Marine and freshwater environments contain some of the most highly evolved animals. These include whales that can communicate using a series of sounds, reptiles that can dive into the deep as they deftly search for their food, and seemingly prehistoric creatures that have developed armor-like external features to endure the tests of time. Of course, these also include rare and commonplace fish containing cocktails of frighteningly potent proteins!
Found in beautiful habitats like coastal reefs and freshwater lakes, poisonous and venomous fish species may have eye-catching morphologies and may be masters in camouflage. With but a few predators, they can be deadly to humans. Simply touching them or mistakenly eating their flesh can cause a painful and irreversible string of consequences. “Venomous” fish are those that can inject their toxic venom into victims, whereas “poisonous” fish are those that cause toxic symptoms only if they are eaten.
Though these fish do pose risks to humans, it’s possible to navigate their habitats safely. While mishaps do occur, especially if they are not sighted beforehand or if they are not prepared properly as ingredients in culinary dishes, incidents of fatalities are rare. Familiarizing oneself with the species listed below is the first step to ensuring that we can responsibly – and safely – explore their native biomes.
1) Lionfish (Pterois spp.)
Given their fascinating morphology and their vibrant colors, lionfish can cast a spell on or be the absolute bane of a scuba diver. These tropical fish have red, orange, or black bands throughout their body and along their spines. These act as a clear visual warning to all other animals around them, depicting the presence of potentially dangerous toxins. Their fan-like fins, lengthy dorsal spines and rays, and fleshy tentacles tend to flare outward, making them appear large – sometimes up to 18 inches (46 cm) long.
A lionfish’s venom is stored in specialized glandular tissues found at the base of its sharp fins. This is comprised of neurotoxins that can cause instant pain, nausea, paralysis, or respiratory problems. Injected into a victim’s bloodstream via puncture wounds caused by the spines, the neurotoxins can be fatal to young children. Anaphylaxis may occur as a response to copious amounts of venom. Healthy adults are more likely to experience mild to moderate discomfort.
In reefs, lionfish act as important predators. These hunters are best spotted in the morning, which is when they are most likely to feed on mollusks, small invertebrates, and smaller fish. As they don’t typically use their spines to immobilize prey, these serve as their defense mechanism against larger predators. Groupers and moray eels do feed on lionfish, aiding in the control of their populations. Outside of their native range, these venomous fish may become dangerously invasive.
2) Bluespotted ribbontail ray (Taeniura lymma)
Bright blue spots often serve as warning features to predators and other reef fish. These are indicative of dangerous toxins, such as those contained in the bluespotted ribbontail ray’s tail. This small stingray, which grows to a mature length of about 14 inches (36 cm), is distinguished by its smooth, circular disc of pectoral fins. Its spots and the blue stripes on its tail contrast their yellowish-brown background. The electric blue hue may be more visible beneath direct sunlight or in the strong light of a torch.
Though this shy stingray tends to flee instead of attack when it is approached or aggravated, it is capable of causing serious damage to humans and marine life. Its venom is contained in sharp spines at the tip of its tail. These are coated with a thin layer of skin, below which run grooves with specialized cells. When threatened and cornered, the ray can quickly lash its tail to inject the venom. The spines, which do regenerate, may break off as they come into contact with the victim’s skin.
The effects of the venom are determined by how much is injected into the victim and where. Rarely fatal, the stinging cells can be broken down by applying direct heat. If you happen to be stung by this otherwise harmless species, you may soak the affected area in hot water. The short-tail stingray (Dasyatis brevicaudata) that killed Steve Irwin was massive, more than 6 feet long, and inflicted damage by repeatedly stabbing Irwin’s chest. This goes to show that these creatures should never be forcibly handled.
3) Pufferfish (Family Tetraodontidae)
Set apart by their quirky way of defending themselves against predators, pufferfish are some of the most distinct and commonly spotted reef fish in tropical locations. Their natural morphology does not actually consist of a “puffed” body; they are elongated, finned fish with an occasionally boxy appearance. Depending on the species, this fish may have characteristic sets of bright spots, well-defined stripes, or elaborate and colorful line patterns throughout its body.
As the pufferfish is a slow swimmer, it has evolved to inflate its body in response to attacks. It does so by filling its stomach, lined with elastic tissues, with air or water. This gives it a spherical shape, one which may be difficult to pack into a prospective predator’s jaws. Its profile as a dangerous fish doesn’t spring from this form of natural defense but from a more deceptive source. Predators that insist on eating this fish may suffer due to the neurotoxins in its liver and intestines.
Tetrodotoxin, the most lethal form of neurotoxin in a pufferfish’s body, is capable of paralyzing victims. Present in varied levels across pufferfish species, this type of chemical is not lethal to sharks and lizardfish, which routinely consume puffers as their natural prey. When prepared correctly, the flesh of some puffers may be wholly free of toxins and may be considered a culinary delicacy (e.g. fugu in Japan).
4) Giant moray (Gymnothorax javanicus)
The giant moray eel is a distinctly slender fish. With an elongated body that grows to about 9 – 10 feet long (2.7 – 3 meters), it resides in hidden crevices. Usually brownish in color, with black speckles found throughout its full length, it can easily remain camouflaged. During its feeding time, it sticks its head out into the surrounding waters to hunt unsuspecting reef fish. It is usually harmless to divers and fishermen, but it may appear menacing because of its toothed jaws.
As the giant moray is found at the top of the food chain, it bioaccumulates toxins found in its prey. For this reason, some specimens may contain high concentrations of ciguatera. Consumption of this group of toxic substances may cause numbness, itchiness, and a host of digestive ailments. In the worst cases, ciguatera fish poisoning (CFP) may have symptoms that last for weeks.
