introduced by Barton (1901) and expanded by Hillis
(1959); Hillis-Colinvaux (1980) also defined five distinct
taxonomic Sections within the genus, based on anatomical
details of the uncalcified nodes (see also “Internal structure” below).
Molecular systematics, first applied to the genus by
Hillis et al. (1998) and expanded by Kooistra et al.
(2002), yielded (i) a well-resolved phylogeny, (ii) confirmation of the five lineages, but requiring some re-organization,
and (iii) demonstration of high cryptic species diversity,
most tropical Halimeda species found consisting of two distantly related, genetic entities, which are indistinguishable
by current methods of morpho-taxonomy. Verbruggen
(2005) provides a detailed account of recent taxonomic progress within the genus.
Distribution
Halimeda is a warm water alga well represented in the
tropics, mainly on coral reefs. There is one species,
H. cuneata, which is strictly subtropical, while a genetically
distinct form of the tropical species H. tuna is the only species found in the temperate Mediterranean Sea.
Of the 30 species then recognized by Hillis Colinvaux
in 1980, 77% occurred in the Indo-Pacific Region and
46% in the Western Atlantic Region (Caribbean). Of the
13 species in the Caribbean, seven also occurred in the
Indo-Pacific. The remaining six endemic species are presumed to have evolved after the rise of the Isthmus of Panama closed the Central American Seaway, separating the
Western Atlantic from the Pacific in the Miocene (circa
3.5 Ma ago).
On coral reefs, Halimeda can occur from protected
lagoons to the shallowest reef flat environments, exposed
seaward reef fronts and as deep as 140 m on fore reef
slopes (Hillis-Colinvaux, 1985; Littler et al., 1986) (see
also “Contribution to carbonate sediments” below).
While Halimeda is the only genus in the family
Halimedaceae, it is closely related to the larger family
Udoteaceae, which contains several other calcified,
green macro algae, such as Udotea, Penicillus, and
Rhipocephalus. These are often conspicuous in Halimeda
habitats, as are several species of the closely related, but
uncalcified, genus Caulerpa (family Caulerpaceae).
Internal structure
Halimeda segments consist of a central medulla of
intertwining, branched, and sometimes anastomosing
siphons (medullary filaments) running the length of the
segment. These segments, with their large vacuoles, are
continuous throughout the plant with a central few
forming the uncalcified nodal region, a flexible joint
between the segments. Within the nodes, the walls of the
filaments may become thickened, fuse for part of their
length, and even develop connecting pores, but there are
no cross walls anywhere in the plant. The entire thallus
is thus one multinucleate cell and is described as
a coenocyte.
Near the segment surface, the medullary filaments
branch vertically to form a cortex consisting of up to four
layers of variously inflated utricles. The top layer (the primary utricles) is closely packed and appears polygonal in
surface view. The size, shape, and branching patterns of
these utricles are important taxonomic characters (HillisColinvaux, 1980).
Growth
Individual Halimeda thalli grow by producing new segments. These develop from small tufts of medullary filaments, which grow out from the distal perimeter of
a segment at or near the tip of a branch. This process
begins in the afternoon and the tuft of filaments becomes
organized into a new soft, uncalcified segment overnight.
This new segment is initially white, but by dawn it is
bright green. A few hours after dawn it is whitish green,
having already begun to calcify (see also “Diurnal color
change (ii)” below). Fast growing branch tips can produce
a new segment every few days.
Calcification
The calcium carbonate endoskeleton is laid down in the
internal spaces between the medullary filaments forming
the segments. This is a light-driven process, requiring only
that the superficial utricles contain functional chloroplasts
and are closely adpressed, effectively isolating the
interfilamental spaces from the external seawater. This
happens within 36 h of initiation of the new segment.
Thereafter, whenever photosynthesis occurs, the resulting
removal of CO 2 from the water in the interfilamental spaces
raises its pH and causes deposition of calcium carbonate as
aragonite crystals (Borowitzka and Larkum, 1976).
The overall degree of calcification varies (i) between
species; (ii) within the algal thallus, with the lower, older
segments more calcified than those at the growing tip;
and (iii) with water depth and levels of irradiance.
Diurnal color change
Halimeda thalli become almost white after dark and then
green again by next morning. This is due to mass relocation of chloroplasts, and probably other organelles, from
the peripheral utricles into the medullary filaments. There
they remain, hidden beneath the calcium carbonate endoskeleton, until re-emergence begins a few hours before
dawn. This mass relocation of organelles, which can only
occur because of the coenocytic structure of the thalli,
involves an extensive cytoskeleton of microtules and actin
fibers and will consume a considerable amount of energy
every day (Drew and Abel, 1990; 1992). This energyintensive endogenous rhythm may, however, give
Halimeda crucial advantages in the dynamic coral reef
environment, including
(i) Daily – minimization of grazing damage because
chloroplasts and other organelles are protected
within the calcium carbonate endoskeleton when surface-scraping grazers are most likely to be active.
