Big Bank Shoals of the Timor Sea
An environmental resource atlas

CORAL ECOSYSTEMS

Reproduction

Corals propagate themselves by a variety of methods, and many species can be considered analogous to plants in this respect. Both asexual and sexual modes of reproduction rapidly produce copies of a colony on the reef of origin, while sexual reproduction increases genetic diversity and provides a mechanism to disperse offspring further afield.

However, vegetative or asexual propagation is also common and can be particularly important for local proliferation in selected species.

A significant ecological dichotomy amongst corals is apparent in the reproductive pathways leading to production of planulae larvae. Planulae may be produced via external fertilisation or brooding. Brooding corals fertilise their eggs within their body cavity, where they develop into planula larvae before being released. It has been shown that some brooded larvae are capable of settling almost immediately upon release, while others can survive for prolonged periods and disperse widely.

Figure 25: The generalised life cycle of a broadcast hard coral.

Symbiotic zooxanthellae, inherited from the parent colony, are thought to help sustain these larvae. Some very important and ubiquitous species in several of the dominant coral families rely on brooded planulae. The majority of corals, however, release eggs and sperm into the water and depend on external fertilisation and development of their larvae (see Mass spawning of corals, p48). Coral eggs released using this method of spawning are rich in fatty yolk. This provides energy reserves during larval development and also makes the eggs highly buoyant. Fertilised eggs gradually develop into planula larvae over the next 24-36 hours.

Figure 26: Hard coral morphologies a: digitate Acropora sp., b: massive Goniastrea sp., c: foliaceous Leptoria sp., d: solitory Fungia sp., e: table Acropora sp., f: corymbose Acropora sp. (AIMS).

Laboratory studies suggest that these larvae will continue to drift in currents for at least 3 days before being fully developed and able to settle onto reefs (see Figure 25). Planula larvae can settle onto a variety of substrates, but recent research suggests that certain chemicals found in reef environments can actively promote attachment of the larvae and stimulate metamorphosis into single coral polyps.

A planula larva (P. Harrison).

Once firmly attached to the substrate the primary polyp builds a skeleton of calcium carbonate then begins to form a growing colony by budding new polyps. The age or size at which sexual maturity is reached varies between each species, but many of the common, reef-building species require 3-5 years before reproducing for the first time.

In addition to sexual planula production, five different modes of asexual reproduction have been identified in hard corals. All of these result in the creation of new colonies that share exactly the same genotype as the parent colony. While several unusual forms of asexual planula and polyp production have been recorded, two types of colony fission are probably much more significant. Fragmentation, common in branching and plate-like colonies, is one of the more significant.

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Last updated - December 18, 2008

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