To avoid CFP, it pays to simply avoid eating reef fish – particularly carnivorous ones. The associated toxins are initially produced by a dinoflagellate (Gambierdiscus toxicus) that is commonly found in tropical reefs. Herbivorous fish may consume this marine organism and, over time, are likely to accumulate low concentrations of its toxins. Carnivores, like moray eels, biomagnify the toxins as they feed on herbivorous fish.
5) Spotted trunkfish (Lactophrys bicaudilis)
Usually found swimming around coral reefs, the spotted trunkfish looks just as its common name suggests. It is generously spotted with well-rounded, brown-black markings and it has a trunk-like body, from which its fins and head protrude. It may look quite similar to boxfish or cowfish, but its body is slightly more slender. Upon close inspection, you’ll find that its plate-like scales are fused and heavily armored, making it too hefty to swim rapidly.
Though the spotted trunkfish can emit toxins (ostracitoxin) from specialized mucus glands on its skin, it is not considered a venomous species. The poisonous toxins have to be ingested for them to cause a cascade of troublesome symptoms. This means that divers who accidentally graze upon or touch this fish are in no danger of being poisoned. Marine animals who attempt to feed on the trunkfish, on the other hand, may be in for a string of unpleasantries.
6) Reef stonefish (Synanceia verrucosa)
A master of disguise, the reef stonefish expertly remains camouflaged on coral beds, the reef floor, and even on the rusted hull of sunken ships because of its irregularly textured and colorful skin. When exposed, its skin often appears to be covered in layers of encrusting algae. Those that remain perched on substrates, waiting to ambush their prey, may cover themselves in a fine layer of sand.
Humans would be wise to keep all fingers and toes away from the rough skin of this stone-like creature. Known for being the world’s most venomous fish, the reef stonefish has fins and an entire dorsal area with sharp, toxin-producing spines. These contain glandular sacs that have evolved to produce a highly potent verrucotoxin. When just a few milligrams of this toxin enter the bloodstream, it can cause cells to burst. Victims may experience pain, convulsions, paralysis, and cardiac arrhythmia.
The effects of stonefish stings can be abated with immediate treatment. Victims should attempt to stop the bleeding (once outside water) before soaking the wound in hot water with a temperature of at least 45˚C (113˚F). This should help neutralize the venom and relieve pain. If symptoms persist, bleeding is uncontrollable, or the wound becomes infected, professional emergency help is advised.
7) Striped poison-fang blenny (Meiacanthus grammistes)
Also known as the line-spot harptail blenny or, simply, the striped blenny, M. grammistes is arguably a surprising addition to this list. Quite common in tropical reefs, with some varieties also fondly raised in aquariums, this venomous fish does not quite have the morphology of a dangerous marine organism. When it opens its mouth, however, it exposes a set of relatively large fangs that arise from its lower jaw! Fang blennies defend themselves from attackers using these distinct canines, which deliver venom.
The striped poison-fang blenny’s venom appears to not have pain as its primary symptom. While, imaginably, the small bite may be mildly painful, it’s the venom’s brief effect on victims that is of interest to researchers. It can apparently affect the victim’s blood pressure, lowering levels by almost 40%, to give the blenny just enough time to search for shelter or to swim as quickly and as far away as it can.
8) Electric stargazers (Astrocopus spp.)
Given their many mechanisms for self-defense and predation, electric stargazers are highly specialized bony fishes. The only members of the Osteichthyes superclass with the anatomical equipment for producing electricity, they are expert hunters. These ambush predators, which measure anywhere between 6 – 36 inches (15 – 91 cm) long at maturity, have venomous spines on the back of their heads. These can be unfolded to deliver a painful sting and folded back down when no threats are present.
A stargazer’s venomous sting is seldom fatal to human victims, though there are cases that have resulted in damaging consequences. The combined effects of an electric shock and a venomous sting are enough to incite fear in potential attackers and unsuspecting victims. Actively avoiding this fish may unfortunately be challenging as it usually hides in the sandy seafloor. It can bury itself quickly and remain immobile as it waits for prey to swim right above its jaws.
9) Scorpionfishes (Scorpaena spp.)
Many true scorpionfishes look like a cross between a lionfish and a stonefish, with which they share a family (Scorpaenidae). These solitary fishes are usually found in rocky zones and coral reefs, where they can easily hide in debris and blend in with the surface of rough substrates. These quirky-looking fish are set apart by their bony heads and their large pectoral and dorsal fins. Coloration and skin patterns are highly varied; some have vibrant stripes and speckles, whereas others have a mottled, brown appearance.
Like their cousins, scorpionfishes defend themselves from predators using their venomous spines. Accidentally stepping on these would be a nightmare for anyone frolicking along the shore. As they are sharp enough to pierce human flesh, they can cause deep and painful wounds. The potency of the venom is not as strong as that of the stonefish, but it is definitely more dangerous than that of the lionfish.
10) Catfish (Order Siluriformes)
Due to their sheer abundance and their remarkable diversity, catfishes belong to the vertebrate order with the highest number of venomous species. Apart from electric catfishes, which belong to the Malapteruridae family, all members of Siluriformes have spine-like rays on their dorsal and pectoral fins. These can be large and sharp enough to cause serious wounds. To make matters worse, they can inject venom into their victims.
The venom of some catfishes, such as that of the striped eel catfish (Plotosus lineatus) and that of members of the Heteropneustes genus, is potent enough to inflict serious harm on human victims. Catfish stings can cause mild to severe tissue necrosis, which has commonly been observed around hand wounds. Initial symptoms include inflammation and severe pain. Though these are usually treatable, catfish stings need to be cleaned and medicated properly to prevent further infection. Before targeting them, anglers and fishermen should learn how to properly hold a catfish without getting stung.
