2
C. den Hartog and J. Kuo
indispensable for a marine water plant. These properties can be listed as follows: (i) the plants must
be adapted to life in a saline medium; (ii) the plants
must be able to grow when fully submerged; (iii)
the plants must have a secure anchoring system; and
(iv) the plants must have a hydrophilous pollination
mechanism. It is obvious that seagrasses fulfil these
requirements; they are able to achieve their vegetative as well as their generative cycle, when fully
submerged in a saline medium. This set of properties is, however, not complete, as there are several
other taxa of aquatic plants that also satisfy the four
criteria listed by Arber, although they do not normally occur in marine habitats; nevertheless, they do
even better in fact than the seagrasses where salinity tolerance is concerned (den Hartog, 1970). They
form the ‘eurysaline’ group (den Hartog, 1981), an
ecological group of aquatic plants, that is characteristic for waters with an unstable salinity, such as
mixo- and hyperhaline brackish waters, continental
salt waters where the dominant anion can be chloride, sulfate or even hydrocarbonate (therefore the
term ‘saline’ is used, to distinguish it from ‘haline’
which refers to chloride dominated waters with a
marine character); some of these taxa can occur in
hard fresh water, and there are observations of some
of them from extremely oligotrophic fresh waters.
It is also known that representatives of this group
can withstand very large and very sudden fluctuations in environmental parameters, such as salinity
and temperature, and in contrast to the true seagrasses their seeds are resistant to protracted desiccation. Although the representatives of this group
may be found in coastal areas their general distribution is not maritime; their altitudinal range is from
sea level up to 4000 m in mountains. The eurysaline
group consists of taxa from three monocotyledonous
families, the Ruppiaceae (with the genus Ruppia),
the Zannichelliaceae (with the genera Zannichellia, Lepilaena, Althenia, and Pseudalthenia), formerly classified as subfamilies of the Potamogetonaceae, and the Potamogetonaceae sensu stricto
of which only Potamogeton subgen. Coleogeton (by
some authors considered to be an independent genus,
Stuckenia) is involved. Several other aquatic plant
families have developed species with a rather wide
salt tolerance, e.g. Najas marina in the Najadaceae
(which recently has been shown to be part of the
Hydrocharitaceae), and Ranunculus baudotii in
the Ranunculaceae, a dicotyledonous family. So the
true seagrasses are characteristic for homoiohaline
marine habitats, while the members of the eurysaline
group occur in poikilosaline waters. It appears, that
these eurysaline species can live under marine circumstances, but are usually not able to compete
successfully with the seagrasses. According to den
Hartog (1970) it is probably a basic rule in ecology
that a wide tolerance for environmental fluctuations
is coupled with a reduced capacity to compete with
more stenobiontic taxa under more or less stable circumstances. The capacity to compete successfully
with other organisms in the marine environment is
thus another basic property of seagrasses.
It has to be pointed out that not all seagrasses
are stenohaline to the same degree. Particularly
some members of the genera Zostera, Cymodocea,
Halodule, and Halophila may penetrate to some
extent into estuaries, and these are the same ones
that extend up to the middle of the intertidal
zone. This means in practice that under estuarine conditions and in the intertidal belt true seagrasses and eurysaline water plants may meet, just
as further upstream eurysaline species may come
into contact with fresh-water plants. In the Baltic
(Samuelsson, 1934; Luther, 1951a,b) and in the
Black Sea (Milchakova, 1999), which both show a
reduced salinity and a considerable salinity gradient,
mixed stands of seagrasses and eurysaline aquatics
have been commonly recorded.
It is our intention to present here the taxonomy of
the seagrasses at the family and the genus level, including also descriptions of the families of the poikilosaline group which have a true marine representative. The author’s names of the species, accepted
as valid, are given in the ‘List of the seagrass species
of the world’ (see Appendix A p. 22–23).
