When threatened, the first line of defense for many reef animals is to hide: "if you can't be seen, you can't be eaten". Some will employ deception, mimicry and camouflage to disguise themselves. Others will hide in holes or crevices. Those that can't hide may distract predators with inks, lights or ejected parts while they make a quick getaway. Others show off bold, bright colours to suggest they are poisonous and to warn predators to keep away. With predators and prey living on the same reef, there is an astounding array of defensive mechanisms and strategies employed by reef creatures, some of which are designed to inflict pain and injury to predators, including humans. Here are some of them:

• Slow-moving creatures like urchins, sea stars and cone shells use chemicals or poisons in combination with darts, spines and needles to protect themselves.
• Stonefish and scorpionfish carry lethal venom on dorsal spines, rays carry venom in their stings, while boxfish, pufferfish, soapfish and toadfish secrete skin poisons.
• Jellyfish, corals, anemones and hydroids have stinging cells which can fire a mass of tiny harpoons (called nematocysts) containing venom to stun and kill their prey.
• Some species of octopus have venom in their saliva. For example, the bite from the tiny blue ringed octopus can be lethal to humans.
• Some innocent-looking reef fishes are equipped with self-defense mechanisms. Squirrelfish and angelfishes have spines on their gill-covers which are venomous in some species of squirrelfish. Surgeonfish have a retractable blade or scalpel at the base of the tail which it uses to slash would-be attackers.
• Some sea cucumbers eject white cuiverian tubules when irritated. These contain toxins that can cause blindness when it comes into contact with the eye - so make sure you wash your hands after handling them. Others are able to cast off parts of their gut as sacrificial bait or emit sticky filaments to entangle would-be predators, including humans. These body parts are regenerated.
• Barracudas, sharks, eels and triggerfishes have powerful jaws for biting while the mantis shrimp uses its deadly claws to spear or smash their victims. Electric rays and electric eels can deliver high-voltage electric shocks to stun its prey.

This golf ball-sized octopus is small, about 7 cm long, shy and hides in coral crevices or under rocks. It may look cute, harmless and attractive but is amongst the most dangerous creatures on the planet. Its venomous bite has claimed a number of human lives as its saliva contains TTX (tetrodotoxin) produced by a bacteria, one of the most potent neurotoxins for which there is no known antidote. TTX is considered to be a hundred times more powerful than potassium cyanide and is also found in certain organs in the pufferfish, porcupinefish, sunfish and a few other animals. The blue rings on its body are only clearly visible and pulsate when the octopus is agitated and about to bite. The bite is slight, reported to be accompanied by little or no pain, but will lead to muscular paralysis and respiratory failure that can lead to death if not treated early. For more information, read Dr. Roy Caldwell's article on blue-ringed octopuses.

Sea snakes are reptiles, have scales and flattened, paddle-like tails with heads that resemble land snakes whereas eels are fishes, have a fish-like face and mouth as well as one long continous dorsal fin. All sea snakes are highly venomous and should not be handled even though they are shy, gentle and do not normally pose a threat unless provoked. When they do bite, venom is injected only in about one-third of all bites. Antidotes are available for treatment. Sea snakes may approach divers or snorkellers out of curiosity. The best defense is to stay calm and not lash out at the snake and it will soon continue on its way. As with other reptiles, they breathe air and have to periodically return to the surface. They are usually found in depths between 5 to 10 metres hunting for food on the seabed or coral reef.

The family of fishes known as Scorpaenidae have venomous spines on their dorsal fins as a defense against rays and sharks. Some species also carry these spines on their pectoral and anal fins. All of them, except for the brightly coloured lionfishes, are well camouflaged, blending with their surroundings and remaining stationary. Hunting mainly at night, they sit and wait until an unwary prey comes near, then they extend their jaws and sucks in the prey at lightning speed. Only the lionfish moves in search of prey. The stationary scorpionfishes are usually given away only by their large pectoral fins and slightly curled up resting position on the sea bed, corals or rocks. They are typically grayish-brown and grow up to about 15 inches in length.

The stonefish, the most venomous fish in the world, looks like encrusted rock or dead coral. Most human victims injure themselves when they accidentally step on it or place their hands on it. 13 hard spines on its back, sharp enough to puncture rubber soled shoes, carry neurotoxic venom into the wound, causing excruciating pain, temporary paralysis and shock, and in rare cases, even death. The best way to avoid stepping on stonefishes and scorpionfishes is to shuffle one's feet when wading in shallow waters and to tread lightly while wearing thick-soled shoes. Also, be very careful where you place your hands - the 'rock' you hold on to might be alive!

This is the only venomous starfish. It is covered with sharp spines all over its body except on its underside. Its skin contains venom-producing glands which release venom on contact. Although not fatal, stings are painful, causes redness and local swelling. In some cases, stings may also cause nausea and vomiting. Upon contact, spines may break off and remain embedded. This starfish can cause widespread damage to coral reefs if their numbers become too large. When that happens, clean-ups are sometimes organized (although such cleanups are widely debated amongst conservationists) where divers collect these starfish (using instruments to avoid hand contact) and then dispose of them on land.

