points of interest

Interest groups

Shark("do I need to be concerned about the sharks?")
A general perception of sharks’ worldwide is that they are nothing other than blood thirsty man killers. In warm tropical waters, especially in shallow reef locations such as where snorkelling activities are safely conducted, the danger of shark attack is negligible. The truth of the matter is most sharks are intimidated by human activities.

Crown of Thorns. ("Are they destroying the reefs? ")
Crowns of thorns (COTS) are starfish closely related to the pincushion starfish. COTS are once again in a state of outbreak. These outbreaks are presently on some parts of the Cairns section of the marine park, however these outbreaks are not exclusive to the Great Barrier Reef. It is in fact an Indo-Pacific problem. The areas affected are from the Red Sea, South Africa, Maldives, Indonesia, Cocos Is, Philippines, Vanuatu to the Cook Is, Fiji and Samoa. Okinawa, Guam and Marianas are also expected to experience problems with record outbreaks. An active outbreak is classified by finding more than 30 mature COTS per hectare. There are reefs that are now major tourist attractions that have recovered from outbreak damage from only 20 years ago.
COTS are prevalent now and previous widespread damage was experienced in the 60's 80's and late 90's can cause wide spread damage to coral reefs when it's numbers reach plague proportions. Covered in poisonous spines it does not have many enemies. It Feeds by dumping its stomach out it's mouth, wrapping it around it's coral prey and digesting the polyps in their own skeletons. The white scaring is soon covered with algae. Many reefs have fully recovered from COTs attacks in as little as Ten years. There are many different opinions why they plague and if it is in fact a threat to the reef ecosystem.

Clown Fish.

On of the many anemone fish inhabiting the Great Barrier Reef. Every reef interpreter’s favourite story for many reasons. They are resident fish living in one location all there life. Therefore they are easy to relocate and some fish in this family are very territorial and will not hide when viewed by snorkellers. The relationship between fish and host is interesting and is an obvious example of symbiosis (animals assisting each other) 28 species of anemone fishes live in 10 species of anemones. Sexual behaviour of the anemone fishes is a story of female domination over a harem of sexually juvenile males and one or two sexually active males. Sounds like fun to us. Interesting is the ability of the males to change sex to sexually active females. This is triggered by the lack of harassment by the female, normally when she is no longer present.

Clownfish are a good example of the complex life a fish undertakes from birth until death. These fish have developed an incredible relationship with sea anemones. This phenomenon is known as ‘symbiosis’, which is simply Latin for two organisms living together and helping each other out. Sea anemones are a simple animal fairly similar to a jellyfish, only living upside down and attached to the bottom with their tentacles extended upwards. They have stinging cells on their tentacles, a mouth in the middle and a simple stomach. They feed on anything that is drifting by, shrimps, prawns and small fish. These are snared using the stinging cells and the tentacles, and then passed via the mouth into the stomach. The clownfish has actually adapted its body so that it is immune to the strong sting that the anemone can issue with its tentacles. This adaptation has enabled an interesting relationship to develop between these two animals.

Juvenile clownfish, as with all fish, have a larval stage. This means that fertilised eggs of fish become planktonic larvae, tiny microscopic beings floating around in the currents. As they grow larger they begin to look for environments that suit them, to settle on. Different fish tend to settle in different habitats, and some fish, such as the clownfish, have extremely specific environments in which they settle. It is still not known how they are able to find the sea anemones but some research has suggested that they are able to ‘smell’ the sea anemones using chemical stimuli. Even more bizarre is that certain species of clownfish seem to settle only in certain species of sea anemone. Once the juvenile clownfish has found an anemone to call it’s home, the fun really begins. You’ve heard of ‘survival of the fittest’, well listen to this.

If the juvenile clownfish does actually manage to survive long enough without being eaten it will hopefully find a sea anemone to settle on. If they do not find one they will certainly be eaten. The juvenile clownfish is not naturally immune to the sea anemone, but it’s body is capable of producing a mucous that will protect it from being severely stung. This mucous must be stimulated to be produced, in other words the juvenile clownfish must repeatedly dive into the tentacles receiving a sting, then hightailing it back out again. This has been viewed by scientists in aquariums after an adult has been removed from its anemone for a period of time. When the adult is reunited with the anemone it must ‘desensitize’ itself all over again. After repeatedly stinging itself for many hours in this fashion, the clownfish's body is covered in a mucous that protects it from stings and it is then able to quite happily nestle down into the otherwise deadly tentacles like they are a comfortable bed. Interestingly, research into the mucous of clownfish will eventually be developed into a cream we can apply before swimming. The idea being that if one was to come into contact with a jellyfish tentacle it would not sting them.