II. Key to the Angiosperm Families
Containing True Marine Species
1a. Leaves differentiated into a sheath and a blade,
without a ligule, or a blade with a clear
petiole. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1b. Leaves differentiated into a sheath and a blade,
with a ligule. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2a. Flowers dioecious, (sometimes monoecious)
with a trimerous perianth. Pollen spherical, free or arranged within a moniliform
string. . . . . . . . . . . . . Marine Hydrocharitaceae
2b. Flowers monoecious, in pairs on a peduncle,
each with two anthers and 4-many ovaries, but
C. den Hartog and J. Kuo
indispensable for a marine water plant. These properties can be listed as follows: (i) the plants must
be adapted to life in a saline medium; (ii) the plants
must be able to grow when fully submerged; (iii)
the plants must have a secure anchoring system; and
(iv) the plants must have a hydrophilous pollination
mechanism. It is obvious that seagrasses fulfil these
requirements; they are able to achieve their vegetative as well as their generative cycle, when fully
submerged in a saline medium. This set of properties is, however, not complete, as there are several
other taxa of aquatic plants that also satisfy the four
criteria listed by Arber, although they do not normally occur in marine habitats; nevertheless, they do
even better in fact than the seagrasses where salinity tolerance is concerned (den Hartog, 1970). They
form the ‘eurysaline’ group (den Hartog, 1981), an
ecological group of aquatic plants, that is characteristic for waters with an unstable salinity, such as
mixo- and hyperhaline brackish waters, continental
salt waters where the dominant anion can be chloride, sulfate or even hydrocarbonate (therefore the
term ‘saline’ is used, to distinguish it from ‘haline’
which refers to chloride dominated waters with a
marine character); some of these taxa can occur in
hard fresh water, and there are observations of some
of them from extremely oligotrophic fresh waters.
It is also known that representatives of this group
can withstand very large and very sudden fluctuations in environmental parameters, such as salinity
and temperature, and in contrast to the true seagrasses their seeds are resistant to protracted desiccation. Although the representatives of this group
may be found in coastal areas their general distribution is not maritime; their altitudinal range is from
sea level up to 4000 m in mountains. The eurysaline
group consists of taxa from three monocotyledonous
families, the Ruppiaceae (with the genus Ruppia),
the Zannichelliaceae (with the genera Zannichellia, Lepilaena, Althenia, and Pseudalthenia), formerly classified as subfamilies of the Potamogetonaceae, and the Potamogetonaceae sensu stricto
of which only Potamogeton subgen. Coleogeton (by
some authors considered to be an independent genus,
Stuckenia) is involved. Several other aquatic plant
families have developed species with a rather wide
salt tolerance, e.g. Najas marina in the Najadaceae
(which recently has been shown to be part of the
Hydrocharitaceae), and Ranunculus baudotii in
the Ranunculaceae, a dicotyledonous family. So the
true seagrasses are characteristic for homoiohaline
marine habitats, while the members of the eurysaline
group occur in poikilosaline waters. It appears, that
these eurysaline species can live under marine circumstances, but are usually not able to compete
successfully with the seagrasses. According to den
Hartog (1970) it is probably a basic rule in ecology
that a wide tolerance for environmental fluctuations
is coupled with a reduced capacity to compete with
more stenobiontic taxa under more or less stable circumstances. The capacity to compete successfully
with other organisms in the marine environment is
thus another basic property of seagrasses.
It has to be pointed out that not all seagrasses
are stenohaline to the same degree. Particularly
some members of the genera Zostera, Cymodocea,
Halodule, and Halophila may penetrate to some
extent into estuaries, and these are the same ones
that extend up to the middle of the intertidal
zone. This means in practice that under estuarine conditions and in the intertidal belt true seagrasses and eurysaline water plants may meet, just
as further upstream eurysaline species may come
into contact with fresh-water plants. In the Baltic
(Samuelsson, 1934; Luther, 1951a,b) and in the
Black Sea (Milchakova, 1999), which both show a
reduced salinity and a considerable salinity gradient,
mixed stands of seagrasses and eurysaline aquatics
have been commonly recorded.
It is our intention to present here the taxonomy of
the seagrasses at the family and the genus level, including also descriptions of the families of the poikilosaline group which have a true marine representative. The author’s names of the species, accepted
as valid, are given in the ‘List of the seagrass species
of the world’ (see Appendix A p. 22–23).
II. Key to the Angiosperm Families
Containing True Marine Species
1a. Leaves differentiated into a sheath and a blade,
without a ligule, or a blade with a clear
petiole. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1b. Leaves differentiated into a sheath and a blade,
with a ligule. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2a. Flowers dioecious, (sometimes monoecious)
with a trimerous perianth. Pollen spherical, free or arranged within a moniliform
string. . . . . . . . . . . . . Marine Hydrocharitaceae
2b. Flowers monoecious, in pairs on a peduncle,
each with two anthers and 4-many ovaries, but