Chapter 1 Taxonomy of Seagrasses
7
of a Phyllospadix specimen along the coast of Baia
California, Mexico.
Heterozostera (Setchell) den Hartog, Sea-grasses
of the world (1970) 114
Type species: Zostera tasmanica Martens ex Ascherson (= Heterozostera tasmanica (Martens ex
Ascherson) den Hartog)
The genus has been originally erected to classify
the rather aberrant Z. tasmanica. All previous researchers (den Hartog, 1970; Tomlinson, 1982; Les
et al., 1997; Tomlinson and Posluszny, 2001) treated
Heterozostera as monotypic and having distinct vegetative wiry erect stems and more than two vascular strands in the rhizome cortex. More material has
now become available for study and the concept of
the monotypic genus has to be reconsidered. The
most recent revision of Heterozostera demonstrates
that it is represented by three distinct taxa in Australia, and a fourth in South America (Kuo, 2005).
All three Australian species have numerous cortical vascular bundles in the rhizome cortex, but only
one of them possesses ‘wiry erect stems’. Within
the genus two distinct species groups can be distinguished which possibly have to be ranked as sections
or subgenera. It is interesting to mention that Les
et al. (2002) observed a low level of molecular divergence (ITS and tmK) between the Heterozostera
population from eastern (one collection) and western
(four collections) Australia. Due to a lack of morphological evidence (which they did not consider)
to support this molecular finding, they concluded
that this molecular difference was due to a relatively
prolonged geographical isolation of the two populations rather than as clear evidence of a speciation
event.
Cymodoceaceae
Cymodoceaceae N. Taylor in N. Amer. Fl. 17 (1909)
31; nom. cons.
Typus: Cymodocea K¨ onig
Dioecious marine plants. Rhizome creeping, either
herbaceous, monopodial, and rooting at the nodes
(Cymodocea, Syringodium, Halodule) or ligneous,
sympodial, and rooting from the internodes (Amphibolis, Thalassodendron). Scales scarious, ovate
or elliptic, marked with more or less small, dark,
longitudinal stripes, and dots (tannin cells). Leaves
distichous. Leaf sheath broad, completely or almost
completely amplexicaulous, leaving open or closed
circular scars when shed, bi-auriculate, ligulate; scarious flaps covered with numerous short dark, longitudinal stripes, and dots (tannin cells). Leaf blade
linear or subulate with three to several parallel or
pseudoparallel (Amphibolis) nerves; parallel with
the nerves more or less, short, dark, longitudinal
stripes, and dots (tannin cells); leaf-tip variable in
outline. ‘Flowers’ without perigone, solitary, either
terminal on a short branch or arranged in a cymose inflorescence (Syringodium). Male ‘flowers’
subsessile or stalked, consisting of two quadrilocular,
extrorsely dehiscent anthers, which are dorsally connate over at least a part of their length and are attached either at the same height or at a slightly different level (Halodule). Pollen confervoid. Female
‘flowers’ sessile or shortly stalked, consisting of two
free ovaries each with either a long style (Halodule) or a short style, which is divided into 2 or 3
loriform stigmata. Ovule 1, suborthotropous, pendulous. Fruit either with a stony pericarp, more or less
compressed (Cymodocea, Halodule, Syringodium)
or with a stony endocarp and a fleshy exocarp from
which four cuneate spreading lobes grow out (Amphibolis) or consisting of a fleshy bract which encloses the fertilized ovaries (Thalassodendron); not
dehiscent. Seed 1. Embryo either consisting for the
larger part of the plumula with a lateral primary root
and a cylindrical hypocotyl, appressed to the upper
part of the plumula (Cymodocea) or consisting of a
long hypocotyl and a short plumula without a primary root (Amphibolis).
The family contains five genera: Halodule,
Cymodocea, Syringodium, Thalassodendron, and
Amphibolis.
From a morphological point of view the family
is homogeneous, and monophyletic. In the past it
has often been combined with the Zannichelliaceae,
e.g. by Hutchinson (1934). The five genera are all
well distinguished and there is no controversy about
their status as is the case in the Zosteraceae. The
family is old, as there are several fossil records of
members of the genus Cymodocea from Eocene and
Miocene deposits. Thalassocharis from the Cretaceous of The Netherlands and Germany has been
considered as being a seagrass by Voigt and Domke
(1955) and den Hartog (1970) did not reject this conclusion, but remarked that the stiff compact stems
and the absence of aerenchymatic tissue show that
Thalassocharis was not yet very well adapted to life
in the aquatic environment. Kuo and den Hartog
7
of a Phyllospadix specimen along the coast of Baia
California, Mexico.
