Chapter 1 Taxonomy of Seagrasses
5
Z. marina has been recorded from both sides of the
northern Atlantic and both sides of the northern Pacific. The species Z. caespitosa and Z. caulescens are
restricted to the Asiatic side of the Pacific; Z. asiatica
described from the north-eastern Pacific may be
identical with the wide-leaved form of Z. marina,
earlier described as Z. latifolia and Z. oregona.
The record of Z. asiatica by Phillips and WilleyEcheverria (1990) for the Pacific coast of North
America may refer to Z. latifolia or Z. oregona.
Regrettably the few distinguishing characters intergrade, and hamper the definition of clear-cut
taxa. Backman (1991) recognized five varieties of
Z. marina along the Pacific coast of North America,
and formally described these. However, he also did
not discuss the relationship of these varieties, if any,
with the taxa related to Z. marina, earlier described
from the same area, such as Z. oregona, Z. latifolia
or Z. pacifica, nor is there any indication that his
Z. marina var. typica has any relation to the original specimens of this species that was described
from Europe. In Europe Z. marina is also not a
sharply defined species. There occur perennial, biennial, and annual populations. The annual form
has been recorded by some authors as a separate
species, Z. angustifolia; others consider it at the variety level or as an example of the variability of the
species, because of the lack of reliable distinguishing
morphological characters. Van Lent and Verschuure
(1994) found that there is a continuum between the
annual and perennial populations; some appear to
be truly annual, in others part of the plants appears
to be biennial, i.e. at least surviving the winter season; and a third group of populations is permanently
green. These differences in life cycle were found
in a small area in the same estuary in The Netherlands. It has also to be mentioned, that various populations along the European coast show considerable
differences in their temperature and salinity tolerance. Further taxonomic research in this subgenus is
necessary.
The subgenus Zosterella is widely distributed in
the warm temperate coastal waters of the seas of the
northern and the southern hemisphere, with some incursions in tropical waters and one species extends
into the cold temperate zone. In the Atlantic, including the Mediterranean only one species, Z. noltii occurs. It has been recorded also from the land-locked
Caspian Sea and the Aral Sea. In the northern Pacific
the subgenus is represented also by only one species,
Z. japonica, which extends from the Siberian east
coast down to Vietnam. Quite recently this species
has colonized the Pacific coasts of Canada and the
USA. In the southern hemisphere Z. capensis occurs from the Cape Province up to Kenya. In temperate Australia three species occur, Z. muelleri,
Z. mucronata, and Z. capricorni, which more or
less exclude each other geographically. Z. muelleri
inhabits the coasts of Victoria, Tasmania and the
eastern part of South Australia, Z. mucronata is
restricted to the eastern coasts of South Australia
and the southern and south-western part of Western
Australia, and Z. capricorni is distributed along the
eastern coast of Australia up to Papua New Guinea.
Moreover, it occurs in New Zealand, together with
Z. novazelandica.
Les et al. (2002) carried out a phylogenetic study
on the Australian and New Zealand Zosteraceae using DNA sequences from nuclear (ITS) and plastid (tmk intron, rbcL) genomes (see also Waycott
et al., Chapter 2). These molecular studies did not
support the distinctness of Zostera capricorni, Z.
mucronata, Z. muelleri, and Z. novazelandica as
four discrete species, but indicated that some isolation by distance had occurred. The matK gene
sequence data of Tanaka et al. (2003) also show
that Z. muelleri, Z. capricorni, Z. novazelandica,
and Z. mucronata belong to the same lineage. However, Tanaka et al. (2003) also unexpectedly found
Z. capensis is not associated with the other above
mentioned Zostera species but belongs to the same
lineage as H. tasmanica. In the meantime, Les et al.
(2002) further conducted a cladistic analysis of 31
morphological, vegetative and reproductive characters, based on data from 15 previous publications of
seven species of Zostera subgen. Zosterella, to conclude that there are no morphological differences between these species. Based on molecular and cladistic evidence Les et al. (2002) recommended that all
Australian Zostera species should be merged taxonomically as a single species, which in that case
by priority should be called Z. muelleri, and not
Z. capricorni, as proposed by Les et al. (2002),
as this latter species has been described nine years
later. Regretfully, this error has already been applied in the most recent literature (Green and Short,
2003). On the other hand, Kato et al. (2003) retained
all described Zostera and Heterozostera species
but wrongly placed these species under the genus
Nanozostera. As discussed above a taxonomic mistake has been made by choosing Nanozostera instead
of Heterozostera.
