About three-quarters of all zooxanthellate corals are hermaphrodite; the remainder have separate male and female colonies or (in solitary species) separately sexed individuals. The sexuality of corals – whether they are hermaphrodite or separately sexed – tends to be generally consistent within species and genera, although there are exceptions. There is sometimes also geographic variation within species.
About three-quarters of all zooxanthellate corals spawn eggs and sperm for external fertilisation rather than brood planulae within their bodies after internal fertilisation. Spawning is associated with high numbers of eggs and planulae, while brooding results in fewer, larger and better developed planulae. This tends to be variable, even within genera. Because of numbers, release of gametes into the ocean facilitates long-distance dispersion and thereby the creation of genetic links between one reef region and another. In contrast, corals that brood planulae can readily concentrate planulae close to the parent colony. Reproductive mode may therefore involve finding balances. These include balancing local abundance (by having planulae that settle rapidly) and long-distance dispersion, self-fertilisation (internally or within egg and sperm bundles) versus outcrossing, and within-species fertilisation versus ‘hybridisation’. When transferred to biogeographic scales, these balances affect the genetic composition of species and how they form geographic patterns. As a result, a single species of coral may have different reproductive modes in different geographic regions. Some species combine brooding with long-distance dispersion by rafting. Others have planulae that have both rapid settling and long-distance dispersal capability. Others release gametes as well as brood planulae.
The long-term control of spawning (control of the maturation of gonads) may be temperature, day length and/or rate of temperature change (either increasing or decreasing). The short-term (getting ready to spawn) control is usually lunar. The final cue (release of spawn) is usually the time of sunset. There are many variations on these controls, probably because synchrony is usually linked to whatever environmental cues work best within a given region. Cues may also be biological as well as physical and synchrony by chemical messengers may not only involve corals, but a host of other marine life as well.
Brooding species, which release planulae and not gametes, can store unfertilised ova for weeks and thus require less synchrony for fertilisation. Spawning species require synchrony within a time frame of hours. This regional synchrony varies geographically: for example, on the Great Barrier Reef, spawning occurs in October/November, in Western Australia it occurs in March and in Japan in June/July. These differences suggest that synchrony, if under a genetic control that is not modified by local environment, could be a barrier to effective long-distance dispersal: a planula larva could successfully make a long journey, only to develop into a colony which is reproductively isolated (spawns asynchronously) from other colonies in the region.
In most major coral regions, not only do different colonies of the same species synchronise their spawning, but colonies of different species have the same synchrony. The outcome, in many regions, is ‘mass’ spawning. When mass spawning occurs, the ocean surface becomes a soup of genetic material creating endless possibilities for cross fertilisation. The extent to which hybridisation occurs – that eggs of one species are fertilised by the sperm of another – is not known. What is known is that different species within the same genus (and rarely between species of different genera) can readily hybridise and that the progeny can be normal-looking corals. This is not what might be expected because if all species within a genus were to hybridise in this way, distinctions between the hybridising species would disappear and there would only be one species. In such a case, maintenance of species distinctions could only be by geographic separation. Geographic separation greatly affects the genetic composition of species, but clearly there must be other mechanisms for maintaining species distinctions within a single region. In many groups of plants and animals, there are behavioural, genetic, or anatomical barriers to hybridisation. Under conditions of mass spawning, such barriers might include reduced fertility or reduced capacity for hybrid survival. They may also include mechanisms that mask genetic change, such as a small number of genes being responsible for major morphological outcomes. Whatever the reality, large amounts of geographic space, evolutionary time, and rare events matter enormously when it comes to creating change. With corals, genetic exchange between one species and another clearly occurs. Over vast amounts of space and time, this creates the interacting dynamic geographic patterns within and between species that we observe today.
An important aspect of coral reproduction, and one that underpins biogeographic patterns, concerns the capacity of corals to undertake extended ocean voyages. It is now known that planulae can spend months being transported by currents and still be competent to settle. It is likely that the planulae of most species of coral make long-distance journeys and probably do so frequently. The likelihood of survival once a distant destination is reached is extremely small, but again, rare events are likely to be the events that matter. Planulae can live on the ocean surface for long periods of time because they have zooxanthellae to provide nutrients. They may even undergo partial metamorphosis while afloat. In some instances, adult colonies may also disperse on floating objects including pumice. This ‘rafting’ of colonies is probably an important mechanism of dispersal in some species, especially of Pocilloporidae.