Sexual Reproduction of Scleractinian Corals Peter L. Harrison Abstract Sexual reproduction by scleractinian reef corals is (1999), Kolinski and Cox (2003), Guest et al. (2005a), important for maintaining coral populations and evolution- Harrison and Booth (2007), and Baird et al. (2009a). ary processes. The ongoing global renaissance in coral repro- Scleractinian reef-building corals are foundation species duction research is providing a wealth of new information on on coral reefs because they provide the complex three- this topic, and has almost doubled the global database on dimensional structure and primary framework of the reef, coral reproductive patterns during the past two decades. and essential habitats and other important resources for many Information on sexual reproduction is now available for 444 thousands of associated species (reviewed by Harrison and scleractinian species, and confirms that hermaphroditic Booth 2007). These corals are distinguished from other broadcast spawning is the dominant pattern among coral spe- members of the Class Anthozoa (Phylum Cnidaria) such as cies studied to date. Relatively few hermaphroditic or gono- soft corals, by their continuous hard calcium carbonate choric brooding species have been recorded. Multispecific crystal exoskeleton, which form the essential building blocks coral spawning has been recorded on many reefs, but the of the reef ecosystem when cemented together by crustose degree of reproductive synchrony varies greatly within and coralline algae. The term “coral” is used in this review to among species at different geographic locations. refer to these hard corals from the Order Scleractinia. Although corals are widely distributed throughout the Keywords world’s seas and deeper ocean environments, they are particu- larly significant in shallow tropical and subtropical seas where the mutualistic symbiosis between the coral polyps and their endosymbiotic dinoflagellates (zooxanthellae) fuels light enhanced calcification and rapid growth of reef-building cor- als, resulting in coral reef development. Corals can be broadly divided ecologically, but not systematically, into reef-building 1 Introduction (hermatypic) corals and non-reef-building (ahermatypic) cor- als. Hermatypic corals create the primary reef framework and This review provides an overview of global knowledge and most hermatypic species in shallow warm-water habitats nor- emerging patterns in the reproductive characteristics of the mally contain millions of zooxanthellae (i.e., zooxanthellate); 444 scleractinian species for which information on sexual in contrast, although ahermatypic corals also secrete complex reproduction is available, and updates the previous review by aragonite exoskeletons they do not usually contribute signifi- Harrison and Wallace (1990), with particular emphasis on cantly to reef formation, and mostly lack zooxanthellae (e.g., new discoveries and data from the past 2 decades. Sexual Yonge 1973; Schuhmacher and Zibrowius 1985; Cairns 2007). reproduction in scleractinian corals has been widely studied However, some azooxanthellate corals do contribute to reef- in many regions of the world, and substantial new informa- building and are therefore hermatypic, including some deeper- tion has become available since earlier detailed reviews on water and deep-sea colonial cold-water corals that form reefs this subject by Fadlallah (1983), Harrison and Wallace (1990), or bioherms, which provide important habitats for many other and Richmond and Hunter (1990), with more recent topic species (e.g., Brooke and Young 2003; Waller and Tyler 2005; reviews provided by Richmond (1997), Harrison and Jamieson Friewald and Roberts 2005). The total number of extant scleractinian “species” is not known, and estimating global coral species richness is com- P.L. Harrison( ) plicated by a number of issues (e.g., Veron 1995, 2000; School of Environmental Science and Management, Southern Cross University, Lismore, NSW, 2480, Australia Cairns 1999, 2007; Harrison and Booth 2007). These include e-mail: [email protected] the limited exploration of deeper reef, mesophotic, and Z. Dubinsky and N. Stambler (eds.), Coral Reefs: An Ecosystem in Transition, 59 DOI 10.1007/978-94-007-0114-4_6, © Springer Science+Business Media B.V. 2011 60 P.L. Harrison deep-sea environments, as well as some shallow tropical reef regions where new species are likely to be found, and imper- fect taxonomic resolution of highly variable species and potential cryptic species. Furthermore, the discovery of hybridization among some morphologically different coral morphospecies (e.g., Willis et al. 1992, 1997; Szmant et al. 1997; van Oppen et al. 2002; Vollmer and Palumbi 2002) challenges the application of the traditional