Box 201 The Coral Mass Spawning Phenomenon

Corals from the majority of species on the GBR develop eggs and sperm once a year and release these for external fertilisation in multispecies spawning events. These occur at night during the week following the full moon in October to February, with November being the major spawning month. The discovery of this extraordinary phenomenon in Australia in the 1980s was one of the major advances in coral reef science of the 20th century. Mass spawning, as it has been dubbed, presents a paradox: on the one hand it ensures ample reproductive material for the cross-fertilisation required to ensure development of the next generation of each species, at the same time, saturating the food supply of predators for a very short period; on the other, it also provides opportunities for mis-matches, in which sperm could fertilise eggs of the wrong species to form hybrids or non-viable embryos. The discovery of mass spawning led to revision of ecological and evolutionary models and predictions and provided unprecedented opportunities for research, management planning and experimentation using gametes. Its impact has been worldwide and the phenomenon has now been reported from most countries bearing coral reefs.

Figure 20.4 Coral colonies spawning during the mass coral spawning event on the Great Barrier Reef. A, Acropora tenuis spawning egg/sperm bundles for external fertilization (note gametes on the water surface); B, female Fungia species spawning eggs (these will be externally fertilised by sperm released by a male); C, Galaxia fascicularis spawning large egg/sperm bundles for external fertilisation. (Photos: GBR.)

Figure 20.4 Coral colonies spawning during the mass coral spawning event on the Great Barrier Reef. A, Acropora tenuis spawning egg/sperm bundles for external fertilization (note gametes on the water surface); B, female Fungia species spawning eggs (these will be externally fertilised by sperm released by a male); C, Galaxia fascicularis spawning large egg/sperm bundles for external fertilisation. (Photos: GBR.)

coelenteric connections between polyps; (2) photo-synthetic activity of the symbiotic dinoflagellates that live in the polyp's tissues (see Chapter 8). Coral polyps are basically carnivorous, with their food source being to some extent determined by polyp, mouth and tentacle size. Food sources include planktonic organisms, incidental detritus and mucous with contained micro-organisms.

The ecology of reef corals is a broad subject that could not be dealt with adequately in this section, but is alluded to in the descriptions of major families below. The combined processes of sexual and asexual reproduction, dispersal, growth and response to physical and chemical conditions on different parts of the reef, play a role in zoning of corals on the reef, such that particular species, genera and even families, can be seen to predominately occur on, and sometimes dominate, particular parts of the reef, such as the inner reef flat, reef crest, mid slope or lagoon floor. Interaction and competition with other corals and occupiers of benthic space, impact of predators, and the impact of unpredictable events, and even coral diseases, also play a role in the presence of particular corals in any particular reef location, as well as in the size and shape of coral colonies.

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