Chapter 1 Taxonomy of Seagrasses
13
did not describe the species on which the genus
was based. Steudel (1840, p. 515) published the
name H. madagascariensis without a description of
the species, thus not validly, but the geographical
indication makes it clear that he must have referred
to the material of Thouars; at that time there were no
other collections of Halophila from Madagascar in
the Paris herbarium. Doty and Stone (1967) validated
the name after 127 years. In 1970 den Hartog
considered this species still as a synonym of H.
ovalis, which is based on Caulinia ovalis from Australia (K, BM), and did not give H. madagascariensis
even the status of a subspecies. After a new study we
now agree that the two taxa indeed show some differences. There is, however, another option for the
typification of the genus, as H. ovata was the first
species described and illustrated within the genus.
In the genus Halophila five sections have been
described, based on differences in the gross vegetative morphology of the plants. These sections do not
show differences with respect to the structure of the
flowers; the variation in the number of styles, for example, may differ between populations of the same
species. Differences in the way of arrangement of the
flowers into inflorescences seem more characteristic
at the species level; the same holds for monoecy and
dioecy.
Most species can be classified within the typical section, Halophila sect. Halophila. This section
contains all Halophila species with one pair of petiolate leaves born on short erect lateral shoots. It
is the morphologically most diverse group and its
geographical distribution coincides with that of the
whole genus. The species of this section occur in various habitats and show a large morphological variability; some of these variants can be treated as independent taxa in their own right. Currently, there are
ten described species: Halophila ovalis (consisting
of four subspecies), H . madagascariensis, H. ovata,
H. minor, H. australis, H. johnsonii, H. decipiens, H.
capricorni, H. stipulacea, and H. hawaiiana. These
species and the subspecies of H. ovalis have recently
been briefly redescribed, with the exception of H.
madagascariensis (Kuo and den Hartog 2001).
Most of the species of the section Halophila are
restricted in their occurrence. Only H . decipiens is
widely distributed occurring in both the northern and
the southern hemisphere, along the tropical and subtropical coasts of the Indian, the Pacific, and the
Atlantic Oceans, but the species has not been
recorded so far from the Mediterranean.
H. ovalis is widely distributed in the tropical IndoWest Pacific and penetrates in some areas beyond
the tropics, e.g. in Western Australia, and in Japan as
indicated by Miki (1934). H. minor has also a wide
area of distribution, from East Africa to the western
Pacific, but it does not extend beyond the tropics. H.
stipulacea, is very common along the eastern coasts
of Africa, in the Red Sea and the Persian Gulf, and
also occurs on Madagascar and along the west coast
of India. This species has invaded the Mediterranean
via the Suez Canal and is there still expanding its
area of distribution (Den Hartog, unpublished). H.
australis has a wide distribution in the temperate
seas of southern Australia, H. capricorni is restricted
to the Coral Sea, and H. ovata occurs only in the
northern part of the western Pacific (Saipan, Guam,
Yap, Manila Bay) (Kuo 2000). H. johnsonii is the
rarest species as it is restricted to one area in Florida.
It appears that various morphologically distinct
species in this section could not be distinguished by
a recent molecular study (Waycott et al. 2002). McDermid et al. (2003) reported that several morphological variations with little genetic variation occur
in populations of H. hawaiiana from different islands of the Hawaiian Archipelago. Procaccini et al.
(1999b) found that the recently established populations of H. stipulacea on Sicily exhibited both significant morphological variations with depth and site as
well as high genetic polymorphism, but these variations appeared to show no correlation. These authors
suggested that this phenomenon might be influenced
by environmental conditions and through vegetative
or other means of asexual reproduction; however,
they did not specify which environmental factors and
did not make an effort for collecting or observing reproductive materials from different populations.
The section Spinulosae Ostenfeld is characterized
by having much elongated, stiff erect lateral shoots
bearing up to 20 pairs of distichously arranged sessile leaves at the nodes. The section consists of only
one species, H. spinulosa, which is morphologically
fairly homogeneous. Japar Sidik et al. (2000) reported, however, that there are some populations in
Malaysia where the leaves are tristichously arranged.
The species is common in Malaya, and extends via
Indonesia where it is rare, to New Guinea and tropical and subtropical Australia. There are no records
of the species from Thailand and Vietnam; the most
northern record is from Luzon, Philippines.