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HALIMEDA
(1959); Hillis-Colinvaux (1980) also defined five distinct
taxonomic Sections within the genus, based on anatomical
details of the uncalcified nodes (see also “Internal structure” below).
Molecular systematics, first applied to the genus by
Hillis et al. (1998) and expanded by Kooistra et al.
(2002), yielded (i) a well-resolved phylogeny, (ii) confirmation of the five lineages, but requiring some re-organization,
and (iii) demonstration of high cryptic species diversity,
most tropical Halimeda species found consisting of two distantly related, genetic entities, which are indistinguishable
by current methods of morpho-taxonomy. Verbruggen
(2005) provides a detailed account of recent taxonomic progress within the genus.
Distribution
Halimeda is a warm water alga well represented in the
tropics, mainly on coral reefs. There is one species,
H. cuneata, which is strictly subtropical, while a genetically
distinct form of the tropical species H. tuna is the only species found in the temperate Mediterranean Sea.
Of the 30 species then recognized by Hillis Colinvaux
in 1980, 77% occurred in the Indo-Pacific Region and
46% in the Western Atlantic Region (Caribbean). Of the
13 species in the Caribbean, seven also occurred in the
Indo-Pacific. The remaining six endemic species are presumed to have evolved after the rise of the Isthmus of Panama closed the Central American Seaway, separating the
Western Atlantic from the Pacific in the Miocene (circa
3.5 Ma ago).
On coral reefs, Halimeda can occur from protected
lagoons to the shallowest reef flat environments, exposed
seaward reef fronts and as deep as 140 m on fore reef
slopes (Hillis-Colinvaux, 1985; Littler et al., 1986) (see
also “Contribution to carbonate sediments” below).
While Halimeda is the only genus in the family
Halimedaceae, it is closely related to the larger family
Udoteaceae, which contains several other calcified,
green macro algae, such as Udotea, Penicillus, and
Rhipocephalus. These are often conspicuous in Halimeda
habitats, as are several species of the closely related, but
uncalcified, genus Caulerpa (family Caulerpaceae).
Internal structure
Halimeda segments consist of a central medulla of
intertwining, branched, and sometimes anastomosing
siphons (medullary filaments) running the length of the
segment. These segments, with their large vacuoles, are
continuous throughout the plant with a central few
forming the uncalcified nodal region, a flexible joint
between the segments. Within the nodes, the walls of the
filaments may become thickened, fuse for part of their
length, and even develop connecting pores, but there are
no cross walls anywhere in the plant. The entire thallus
is thus one multinucleate cell and is described as
a coenocyte.
Near the segment surface, the medullary filaments
branch vertically to form a cortex consisting of up to four
layers of variously inflated utricles. The top layer (the primary utricles) is closely packed and appears polygonal in
surface view. The size, shape, and branching patterns of
these utricles are important taxonomic characters (HillisColinvaux, 1980).
Growth
Individual Halimeda thalli grow by producing new segments. These develop from small tufts of medullary filaments, which grow out from the distal perimeter of
a segment at or near the tip of a branch. This process
begins in the afternoon and the tuft of filaments becomes
organized into a new soft, uncalcified segment overnight.
This new segment is initially white, but by dawn it is
bright green. A few hours after dawn it is whitish green,
having already begun to calcify (see also “Diurnal color
change (ii)” below). Fast growing branch tips can produce
a new segment every few days.
Calcification
The calcium carbonate endoskeleton is laid down in the
internal spaces between the medullary filaments forming
the segments. This is a light-driven process, requiring only
that the superficial utricles contain functional chloroplasts
and are closely adpressed, effectively isolating the
interfilamental spaces from the external seawater. This
happens within 36 h of initiation of the new segment.
Thereafter, whenever photosynthesis occurs, the resulting
removal of CO 2 from the water in the interfilamental spaces
raises its pH and causes deposition of calcium carbonate as
aragonite crystals (Borowitzka and Larkum, 1976).
The overall degree of calcification varies (i) between
species; (ii) within the algal thallus, with the lower, older
segments more calcified than those at the growing tip;
and (iii) with water depth and levels of irradiance.
Diurnal color change
Halimeda thalli become almost white after dark and then
green again by next morning. This is due to mass relocation of chloroplasts, and probably other organelles, from
the peripheral utricles into the medullary filaments. There
they remain, hidden beneath the calcium carbonate endoskeleton, until re-emergence begins a few hours before
dawn. This mass relocation of organelles, which can only
occur because of the coenocytic structure of the thalli,
involves an extensive cytoskeleton of microtules and actin
fibers and will consume a considerable amount of energy
every day (Drew and Abel, 1990; 1992). This energyintensive endogenous rhythm may, however, give
Halimeda crucial advantages in the dynamic coral reef
environment, including
(i) Daily – minimization of grazing damage because
chloroplasts and other organelles are protected
within the calcium carbonate endoskeleton when surface-scraping grazers are most likely to be active.
536
HALIMEDA