Heterozostera (Setchell) den Hartog, Sea-grasses
of the world (1970) 114
Type species: Zostera tasmanica Martens ex Ascherson (= Heterozostera tasmanica (Martens ex
Ascherson) den Hartog)
The genus has been originally erected to classify
the rather aberrant Z. tasmanica. All previous researchers (den Hartog, 1970; Tomlinson, 1982; Les
et al., 1997; Tomlinson and Posluszny, 2001) treated
Heterozostera as monotypic and having distinct vegetative wiry erect stems and more than two vascular strands in the rhizome cortex. More material has
now become available for study and the concept of
the monotypic genus has to be reconsidered. The
most recent revision of Heterozostera demonstrates
that it is represented by three distinct taxa in Australia, and a fourth in South America (Kuo, 2005).
All three Australian species have numerous cortical vascular bundles in the rhizome cortex, but only
one of them possesses ‘wiry erect stems’. Within
the genus two distinct species groups can be distinguished which possibly have to be ranked as sections
or subgenera. It is interesting to mention that Les
et al. (2002) observed a low level of molecular divergence (ITS and tmK) between the Heterozostera
population from eastern (one collection) and western
(four collections) Australia. Due to a lack of morphological evidence (which they did not consider)
to support this molecular finding, they concluded
that this molecular difference was due to a relatively
prolonged geographical isolation of the two populations rather than as clear evidence of a speciation
event.
Cymodoceaceae
Cymodoceaceae N. Taylor in N. Amer. Fl. 17 (1909)
31; nom. cons.
Typus: Cymodocea K¨ onig
Dioecious marine plants. Rhizome creeping, either
herbaceous, monopodial, and rooting at the nodes
(Cymodocea, Syringodium, Halodule) or ligneous,
sympodial, and rooting from the internodes (Amphibolis, Thalassodendron). Scales scarious, ovate
or elliptic, marked with more or less small, dark,
longitudinal stripes, and dots (tannin cells). Leaves
distichous. Leaf sheath broad, completely or almost
completely amplexicaulous, leaving open or closed
circular scars when shed, bi-auriculate, ligulate; scarious flaps covered with numerous short dark, longitudinal stripes, and dots (tannin cells). Leaf blade
linear or subulate with three to several parallel or
pseudoparallel (Amphibolis) nerves; parallel with
the nerves more or less, short, dark, longitudinal
stripes, and dots (tannin cells); leaf-tip variable in
outline. ‘Flowers’ without perigone, solitary, either
terminal on a short branch or arranged in a cymose inflorescence (Syringodium). Male ‘flowers’
subsessile or stalked, consisting of two quadrilocular,
extrorsely dehiscent anthers, which are dorsally connate over at least a part of their length and are attached either at the same height or at a slightly different level (Halodule). Pollen confervoid. Female
‘flowers’ sessile or shortly stalked, consisting of two
free ovaries each with either a long style (Halodule) or a short style, which is divided into 2 or 3
loriform stigmata. Ovule 1, suborthotropous, pendulous. Fruit either with a stony pericarp, more or less
compressed (Cymodocea, Halodule, Syringodium)
or with a stony endocarp and a fleshy exocarp from
which four cuneate spreading lobes grow out (Amphibolis) or consisting of a fleshy bract which encloses the fertilized ovaries (Thalassodendron); not
dehiscent. Seed 1. Embryo either consisting for the
larger part of the plumula with a lateral primary root
and a cylindrical hypocotyl, appressed to the upper
part of the plumula (Cymodocea) or consisting of a
long hypocotyl and a short plumula without a primary root (Amphibolis).
The family contains five genera: Halodule,
Cymodocea, Syringodium, Thalassodendron, and
Amphibolis.
From a morphological point of view the family
is homogeneous, and monophyletic. In the past it
has often been combined with the Zannichelliaceae,
e.g. by Hutchinson (1934). The five genera are all
well distinguished and there is no controversy about
their status as is the case in the Zosteraceae. The
family is old, as there are several fossil records of
members of the genus Cymodocea from Eocene and
Miocene deposits. Thalassocharis from the Cretaceous of The Netherlands and Germany has been
considered as being a seagrass by Voigt and Domke
(1955) and den Hartog (1970) did not reject this conclusion, but remarked that the stiff compact stems
and the absence of aerenchymatic tissue show that
Thalassocharis was not yet very well adapted to life
in the aquatic environment. Kuo and den Hartog