The section Microhalophila Aschers. is characterized by the possession of distinct erect lateral shoots,
13
did not describe the species on which the genus
was based. Steudel (1840, p. 515) published the
name H. madagascariensis without a description of
the species, thus not validly, but the geographical
indication makes it clear that he must have referred
to the material of Thouars; at that time there were no
other collections of Halophila from Madagascar in
the Paris herbarium. Doty and Stone (1967) validated
the name after 127 years. In 1970 den Hartog
considered this species still as a synonym of H.
ovalis, which is based on Caulinia ovalis from Australia (K, BM), and did not give H. madagascariensis
even the status of a subspecies. After a new study we
now agree that the two taxa indeed show some differences. There is, however, another option for the
typification of the genus, as H. ovata was the first
species described and illustrated within the genus.
In the genus Halophila five sections have been
described, based on differences in the gross vegetative morphology of the plants. These sections do not
show differences with respect to the structure of the
flowers; the variation in the number of styles, for example, may differ between populations of the same
species. Differences in the way of arrangement of the
flowers into inflorescences seem more characteristic
at the species level; the same holds for monoecy and
dioecy.
Most species can be classified within the typical section, Halophila sect. Halophila. This section
contains all Halophila species with one pair of petiolate leaves born on short erect lateral shoots. It
is the morphologically most diverse group and its
geographical distribution coincides with that of the
whole genus. The species of this section occur in various habitats and show a large morphological variability; some of these variants can be treated as independent taxa in their own right. Currently, there are
ten described species: Halophila ovalis (consisting
of four subspecies), H . madagascariensis, H. ovata,
H. minor, H. australis, H. johnsonii, H. decipiens, H.
capricorni, H. stipulacea, and H. hawaiiana. These
species and the subspecies of H. ovalis have recently
been briefly redescribed, with the exception of H.
madagascariensis (Kuo and den Hartog 2001).
Most of the species of the section Halophila are
restricted in their occurrence. Only H . decipiens is
widely distributed occurring in both the northern and
the southern hemisphere, along the tropical and subtropical coasts of the Indian, the Pacific, and the
Atlantic Oceans, but the species has not been
recorded so far from the Mediterranean.
H. ovalis is widely distributed in the tropical IndoWest Pacific and penetrates in some areas beyond
the tropics, e.g. in Western Australia, and in Japan as
indicated by Miki (1934). H. minor has also a wide
area of distribution, from East Africa to the western
Pacific, but it does not extend beyond the tropics. H.
stipulacea, is very common along the eastern coasts
of Africa, in the Red Sea and the Persian Gulf, and
also occurs on Madagascar and along the west coast
of India. This species has invaded the Mediterranean
via the Suez Canal and is there still expanding its
area of distribution (Den Hartog, unpublished). H.
australis has a wide distribution in the temperate
seas of southern Australia, H. capricorni is restricted
to the Coral Sea, and H. ovata occurs only in the
northern part of the western Pacific (Saipan, Guam,
Yap, Manila Bay) (Kuo 2000). H. johnsonii is the
rarest species as it is restricted to one area in Florida.
It appears that various morphologically distinct
species in this section could not be distinguished by
a recent molecular study (Waycott et al. 2002). McDermid et al. (2003) reported that several morphological variations with little genetic variation occur
in populations of H. hawaiiana from different islands of the Hawaiian Archipelago. Procaccini et al.
(1999b) found that the recently established populations of H. stipulacea on Sicily exhibited both significant morphological variations with depth and site as
well as high genetic polymorphism, but these variations appeared to show no correlation. These authors
suggested that this phenomenon might be influenced
by environmental conditions and through vegetative
or other means of asexual reproduction; however,
they did not specify which environmental factors and
did not make an effort for collecting or observing reproductive materials from different populations.
The section Spinulosae Ostenfeld is characterized
by having much elongated, stiff erect lateral shoots
bearing up to 20 pairs of distichously arranged sessile leaves at the nodes. The section consists of only
one species, H. spinulosa, which is morphologically
fairly homogeneous. Japar Sidik et al. (2000) reported, however, that there are some populations in
Malaysia where the leaves are tristichously arranged.
The species is common in Malaya, and extends via
Indonesia where it is rare, to New Guinea and tropical and subtropical Australia. There are no records
of the species from Thailand and Vietnam; the most
northern record is from Luzon, Philippines.
The section Microhalophila Aschers. is characterized by the possession of distinct erect lateral shoots,