Marine Biology by Philip V. Mladenov

Marine Biology by Philip V. Mladenov

Author:Philip V. Mladenov
Language: eng
Format: epub
ISBN: 9780192578792
Publisher: OUP Oxford
Published: 2020-01-20T16:00:00+00:00


Sexual reproduction in corals

Reef-building corals can disperse from the parent colony and colonize new habitats through sexual reproduction. Most species of coral are hermaphroditic—able to produce both eggs and sperm on the same colony. In some species, separate male and female colonies are the norm. Most coral species employ what is termed broadcast spawning—they eject huge numbers of sperm and eggs into the ocean where fertilization takes place. The fertilized eggs develop into tiny ciliated larval forms, called planulae, which are transported by currents for days or weeks, depending on the species. When the planulae detect favourable conditions they swim to the bottom, where they attach themselves and start a new colony.

On coral reefs throughout the world, the coral colonies over a large portion of a reef often spawn simultaneously in a spectacular mass spawning event in which the surrounding seawater becomes saturated with coral gametes, which form distinct slicks on the ocean surface. On parts of the Great Barrier Reef, for instance, millions of colonies consisting of many different species of coral spawn together on a single night, or over the course of a few nights, during the southern hemisphere spring or early summer after a full moon. Mass spawning in corals has most likely evolved to create very dense concentrations of sperm and eggs in the seawater to ensure high rates of successful fertilization. The coral eggs can distinguish among the different kinds of sperm present in the seawater during a mass spawning, rejecting sperm of a different species, and so limit the chances of hybridization between species.

The breeding season of coral colonies is controlled by factors such as seasonal changes in ocean temperature or day length, which serve to bring the corals into breeding condition at the same time. The actual spawning of the ripe colonies is triggered by different factors, including lunar periodicity, a decrease in light levels at sunset, and chemical cues released into the water by other colonies of the same species. So-called ‘primitive’ corals can thus detect light levels, distinguish the phases of the moon, and communicate with each other chemically.



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