biological- species concept based on reproductive isolation between different species, for some corals. If we assume that the current primarily morphologically based taxonomy provides an appropriate indication of global coral species richness, there are probably at least 900 extant hermatypic scleractinian species (e.g., Wallace 1999; Veron 2000; J. Veron, personal communication). Of these, 827 Fig. 1 A brooded Isopora cuneata planula searching reef substratum covered with crustose coralline algae for a suitable site for initial attach- zooxanthellate hermatypic coral species have been assessed ment and metamorphosis into a juvenile polyp (Photo: author) for their conservation status (Carpenter et al. 2008). In addi- tion, at least 706 azooxanthellate scleractinian species are known, including 187 colonial and 519 solitary coral species, that initiates the formation of the calcium carbonate exoskel- with their most common depth range being 200–1,000 m eton. Subsequent growth during an initial presexual juvenile (Cairns 2007). Of the more than 1,500 recognized coral phase leads to development of the adult form that becomes species, aspects of sexual reproduction have now been sexually reproductive, which completes the life cycle recorded in at least 444 species, the vast majority being shal- (Harrison and Wallace 1990). low-water zooxanthellate hermatypic coral species. This Asexual reproduction produces genetically identical global knowledge base provides a wealth of information on modules that may prolong the survival of the genotype, the biology and ecology of coral reproduction that surpasses whereas sexual reproduction enables genetic recombination most other marine invertebrate groups, and corals therefore and production of new coral genotypes that may enhance fit- provide an important model for assessing life history and ness and survival of the species. Four basic patterns of sexual evolutionary theory. reproduction are evident among corals, which include: This chapter focuses mainly on sexual reproductive char- hermaphroditic broadcast spawners, hermaphroditic brood- acteristics in hermatypic species because these corals have ers, gonochoric broadcast spawners, or gonochoric brooders. been more extensively studied and are foundation species on Hermaphroditic corals have both sexes developed in their coral reefs (Harrison and Booth 2007). Additional reference polyps and colonies, whereas gonochoric corals have sepa- is made to sexual reproduction in some ahermatypic sclerac- rate sexes; and corals with these sexual patterns either broad- tinian species, and an overview of asexual reproduction in cast spawn their gametes for external fertilization and corals is provided below to highlight the diversity of repro- subsequent embryo and larval development, or have internal ductive processes exhibited by scleractinians. fertilization and brood embryos and planula larvae within their polyps (reviewed by Harrison and Wallace 1990; Richmond and Hunter 1990). However, not all coral species are readily classified into these basic patterns, as mixed 2 Coral Life Cycle and Reproduction sexual patterns or both modes of development are known to occur in some species. Corals have a relatively simple life cycle involving a domi- nant benthic polyp phase and a shorter planula larval phase. The polyp phase is characterized by growth of tissues and 2.1 Asexual Budding and Reproduction skeleton that often includes one or more forms of asexual budding or reproduction; and repeated cycles of sexual repro- duction (iteroparity) involving the production of gametes, Different modes of asexual production can be distinguished; fertilization, embryo development, and a larval phase that is asexual budding of polyps leads to the formation of coral usually planktonic and dispersive to some degree (Harrison colonies, while various forms of asexual reproduction result and Wallace 1990). If the planula survives and successfully in the production of new modules that form physically sepa- attaches and settles permanently on hard substratum (Fig. 1), rate but genetically identical clones (ramets) (reviewed by it metamorphoses from the larval form into a juvenile polyp Highsmith 1982; Cairns 1988; Harrison and Wallace 1990; Sexual Reproduction of Scleractinian Corals 61 Richmond 1997). Most hermatypic coral species (Wallace 1999; Veron 2000) and about 26% of the known ahermatypic azooxanthellate coral species (Cairns 2007) form colonies via asexual budding of polyps and are therefore modular, iterative colonial organisms. Budded polyps usually form by growth and internal division of existing polyps (intratentacu- lar budding) or development of new polyps from tissues adjacent to, or between, existing polyps (extra-tentacular budding) (reviewed