Skip to main content

Fish

Fish of the Great Barrier Reef

The Great Barrier Reef World Heritage area is home to over 1600 different species fish. There are a unique range of ecological habitats, communities and species within the Great Barrier Reef Marine Park – all of which make the Reef one of the most complex natural ecosystems in the world.

Sea Butterfly from the front

Sea Butterfly

Sea butterflies, or flapping snails, are holoplanktonic mollusks (Mollusca, Gasteropoda), belonging to the suborder Thecosomata (Blainville, 1824). Holoplanktonic means that they are living in the water column between bottom and surface.

They float freely in the water, along with the currents. This has led to a number of adaptations in their bodies. The shell and the gill have disappeared in several families. Their foot has taken the form of two wing-like lobes, or parapodia, which propel this little animal through the sea by slow flapping movements. At times, they just float along, ventral-side up, with the currents.

They are rather difficult to observe, since the shells are mostly colorless, very fragile and usually less than 1 cm in length. Their calcareous shells are bilaterally symmetric and can vary widely in shape: coiled, needle-like, triangular, globulous. These shells are very sharp, and will stick into exposed human skin.

Little is known about the behaviour of sea butterflies. They have a peculiar way of feeding. They are mostly passive plankton feeders, but at times they can be real hunters. They entangle planktonic food through a mucous web than can be up to 5 cm wide, many times larger than themselves. If disturbed, they dump the net and flap slowly away. When descending to deeper water, they hold their wings up.

Sometimes, they swarm in large numbers in flotsam along the coast of eastern Australia. Every day, they migrate vertically in the water column, following their planktonic prey. At night they hunt at the surface and return to deeper water in the morning. They also used to be called pteropods. This term, however, is applied to the suborder Thecosomata as well as to the suborder Gymnosomata. Mollusks of the suborder Thecosomata have a shell, while the Gymnosomata lack a shell.

Closeup of blue parrotfish

Parrotfish

Of the approximately 1600 species of fish along the length of the Great Barrier Reef, the ubiquitous Parrotfish family is probably the most easily recognized. On any given visit to the reef, you are guaranteed to see these magnificently coloured creatures swimming over the coral zones.

There are over 30 species of parrotfish on the Great Barrier Reef, and all share a few common traits. The most obvious of all relates to their teeth. Parrotfish evolved from the carnivorous wrasse family, but through the course of time, their teeth have moved forward on the jawbone and reduced in size. Eventually, the teeth fused together, creating a very useful tool for scratching algae off the surface of the coral. Usually they will scratch a thin layer of limestone off the coral along with the algae, and this helps with the breakdown of the cellulose. Often parrotfish will be seen passing the broken down sand as a fine stream of sediments, and some scientists have calculated that up to 30% of the coral sand you see at the reef is in fact old parrotfish poo! As always, however, there are a few exceptions to the rules, with some of the larger species actually feeding in part on live corals.

Reproduction in parrotfish is usually conducted in one of two ways. Either a group spawning, where there are many more very excited males than females (practiced regularly by the aptly named Sordid green parrotfish), or pair spawning, where there is often an elaborate courtship display, where the male woos the female with gentle brushes of his fins down her side and a sexy dance. A fine example is the superb Bicolour parrotfish.
Parrotfish are generally thought to be protogynous, where they are born females, but may change sex to the terminal phase. The reason for this is probably based around population dynamics. One male can produce thousands of sperms to fertilise a lot of females, so there’s not a great need to have plenty of males around. If there is a dramatic loss of males in the local populations, the initial phase females can actually change sex and turn into fully functional males. This is usually associated with a change in colour to the brilliant greens and blues we see daily on the reef.

Parrotfish form large mixed schools, with up to 10 different species of parrotfish, and often other families of fish such as surgeonfish and wrasses. These completely unrelated species often benefit each other, with the parrotfish scratching the surface, sending fine algal strands into the water column, which the surgeonfish swoop on, and the wrasses pick off the disturbed invertebrates.

