Acropora embryos are fragile ... by design!
Acropora millepora is an amazing species, and we're not just talking about their beauty. Photo by Dana Riddle
Forming a unique part of the animal kingdom, corals have built the only living entity visible from space; the Great Barrier Reef. Scientists from the Australian Institute of Marine Science (AIMS) have recently discovered a previously unknown reproductive strategy in corals, adding another dimension to our understanding of their complex life cycles.
A study published today in the prestigious international journal Science shows for the first time that coral offspring have the unique ability to form genetic clones of themselves before they settle and develop into adult corals.
Coral ‘offspring' are usually the result of sexual reproduction - eggs are fertilised either before or after being released by the parent coral into the surrounding water. These fertilised eggs are carried by ocean currents before settling at new locations.
Coral eggs are rich in waxy fat, which provides energy during development. Consequently the eggs they float to the sea surface, commonly as a bundle of eggs and sperm for each polyp. At the surface the individual eggs separate and become fertilised. It is typical for broadcast spawned coral eggs to be pastel coloured, often pink, red or orange, and quite uniform in size for each colony and species. Because they are buoyant the spanned eggs and embryos get pushed about like floatsam, sometimes forming pink slicks on the ocean containing billions of embryos. At the same time they are exposed to currents and waves. Image: Andrew Heyward and Andrew Negri
Coral "clones", on the other hand, are genetic replicas of the parent coral. For example, if waves generated in a storm break up a coral colony, the remnant parts may continue to survive as independent but genetically identical individuals; a faculty that most animals do not possess.
Dr Andrew Heyward and Dr Andrew Negri suspected that fertilised coral eggs (embryos) might also break up because, unlike most animal embryos, coral embryos lack a protective outer-layer or membrane; they are so called ‘naked' embryos.
"As the early stage embryo develops it divides into a cluster of cells," explains Dr Heyward, "because this ball of cells lacks a protective outer-layer we wondered whether subjecting them to a little turbulence might cause them break up,"
It did, but what happened next was even more astonishing.
"To our surprise many of the fragmented coral embryos later began to develop and settle in just the same way as their siblings that had remained intact," continues Dr Heyward. "Interestingly, these fragmented embryos became smaller versions of baby corals than the complete embryos". The scientists were able to create these turbulent conditions in the laboratory simply by pouring embryos floating in seawater over a vertical distance of 30 cm.
"This effectively mimics the kind of wave height generated by moderate wind speeds where small breaking waves, commonly called whitecaps, occur. That sort of weather is often encountered during a night of coral spawning on the Great Barrier Reef," says Dr Negri. "So it's highly likely that this fragmentation occurs regularly on nights when corals release their eggs.
"It appears that the lack of protective membrane is no accident. Almost half of all these naked embryos fragmented in our experiments, suggesting that this has long been part of the corals' repertoire for maximising the impact of their reproductive efforts".
Dr Heyward explains why discovery of this novel reproductive strategy is so significant. "This mixed breeding system means colonising corals benefit simultaneously from the advantages of both sexual and asexual reproduction.
"Much like humans, it's important that the offspring of corals have genetically distinct parents, but these embryos also readily clone to form multiple versions of themselves, and helps to explain how coral maximise their chances of finding a suitable habitat in which to settle and survive.
In human terms this is the equivalent of giving birth to identical twins, triplets, quadruplets and so on.
"This is another example of the complexity of these incredible animals and suggests that there may be more to learn about the lives of corals and their interaction with the environment."
For further information:
Dr Andrew Heyward: Principal Research Scientist, AIMS (Perth) 0417 400 273; Sat phone in Australia: 0011 8707 6115 2693; RV Solander mobile: + 61 488 720 858; Email: firstname.lastname@example.org
Dr Andrew Negri: Senior Research Scientist, AIMS (Townsville) 07 4753 4322, email@example.com
Wendy Ellery, AIMS media liaison, (07) 4753 4409; 0418 729 265; firstname.lastname@example.org