Life History And Ecology

All corals have the ability to reproduce sexually, by producing male and female gametes for fertilisation and subsequent development of large numbers of ciliated larvae known as planula larvae (or planulae), which settle on the reef surface, transform into individual polyps and begin to lay down skeleton. The coral colony is then formed by division of this founder polyp. Individual polyps in a colony may all be male or female (the gonochoric or dioecious condition) or both sexes can be developed within each polyp (hermaphroditic or monoecious condition). There are two very different modes of production of larvae, usually characteristic of a genus or family. Sperm may be released by the polyps of one colony to find their way into the body cavity of other colonies, where eggs are fertilised and develop in or on the polyps (Fig. 20.3A). This is the brooding condition, with corals having this life cycle being referred to as 'brooders'; until the 1980s, this was thought to be the only type of life cycle in corals (see Box 20.1). In an alternative life cycle, shown by corals referred to as 'broadcast spawners', eggs and sperm are released into the water column for external fertilisation and development (Fig. 20.3B). Since gametes from same colonies do not usually self-fertilise, this second mode requires many colonies of the same species to release gametes at the same time (see Box 20.1).

The larvae of brooding corals are released fully developed and ready to settle on the reef, whereas embryogenesis and development of larvae by broadcast-spawning corals requires a developmental period of up to ten days before the planula is ready to settle. Dispersal of larvae outside the home reef is thus far more likely in broadcasters than brooders. Some corals are also subject to asexual reproduction by breakage and redistribution of branches, formation of polyp balls, and other mechanisms, and this may contribute considerably to population size and biomass.

Feeding and nutrition with a coral colony generally involves two sources: (1) acquisition of food by individual polyps, digestion in the polyp's gut and distribution of digestate through the colony via the

Sperm separates from the bundles and fertilises other eggs

Sperm separates from the bundles and fertilises other eggs

Adult polyps release egg and sperm bundles Into the water once each year

Adult polyps release egg and sperm bundles Into the water once each year

Fertilised egg develops Into a planutalaiva

Ptarxila finds a suitable site on the reef and settles, becoming a founder pc+yp

Ptarxila finds a suitable site on the reef and settles, becoming a founder pc+yp

Founder poiyp grows and divides, producing a new colony of pofyps

Figure 20.3 Reproductive cycles in corals. A, cycle in brooding species; B, cycle in hermaphroditic broadcast-spawning species. Other variations such as broadcast spawning in gonochoric species, occur in some corals. (Image adapted from Wallace and Aw 2000.)

Founder poiyp grows and divides, producing a new colony of pofyps

Figure 20.3 Reproductive cycles in corals. A, cycle in brooding species; B, cycle in hermaphroditic broadcast-spawning species. Other variations such as broadcast spawning in gonochoric species, occur in some corals. (Image adapted from Wallace and Aw 2000.)

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