When parrotfish sleep at night, they often secrete a mucus sleeping bag around themselves to protect themselves from predators. The bags also have antibiotic properties that are beneficial for the fish.

Coraltrout swimming above sand

Coral Trout

Enter any seafood restaurant during your stay in Cairns and you are likely to find Coral Trout on the menu. The firm pearly white flesh is considered to be the finest eating of all of our reef fish.

Coral Trout are generally pinky/red in colour with a dense scattering of electric blue spots, though individuals from shallow waters are more likely to be Olive/Brown in colour. Don’t be fooled by the name however, the Coral trout is not a trout at all. These fish are actually a member of the Grouper and Cod family possessing a large mouth and sharp conical teeth.

Coral trout are an ambush predator that lurks in the shady over hangs and crevices of the reef employing an explosive rush to capture prey. They feed on a variety of reef fish species from tiny Hardy head bait fish to sizable Parrott fish. The difference in colour between individuals is a camouflage strategy with pink more suited to the duller light conditions of deeper waters. In the absence of the red wave lengths of light at depth the pink colour makes the fish appear olive brown in colour which is ideal for hiding in the shadows. Look for the Coral trout lurking below dense schools of bait fish when you are diving or snorkeling on the Great Barrier Reef.

There are 4 species of coral trout found commonly on the Great Barrier Reef with the most obvious difference between species being the size and shape of the spots. A relatively fast growing fish Coral Trout achieve a maximum size of 120cm and a weight of 26 kg by which time they would be 16 to 18 years of age. Growing to 20cm in their first year coral trout mature sexually at 2 years of age at a size of 30cm. The minimum legal size at which coral trout can be kept by a fisherman is 38cm at which time they would be close to 4 years old.

Though this may sound a little weird the coral trout begins life as a female before changing sex at 4 to 6 years of age to become a male. Sex change in reef fish species is quite common and involves hormonal rather than physical changes. This strategy allows populations to optimize the female/male ratio maximizing reproductive output. Coral trout spawn during the late spring (September to November).

Coral trout are the dominant species in the commercial reef line fishery and one of the prime target species for recreational anglers. Economically Coral trout are one of the most valuable fish species on the Great Barrier reef with a commercial catch of 1500+ tones per year. The management of this fishery is seen as priority by the management authorities involved and uses world recognized best practice strategies including reef closures, spawning closures and bag limits to ensure the sustainability of this fishery.

Clownfish closeup surrounded by purple anemone

Clownfish

Clown fish, or anemone fish as they are sometimes known, belong to the Damselfish family that includes sergeant majors and chromids. There are 28 species in the world, they are generally small in size (the largest species attaining 16cm maximum in length), and usually have some orange coloration on their bodies. The most famous, and most sought after, species is the clown anemone fish, Amphiprion percula, which has three white bars narrowly edged in black across their bodies. They are generally shallower water dwellers, existing in the top 15-20m of the world’s oceans, and are opportunistic planktovores.

The term anemone fish relates to the special relationship that this group of fish has developed with sea anemones. Sea anemones are cnidarians, which means they are related to corals and jellyfish, and the common denominator is that they all bear tentacles covered in millions of tiny stinging cells called nematocysts. Any “normal” fish likely to come in contact with the tentacles of an anemone is likely to be stung, killed and eaten. Of the more than 1000 species of anemones, only 10 act as hosts for the clown fish.
How a clown fish can live amongst the poisonous tentacles without injuring itself has been the subject of much debate. It appears that the fish will slowly brush itself against the tentacles, giving itself a small sting, but in the process covering itself in the anemones mucus. When fully coated, it can then swim through the tentacles at ease, creating the perfect fortress against invasion by predators. There are some reports that the clown fish will also feed upon the leftover bits of food that the anemone misses. In return, the clown fish will defend its’ home from species likely to harm the host anemone, such as butterfly fish (well known as polyp feeders), and provide a source of food through it’s faeces. This interaction is known as a symbiotic relationship, where both parties benefit.
A single anemone can host lots of clown fish, depending on its size. We know of one where there are over 40 individuals, living harmoniously together. The largest member of the group is nearly always a female, and the rest are generally immature males. If the female dies, or is removed from the anemone, the next most dominant male (usually the largest, but not always) will assume custody of the clan, and over a week or two will change sex into a fully functional, fertile female . Many types of fish have sex change, such as parrotfish and wrasses, but the vast majority change from female to male.
When females are ready to lay their eggs, they show an amazing degree of maternal care. The females have been observed to move out towards the perimeter of the anemone, and start to nuzzle and chew on the tentacles. This causes the anemone to curl up into a ball for a few minutes, and during that time the female rushes down and lays her eggs on the substrate. When the anemone relaxes again, it extends back over the eggs, providing a degree of protective cover. When the female wants to tend to the eggs, she’ll go through the same process, and will mouth the eggs, and use her fins to fan them, washing away waste products and delivering oxygenated sea water.

