Chapter 1 Taxonomy of Seagrasses
17
species, H. australis, occurring along the southern
shores of Australia, and the widespread Indo-West
Pacific species, H. ovalis, extends its area in East
Asia far beyond the tropics. Thalassodendron has
one species with a wide distribution in the tropical
Indo-West-Pacific and a second species with a very
small area of distribution along the warm-temperate
south-western coast of Australia. In all other cases
where tropical species cross the Tropic of Cancer (e.g. in the northern Red Sea, and the Ryukyu
Islands) or the Tropic of Capricorn (Syringodium
isoetifolium along the Australian West coast) warm
currents make this possible. It is striking that the
tropical Atlantic seagrass flora has no genera of its
own, although it is well distinguished at the species
level. The only supraspecific taxon characteristic
for the tropical Atlantic, with one outlying record
for Pacific Central America (den Hartog, 1960), is
Halophila sect. Americanae.
Five genera are mainly distributed along the
world’s temperate shores, viz. Zostera, Phyllospadix,
Heterozostera, Posidonia and Amphibolis. Two of
these genera have their distribution in the northern as well as in the southern hemisphere. Zostera
subgen. Zosterella has representatives in temperate
Australia, New Zealand, southern Africa, as well as
in eastern Asia and along the Atlantic and Mediterranean coasts of Europe, and northern Africa. Z.
noltii is the only species occurring in the continental
relict seas of western Asia (Caspian Sea and Aral
Sea). It is striking that up to very recently this subgenus has been absent from the Atlantic as well as
the Pacific coasts of the Americas. Zostera subgen.
Zostera is restricted to the northern Pacific and the
northern Atlantic, and also occurs in the Mediterranean and the Black Sea. Posidonia is the second
genus with a bipolar distribution, but in contrast to
Zostera the northern hemisphere distribution is restricted to one species in the Mediterranean, and in
the southern hemisphere it occurs with eight species
in Australia. Heterozostera and Amphibolis are restricted to temperate Australia, apart from a small
area occupied by the endemic H. chilensis in Chile.
Only some species of Zostera subgen. Zosterella
penetrate here and there within the tropics.
The Arctic Sea has no species of its own, although
the cold temperate species Zostera marina crosses
the Arctic Circle in Europe as well in the northern
Pacific. Antarctica is devoid of seagrasses.
In spite of shipping, fishing, culturing marine organisms, and other human activities over the centuries, the areas of distribution of the seagrasses
are still reasonably intact. The only changes which
are of an anthropogenous nature relate to Z. japonica which has been accidentally introduced in
Pacific North America and is still expanding its area
(Harrison, 1976; Bigley and Barreca, 1982), and
Halophila stipulacea which has passed through the
Suez Canal and has conquered the eastern Mediterranean. Of course, within the known areas large
changes in abundance of species have taken place,
of which many are indicated in the atlas of Green
and Short (2003).
In the reviews by Larkum and den Hartog (1989)
and Kuo and den Hartog (2000) it is already stated
that the origin of the seagrasses is still very unclear.
Fossil material is extremely rare. Most of the fossils thought to be seagrass appeared not to be seagrass at all. The original material of Archeozostera,
described from the Cretaceous of Japan, appeared
after a profound study by Kuo et al. (1989) not to
be a seagrass and probably not even a plant. There
was already some doubt about Thalassocharis, described from the Cretaceous of Westfalen (Germany)
and Maastricht (The Netherlands), as the stems had
no seagrass morphology; the absence of air lacunae
leads to the conclusion that it is not an aquatic plant.
The only seagrass genus of which the Cretaceous
origin can be confirmed appears to be Posidonia.
P. cretacea Hos. et v. d. Mark, described from Germany, is not very well known, but recently a very fine
specimen, consisting of a bundle of leaf-sheaths fully
comparable with the ‘shaving brush’ of the mediterranean P. oceanica, has been collected from the
Cretaceous of Maastricht.