Cone shells are highly sought after by shell collectors due to their attractive and intricate markings. Behind this attractive facade is a highly venomous harpoon which is used to paralyze prey, typically fish, molluscs or worms, before drawing it back to their mouths. The venom is contained in the tongue-like proboscis (radula) equipped with harpoon-shaped teeth. The harpoon can be fired numerous times if needed and with such force that it can pierce wetsuits and clothing. Many human victims get stung when they carry them in pockets or in mesh bags close to the body. The venom contains neurotoxins which cause weakness, loss of coordination, and in serious cases, respiratory failure and even death. For more information, visit the Cone Shells and Conotoxins site.

Stingrays are shy and frequently hide in shallow waters under rocks or buried under the sand with only their eyes slightly exposed. The front half of the tail may have up to 7 barbs or spines located on the top side which the ray can use to inflict a painful, venomous sting to any aggressor by whipping its tail upwards in an arc, much like the way a scorpion stings. The barb, which can be up to 25cm long, has razor-sharp serrations and is sometimes hidden beneath a sheath and exposed only when striking. The barb may break off after a strike and become lodged in the victim. Rays can grow new barbs and do so whether they use it or not. Not all rays have stings. Devil rays, butterfly rays and some that are sometimes called 'rays' (like guitarfishes, electric rays and certain skates) do not have stings.

Rays are gentle creatures which is probably why they are commonly found in touch-pools at public marine aquariums. They forage for their food by flapping and uncovering prey hiding in the sandy bottom and never use their sting when hunting. Their barbs are used only for defense especially against their predators (mainly sharks). Most human victims are stung around the ankles or feet when they accidentally step on the ray, and fishermen have been known to be stung while sorting out their catch, which includes sting rays, on the boat deck. Fatalities involving ray stings are rare but in what is probably the most publicised incident involving a stingray, well-known Australian conservationist Steve Irwin, better known as the Crocodile Hunter, died after being stung in the chest by a large bull ray. As with the stonefish, the best way to avoid stepping on rays is to shuffle one's feet when wading in shallow water. Divers should avoid cornering rays and not swim close to and directly on top of the ray where they may be perceived as a threat.

Jellyfish are not fishes, but are closely related to corals, hydroids and anemones (known collectively as cnidarians which literally means 'stinging creatures'). All possess stinging cells which are used to capture prey. Jellyfish are related to coral polyps (see What are corals?) and are the free-floating medusa-form of cnidarians, while anemone and coral polyps are the fixed polypoid form.

Jellyfish are composed mainly of water and absorb oxygen directly from the surrounding water. Their tentacles range from 1 cm to 36 metres in length, and contain hundreds of thousands of nematocysts. The bell does not contain stinging cells. Nematocysts can still sting even when the tentacles are severed from the jellyfish so beware of touching jellyfish washed up on beaches or broken tentacles floating in the water.

Most varieties of jellyfish stings are harmless to humans (other than causing painful stings and welts). Of about 2000 known species, only about 70 can seriously harm humans, including the box jelly Chironex, also known as the Sea Wasp, and the tiny Irukandji jellyfish. Some species of jellyfish don't even sting at all, like those found in Jellyfish Lake, Palau and Kabakan island, east Kalimantan, Indonesia.

The box jelly Chironex fleckeri is the world's most venomous jellyfish and possibly the world's most venomous creature, causing more fatalities than stonefish, cone shells and blue-ring octopus combined. It is found mainly in the northern coastal waters of Australia and in some parts of the Indo-Pacific. It prefers coastal beaches and sheltered inlets and is rarely encountered on coral reefs. Despite its fearsome reputation, green turtles have been observed eating Chironex fleckeri without any problems.

Covering up exposed skin by wearing a full-length wetsuit, lycra swimwear or clothes is the best protection against jellyfish as the stinging cells are triggered by the presence of chemicals on the skin. For this reason, some Australian surf lifeguards wear nylon panty hose on their arms and legs. Lotions like Safesea from Nidaria have also been proven to be effective in preventing these stinging cells from firing.

Fire corals (millepora) and hydroids are hydrozoans, closely related to jellyfish, anemones and other corals. All possess stinging cells called nematocysts, which release thousands of tiny spring-loaded venomous darts upon contact with the skin, causing a painful rash and burning sensation. Despite the pain, stings from fire coral and stinging hydroids are not dangerous unless accompanied by allergic reaction.

Fire coral looks like dead branching hard coral with no visible tissue, mucus or polyps. All species have a characteristic whitish or yellowish tip on each branch and also have a soft, hairy, rounded or fine texture, unlike other hard corals which have a rougher appearance. Most common species of fire coral are typically brown or mustard yellow in colour.

Lace coral looks like hard coral fans with beautiful branches usually adorning ceilings of caves and over-hangs.

Stinging hydroids are commonly found attached to rocks, wrecks, shells, sponges and corals and look like ferns with a central stalk and many side branches. Some crabs, like hermit crabs, and other molluscs may carry hydroids on their shells as a form of protection.

Most cases of stings are caused by accidentally brushing against these stinging corals. As with jellyfish, covering up exposed skin will prevent stings.