Chapter 1 Taxonomy of Seagrasses
15
hyperhaline waters where it tolerates salinities up
to three times the salinity of the sea. It has also
been found under marine conditions, but only in
very sheltered places, where also very large temperature fluctuations may occur. Its occurrence in
the tropics is very local, probably because the environments where representatives of this genus would
abound are ephemeral under tropical conditions.
Lagoons, which become detached from the sea, will
be brackish only very temporarily; in the wet tropics they will develop into freshwater marshes due
to dilution with rain, and in the dry tropics they
will become desiccated and transformed into a salty
desert.
Circa 10 species are known. As a consequence of
the great morphological variation between populations, partly due to environmental differences and
partly genetically determined, the taxonomy of the
genus is still unsatisfactory. Another difficulty is that
in the past it was not recognized that in the herbarium material the flowering and fruiting organs were
not always in the same stage, so the number of described varieties is large; most of them cannot be
maintained. However, many investigators have concluded from the chaotic taxonomic situation that the
best solution to the problem was to consider the
genus as containing one very variable species. This
has given cause to the uncritical ‘traditional’ identification of specimens of this genus as R. maritima.
Studies based on the investigation of living plants,
herbarium material and chromosome analyses in
Europe (Reese, 1962; Verhoeven, 1979), Australia (Jacobs and Brock, 1982) and New Zealand
(Mason, 1967) have shown that this is not correct.
It is obvious that the genus is in an urgent need of
revision on a world scale. This revision should not
only be based on herbarium material, but also on
the study of living material cultured under various
ecological circumstances; further chromosome and
isozyme studies should be included.
The genus Ruppia has been classified in the past
in various ways; several authors considered it as a
family on its own, the Ruppiaceae, but it has also
been regarded as a subfamily of the Potamogetonaceae. According to Jacobs and Brock (1982) the
differences with Potamogeton are not sufficient to
warrant a separate position within the Potamogetonaceae sensu stricto. Les et al. (1997) have shown
that molecular rbcL data indicate that Ruppia is
phylogenetically much closer to Posidonia than to
Potamogeton.
The only exclusively marine species of the genus
has not yet been formally described, and is indicated
as R. aff. tuberosa (den Hartog, in preparation)
Zannichelliaceae
Zannichelliaceae Dumortier, Anal. Fam. Pl. (1829)
59, 61; nom. cons.
Typus: Zannichellia L.
Monocious or dioecious, annual or rarely perennial
aquatic herbs. Rhizome creeping, sympodial, often
poorly developed, herbaceous, rooting at the nodes.
Roots not branched, 1-several per node. Scales on the
rhizome membranous, caducous. Erect shoots arising from each node, sympodial, branching profusely
and producing flowers arranged in a rather complex
inflorescence. Leaves distichously arranged, sometimes alternate or in a pseudo-whorl, linear, green,
without tannin cells, with a single central vein, on
each side of it with 1–2 parallel air channels; margin
entire; leaves with a ligule, sheathing at the base, or
without a ligule, but with free stipules at the base of
the leaves (Zannichellia). Tannin cells absent. Flowers terminal. Male flower with or without perianth,
with one stamen consisting of 2–12 longitudinally
dehiscent, sporangiate cells, with or without connective appendages. Stamens generally submerged, but
in at least one species emerging above the water surface. Pollen spherical. Female flower consisting of
a cup-shaped structure, the cupula, with 1–8 sessile
or shortly pedicellate free carpels. Cupula consisting of three tepals, which are free (Lepilaena, Althenia), or united to form a closed tube (Zannichellia,
Pseudalthenia). Carpels with a short (Zannichellia,
Pseudalthenia) or a long style (Lepilaena, Althenia);
stigma funnel-shaped or peltate with a smooth, lobate, or feathery appearance. Ovule 1, anatropous,
pendulous. Fruit an achene, with a hard endocarp,
a soft mesocarp and a membranous exocarp, and a
terminal beak. In some species a short podogynium
is developed after fertilization, forming an integral
part of the fruit without an abscission zone. Fruit
wall smooth or tuberculate. No endosperm.
