16
C. den Hartog and J. Kuo
genus occur in brackish as well as hard fresh water, in
standing ponds and lakes as well as in streams, but
do not inhabit marine sites. Most of these species
are also tolerant to large temperature fluctuations,
and their seeds can stand protracted desiccation. In
the past the genus has been considered to consist of
one extremely variable species, Z. palustris. This has
often led to an uncritical ‘traditional’ identification
of plant material of this genus. It has, however, turned
out that a number of well-defined species can be
recognized. In Europe at least six species occur (Van
Vierssen, 1982; Van Vierssen and Van Wijk, 1982;
Uotila et al., 1983; Talavera et al., 1986), and more
species are expected to be recognized, as the material from the Ponto-Caspian area and Central Asia
has been subjected to a critical study. Haynes and
Holm-Nielsen (1987) described a species from the
Andes. The genus is urgently in need of a revision.
Althenia, with one or two, but possibly more species,
is distributed from the Mediterranean area to Central Asia, and has further been recorded from South
Africa. It occurs in saline waters of salt marshes
along the coast, but is also widely distributed in continental salt waters. The genus Pseudalthenia (also
known under the name Vleisia) is monotypic and is
restricted in its occurrence to a few brackish-water
environments in the Cape Province of South Africa.
Lepilaena is the only genus in this family with a true
marine species.
Lepilaena Drummond ex Harvey, J. Bot. Kew Gard.
Misc. 7 (1855) 57
Type species: Lepilaena australis Drummond ex
Harvey
The genus contains at least six species which all
are widely distributed in temperate Australia; one of
these, L. bilocularis, is shared with New Zealand.
Robertson (1986) gives a key to the presently described species of the genus. Its representatives occur in all kinds of brackish water habitats, but also
in continental salt waters; further they inhabit hard
fresh water environments such as shallow lakes and
ponds, and are often found in temporary pools. A still
undescribed species has been collected in a shallow
temporary soft water environment in south-western
Australia. One species, L. marina, has been found so
far only in sheltered marine environments; in vegetative condition this species can be confused with
L. bilocularis. Marine records of the latter almost
certainly refer to L. marina, e.g. the records by den
Hartog (1970).
IV. Biogeography
The first attempt to analyze the geographical distribution of the seagrasses was by Ascherson (1871),
who produced also the first world map of their
distribution. At present this paper has only historical value, as taxonomy of the seagrasses and
knowledge of their distribution has increased enormously since, not least by the continuous efforts
of Ascherson himself to obtain material from all
over the world, and to publish updates (Ascherson,
1875, 1906). His work has been continued by
the publication of the fine essays by Ostenfeld
(1915) and Setchell (1920, 1935). Ostenfeld published also a number of detailed maps of the areas of distribution of the seagrass species known
at that time in the prestigious series ‘Pflanzenareale’
(Ostenfeld, 1927a, marine Hydrocharitaceae; 1927b,
marine Potamogetonaceae, i.e. representatives of the
Zosteraceae, Posidoniaceae and Cymodoceaceae).
In his monograph den Hartog (1970) described the
areas of all known species, based on the collections
available at that time, and produced maps of the distribution of all 12 genera. These maps show also
perfectly that the pattern of seagrass distribution, already suggested by Ascherson (1906), has become
more and more prominent. Later publications show a
further increase in knowledge; L¨ uning (1990, p. 204)
amended the genus maps of den Hartog. Larkum
and den Hartog (1989) discussed the evolution of
seagrass genera and events that gave rise to their
current distribution. Regrettably the species maps
given by Green and Short (2003) in their world atlas
of seagrasses (2003 Appendix 3) are not always fully
accurate.
