Rapid paling of the green segments also occurs in
response to damage during the day.
(ii) Every few days – relocation of chloroplasts beyond
their parent segment allows the new segments, which
develop overnight, to be populated with functional
chloroplasts before dawn, via the nodal filaments.
This allows calcification to start in the new segments
at first light, rather than awaiting subsequent activation of its proto-chloroplasts by light-dependent de
novo synthesis of chlorophyll.
(iii) Only once – immediately before death – facilitation of rapid overnight redistribution of the entire
contents of the medullary filaments into the superficial gametangia in preparation for holocarpic sexual
reproduction (see also Reproduction below).
Grazing
Halimeda is often found in abundance in areas on coral
reefs where herbivorous fish exert high grazing pressure,
but it is among the least preferred food items there. This
is probably because of the high levels of calcification,
a major physical defense against predation. In addition,
the new segments, which are very vulnerable until calcified, are always produced at night when grazing pressure
is low (Hay et al., 1988).
Halimeda also has chemical defenses against grazing. It
synthesizes two secondary natural products, the
diterpenoids halimedatetracetate and halimedatrial, which
have considerable grazing deterrence properties, but are
also potentially toxic to Halimeda itself. However,
although both are present at high concentrations in the
young, uncalcified segments, only reduced concentrations
of the less toxic halimedatetracetate occur in the older segments, which are now heavily calcified, and so less vulnerable (Paul and Alstyne, 1988a).
Although these mechanisms can help to minimize grazing on Halimeda, it can still be heavily grazed, especially
on seaward reef fronts where the lower parts of thalli,
which have been severely cropped by herbivorous fishes
are often found in inaccessible crevices. Once parts of
a Halimeda thallus have been damaged, its coenocytic
structure makes it vulnerable to catastrophic loss of cell
contents. However, constrictions at branching points in
the medullary filaments, and between the layers of cortical
utricles, can quickly become plugged by organelles
flowing toward the leak. This will retain the cell contents
while the rapid wound repair processes temporarily seal
the damaged area within a minute and construct a new
wall within an hour (Menzel, 1988).
A very specific and widespread grazer on Halimeda is
Elysia halimedea, a small Saccoglossan Ophistobranch
mollusc. It is a surface scraper completely undeterred by
the diterpenoids, which help keep other grazers away.
Instead, it actually modifies and accumulates the deterrent
chemicals up to 7% of its body weight and then, when irritated, it secretes a defensive mucilage containing them
(Paul and Alstyne, 1988b).
Elysia halimedea is about 20 mm long and extremely
cryptic as it is exactly the same color as the plants it feeds
on. This is because it uses modified radula teeth to harvest
intact chloroplasts from the outer utricles of Halimeda and
stores them in specialized “digestive cells” within its body.
This process is called kleptoplasty). The chloroplasts are
unable to divide within the animal but can continue to photosynthesize and supply it with nutrition for weeks or even
months (Trench, 1973; Evertsten et al., 2007). A number
of other saccoglossan molluscs also use chloroplasts from
other bryosidalean algae such as Caulerpa and Codium.
Another small animal closely associated with Halimeda
is the crab Huenia heraldica, which grazes on the filamentous epiphytes growing on the segments. It is about 25 mm
long, green in color, and has a wide, segment-like carapace. It does graze on Halimeda itself to a limited extent,
removing segments, and holding them in small specialized
claws on its upper body to complete its camouflage.
Reproduction
Halimeda plants bearing gametangia were first reported
by Derbes and Solier (1856) and Schmitz (1880). Both
also reported the release of biflagellate zooids, although
these were not demonstrated to be gametes until Nasr
(1947) reported sexual fusion. However, collection of fertile plants has been so infrequent that Hillis Colinvaux
(1980) still considered sexual reproduction to be of minor
significance in the production of new Halimeda plants
compared with asexual cloning involving rhizoidal runners and also the continued growth of dispersed plant
fragments.
The paucity of field observations of sexual reproduction in Halimeda, even after the advent of scuba diving,
is probably because it is completed in only 36 h. Healthy
green plants transfer their entire protoplasm overnight into
large numbers of newly formed, stalked bunches of gametangia that developed on the edges and surface of the segments the day before. The segments themselves are empty
and white by dawn. These conspicuous green gametangia
mature during that first day and explosively release their
contents as gametes soon after dawn on the following
day. This can produce a spectacular green soup of gametes
in an aquarium tank, leaving a white thallus that is dead
and soon disintegrates into a pile of calcified segments.
This process, whereby the plant dies after the whole
thallus has taken part in simultaneous conversion of the
entire cell contents to gametes, is called holocarpy.
Sexual reproduction in Halimeda is anisogamous and
dioecious, two types of gametes being produced, each on
different individuals. The larger “female” macro- gametes
are bright green and contain a distinct red eyespot while
the smaller ‘male’ microgametes are golden green and
have no eyespot (Hillis-Colinvaux, 1980). Gametangiabearing material is now known for most Halimeda species
and gamete release and fusion have been reported upon
in several, but development of zygotes has only been
studied in the laboratory in one species, H.tuna from the
HALIMEDA
537
response to damage during the day.