The family has a world wide distribution, and consists of four genera. The taxonomy of the family at
the genus level has been worked out by Tomlinson
and Posluszny (1976). Zannichellia has a very wide
distribution, almost covering the area of the family, and has been recorded from sea level up to the
high Andes and the Himalayas. The species of this
15
hyperhaline waters where it tolerates salinities up
to three times the salinity of the sea. It has also
been found under marine conditions, but only in
very sheltered places, where also very large temperature fluctuations may occur. Its occurrence in
the tropics is very local, probably because the environments where representatives of this genus would
abound are ephemeral under tropical conditions.
Lagoons, which become detached from the sea, will
be brackish only very temporarily; in the wet tropics they will develop into freshwater marshes due
to dilution with rain, and in the dry tropics they
will become desiccated and transformed into a salty
desert.
Circa 10 species are known. As a consequence of
the great morphological variation between populations, partly due to environmental differences and
partly genetically determined, the taxonomy of the
genus is still unsatisfactory. Another difficulty is that
in the past it was not recognized that in the herbarium material the flowering and fruiting organs were
not always in the same stage, so the number of described varieties is large; most of them cannot be
maintained. However, many investigators have concluded from the chaotic taxonomic situation that the
best solution to the problem was to consider the
genus as containing one very variable species. This
has given cause to the uncritical ‘traditional’ identification of specimens of this genus as R. maritima.
Studies based on the investigation of living plants,
herbarium material and chromosome analyses in
Europe (Reese, 1962; Verhoeven, 1979), Australia (Jacobs and Brock, 1982) and New Zealand
(Mason, 1967) have shown that this is not correct.
It is obvious that the genus is in an urgent need of
revision on a world scale. This revision should not
only be based on herbarium material, but also on
the study of living material cultured under various
ecological circumstances; further chromosome and
isozyme studies should be included.
The genus Ruppia has been classified in the past
in various ways; several authors considered it as a
family on its own, the Ruppiaceae, but it has also
been regarded as a subfamily of the Potamogetonaceae. According to Jacobs and Brock (1982) the
differences with Potamogeton are not sufficient to
warrant a separate position within the Potamogetonaceae sensu stricto. Les et al. (1997) have shown
that molecular rbcL data indicate that Ruppia is
phylogenetically much closer to Posidonia than to
Potamogeton.
The only exclusively marine species of the genus
has not yet been formally described, and is indicated
as R. aff. tuberosa (den Hartog, in preparation)
Zannichelliaceae
Zannichelliaceae Dumortier, Anal. Fam. Pl. (1829)
59, 61; nom. cons.
Typus: Zannichellia L.
Monocious or dioecious, annual or rarely perennial
aquatic herbs. Rhizome creeping, sympodial, often
poorly developed, herbaceous, rooting at the nodes.
Roots not branched, 1-several per node. Scales on the
rhizome membranous, caducous. Erect shoots arising from each node, sympodial, branching profusely
and producing flowers arranged in a rather complex
inflorescence. Leaves distichously arranged, sometimes alternate or in a pseudo-whorl, linear, green,
without tannin cells, with a single central vein, on
each side of it with 1–2 parallel air channels; margin
entire; leaves with a ligule, sheathing at the base, or
without a ligule, but with free stipules at the base of
the leaves (Zannichellia). Tannin cells absent. Flowers terminal. Male flower with or without perianth,
with one stamen consisting of 2–12 longitudinally
dehiscent, sporangiate cells, with or without connective appendages. Stamens generally submerged, but
in at least one species emerging above the water surface. Pollen spherical. Female flower consisting of
a cup-shaped structure, the cupula, with 1–8 sessile
or shortly pedicellate free carpels. Cupula consisting of three tepals, which are free (Lepilaena, Althenia), or united to form a closed tube (Zannichellia,
Pseudalthenia). Carpels with a short (Zannichellia,
Pseudalthenia) or a long style (Lepilaena, Althenia);
stigma funnel-shaped or peltate with a smooth, lobate, or feathery appearance. Ovule 1, anatropous,
pendulous. Fruit an achene, with a hard endocarp,
a soft mesocarp and a membranous exocarp, and a
terminal beak. In some species a short podogynium
is developed after fertilization, forming an integral
part of the fruit without an abscission zone. Fruit
wall smooth or tuberculate. No endosperm.
The family has a world wide distribution, and consists of four genera. The taxonomy of the family at
the genus level has been worked out by Tomlinson
and Posluszny (1976). Zannichellia has a very wide
distribution, almost covering the area of the family, and has been recorded from sea level up to the
high Andes and the Himalayas. The species of this