Mediterranean (Meinesz, 1980). Development was reportedly very slow and had not progressed beyond a small,
uncalcified filamentous plant after 13 months. The characteristic Halimeda plant is presumed to be a haploid gametophyte, but it is not known when meiosis occurs, leading
to speculation that a second, spore-producing, phase may
be involved.
Some species of Halimeda may reproduce sexually at
species-specific times of the year, while synchrony
between field and laboratory populations, and also over
considerable areas of Halimeda meadows, has been
observed on the Great Barrier Reef (Drew and Abel,
1988b).
Recent observations show conclusively that sexual
reproduction in seven species of Halimeda as well as 15
other Bryopsidalean algae from four genera, is an organized and pervasive phenomenon of coral reefs (Clifton
and Clifton, 1999). It is comparable in synchrony and
precision with the mass spawning of the corals themselves. Mass spawning of these green algae is also equally
spectacular, releasing so many gametes that visibility on
shallow reefs can be greatly reduced. Unlike the corals,
however, in which entire populations spawn simultaneously on a single day, and live to spawn again next
year, only a small part of the algal populations spawn each
day, so that spawning can continue for several months, but
the participating plants die and the population is progressively reduced. The lifespan of Halimeda thalli may be
only months, to a maximum of a year, depending on
species.
Contribution to carbonate sediments
Halimeda segments may be shed intermittently by
living plants or in toto after holocarpic sexual reproduction. They contribute much to the structure of coral reefs
and atolls as they accumulate between the branching
framework produced by the corals themselves. In
some reefs, they also cascade down steep fore-reef
slopes to form thick sediment deposits on the seabed
(Freile et al., 1995).
In the outer lagoon of the Great Barrier Reef, Halimeda
can also form extensive “meadows” between reefs, growing atop mounds of Halimeda-rich gravels, which have
been deposited in situ by the meadows themselves (Drew
and Abel, 1988a). The substratum, of loose dead segments, is stable enough to support Halimeda vegetation
dominated by the same lithophytic species found on
nearby coral reefs, rather than species with holdfasts better
adapted to soft substrata. The individual mounds are typically 200–300 m in diameter, coalescing laterally to form
30 m thick banks several kilometers wide. These banks are
easily identified by their sinusoidally undulating surface
and characteristic seismic signature. They are usually
associated with up-welling of cooler, nutrient-rich water
from below the thermocline in adjacent oceanic waters
(Wolanski et al., 1988), and have accumulated since the
underlying carbonate rock was exposed during the last
low sea-level event 10,000 years ago. They have actually
grown faster than the nearby coral reefs. Pinnacles of the
underlying Pleistocene-age bedrock frequently occur in
the hollows between the mounds of Halimeda-rich gravels
(Phipps et al., 1985). Similar Halimeda banks have been
found in Indonesian waters (Phipps and Roberts, 1988).
The banks closely resemble geological structures
known as “bioherms” – dome-shaped structures composed mainly of the remains of one type of animal or plant
(Marshall and Davies, 1988). Fossilized Halimeda
bioherms are known from India, the Caribbean, and Spain.
The latter form part of aerially exposed reefs and are
described as “segment reefs.”
Fossil Halimeda
The unlithified banks of Halimeda-rich gravels described
above are potential precursors of the carbonate
wackestones, rocks composed mainly of fossilized
Halimeda segments, which occurred in fossil reefs as far
back as the early Jurassic, circa 190 Ma ago. The carbonate structure is often very well defined and the rocks are
relatively porous, due to voids between the loosely packed
segments deposited in situ. There are also micro-voids
within the segments reflecting the internal anatomy of
the algae themselves.
These deposits are very similar to much earlier calcareous fossil algae (collectively known as Phylloid algae),
which are widespread in ancient reefs in many parts of
the world. Here, their considerable porosity has often
allowed the accumulation of hydrocarbons (Wray, 1977).
Bibliography
Author’s Halimeda website: http://members.iinet.net.au/$edrew1/
halimeda/halimeda.htm
Barton, E. S., 1901. The Genus Halimeda. Leiden. Monographs of
the Siboga Expedition Vol. 60, p. 32.
Borowitzka, M. M., and Larkum, A. W. D., 1976. Calcification in
the green alga Halimeda:.III. The sources of inorganic carbon
for photosynthesis and calcification and a model for the mechanism of calcification. Journal of Experimental Botany, 27,
879–893.
Clifton, K. E, and Clifton, L. M., 1999. The phenology of sexual
reproduction by green algae (Bryopsidales) on Caribbean coral
reefs. Journal of Phycology, 35, 24–34.
Derbes, A. A., and Solier, J. J., 1856. Memoire sur quelques points
de la physiologie des algues. Comptes Rendus Hebdomadaires
des Seances de l’ Academie des Sciences Paris, 1(suppl.),
1–120.
Drew, E. A., and Abel, K. M., 1988a. Studies on Halimeda I. The
distribution and species composition of Halimeda meadows
throughout the Great Barrier Reef Province. Coral Reefs, 6,
195–205.
Drew, E. A., and Abel, K. M., 1988b. Studies on Halimeda II.
Reproduction, and particularly the seasonality of gametangia
formation, in a number of species from the Great Barrier Reef
Province. Coral Reefs, 6, 207–218.
Drew, E. A., and Abel, K. M., 1990. Studies on Halimeda III.
A daily cycle of chloroplast migration within segments. Botanica Marina, 33, 31–45.