The trials of the juvenile clownfish do not finish here however. Once it is immune to the tentacle stings it then has to deal with the constant bullying it receives from other clownfish that reside in the anemone. The degree of bullying depends on the size of the sea anemone and the number of other clownfish already residing there. There is a definite hierarchical relationship between the clownfish residing within the anemone. Funnily enough the dominant fish is actually a female and she is the largest and usually most aggressive of all the fish. I have often been accosted by these females while examining an anemone, some of them are brave enough to actually bite, very brave considering their size. Next is her mate, the largest of the males. She constantly chases and bullies the smaller male. His frustration at being treated in this way is vented on the next largest male, who in turn vents his frustration on the next male in line, and so on and so on. There is only ever one female, and there can be numerous males, sometimes as many as twenty. The juvenile clownfish joins the community at the bottom of the rung, he is delegated the outskirts of the anemone considered to be poor real estate due to its lack of protection. Being at the bottom of the rung, he receives his share of bullying from the next smallest male. This bullying can be so severe that the juvenile is sometimes forced from the anemone where it will usually be eaten not long afterwards. The bullying also takes its toll on the size of the smallest member, as his time is spent fleeing rather than feeding. If he does survive then he eventually moves up the ladder. Which leads us to an interesting question. Why is there always only one female in every sea anemone?

The answer is simple, they change sex. Strange but true, and the trigger for sex reversal is in the bullying. In every anemone the female will eventually die and the bullying of her mate, the largest male, ceases. The hierarchy continues with the largest male bullying the next largest and so on down the line, but the largest male is no longer being bullied. And this is the trigger for him to make the transition from male to female. If you think about it, its quite clever. This ensures that there is always a breeding pair in every anemone.

Life continues as normal only now it is the new females turn to lay the eggs, which are fertilised by her new mate. The mating process is an interesting one that the female plays little part in. The male is responsible for clearing a piece of rock close to the anemone so that it is smooth and free of algae and other growth. The female will then lay two hundred tiny pink eggs on this patch and that is the extent of her involvement in the process. The male fertilises the eggs and his care over the eggs is extensive. He constantly blows and fans oxygenated water over the eggs to keep them healthy, he will also clear any infringing algal growth from the eggs and discard any eggs that have died. This process is constantly carried out for two weeks. Once the eggs have fully developed they hatch at night and the larvae are attracted upwards toward the moon where they begin their life as planktonic larvae and the whole process begins again.

Ciguatera. (Food poisoning by tropical fish)
Large reef fish are a regular source of ciguatera. Trout, Red Bass, Chinaman and in southern reef waters, Mackerel are often carrying the Ciguatera toxin. This Toxin is produced by a primitive life form called a dinoflagellate a whip propelled protozoan. See the phylogenic tree. It lives in and on algae eaten by smaller herbivores. Predators higher in the food chain store and carry the toxin, especially in the liver. Families dependent on fish as a food source feed the cat first as a safety measure similar to the canary in the mineshaft.

Turtles
Contact with sea turtles on the reef is not uncommon and is a memorable experience. Needing to breathe at the surface leaves them easy to find while at the surface. Many snorkellers will swim right over turtles feeding on the bottom and not notice the turtles due to the shell being well camouflaged. During the nesting seasons the females climb out of the water to lay eggs above the high tide line in body pits then bury the eggs to fend for them self. Islands with soft sandy beaches are the best place to view turtles nesting.

