Toxic Marine Life

Information about dangerous marine life

Toxic Marine Life

Did You Know?

There are more poisonous fish than venomous snakes in the world..!!

There are 1,200 or so species of venomous fish including the stonefish, lionfish, scorpionfish, stargazer, weever and toadfish. Most marine creatures are not hostile, it’s mainly humans who provoke them leading to stings and bites.

You don’t hear much about poisonous fish, but there are actually more poisonous fish in the world than venomous snakes. These fish usually release their venom through spines on their fins and tails. If you step on them or brush up against them, watch out! You’re in for a painful – and sometimes deadly – surprise.

Poisonous vs Venomous – What’s The Difference?

Poisonous and venomous are terms often used interchangeably but they have different meanings. A venomous animal has a means of injecting their toxin into another animal, whereas a poisonous animal can only deliver their toxin in a more passive manner (by being touched or eaten).

A common example given to clarify this difference is that frogs are poisonous while snakes are venomous.

It is also important to note that the affects from a venomous fish sting can often be relieved by immersing the wound in hot water (40-50° C) because these toxins can be destroyed or altered by heat.

Toxic Fish on Koh Tao

So let’s have a looks at some of the toxic fish we can find diving Koh Tao….


Stonefish have usurped the title of ‘Most Venomous Fish’ in recent years. They often resemble encrusted stones (hence the name), blending into their natural environment with ease. They deliver their venom through a row of spines on their back that can be extended when threatened (or stepped on).

Venom is involuntarily expelled when pressure is placed on the fish and the more pressure the more venom. They reside in the Indo-Pacific region and northern Australia. A sting from one of these fish can cause excruciating pain, rapid swelling, tissue death, muscle weakness, temporary paralysis, and in very rare cases death.

Indian Ocean Walkman

The Indian Walkman, also known as the Devil Stinger or Spiny Devilfish is a member of the Inimicus genus of venomous fish and closely related to the Stonefish. They grow to 25cm’s in length and, like the Stonefish, have a kobbly appearance with venomous spines to ward off enemies.

They are nocturnal, often digging themselves into the sandy seabeds during daytime. They are red/yellow coloured and blend in well with sandy and coral seabeds. This particular one, was photographed at Pottery Pinnacle on a night dive by PADI MSDT Kevin He on the 20th July 2016.

The Indian Walkman as we call it on Koh Tao has no known natural predators, and once ‘dug in’ in the sand is often very reluctant to leave its hiding place. When it does move, it crawls slowly along the bottom, employing the four lower rays (two on each side) of its pectoral fins as legs!

When disturbed, it fans out its brilliant coloured pectoral and caudal fins as a warning to potential predators or scuba divers! It has a poisonous dorsal fish spines that can cause a painful sting.


Lionfish were thought to be the most venomous fish until recent years when stonefish stole the title. These conspicuous fish have venomous dorsal, anal, and pelvic spines covered by a loose sheath that moves down and compresses venom glands when the spine punctures tissue. A sting from these fish can cause extreme pain, swelling, and in very severe cases, cardiovascular collapse.

Most lionfish naturally reside in the Indo-Pacific but they have become an invasive species in recent years; most notably along the Atlantic coast of the United States where they are having a major impact on Atlantic coral-reef communities. Lionfish were likely introduced along the Atlantic coast through aquarium releases.


Pufferfish (some species are also called toadfish) have been given the title ‘Most Poisonous Fish’ and have also been labeled the second most poisonous vertebrate in the world. The toxin responsible for ranking this fish so high in the ‘danger zone’ is called tetrodotoxin.

Tetrodotoxin is neurotoxic and inhibits neural transmission leading to weakness, paralysis, and even death at relatively low concentrations. This toxin is found in the fish’s liver, ovaries, intestines and skin, leaving muscle tissue with relatively low and somewhat safe levels to eat.

However, only highly trained and certified chefs are allowed to prepare this fish for consumption; a common practice in Japan where pufferfish are considered a delicacy called ‘Fugu’.

Crown of Thorns

Among starfishes in the world, the Crown of Thorns is the only starfish that is poisonous. Its entire body barring the underside is covered with spines which are venomous. Whenever the Crown of Thorns stings a person using these spines, it releases venom that causes intense pain, redness and swelling.

These crafty creatures also poison the coral as it crawls across destroying and eating everything it passes.

The Box Fish

The boxfish has a wicked chemical defense system. Being closely related to the Pufferfish this is no surprise, however the toxins they hold ‘Pahutoxin’ also known as ‘Ostracitoxin’ works in a way like no other known fish toxins. When in danger the Boxfish secretes this toxin from special cells covering their entire skin and quickly disperses this clear mucus in what is effectively a cloud of regret for any engaged predator!

The pahutoxin has what is known as a ‘haemolytic’ effect, meaning it bursts red blood cells. The toxic mucus binds to the gills and slowly asphyxiates its victim all at a concentration of 10 parts per million. To put it into more relative context of how potent pahutoxin is, 10 parts per million is roughly the same as you spitting your mouthwash out on a morning but instead of the sink, into a 2.5million litre sized Olympic swimming pool!

The Surgeon Fish

Surgeon Fish essentially is a reef fish related to Cihclids. One famous Surgeon Fish you might be familiar with is ‘Dory’, originally from ‘Finding Nemo‘ and now the star of her own movie, the recently released ‘Finding Dory’. They attack with their tail which has spines at its base. These spines remain dormant within the tail until the fish feels like that there is no danger.

Whenever aggravated the surgeon fish stick out their spines that are like small knives. These highly poisonous spines can even cause death by hypovolemia. The other symptoms are hypertension and extreme pain. But on the bright side they won’t come near you because they have probably forgotten their own name..!!

None of these fish will seek you out, and attack, like wasps do, but as divers we must make sure that we respect them, use good buoyancy and look but don’t touch. As scary as they might sound, all these creatures of the deep are beautiful, interesting and a real treat to see underwater.

Author: Victor Szymanik (PADI MSDT #305957)

Crystal Dive Koh Tao

7/1 Moo 2
Tambon Ko Tao
Koh Tao
Surat Thani

E-mail: [email protected]

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