Chapter 1 Taxonomy of Seagrasses
19
both the vegetative and the reproductive organs
of seagrasses; this holds in fact for many other
aquatic plants as well (see Chapter 3, Kuo and
den Hartog). So, another powerful tool, that of the
cladistic analysis, which often is employed in terrestrial plant taxonomy may not be very suitable for
seagrass taxonomy, as demonstrated by Les et al.
(2002).
It is true that the currently available knowledge
of seagrass taxonomy, in particular where the definition of species is concerned, is not always adequate and requires an urgent improvement. Basic
morphological and anatomical studies on as many
samples as possible from wide geographic areas
and growing under as many different habitat conditions as possible should be conducted. It is very
important that good documentation of morphological and anatomical variations within the species
from the various study areas becomes available;
it has to be encouraged that samples of material
used for molecular, physiological, phytochemical
and morphological research are deposited in the recognized herbaria for future study and to improve
the descriptions of species. Without such fundamental studies, the queries on identification of variable species such as those of Zostera, Halodule, and
Halophila will continue to persist. A stable taxonomy is a necessary base for all botanical research.
Therefore, it is also recommended that, if possible,
type material of the various taxa is included in the
analyses.
Finally we have come to the conclusion, in agreement with Tomlinson (1982), that there are no special
morphological characters that distinguish the seagrasses from other aquatic plants. The only character
in which most of them differ from the other aquatic
plants is the filiform pollen (Zosteraceae, Posidoniaceae, Cymodoceaceae) or the strings of spherical
pollen (Thalassia, Halophila); however, we do not
see what the special advantage of these may be for
life in the marine environment.
The seagrasses as well as the aquatics of the
eurysaline group, obviously, differ from the other
aquatic plants by their ability to live in waters with
a high salinity. It is at present still not clear how
they cope with this situation. The fact that the seagrasses inhabit generally the homoiohaline waters,
where the environmental conditions are relatively
stable, and the eurysaline species the poililosaline
waters where they are generally subjected to large
fluctuations in salinity, indicates that these plants
must have evolved very special physiological mechanisms to deal with these problems. So far very little
is known about these mechanisms; it is not even clear
whether there is only one mechanism or whether
each family involved has developed its own way to
deal with salinity, as the families are not closely related. These mechanisms are certainly firmly fixed
in the genomes of these plants. Therefore, we recommend a thorough study of the physiology of both
the seagrasses and the eurysaline aquatics in order
to solve this basic problem.
References
Agardh CA (1823) Species Algarum part 2 (Fasc. A. Asher Co.,
Amsterdam, 1969)
Agardh CA (1824) Systema Algarum (Fasc. A. Asher Co.,
Amsterdam, 1965)
Arber A (1920) Water Plants. A Study of Aquatic Angiosperms.
Cambridge University Press, Cambridge
Ascherson P (1871) Die geographische Verbreitung der
Seegr¨ aser. In: Petermann’s Geographische Mittheilungen 17:
241–248
Ascherson P (1875) Die geographische Verbreitung der
Seegr¨ aser. In: Neumayer G (ed), Anleitung zu wissenschaftlichen Beobachtungen auf Reisen, mit besondere
R¨ ucksicht auf die Bed¨ urfnisse der kaiserlichen Marine.
pp 359–373. Robert Oppenheim Verlag, Berlin
Ascherson P (1906) Die geographische Verbreitung der
Seegr¨ aser. In: Neumayer G (ed), Anleitung zu wissenschaftlichen Beobachtungen auf Reisen. 3rd Ed, Band 2, pp 389–
413. Dr. Max J¨ anecke Verlagsbuchhandlung, Hannover
Ascherson P and Graebner P (1907) Potamogetonaceae. In: Engler A (ed) Das Pflanzenreich Heft 31, 1–184. W. Engelmann,
Leipzig
Ascherson P and G¨ urke M (1889) Hydrocharitaceae, In: Engler
A and Prantl K (eds) Die nat¨ urlichen Pflanzenfamilien. 2, 238–
258. W. Engelmann, Leipzig
Backman TWH (1991) Genotypic and phenotypic variability of
Zostera marina on the west coast of North America. Can J Bot
69: 1361–1371
Bigley RE and Barreca JL (1982) Evidence for synonymizing
Zostera americana den Hartog and Zostera japonica Aschers.
and Graebn. Aquat Bot 14: 349–356
Cambridge ML and Kuo J (1979) Two new species of seagrasses
from Australia, Posidonia sinuosa and P. angustifolia (Posidoniaceae). Aquat Bot 6: 307–328
Campey ML, Waycott M and Kendrick GA (2000) Re-evaluating
species boundaries among members of the Posidonia ostenfeldii species complex (Posidoniaceae)—morphological and
genetic variation. Aquat Bot 66: 41–56
Cook CDK (1990) Aquatic Plant Book. SPB Academic Publishing, The Hague
Cook CDK (1998) Hydrocharitaceae In: Kubitzki K (ed) The
Families and Genera of Vascular Plants, Vol. IV. Flowering Plants, Monocotyledons: Alismatanae and Commelinanae
(except Gramineae), pp 234–248. Springer Verlag, Berlin
19
both the vegetative and the reproductive organs
of seagrasses; this holds in fact for many other
aquatic plants as well (see Chapter 3, Kuo and
den Hartog). So, another powerful tool, that of the
cladistic analysis, which often is employed in terrestrial plant taxonomy may not be very suitable for
seagrass taxonomy, as demonstrated by Les et al.
