Corals have evolved a remarkable range of reproductive strategies to survive in their dynamic environment. Budding—division into clones—and fragmentation are examples of asexual reproduction. Fragmentation occurs naturally when coral pieces are broken off a colony as a result of wave action, storms or animal activities. Under favorable conditions, these fragments can attach and develop into new colonies. Sexual reproduction is important to maintain genetic diversity of a species, which may be especially important for coral restoration efforts ('sexual coral restoration'). Most coral species are hermaphrodites (polyps are both male and female), while about one third have separate sexes (gonochoric). There are two modes of sexual coral reproduction: broadcast spawning and brooding.
Broadcast spawners usually release their eggs and sperm in mass spawning events once a year (elkhorn coral spawning, Limones, Mexico 2015, Porites spawning, Camiguin Island, Philippines, 2015). The released gametes drift to the water surface where fertilization takes place. Commonly after a few days, the embryos will have developed into coral larvae (planulae). By contrast, brooding coral species have internal fertilization and embryogenesis before releasing settlement competent larvae. Brooders show extended reproductive seasons from a few months to almost continuously throughout the year. In general, they produce relatively big larvae that already contain zooxanthellae—contrary to broadcast spawners' larvae that generally take up their zooxanthellae shortly after settlement.
Larvae from broadcast spawning may reside as plankton in the water column for up to several weeks, whereas brooded larvae may be immediately ready for settlement. Either way, the larvae eventually swim to the sea floor and search for a suitable settlement substrate. When a larva finds an appropriate place, it settles and undergoes metamorphosis (larval settlement), changing into a tiny coral polyp not bigger than one millimeter. The following weeks to month are a crucial period for the minute polyp. In order to avoid an overgrowth by algae or being buried by sedimentation, the little coral has to grow quickly. Soon, the surviving coral recruits start to form a colony (early life stages) by dividing into clones, each a distinct polyp, yet closely connected with each other.
Many coral species, especially broadcast spawners, release huge quantities of gametes to compensate for the enormous loss of offspring during the early developmental stages. In some regions, beaches turn red after a night of coral spawning from the gametes that are washed ashore. This evolutionary strategy of excessive gamete production has helped corals to ensure their viability over millions of years. Today, however, corals' reproductive success is diminished due to stressed and weakened spawning colonies, insurmountable distances between spawning colonies, as well as unbearable environmental conditions for larval development and the survival of recruits.
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