Reproduction of Corals
291
The size of planulae hatched by viviparous corals varies between 0.5-1
and 0.8-3 mm. They have a large stock of steroid lipids which in the
newborn cells comprises up to 70% of the dry weight. This stock of material
of high caloric value serves as a source of energy enabling them to survive
until they settle. Another source of energy is photosynthetic products,
translocated by symbiotic zooxanthellae, which they acquire from their
parents via eggs, or through infection by free-living zooxanthellae cells after
hatching (Richmond 1981; Babcock et al. 1986). Hatched planulae swim in
the water column, part of them in populations of the same species crawl on
the bottom (Lewis 1974b). Often they come together and form aggregates.
The new-born planulae have positive phototaxis which keeps them in the
water column (Kawaguti 1941; Harrigan 1972). Several days later their taxis
changes to negative. Then the planulae settle on the bottom and start to
seek place for recruiting. They try to find shadowed hidden places of rocky
ground. The planulae of different corals display definite selectivity with
respect to substrates to settle on, being able to select the biotopes where the
adult corals prefer to live (Yonge 1973). They seem to be able recognize the
young colonies of the same species near which they prefer to settle (Lewis
1974a). The planulae of free-living fungiid corals, living in adult age on a
soft or rubble bottom, settle first on hard rocks and grow up attached to the
surface by stalks like mushrooms. Later, the stalks break and the polyps on
being set free are transferred to their appropriate places by wave action.
Upon being settled, the planulae start their metamorphosis. Within 1530 h the apical end of the mouth divides and six initial tentacles appear. In
its mesentery new septae appear and their total number will soon reach 12.
After 1-3 days the formation of the exoskeleton starts. After 4-6 days the
walls of the corallite start to grow and in the course of 10-20 days the
planula transforms into corallite with a single polyp in it. Within 2-3 weeks
this corallite reaches the size of 2-5 mm and in its ectoderm around its edges
the buds appear, which after passing through the same stages of
development as the first corallite, give start to the formation of a new
colony. Within a year the settled young colony reaches a size of 3-8 mm in
spawning corals, and up to 10-15 mm in the viviparous corals Stylophora
and Porites asteroids. The fast-growing spat of Pocillopora reaches the
above size in 3-5 months (Coles 1985; Sato 1985).
The planulae of Pocillopora damicornis were found to be able to reverse
their morphogenesis, which is a unique feature among the larvae of
invertebrates (Richmond 1985). These planulae also settled and formed the
first (larval) corallites, which on being stressed (by excess of light, for
example) may abandon them and return again to the planktonic stage,
whence they are able to settle again, thus escaping the possibility to die in
unfavorable conditions. Among the group of planulae hatched by this coral
there could be several non settling specimens, which pass through their
metamorphosis up to the stage of polyp, remaining still in the planktonic
stage. The-free living polyps thus formed spend some time in the water
291
The size of planulae hatched by viviparous corals varies between 0.5-1
and 0.8-3 mm. They have a large stock of steroid lipids which in the
newborn cells comprises up to 70% of the dry weight. This stock of material
of high caloric value serves as a source of energy enabling them to survive
until they settle. Another source of energy is photosynthetic products,
translocated by symbiotic zooxanthellae, which they acquire from their
parents via eggs, or through infection by free-living zooxanthellae cells after
hatching (Richmond 1981; Babcock et al. 1986). Hatched planulae swim in
the water column, part of them in populations of the same species crawl on
the bottom (Lewis 1974b). Often they come together and form aggregates.
The new-born planulae have positive phototaxis which keeps them in the
water column (Kawaguti 1941; Harrigan 1972). Several days later their taxis
changes to negative. Then the planulae settle on the bottom and start to
seek place for recruiting. They try to find shadowed hidden places of rocky
ground. The planulae of different corals display definite selectivity with
respect to substrates to settle on, being able to select the biotopes where the
adult corals prefer to live (Yonge 1973). They seem to be able recognize the
young colonies of the same species near which they prefer to settle (Lewis
1974a). The planulae of free-living fungiid corals, living in adult age on a
soft or rubble bottom, settle first on hard rocks and grow up attached to the
surface by stalks like mushrooms. Later, the stalks break and the polyps on
being set free are transferred to their appropriate places by wave action.
Upon being settled, the planulae start their metamorphosis. Within 1530 h the apical end of the mouth divides and six initial tentacles appear. In
its mesentery new septae appear and their total number will soon reach 12.
After 1-3 days the formation of the exoskeleton starts. After 4-6 days the
walls of the corallite start to grow and in the course of 10-20 days the
planula transforms into corallite with a single polyp in it. Within 2-3 weeks
this corallite reaches the size of 2-5 mm and in its ectoderm around its edges
the buds appear, which after passing through the same stages of
development as the first corallite, give start to the formation of a new
colony. Within a year the settled young colony reaches a size of 3-8 mm in
spawning corals, and up to 10-15 mm in the viviparous corals Stylophora
and Porites asteroids. The fast-growing spat of Pocillopora reaches the
above size in 3-5 months (Coles 1985; Sato 1985).
The planulae of Pocillopora damicornis were found to be able to reverse
their morphogenesis, which is a unique feature among the larvae of
invertebrates (Richmond 1985). These planulae also settled and formed the
first (larval) corallites, which on being stressed (by excess of light, for
example) may abandon them and return again to the planktonic stage,
whence they are able to settle again, thus escaping the possibility to die in
unfavorable conditions. Among the group of planulae hatched by this coral
there could be several non settling specimens, which pass through their
metamorphosis up to the stage of polyp, remaining still in the planktonic
stage. The-free living polyps thus formed spend some time in the water