824, p. 192) recognized A. zosteraefolia as synonymous with Ruppia antarctica Labill., which was originally described by Labillardiere (1806) as a flowering plant from the shore of Western Australia. Ducker et al. (1977) selected A. zosteraefolia instead of A. bicornis as the type of the Amphibolis, because, according to them, the type specimen of A. zosteraefolia has the unique grappling apparatus of which Agardh made the first description.
This genus is fully restricted to the temperate south and west coasts of Australia. It consists of two species. A. antarctica is distributed from Victoria and Tasmania in the east along the whole southern and western coast of Australia north to Carnarvon. The area of distribution of A. griffithii is more restricted; it occurs from Spencer Gulf in South Australia to the Geraldton area in Western Australia.
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