Due to the success of the film, there has been an unprecedented rush to purchase clown fish and anemones. This could result in a decline of clown fish in the wild, so if you are considering a little Nemo for your salt water tank, do the right thing and investigate your sources and buy a captive bred specimen. Better still, leave them where they are, and come out to the Great Barrier Reef and see them in their natural environment. Nothing like the real thing!

School of black surgeonfish above coral

Surgeonfish

Surgeonfish (or Tangs) are found throughout the world’s temperate and tropical oceans, and there are approximately 80 species, many of which are found on the Great Barrier Reef. The body is basically flat, and oval in shape, with a small mouth for grazing on algae (although some feed on zooplankton).

They get their name from highly modified scales that have evolved into weapon-like spines at the base of the tail. These spines are either fixed or mobile (like a flick knife) and there can be up to 6 per side, depending on the species. They are mainly used when the fish feels threatened over territorial disputes, but fishermen have often been slashed when removing them carelessly from nets.

One species, the mimic surgeonfish, has a unique strategy, where the juvenile assumes the colours of angelfish (colours that are good for camouflage). When the fish becomes larger than the angelfish, and it no longer needs the protection, it changes colour so that other mimic surgeonfish know that it is a reproductive adult.

Within the family is a group called the Unicorn fish. These intriguing creatures are so named because of the enlarged proboscis or “horn” that emerge from their foreheads. It’s believed by many marine biologists that the horn is used like a rudder, so that these unicorn fish can maintain their position in the water currents when they are feeding on plankton.
Like some other families of fish, they have the remarkable ability to change their colour according to their mood. The presence of a predator can cause a near instant change, and when they sleep at night the colour can change again (often referred to as the pajama colour). Also, within species variation can be found (the yellow tang is bright yellow around Hawaii, but can often be more brown in other locations).

The males of most species go through another colour change when they are about to spawn (which occurs during a full moon). All sexually mature fish form large swarms, and get highly agitated. They quickly release their eggs and sperm, and the fertilised eggs float to the surface of the water (they do this due to the presence of a tiny bubble of oil, which makes them positively buoyant). The eggs hatch within a day, and feeding generally commences within a week.

The most well known surgeonfish is the blue tang, made famous by that goofy character Dory, from “Finding Nemo”. Blue tangs are generally found on the seaward edge of coral reefs, or on the corners, as they prefer high current areas.

Some other species

Eel looking at camera

Eels

Although eels aren’t the most obvious fish you’re likely to encounter on your visit to the Great Barrier Reef (GBR), they are certainly one of the most fascinating. There are more than 700 species of eels worldwide, and a large number are found on the GBR.

One perennial favorite are the moray eels. This generally large eels lack pelvic and pectoral fins, but have a dorsal and ventral fin that runs the length of their body. They have very large canine teeth, designed for gripping prey, and then swallowing them whole (although a few species have evolved a taste foe crustaceans, and have small conical teeth so they can crush their prey). These gentle giants can reach up to 2.4m long.