5
Z. marina has been recorded from both sides of the
northern Atlantic and both sides of the northern Pacific. The species Z. caespitosa and Z. caulescens are
restricted to the Asiatic side of the Pacific; Z. asiatica
described from the north-eastern Pacific may be
identical with the wide-leaved form of Z. marina,
earlier described as Z. latifolia and Z. oregona.
The record of Z. asiatica by Phillips and WilleyEcheverria (1990) for the Pacific coast of North
America may refer to Z. latifolia or Z. oregona.
Regrettably the few distinguishing characters intergrade, and hamper the definition of clear-cut
taxa. Backman (1991) recognized five varieties of
Z. marina along the Pacific coast of North America,
and formally described these. However, he also did
not discuss the relationship of these varieties, if any,
with the taxa related to Z. marina, earlier described
from the same area, such as Z. oregona, Z. latifolia
or Z. pacifica, nor is there any indication that his
Z. marina var. typica has any relation to the original specimens of this species that was described
from Europe. In Europe Z. marina is also not a
sharply defined species. There occur perennial, biennial, and annual populations. The annual form
has been recorded by some authors as a separate
species, Z. angustifolia; others consider it at the variety level or as an example of the variability of the
species, because of the lack of reliable distinguishing
morphological characters. Van Lent and Verschuure
(1994) found that there is a continuum between the
annual and perennial populations; some appear to
be truly annual, in others part of the plants appears
to be biennial, i.e. at least surviving the winter season; and a third group of populations is permanently
green. These differences in life cycle were found
in a small area in the same estuary in The Netherlands. It has also to be mentioned, that various populations along the European coast show considerable
differences in their temperature and salinity tolerance. Further taxonomic research in this subgenus is
necessary.
The subgenus Zosterella is widely distributed in
the warm temperate coastal waters of the seas of the
northern and the southern hemisphere, with some incursions in tropical waters and one species extends
into the cold temperate zone. In the Atlantic, including the Mediterranean only one species, Z. noltii occurs. It has been recorded also from the land-locked
Caspian Sea and the Aral Sea. In the northern Pacific
the subgenus is represented also by only one species,
Z. japonica, which extends from the Siberian east
coast down to Vietnam. Quite recently this species
has colonized the Pacific coasts of Canada and the
USA. In the southern hemisphere Z. capensis occurs from the Cape Province up to Kenya. In temperate Australia three species occur, Z. muelleri,
Z. mucronata, and Z. capricorni, which more or
less exclude each other geographically. Z. muelleri
inhabits the coasts of Victoria, Tasmania and the
eastern part of South Australia, Z. mucronata is
restricted to the eastern coasts of South Australia
and the southern and south-western part of Western
Australia, and Z. capricorni is distributed along the
eastern coast of Australia up to Papua New Guinea.
Moreover, it occurs in New Zealand, together with
Z. novazelandica.
Les et al. (2002) carried out a phylogenetic study
on the Australian and New Zealand Zosteraceae using DNA sequences from nuclear (ITS) and plastid (tmk intron, rbcL) genomes (see also Waycott
et al., Chapter 2). These molecular studies did not
support the distinctness of Zostera capricorni, Z.
mucronata, Z. muelleri, and Z. novazelandica as
four discrete species, but indicated that some isolation by distance had occurred. The matK gene
sequence data of Tanaka et al. (2003) also show
that Z. muelleri, Z. capricorni, Z. novazelandica,
and Z. mucronata belong to the same lineage. However, Tanaka et al. (2003) also unexpectedly found
Z. capensis is not associated with the other above
mentioned Zostera species but belongs to the same
lineage as H. tasmanica. In the meantime, Les et al.
(2002) further conducted a cladistic analysis of 31
morphological, vegetative and reproductive characters, based on data from 15 previous publications of
seven species of Zostera subgen. Zosterella, to conclude that there are no morphological differences between these species. Based on molecular and cladistic evidence Les et al. (2002) recommended that all
Australian Zostera species should be merged taxonomically as a single species, which in that case
by priority should be called Z. muelleri, and not
Z. capricorni, as proposed by Les et al. (2002),
as this latter species has been described nine years
later. Regretfully, this error has already been applied in the most recent literature (Green and Short,
2003). On the other hand, Kato et al. (2003) retained
all described Zostera and Heterozostera species
but wrongly placed these species under the genus
Nanozostera. As discussed above a taxonomic mistake has been made by choosing Nanozostera instead
of Heterozostera.