Go to Turtles

Dolphins and Whales
Dolphins and whales are easily seen from boats travelling to and from the reef. In winter months humpbacks give birth in the shelter of the reef. During the years of colder waters on the eastern coast of Australia (El Niño) larger numbers of Humpback whales were venturing above the 16? latatude line and further north. The (La Niña) leaves warmer than normal along the far north Queensland. On the calm days at sea they are easier to see. If whale watching is desired over swimming on the reef Harvey bay is well organised during the late winter months and provides some of the most consistent whale watching anywhere in the world. Snorkelling with whales is not encouraged and indeed may be contrary to the whale watching regulations on the eastern coast of Australia. Humpback whales are seen off southern reefs late June and the northern reefs past Cairns mid July onwards to September.

Dolphins are feed at Tangalooma in Morton Bay south of the Great Barrier Reef. Swimming with dolphins is rare but does happen on the reef.

Minke whales frequent the reefs north of Cairns during July to August. Snorkellers are a often followed by inquisitive Minke whales very near the reef edges off tourist vessels. Minke whales are much smaller than the Humpback whales but are also in the sub-order Mysticaetae, which means that instead of teeth they have long rows of black bristles literally hanging from the roof of their mouth. When whales are feeding they will open their mouths and take a huge gulp of water and then filter out the plankton they feed on by using their tongue to force the water through these plates and trapping the food. The tongue of some other species of whales can be the size of a car. The Minke whales in these waters are actually a sub-species called Dwarf Minkes, as they are smaller than the Minke found in other areas of the world. They are known to be a very curious species and there are many reports annually of these whales coming up to play with boats and swimmers.

Go to Marine Mammals

Box JellyfishJellyfish (Chironex fleckeri) and (Chriopsalmus quadrigatus)
Box Jellyfish are named because of their distinct cubiod/box shape. Chrinonex is considered to be the more dangerous. Present along the coastal beaches and are prevalent following local rain, especially near river and creak outlets, but are usually absent during while the coastal seas are rough but not always. It is considered highly unlikely over coral reefs where snorkelling is a commercial recreational activity well off the coast. These nasty little animals are closely related to corals this may help you understand why touching corals with stinging cells maybe a bad idea. Smaller animals have bells as small as 2-5 cm in diameter larger mature specimens are up to 20 cm and can extend tentacles over 3 meters. Estimations of the number of stinging cells (nematocysts) are in the 4 - 5 thousand million.
Far North Queensland has higher numbers in the warmer (October to April) the risk can vary greatly from year to year. My advice is swim on the reef not along the beaches in summer.

Coral Bleaching.

What is coral bleaching? Coral bleaching is a condition that can seriously damage or kill entire reef systems. Corals contain microscopic plants called zooxanthellae that colour their tissues and provide them with food by photosynthesis - the same process that plants manufacture food from light. Without these tiny plants corals cannot survive or lay down the huge amounts of limestone in their skeletons. When corals become stressed, the zooxanthellae are the first to go. Stressed corals expel the zooxanthellae and turn white or "bleach". If zooxanthellae do not return to the coral's tissue, the coral will die. Generally speaking at present the south pacific has been spared from previous bleaching events. The 1998-1999 summer has seen some of warmest water temperatures in living memory. Corals can recover from stress and having dumped the symbiotic algae and reabsorb them later but repeated stresses will normally terminate the entire colony eventually. There is evidence of bleaching effects on the Great Barrier Reef. Because of the increasing intensity and geographic scale of recent bleaching events, mass bleaching is considered by most reef scientists to be a serious challenge to the health of the world's coral reefs. The worst coral bleaching ever was recorded in 1998. Every reef system in the world's tropical oceans was affected. In some places, such as the Indian Ocean, entire reef systems died.

Increasing ocean temperatures is the main cause of bleaching and ocean temperatures are expected to continually clime 1-2º C per century.

SST Anomaly and Coral Bleaching Hotspots

Coral Spawning. (Broadcasting eggs and sperm)
Does the reef spawn at the same time?
The Great Barrier Reef corals spawn during the Beginning of summer. Spawning is generally 4-6 days after the full moon in October, November or December in the earlier hours of the night. Different Species tend to spawn at different times. Also because the reef is over 2000 km from top to bottom, the spawning times of the same coral species can be apart by days and even months. Corals spawn throughout the year if you travel the globe. Every month there will be a reef spawning somewhere.

more info on spawning

Cleaner Wrasse

Giant clams

Irukanji the tropical marine stinger

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If you have comments or suggestions, e-mail me: pete@reefpix.com.au

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