If you get close to a moray (but not too close!), you may be able to make out some tube like extensions to their nose. These highly sensitive structures allow the moray to “taste” the water for chemical cues. This is a great advantage, as their eye sight is generally quite poor. Their poor vision has earned them an undeserved reputation for being aggressive, because they have often bitten the hands of divers who have tried to hand feed them (not a good practice!). The eels, unfortunately, can’t determine where the food stops, and the hand starts, and their raking jaws can cause nasty, and easily infected, wounds.

Morays have the ability to tie their bodies in knots and use this to gain leverage when tearing food. It was long thought that this was only a story, not based on fact, but has since been verified. Long, flexible fishes such as eels swim by flexing the whole body into lateral waves. This is known as anguilliform swimming. Most eels (including the Green Moray) have laterally compressed tails. The Green Moray also has well a developed dorsal fin which results in further compression of the body.

Although it appears that eels have no scales, they are generally present as minute, conical structures. The eels that you see on the menu at a fancy restaurant do not come from the ocean – they are all freshwater eels, and Australia produces some of the highest quality eels for the domestic and international market. All saltwater eels are prone to harboring ciguatera, a toxin that can cause severe poisoning in humans.

If you go out to the reef looking for eels, your best chance is to swim along the inner edge of a reef, and observe carefully, and you might encounter one swimming from hole to hole, trying to find its next meal.

Eel Facts:

- There are over 400 species worldwide
- Young eels are referred to as “elvers”
- Eels can swim backwards as well as they can forwards
- Eels can absorb oxygen through their skin as well as through their gills
- Eels can survive for up to 2 days out of water, relying on the oil in their skin

Sea Snake

How many species occur along the Great Barrier Reef?
Around the world, there are nearly 60 species of sea snakes. 14 species are found along the Great Barrier Reef, including the large Olive sea snake, and the black and white banded sea krait. It is thought that sea snakes evolved from Australia’s elapid (front fanged) snakes about 30 million years ago. Today, there are 2 distinct groups, the Laticaudids and the Hydophiids.

What adaptations make them special?
To aid swimming, their tails have evolved from a long, cylindrical shape to a paddle shaped structure, which allows them to use their natural undulating movements. Because they often spend long periods (30 minutes to 2 hours) foraging on the sea floor, their lungs are greatly modified. One lung extends nearly the full length of the snake, and is very efficient in gas exchange. Also, they are able to extract oxygen from the sea water through their skin.

Sea snakes have a gland under their tongue which allows them to excrete excessive salt. Lastly, they have to shed their skins (sloughing) more regularly than land snakes, as they are more prone to fouling by other marine organisms, such as barnacles and algae.

How do they reproduce?
Breeding is conducted in the ocean, which in itself produces a few problems. Couples will form during large aggregations, and the snakes will intertwine. Reproduction can last for hours, and the female controls when they surface to breathe (so the male better be nice!). Due to the shape and design of the male snakes’ penis, the male is unable to disengage from the female until mating has finished.
Females go though a gestation period between 4 and 11 months, and the young are born alive and underwater (except for the laticaududs, which go onto land to lay eggs). Once born, they must be able to fend for themselves as there is no parental care.

Sea snakes are all incredibly venomous, but are usually very placid and reluctant to bite. The only time they get really inquisitive is when they are breeding. It is not known why they aggregate in large numbers on some reefs and are rarely seen on others.

Another common story is that “sea snakes can’t bite you, as they have very small mouths”. This may be true for some species, but the majority are certainly capable of delivering a dangerous bite.

Blue Spotted Ray viewed from the side

Marine Rays

Along the length of the Great Barrier Reef, there are over 35 species of marine rays. Rays play a very special part of Australia’s early discovery by Europeans. Lieutenant James Cook recorded numerous large rays caught in Botany Bay (so many, in fact, that Cook originally named it Stingray Bay).