Other seagrass fossils are known from the Eocene
of the Basin of Paris; these have been assigned
to Posidonia parisiensis (Brongt.) Fritel, Cymodocea serrulata and C. nodosa (den Hartog. 1970;
Larkum and den Hartog, 1989). An other set of
Eocene seagrass fossils has been collected from
the Avon Park formation in Florida, viz. Thalassodendron auricula-leporis den Hartog, Cymodocea
floridana den Hartog and Thalassia testudinum
(Lumbert et al. 1984). It is remarkable that most of
these Eocene fossils can be identified as, or are morphologically very similar to, presently still extant
species. C. floridana shows a striking resemblance
to C. angustata, which has a very small area of distribution in northwestern Australia (Kuo and den
Hartog, 2000). The Eocenous Thalassodendron
species is rather different from the modern species,
17
species, H. australis, occurring along the southern
shores of Australia, and the widespread Indo-West
Pacific species, H. ovalis, extends its area in East
Asia far beyond the tropics. Thalassodendron has
one species with a wide distribution in the tropical
Indo-West-Pacific and a second species with a very
small area of distribution along the warm-temperate
south-western coast of Australia. In all other cases
where tropical species cross the Tropic of Cancer (e.g. in the northern Red Sea, and the Ryukyu
Islands) or the Tropic of Capricorn (Syringodium
isoetifolium along the Australian West coast) warm
currents make this possible. It is striking that the
tropical Atlantic seagrass flora has no genera of its
own, although it is well distinguished at the species
level. The only supraspecific taxon characteristic
for the tropical Atlantic, with one outlying record
for Pacific Central America (den Hartog, 1960), is
Halophila sect. Americanae.
Five genera are mainly distributed along the
world’s temperate shores, viz. Zostera, Phyllospadix,
Heterozostera, Posidonia and Amphibolis. Two of
these genera have their distribution in the northern as well as in the southern hemisphere. Zostera
subgen. Zosterella has representatives in temperate
Australia, New Zealand, southern Africa, as well as
in eastern Asia and along the Atlantic and Mediterranean coasts of Europe, and northern Africa. Z.
noltii is the only species occurring in the continental
relict seas of western Asia (Caspian Sea and Aral
Sea). It is striking that up to very recently this subgenus has been absent from the Atlantic as well as
the Pacific coasts of the Americas. Zostera subgen.
Zostera is restricted to the northern Pacific and the
northern Atlantic, and also occurs in the Mediterranean and the Black Sea. Posidonia is the second
genus with a bipolar distribution, but in contrast to
Zostera the northern hemisphere distribution is restricted to one species in the Mediterranean, and in
the southern hemisphere it occurs with eight species
in Australia. Heterozostera and Amphibolis are restricted to temperate Australia, apart from a small
area occupied by the endemic H. chilensis in Chile.
Only some species of Zostera subgen. Zosterella
penetrate here and there within the tropics.
The Arctic Sea has no species of its own, although
the cold temperate species Zostera marina crosses
the Arctic Circle in Europe as well in the northern
Pacific. Antarctica is devoid of seagrasses.
In spite of shipping, fishing, culturing marine organisms, and other human activities over the centuries, the areas of distribution of the seagrasses
are still reasonably intact. The only changes which
are of an anthropogenous nature relate to Z. japonica which has been accidentally introduced in
Pacific North America and is still expanding its area
(Harrison, 1976; Bigley and Barreca, 1982), and
Halophila stipulacea which has passed through the
Suez Canal and has conquered the eastern Mediterranean. Of course, within the known areas large
changes in abundance of species have taken place,
of which many are indicated in the atlas of Green
and Short (2003).
In the reviews by Larkum and den Hartog (1989)
and Kuo and den Hartog (2000) it is already stated
that the origin of the seagrasses is still very unclear.
Fossil material is extremely rare. Most of the fossils thought to be seagrass appeared not to be seagrass at all. The original material of Archeozostera,
described from the Cretaceous of Japan, appeared
after a profound study by Kuo et al. (1989) not to
be a seagrass and probably not even a plant. There
was already some doubt about Thalassocharis, described from the Cretaceous of Westfalen (Germany)
and Maastricht (The Netherlands), as the stems had
no seagrass morphology; the absence of air lacunae
leads to the conclusion that it is not an aquatic plant.
The only seagrass genus of which the Cretaceous
origin can be confirmed appears to be Posidonia.
P. cretacea Hos. et v. d. Mark, described from Germany, is not very well known, but recently a very fine
specimen, consisting of a bundle of leaf-sheaths fully
comparable with the ‘shaving brush’ of the mediterranean P. oceanica, has been collected from the
Cretaceous of Maastricht.
Other seagrass fossils are known from the Eocene
of the Basin of Paris; these have been assigned
to Posidonia parisiensis (Brongt.) Fritel, Cymodocea serrulata and C. nodosa (den Hartog. 1970;
Larkum and den Hartog, 1989). An other set of
Eocene seagrass fossils has been collected from
the Avon Park formation in Florida, viz. Thalassodendron auricula-leporis den Hartog, Cymodocea
floridana den Hartog and Thalassia testudinum
(Lumbert et al. 1984). It is remarkable that most of
these Eocene fossils can be identified as, or are morphologically very similar to, presently still extant
species. C. floridana shows a striking resemblance
to C. angustata, which has a very small area of distribution in northwestern Australia (Kuo and den
Hartog, 2000). The Eocenous Thalassodendron
species is rather different from the modern species,