538
HALIMEDA
uncalcified filamentous plant after 13 months. The characteristic Halimeda plant is presumed to be a haploid gametophyte, but it is not known when meiosis occurs, leading
to speculation that a second, spore-producing, phase may
be involved.
Some species of Halimeda may reproduce sexually at
species-specific times of the year, while synchrony
between field and laboratory populations, and also over
considerable areas of Halimeda meadows, has been
observed on the Great Barrier Reef (Drew and Abel,
1988b).
Recent observations show conclusively that sexual
reproduction in seven species of Halimeda as well as 15
other Bryopsidalean algae from four genera, is an organized and pervasive phenomenon of coral reefs (Clifton
and Clifton, 1999). It is comparable in synchrony and
precision with the mass spawning of the corals themselves. Mass spawning of these green algae is also equally
spectacular, releasing so many gametes that visibility on
shallow reefs can be greatly reduced. Unlike the corals,
however, in which entire populations spawn simultaneously on a single day, and live to spawn again next
year, only a small part of the algal populations spawn each
day, so that spawning can continue for several months, but
the participating plants die and the population is progressively reduced. The lifespan of Halimeda thalli may be
only months, to a maximum of a year, depending on
species.
Contribution to carbonate sediments
Halimeda segments may be shed intermittently by
living plants or in toto after holocarpic sexual reproduction. They contribute much to the structure of coral reefs
and atolls as they accumulate between the branching
framework produced by the corals themselves. In
some reefs, they also cascade down steep fore-reef
slopes to form thick sediment deposits on the seabed
(Freile et al., 1995).
In the outer lagoon of the Great Barrier Reef, Halimeda
can also form extensive “meadows” between reefs, growing atop mounds of Halimeda-rich gravels, which have
been deposited in situ by the meadows themselves (Drew
and Abel, 1988a). The substratum, of loose dead segments, is stable enough to support Halimeda vegetation
dominated by the same lithophytic species found on
nearby coral reefs, rather than species with holdfasts better
adapted to soft substrata. The individual mounds are typically 200–300 m in diameter, coalescing laterally to form
30 m thick banks several kilometers wide. These banks are
easily identified by their sinusoidally undulating surface
and characteristic seismic signature. They are usually
associated with up-welling of cooler, nutrient-rich water
from below the thermocline in adjacent oceanic waters
(Wolanski et al., 1988), and have accumulated since the
underlying carbonate rock was exposed during the last
low sea-level event 10,000 years ago. They have actually
grown faster than the nearby coral reefs. Pinnacles of the
underlying Pleistocene-age bedrock frequently occur in
the hollows between the mounds of Halimeda-rich gravels
(Phipps et al., 1985). Similar Halimeda banks have been
found in Indonesian waters (Phipps and Roberts, 1988).
The banks closely resemble geological structures
known as “bioherms” – dome-shaped structures composed mainly of the remains of one type of animal or plant
(Marshall and Davies, 1988). Fossilized Halimeda
bioherms are known from India, the Caribbean, and Spain.
The latter form part of aerially exposed reefs and are
described as “segment reefs.”
Fossil Halimeda
The unlithified banks of Halimeda-rich gravels described
above are potential precursors of the carbonate
wackestones, rocks composed mainly of fossilized
Halimeda segments, which occurred in fossil reefs as far
back as the early Jurassic, circa 190 Ma ago. The carbonate structure is often very well defined and the rocks are
relatively porous, due to voids between the loosely packed
segments deposited in situ. There are also micro-voids
within the segments reflecting the internal anatomy of
the algae themselves.
These deposits are very similar to much earlier calcareous fossil algae (collectively known as Phylloid algae),
which are widespread in ancient reefs in many parts of
the world. Here, their considerable porosity has often
allowed the accumulation of hydrocarbons (Wray, 1977).
Bibliography
Author’s Halimeda website: http://members.iinet.net.au/$edrew1/
halimeda/halimeda.htm
Barton, E. S., 1901. The Genus Halimeda. Leiden. Monographs of
the Siboga Expedition Vol. 60, p. 32.
Borowitzka, M. M., and Larkum, A. W. D., 1976. Calcification in
the green alga Halimeda:.III. The sources of inorganic carbon
for photosynthesis and calcification and a model for the mechanism of calcification. Journal of Experimental Botany, 27,
879–893.
Clifton, K. E, and Clifton, L. M., 1999. The phenology of sexual
reproduction by green algae (Bryopsidales) on Caribbean coral
reefs. Journal of Phycology, 35, 24–34.
Derbes, A. A., and Solier, J. J., 1856. Memoire sur quelques points
de la physiologie des algues. Comptes Rendus Hebdomadaires
des Seances de l’ Academie des Sciences Paris, 1(suppl.),
1–120.
Drew, E. A., and Abel, K. M., 1988a. Studies on Halimeda I. The
distribution and species composition of Halimeda meadows
throughout the Great Barrier Reef Province. Coral Reefs, 6,
195–205.
Drew, E. A., and Abel, K. M., 1988b. Studies on Halimeda II.
Reproduction, and particularly the seasonality of gametangia
formation, in a number of species from the Great Barrier Reef
Province. Coral Reefs, 6, 207–218.
Drew, E. A., and Abel, K. M., 1990. Studies on Halimeda III.
A daily cycle of chloroplast migration within segments. Botanica Marina, 33, 31–45.
538
HALIMEDA