(ii) Every few days – relocation of chloroplasts beyond
their parent segment allows the new segments, which
develop overnight, to be populated with functional
chloroplasts before dawn, via the nodal filaments.
This allows calcification to start in the new segments
at first light, rather than awaiting subsequent activation of its proto-chloroplasts by light-dependent de
novo synthesis of chlorophyll.
(iii) Only once – immediately before death – facilitation of rapid overnight redistribution of the entire
contents of the medullary filaments into the superficial gametangia in preparation for holocarpic sexual
reproduction (see also Reproduction below).
Grazing
Halimeda is often found in abundance in areas on coral
reefs where herbivorous fish exert high grazing pressure,
but it is among the least preferred food items there. This
is probably because of the high levels of calcification,
a major physical defense against predation. In addition,
the new segments, which are very vulnerable until calcified, are always produced at night when grazing pressure
is low (Hay et al., 1988).
Halimeda also has chemical defenses against grazing. It
synthesizes two secondary natural products, the
diterpenoids halimedatetracetate and halimedatrial, which
have considerable grazing deterrence properties, but are
also potentially toxic to Halimeda itself. However,
although both are present at high concentrations in the
young, uncalcified segments, only reduced concentrations
of the less toxic halimedatetracetate occur in the older segments, which are now heavily calcified, and so less vulnerable (Paul and Alstyne, 1988a).
Although these mechanisms can help to minimize grazing on Halimeda, it can still be heavily grazed, especially
on seaward reef fronts where the lower parts of thalli,
which have been severely cropped by herbivorous fishes
are often found in inaccessible crevices. Once parts of
a Halimeda thallus have been damaged, its coenocytic
structure makes it vulnerable to catastrophic loss of cell
contents. However, constrictions at branching points in
the medullary filaments, and between the layers of cortical
utricles, can quickly become plugged by organelles
flowing toward the leak. This will retain the cell contents
while the rapid wound repair processes temporarily seal
the damaged area within a minute and construct a new
wall within an hour (Menzel, 1988).
A very specific and widespread grazer on Halimeda is
Elysia halimedea, a small Saccoglossan Ophistobranch
mollusc. It is a surface scraper completely undeterred by
the diterpenoids, which help keep other grazers away.
Instead, it actually modifies and accumulates the deterrent
chemicals up to 7% of its body weight and then, when irritated, it secretes a defensive mucilage containing them
(Paul and Alstyne, 1988b).
Elysia halimedea is about 20 mm long and extremely
cryptic as it is exactly the same color as the plants it feeds
on. This is because it uses modified radula teeth to harvest
intact chloroplasts from the outer utricles of Halimeda and
stores them in specialized “digestive cells” within its body.
This process is called kleptoplasty). The chloroplasts are
unable to divide within the animal but can continue to photosynthesize and supply it with nutrition for weeks or even
months (Trench, 1973; Evertsten et al., 2007). A number
of other saccoglossan molluscs also use chloroplasts from
other bryosidalean algae such as Caulerpa and Codium.
Another small animal closely associated with Halimeda
is the crab Huenia heraldica, which grazes on the filamentous epiphytes growing on the segments. It is about 25 mm
long, green in color, and has a wide, segment-like carapace. It does graze on Halimeda itself to a limited extent,
removing segments, and holding them in small specialized
claws on its upper body to complete its camouflage.
Reproduction
Halimeda plants bearing gametangia were first reported
by Derbes and Solier (1856) and Schmitz (1880). Both
also reported the release of biflagellate zooids, although
these were not demonstrated to be gametes until Nasr
(1947) reported sexual fusion. However, collection of fertile plants has been so infrequent that Hillis Colinvaux
(1980) still considered sexual reproduction to be of minor
significance in the production of new Halimeda plants
compared with asexual cloning involving rhizoidal runners and also the continued growth of dispersed plant
fragments.
The paucity of field observations of sexual reproduction in Halimeda, even after the advent of scuba diving,
is probably because it is completed in only 36 h. Healthy
green plants transfer their entire protoplasm overnight into
large numbers of newly formed, stalked bunches of gametangia that developed on the edges and surface of the segments the day before. The segments themselves are empty
and white by dawn. These conspicuous green gametangia
mature during that first day and explosively release their
contents as gametes soon after dawn on the following
day. This can produce a spectacular green soup of gametes
in an aquarium tank, leaving a white thallus that is dead
and soon disintegrates into a pile of calcified segments.
This process, whereby the plant dies after the whole
thallus has taken part in simultaneous conversion of the
entire cell contents to gametes, is called holocarpy.
Sexual reproduction in Halimeda is anisogamous and
dioecious, two types of gametes being produced, each on
different individuals. The larger “female” macro- gametes
are bright green and contain a distinct red eyespot while
the smaller ‘male’ microgametes are golden green and
have no eyespot (Hillis-Colinvaux, 1980). Gametangiabearing material is now known for most Halimeda species
and gamete release and fusion have been reported upon
in several, but development of zygotes has only been
studied in the laboratory in one species, H.tuna from the
HALIMEDA
537