198
C. den Hartog and R.C. Phillips
The broad occurrence of seagrasses over a wide range of latitudes and in a
diversity of habitats suggests genetic differentiation among the species populations (McMillan and Phillips 1979). Laboratory and field experimental
results imply that local habitat conditions have had a selective influence.
In the temperate regions of the southern hemisphere, Zostera species are
also prominent in some areas. In South Africa, Z. capensis forms dense beds in
suitable locations in the intertidal zone, but it is also able to occupy the upper
part of the sublittoral zone, possibly because there is no counterpart for
Z. marina. Sublittoral morphs of Z. capensis are considerably coarser than
those from the intertidal. The intertidal morph of this species extends along
the east coast of Africa up to the equator. Its beds do not intermix with those
of the tropical species.
Along the temperate coasts of Australia the situation is more complex.
There are three species of Zostera, each with its own area of distribution.
Z. mucronata is confined to the southwestern part of Australia. Z. muellerii is
confined to the southeastern part, while Z. capricorni is confined to the
eastern part of Australia. Only the latter species is able to establish beds in the
sublittoral zone where its morphs are very robust. The other two species are
mainly restricted to the intertidal zone. Z. muellerii and Z. mucronata can
border on beds of Heterozostera tasmanica, and even intermingle with them
around the low-water mark. However, the sublittoral seagrass beds in temperate Australia are not composed of Zostera species, but of species of Posidonia and Amphibolis. These communities show a considerably higher diversity in species than do those of Zostera, and they are structurally more
complex. In disturbed places in the sublittoral, H. tasmanica and Halophila
ovalis may dominate.
In New Zealand the seagrass vegetation is not as developed as in Australia.
There is one species, Z. novozelandica, which occurs in the intertidal and
upper sublittoral zones. Z. capricorni is present, but only in the extreme
north.
The temperate west coast of South America is almost devoid of seagrasses.
Three beds of H. tasmanica have been found near Coquimbo, Chile. No
seagrass beds have been found from the temperate east coast of South
America. There is a record of a Zostera leaf washed ashore near Montevideo,
Uruguay (SetchellI935).
In the tropical waters, the place of Zostera is taken by species of other
genera. In the lower littoral zone, there are narrow-leaved morphs of species
of Halodule, and here and there tiny species of Halophila. In the sublittoral,
dense beds of species of Cymodocea, Syringodium and Thalassia occur.
In the Caribbean and the Gulf of Mexico, beds of Halodule wrightii or H.
beaudettei occur near the low-water mark bordering on sublittoral beds of
Thalassia testudinum. Syringodium filiforme may be interspersed in the
Thalassia bed, but it may also form stands of its own. The same holds for the
C. den Hartog and R.C. Phillips
The broad occurrence of seagrasses over a wide range of latitudes and in a
diversity of habitats suggests genetic differentiation among the species populations (McMillan and Phillips 1979). Laboratory and field experimental
results imply that local habitat conditions have had a selective influence.
In the temperate regions of the southern hemisphere, Zostera species are
also prominent in some areas. In South Africa, Z. capensis forms dense beds in
suitable locations in the intertidal zone, but it is also able to occupy the upper
part of the sublittoral zone, possibly because there is no counterpart for
Z. marina. Sublittoral morphs of Z. capensis are considerably coarser than
those from the intertidal. The intertidal morph of this species extends along
the east coast of Africa up to the equator. Its beds do not intermix with those
of the tropical species.
Along the temperate coasts of Australia the situation is more complex.
There are three species of Zostera, each with its own area of distribution.
Z. mucronata is confined to the southwestern part of Australia. Z. muellerii is
confined to the southeastern part, while Z. capricorni is confined to the
eastern part of Australia. Only the latter species is able to establish beds in the
sublittoral zone where its morphs are very robust. The other two species are
mainly restricted to the intertidal zone. Z. muellerii and Z. mucronata can
border on beds of Heterozostera tasmanica, and even intermingle with them
around the low-water mark. However, the sublittoral seagrass beds in temperate Australia are not composed of Zostera species, but of species of Posidonia and Amphibolis. These communities show a considerably higher diversity in species than do those of Zostera, and they are structurally more
complex. In disturbed places in the sublittoral, H. tasmanica and Halophila
ovalis may dominate.
In New Zealand the seagrass vegetation is not as developed as in Australia.
There is one species, Z. novozelandica, which occurs in the intertidal and
upper sublittoral zones. Z. capricorni is present, but only in the extreme
north.
The temperate west coast of South America is almost devoid of seagrasses.
Three beds of H. tasmanica have been found near Coquimbo, Chile. No
seagrass beds have been found from the temperate east coast of South
America. There is a record of a Zostera leaf washed ashore near Montevideo,
Uruguay (SetchellI935).
In the tropical waters, the place of Zostera is taken by species of other
genera. In the lower littoral zone, there are narrow-leaved morphs of species
of Halodule, and here and there tiny species of Halophila. In the sublittoral,
dense beds of species of Cymodocea, Syringodium and Thalassia occur.
In the Caribbean and the Gulf of Mexico, beds of Halodule wrightii or H.
beaudettei occur near the low-water mark bordering on sublittoral beds of
Thalassia testudinum. Syringodium filiforme may be interspersed in the
Thalassia bed, but it may also form stands of its own. The same holds for the