If you meet a coral researcher this week and they look a little tired and stressed - be nice. Chances are they were working around the clock last week to make the most of the annual coral spawning on Australia' Great Barrier Reef.
Coral spawning occurs in Spring (October and November) on the Great Barrier Reef, the world's largest coral reef ecosystem. In a spectacular display, hundreds of hard coral species across a vast area release packages of egg and sperm into the water column at the same time. The water becomes filled with tiny floating balls of red, white and pink as the gametes (egg and sperm) rise to the surface to fertilise, and develop into planktonic coral larvae, or planula.
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| Coral Spawning on the Great Barrier Reef - one of the planet's most incredible reproductive events |
Richard was out filming the event at Agincourt Reef out of Cairns, and reported quickly that the biggest nights were on Thursday and Friday, and the fun kicked off each night around 8:30pm.
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| Richard Fitzpatrick of Biopixel and OceansIQ filming coral spawning on Agincourt Reef, Far Northern Great Barrier Reef |
You will find plenty of information on mass spawning all over the inter webs, including an OIQ video here and information from the Great Barrier Reef Marine Park Authority here, so we'd thought we'd bring you a few lesser known facts about the Great Barrier Reef's mass coral spawning event.
1. The world's largest breeding event was not discovered until 1981!
While it was well known that the corals were doing something around this time of year, researchers did not document the mass spawning event until the 1984 publication in Science. Researchers recorded in 1981 and 1982, for the first time anywhere, that over 32 species of corals had released eggs and sperm into the water column simultaneously at 3 locations along the GBR. Until then, it was thought that most corals were brooders (not in the emotional sense of the word) in that they had internally fertilised and developed larvae. While there are many corals that take this brooder approach, the corals involved in mass spawning are classified as broadcast spawners.
2. Not all parts of the Great Barrier Reef are on the same timetable!
The timing of coral spawning is dictated by the seasons. On the Great Barrier Reef, it occurs in the Australian Spring (October and November), as water temperatures are starting to rise. Due to the enormous size of the GBR, the water warms at different rates. Reefs closer to the coast (inshore reefs) tend to warm faster than those located further out to sea, or in the cooler south.
Because of these temperature differences, corals on the inner reefs will generally spawn a few nights after the full moon in October, while the rest of the reef 'goes off' in November.
3. Research involving coral spawning is hard work!
Firstly - it only happens once a year, for goodness sake! So if you miss it, well then hang around for another 12 months, because that's all you're getting!
And secondly - while it seems spawning should be completely predictable, nature doesn't mind screwing with the timetable. With the hottest September on record and above average water temperatures this year, Dr Selina Ward at University of Queensland thought she should be ready for an early spawning at Heron Island, just in case. Turns out, that was a good idea.
"I figured that the corals might go in October because it was the hottest September on record so I got all set for an October spawning. We found plenty of gravid corals and did our work then".
AND another thing - rearing coral larvae is the next tough job after the corals have spawned. The gametes are taken from their birth suite (small aquaria) into the nursery (loads of 200L tubs) with lots of fresh seawater, which is in plentiful supply at Heron Island Research Station where Dr Ward conducted her field work.
4. Researchers get hints of what colonies will spawn early on
Compared to the size of the coral polyp, the egg and sperm bundles they produce are HUGE! Researchers get an idea of what colonies are ready by peeking into just a few colonies. If the colony is gravid (official word for polyp with eggs), it will have a red or pink bundle of joy sitting at the polyps centre. This becomes bigger and therefore easier to see close to spawning.
When gravid colonies are identified, researchers will bring them into the aquarium under permits issued by GBRMPA. This way, researchers can easily monitor the colonies, alter variables if required and collect clean eggs and sperm once the spawning starts.
It is also easier to monitor the colonies when on land - diving for coral spawning can mean many nights under the waves, waiting for something to happen.
While the coral's appearance gives clues as to whether spawning may occur in that colony, it doesn't necessarily indicate they will fertilise successfully. One species of many that Dr Ward studies (Acropora aspera) looked set for a good season, but showed virtually no fertilisation after spawning. These colonies were from the reef flat, a particularly harsh environment for corals to live. Definitely a case of 'don't count your chickens...'
5. Corals synchronise their spawning by the light of the moon
While temperature is a major driver of the timing of spawning, corals need to fine-tune the event - there is little use in spawning a week or 2 apart. So when the time of the year is right, mass coral spawning is synchronised by the moon, occurring just a few nights after.
But without eyes of any sort, how do corals check their lunar calendar? Researchers a few years ago published their findings in Science that, despite having no eyes, corals respond to the intensity of light, specifically blue light, from the moon using proteins called cryptochromes. Cryptochromes are found across the animal kingdom and help set circadian rhythms from insects to mammals. With this physiological response to changes in blue light intensity, corals many kilometres away from each other can look to the moon (metaphorically speaking of course) and sync their biological watches.
Coral spawning is an amazing event and is on many diver's bucket lists. Any of our readers lucky enough to have witnessed it?
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