(2002).
It is true that the currently available knowledge
of seagrass taxonomy, in particular where the definition of species is concerned, is not always adequate and requires an urgent improvement. Basic
morphological and anatomical studies on as many
samples as possible from wide geographic areas
and growing under as many different habitat conditions as possible should be conducted. It is very
important that good documentation of morphological and anatomical variations within the species
from the various study areas becomes available;
it has to be encouraged that samples of material
used for molecular, physiological, phytochemical
and morphological research are deposited in the recognized herbaria for future study and to improve
the descriptions of species. Without such fundamental studies, the queries on identification of variable species such as those of Zostera, Halodule, and
Halophila will continue to persist. A stable taxonomy is a necessary base for all botanical research.
Therefore, it is also recommended that, if possible,
type material of the various taxa is included in the
analyses.
Finally we have come to the conclusion, in agreement with Tomlinson (1982), that there are no special
morphological characters that distinguish the seagrasses from other aquatic plants. The only character
in which most of them differ from the other aquatic
plants is the filiform pollen (Zosteraceae, Posidoniaceae, Cymodoceaceae) or the strings of spherical
pollen (Thalassia, Halophila); however, we do not
see what the special advantage of these may be for
life in the marine environment.
The seagrasses as well as the aquatics of the
eurysaline group, obviously, differ from the other
aquatic plants by their ability to live in waters with
a high salinity. It is at present still not clear how
they cope with this situation. The fact that the seagrasses inhabit generally the homoiohaline waters,
where the environmental conditions are relatively
stable, and the eurysaline species the poililosaline
waters where they are generally subjected to large
fluctuations in salinity, indicates that these plants
must have evolved very special physiological mechanisms to deal with these problems. So far very little
is known about these mechanisms; it is not even clear
whether there is only one mechanism or whether
each family involved has developed its own way to
deal with salinity, as the families are not closely related. These mechanisms are certainly firmly fixed
in the genomes of these plants. Therefore, we recommend a thorough study of the physiology of both
the seagrasses and the eurysaline aquatics in order
to solve this basic problem.
References
Agardh CA (1823) Species Algarum part 2 (Fasc. A. Asher Co.,
Amsterdam, 1969)
Agardh CA (1824) Systema Algarum (Fasc. A. Asher Co.,
Amsterdam, 1965)
Arber A (1920) Water Plants. A Study of Aquatic Angiosperms.
Cambridge University Press, Cambridge
Ascherson P (1871) Die geographische Verbreitung der
Seegr¨ aser. In: Petermann’s Geographische Mittheilungen 17:
241–248
Ascherson P (1875) Die geographische Verbreitung der
Seegr¨ aser. In: Neumayer G (ed), Anleitung zu wissenschaftlichen Beobachtungen auf Reisen, mit besondere
R¨ ucksicht auf die Bed¨ urfnisse der kaiserlichen Marine.
pp 359–373. Robert Oppenheim Verlag, Berlin
Ascherson P (1906) Die geographische Verbreitung der
Seegr¨ aser. In: Neumayer G (ed), Anleitung zu wissenschaftlichen Beobachtungen auf Reisen. 3rd Ed, Band 2, pp 389–
413. Dr. Max J¨ anecke Verlagsbuchhandlung, Hannover
Ascherson P and Graebner P (1907) Potamogetonaceae. In: Engler A (ed) Das Pflanzenreich Heft 31, 1–184. W. Engelmann,
Leipzig
Ascherson P and G¨ urke M (1889) Hydrocharitaceae, In: Engler
A and Prantl K (eds) Die nat¨ urlichen Pflanzenfamilien. 2, 238–
258. W. Engelmann, Leipzig
Backman TWH (1991) Genotypic and phenotypic variability of
Zostera marina on the west coast of North America. Can J Bot
69: 1361–1371
Bigley RE and Barreca JL (1982) Evidence for synonymizing
Zostera americana den Hartog and Zostera japonica Aschers.
and Graebn. Aquat Bot 14: 349–356
Cambridge ML and Kuo J (1979) Two new species of seagrasses
from Australia, Posidonia sinuosa and P. angustifolia (Posidoniaceae). Aquat Bot 6: 307–328
Campey ML, Waycott M and Kendrick GA (2000) Re-evaluating
species boundaries among members of the Posidonia ostenfeldii species complex (Posidoniaceae)—morphological and
genetic variation. Aquat Bot 66: 41–56
Cook CDK (1990) Aquatic Plant Book. SPB Academic Publishing, The Hague
Cook CDK (1998) Hydrocharitaceae In: Kubitzki K (ed) The
Families and Genera of Vascular Plants, Vol. IV. Flowering Plants, Monocotyledons: Alismatanae and Commelinanae
(except Gramineae), pp 234–248. Springer Verlag, Berlin