3.2 Reproduction in Scleractinian Corals
57
planktonic larval dispersal, larval settlement, post-settlement growth, and survival
(Ceh et al. 2012).
Corals depict a simple life cycle involving two phases: a dominant benthic polyp
phase and a shorter planula larval phase. The polyp phase is characterized by growth
of tissues and skeleton that often includes one or more forms of asexual budding or
reproduction; and repeated cycles of sexual reproduction involving the production
of gametes, fertilization, embryo development, and a larval phase that is usually
planktonic and dispersive to some degree (Harrison and Wallace 1990). Survival of
the planula and successful attachment on a hard substratum enables it to form into
a juvenile polyp which initiates the formation of the calcium carbonate exoskeleton.
Subsequent growth during an initial presexual juvenile phase leads to development
of the adult form that becomes sexually reproductive, which completes the life cycle
(Harrison and Wallace 1990).
Majority of the reef-building corals reproduce sexually and asexually (Combosch
and Vollmer 2013). Sexual reproduction allows genetic recombination and production of new coral genotypes, thus enhancing fitness and survival of the species
(Harrison 2011). Sexual reproduction occurs by either free spawning eggs and sperm
or internally brooding larvae inside the coral polyp (Harrison 2011) with the resulting
planula dispersing, settling and metamorphosing to form new coral polyps; which
grow by asexual fission of individual polyps (Combosch and Vollmer 2013).
Asexual reproduction; referred to as fragmentation; occurs when new colonies
form from branching coral, which are broken by physical forces such as wave action.
Asexual reproduction produces genetically identical modules that may prolong the
survival of the genotype (Harrison 2011).
Corals could be hermaphroditic or gonochoric in nature and thus Harrison
(2011) reported that four basic patterns of sexual reproduction are evident among
corals, which include: hermaphroditic broadcast spawners, hermaphroditic brooders,
gonochoric broadcast spawners and gonochoric brooders.
Hermaphroditic corals consist of both sexes developed in their polyps and
colonies, whereas gonochoric corals have separate sexes; and corals with these sexual
patterns either broadcast spawn their gametes for external fertilization and subsequent
embryo and larval development, or have internal fertilization and brood embryos and
planula larvae within their polyps (reviewed by Harrison and Wallace 1990). In case
of hermaphroditic coral species, eggs and spermaries develop either on the same
mesenteries (such as in the faviids and mussids) or on different mesenteries within
the same polyp (as in most pocilloporids and acroporids) or in different polyps within
the same colony (Fadlallah 1983, Harrison and Wallace 1990). For example, Ward
and Harrison (2000) mentioned that Acroporids are hermaphroditic with eggs and
spermaries on different mesenteries within the same polyp.
Harrison (2011) and Kerr et al. (2011) highlighted that approximately 70–75% of
corals are hermaphroditic, while remaining 25–30% gonochoric. Oocytes and testes
develop within the same polyp in hermaphroditic corals whilst in gonochoric corals
male colonies produce sperm and female colonies produce oocytes (Hidaka 2016).
Many scleractinian corals reproduce by broadcast spawning and are referred to
as broadcast spawners. There are release of gamete bundles from different coral
57
planktonic larval dispersal, larval settlement, post-settlement growth, and survival
(Ceh et al. 2012).
Corals depict a simple life cycle involving two phases: a dominant benthic polyp
phase and a shorter planula larval phase. The polyp phase is characterized by growth
of tissues and skeleton that often includes one or more forms of asexual budding or
reproduction; and repeated cycles of sexual reproduction involving the production
of gametes, fertilization, embryo development, and a larval phase that is usually
planktonic and dispersive to some degree (Harrison and Wallace 1990). Survival of
the planula and successful attachment on a hard substratum enables it to form into
a juvenile polyp which initiates the formation of the calcium carbonate exoskeleton.
Subsequent growth during an initial presexual juvenile phase leads to development
of the adult form that becomes sexually reproductive, which completes the life cycle
(Harrison and Wallace 1990).
Majority of the reef-building corals reproduce sexually and asexually (Combosch
and Vollmer 2013). Sexual reproduction allows genetic recombination and production of new coral genotypes, thus enhancing fitness and survival of the species
(Harrison 2011). Sexual reproduction occurs by either free spawning eggs and sperm
or internally brooding larvae inside the coral polyp (Harrison 2011) with the resulting
planula dispersing, settling and metamorphosing to form new coral polyps; which
grow by asexual fission of individual polyps (Combosch and Vollmer 2013).
Asexual reproduction; referred to as fragmentation; occurs when new colonies
form from branching coral, which are broken by physical forces such as wave action.
Asexual reproduction produces genetically identical modules that may prolong the
survival of the genotype (Harrison 2011).
Corals could be hermaphroditic or gonochoric in nature and thus Harrison
(2011) reported that four basic patterns of sexual reproduction are evident among
corals, which include: hermaphroditic broadcast spawners, hermaphroditic brooders,
gonochoric broadcast spawners and gonochoric brooders.
Hermaphroditic corals consist of both sexes developed in their polyps and
colonies, whereas gonochoric corals have separate sexes; and corals with these sexual
patterns either broadcast spawn their gametes for external fertilization and subsequent
embryo and larval development, or have internal fertilization and brood embryos and
planula larvae within their polyps (reviewed by Harrison and Wallace 1990). In case
of hermaphroditic coral species, eggs and spermaries develop either on the same
mesenteries (such as in the faviids and mussids) or on different mesenteries within
the same polyp (as in most pocilloporids and acroporids) or in different polyps within
the same colony (Fadlallah 1983, Harrison and Wallace 1990). For example, Ward
and Harrison (2000) mentioned that Acroporids are hermaphroditic with eggs and
spermaries on different mesenteries within the same polyp.
Harrison (2011) and Kerr et al. (2011) highlighted that approximately 70–75% of
corals are hermaphroditic, while remaining 25–30% gonochoric. Oocytes and testes
develop within the same polyp in hermaphroditic corals whilst in gonochoric corals
male colonies produce sperm and female colonies produce oocytes (Hidaka 2016).
Many scleractinian corals reproduce by broadcast spawning and are referred to
as broadcast spawners. There are release of gamete bundles from different coral