Rays are generally broad, flat fishes that have evolved from sharks, and as such, have a cartilaginous skeleton. Like sharks, they have open gill slits, spiracles (holes for inhaling water) and all exhibit internal fertilization. The majority of species have evolved a bottom dwelling life, using their highly modified crushing jaws to feed on molluscs.

The most well known part of a sting ray is the tail. This structure is usually no more than a flimsy filament, although in some species it has evolved to be a more robust tool. Many species (but not all) have a dangerous and poisonous barb, which is only used in self defence. The barbs have been used by aborigines as the tips for spears for thousands of years. These structures are intricately designed, and very difficult to remove without causing major tissue damage (stories circulate about fishermen having to push the barb all the way through a limb to remove it!). Also, there is toxic venom around the barb that can cause complications.

Of course, the chances of a nasty encounter with a ray whilst swimming around are about nil, as the animal will always retreat unless trapped or physically disturbed. Like all creatures on the reef; observe and enjoy!
The most commonly seen ray is the blue spotted lagoon ray, which reaches a disc length of 50cm. It is usually seen half buried in the sand with only the eyes protruding. The thorny ray is so named due to the extremely rough dorsal surface. It also has an unusually short tail with no stinging spines.

Eagle rays are a diverse group that are much broader than they are long. They produce 2-6 young at a time, and are well known aerial acrobats, leaping out of the water to considerable heights. They spend a lot of their time in groups near the surface of the water, but still feed on shellfish on the sea floor.

Another group not instantly recognized as a ray are the guitarfishes. They have 2 large dorsal fins, and a shark shaped torso, which confuses some fishermen. They can reach over 3m in length, and are quite inquisitive, and will approach divers underwater.

The most impressive of the rays, in appearance and size, are the devil rays. The largest ever specimen reliably measured was over 9m in width! They have extensions of their pectoral fins, called cephalic lobes, which look like horns on their heads (hence the term devil rays). These are believed to aid the collection of plankton, which they feed upon. Their tails are short and filamentous, and most species in the group lack a spine, such as the beautiful Manta ray.

Lionfish

Poisonous / Venomous Fish

What’s the difference?
A poisonous fish is one that is toxic to eat, such as puffer fish and toadfish. These colourful characters harbour a strong poison in glands in their bodies, and when they are eaten by other fish (or people!), they can cause illness or even death. Puffer fish have an alternative defensive mechanism where they can inflate their bodies to create the illusion that they are larger than they actually are, which will often scare of potential enemies. However, if they over inflate their bodies, they can induce heart failure (in themselves!). In Japan, highly skilled chefs use parts of the blowfish (a closely related species) to create a dish called “fugu”, with just enough of the tetrodotoxin poison to give the meal that “special flavour”!

Venomous fish, on the other hand, are ones that cause an envenomation, usually through a penetration wound, such as a spine. Some fish, such as the infamous stone fish, have highly toxic glands at the base of their spines, and when people stand on them, the pressure forces the venom into the victim. Although it is extremely painful, it is rarely lethal, and an antivenom is available for severe stings. Stonefish stings are generally the most severe, as they have neurotoxins in their venom, but there are others, such as lionfish and stingrays. Lionfish have elaborate pectoral fins, attired in the warning colours or orange and red. When they are actively hunting prey, they herd their victims up against the coral, spreading their venomous arrangement of spines in a rainbow of colour so they can’t escape.

Stingray envenomations usually occur when theyare trodden on or handled by fishermen. The natural reaction for a stingray that is used as a doormat is to thrust backwards with the spine located on its tail. The spine is designed to easily penetrate but not to be easily removed. The venom around the spine can cause infections and sometimes necrosis (death) of the surrounding tissues.

The best treatment for an impaling wound is heat, as heat denatures the protein of the toxin, and converts it to a less dangerous compound (and it also helps alleviate the pain!). It also increases blood flow and helps disperse the venom. The water temperature shouldn’t be any more than 50°C, as anything above this could burn your skin. Keep a close watch over the next few days for possible infection, and seek medical help if necessary. The victim should be OK in a few days.