Seven genera appear to be mainly distributed
along tropical coasts, viz. Thalassia, Halophila,
Syringodium, Halodule, Cymodocea, Thalassodendron and Enhalus. The first four genera have representatives in the tropical Atlantic as well as in the
Indo-Pacific. The other three genera are restricted
to the Indo-West Pacific. However, with respect to
Cymodocea, Halophila and Thalassodendron the
picture is somewhat more complicated. One of the
Cymodocea species, C. nodosa, has a more warmtemperate distribution in the Mediterranean, and
fans out into the Atlantic along the Iberian Peninsula and the north-western coast of Africa where it
just passes the Tropic of Cancer. Halophila, with
15 described species the most species-rich genus
of the seagrasses, has developed a warm-temperate
C. den Hartog and J. Kuo
genus occur in brackish as well as hard fresh water, in
standing ponds and lakes as well as in streams, but
do not inhabit marine sites. Most of these species
are also tolerant to large temperature fluctuations,
and their seeds can stand protracted desiccation. In
the past the genus has been considered to consist of
one extremely variable species, Z. palustris. This has
often led to an uncritical ‘traditional’ identification
of plant material of this genus. It has, however, turned
out that a number of well-defined species can be
recognized. In Europe at least six species occur (Van
Vierssen, 1982; Van Vierssen and Van Wijk, 1982;
Uotila et al., 1983; Talavera et al., 1986), and more
species are expected to be recognized, as the material from the Ponto-Caspian area and Central Asia
has been subjected to a critical study. Haynes and
Holm-Nielsen (1987) described a species from the
Andes. The genus is urgently in need of a revision.
Althenia, with one or two, but possibly more species,
is distributed from the Mediterranean area to Central Asia, and has further been recorded from South
Africa. It occurs in saline waters of salt marshes
along the coast, but is also widely distributed in continental salt waters. The genus Pseudalthenia (also
known under the name Vleisia) is monotypic and is
restricted in its occurrence to a few brackish-water
environments in the Cape Province of South Africa.
Lepilaena is the only genus in this family with a true
marine species.
Lepilaena Drummond ex Harvey, J. Bot. Kew Gard.
Misc. 7 (1855) 57
Type species: Lepilaena australis Drummond ex
Harvey
The genus contains at least six species which all
are widely distributed in temperate Australia; one of
these, L. bilocularis, is shared with New Zealand.
Robertson (1986) gives a key to the presently described species of the genus. Its representatives occur in all kinds of brackish water habitats, but also
in continental salt waters; further they inhabit hard
fresh water environments such as shallow lakes and
ponds, and are often found in temporary pools. A still
undescribed species has been collected in a shallow
temporary soft water environment in south-western
Australia. One species, L. marina, has been found so
far only in sheltered marine environments; in vegetative condition this species can be confused with
L. bilocularis. Marine records of the latter almost
certainly refer to L. marina, e.g. the records by den
Hartog (1970).
IV. Biogeography
The first attempt to analyze the geographical distribution of the seagrasses was by Ascherson (1871),
who produced also the first world map of their
distribution. At present this paper has only historical value, as taxonomy of the seagrasses and
knowledge of their distribution has increased enormously since, not least by the continuous efforts
of Ascherson himself to obtain material from all
over the world, and to publish updates (Ascherson,
1875, 1906). His work has been continued by
the publication of the fine essays by Ostenfeld
(1915) and Setchell (1920, 1935). Ostenfeld published also a number of detailed maps of the areas of distribution of the seagrass species known
at that time in the prestigious series ‘Pflanzenareale’
(Ostenfeld, 1927a, marine Hydrocharitaceae; 1927b,
marine Potamogetonaceae, i.e. representatives of the
Zosteraceae, Posidoniaceae and Cymodoceaceae).
In his monograph den Hartog (1970) described the
areas of all known species, based on the collections
available at that time, and produced maps of the distribution of all 12 genera. These maps show also
perfectly that the pattern of seagrass distribution, already suggested by Ascherson (1906), has become
more and more prominent. Later publications show a
further increase in knowledge; L¨ uning (1990, p. 204)
amended the genus maps of den Hartog. Larkum
and den Hartog (1989) discussed the evolution of
seagrass genera and events that gave rise to their
current distribution. Regrettably the species maps
given by Green and Short (2003) in their world atlas
of seagrasses (2003 Appendix 3) are not always fully
accurate.
Seven genera appear to be mainly distributed
along tropical coasts, viz. Thalassia, Halophila,
Syringodium, Halodule, Cymodocea, Thalassodendron and Enhalus. The first four genera have representatives in the tropical Atlantic as well as in the
Indo-Pacific. The other three genera are restricted
to the Indo-West Pacific. However, with respect to
Cymodocea, Halophila and Thalassodendron the
picture is somewhat more complicated. One of the
Cymodocea species, C. nodosa, has a more warmtemperate distribution in the Mediterranean, and
fans out into the Atlantic along the Iberian Peninsula and the north-western coast of Africa where it
just passes the Tropic of Cancer. Halophila, with
15 described species the most species-rich genus
of the seagrasses, has developed a warm-temperate