Deadly Sea Urchins and Mind-Altering Jellyfish: 7 Tiny Terrors of the Tides

It turns out you don't have to be a great white shark to be one of the deadliest creatures in the sea. This collection of pint-sized poisoners can do a number on the human body - and in one case, the human mind. Let's dive right in and take a look at the...

• Flower Urchin
• Fire Coral
• Australian Box Jellyfish
• Blue Dragon Sea Slug
• Geography Cone Snail
• Blue-Ringed (and Blue-Lined) Octopus
• Irukandji Jellyfish

Flower Urchin

Did you know that there's a deadly sea urchin? The casing of the flower urchin is covered in small pincer-like organs that can deliver paralytic poison to an unwary limb. Those experiencing the effects find the smooth muscle in their organs contracting, resulting in difficulty breathing, severe pain and paralysis.

Though there are no scientifically confirmed human deaths due to these urchins, it certainly has the potential to be lethal and there have been several anecdotal cases of fatal encounters. Even if the venom itself wasn't enough to kill a human, partial or total paralysis while swimming is pretty much a death sentence!

Fire Coral

Beyond a potential scrape, you wouldn't really consider coral to be dangerous, would you? Fire coral (technically not a true coral but a "hydrocoral") might be able to change your mind - touch some and you'll soon know it, as the sting will start a burning sensation that can persist for hours. A rash will soon follow, lasting for days and potentially reoccurring a few times over the next few weeks.

While the venom is generally considered non-lethal to humans, it can cause dangerous (even life-threatening) reactions in some victims. The nastiest results happen when the fire coral causes a laceration and stings get into a wound. The venom is dermonecrotic, which means that it can start killing off skin around the injury. Areas of dead skin can become a breeding ground for all sorts of nasty bacteria... and can potentially become gangrenous as a result!

Australian Box Jellyfish

Despite having a body that only grows about as large as a basketball, the Australian box jellyfish (or sea wasp) is one of the most dangerous creatures to humans haunting the waters. In fact, the creature's scientific name can be interpreted as "hand of death."

This jellyfish is considered the most venomous marine animal in the world and a sting can cause paralysis or cardiac arrest in short order. To make things even more disturbing, these jellyfish have eyes (24 of them, as it happens) and are capable of swimming at a rate of almost four knots (that's around 4.6mph) when most jellyfish just drift with the current. Between the eyes and swimming, some researchers believe that these jellies actively hunt prey rather than just waiting for it to brush their tentacles!

Blue Dragon Sea Slug

Despite having broad wing-like structures and a neon blue body, the blue dragon is no reptile - in fact, it is a small sea slug (up to 3cm or so long) that floats upside down just beneath the water's surface.

So how does this creature pose a threat? Well, it lacks venom of its own, but it does feed on creatures like the Portuguese man-o'-war... and subsequently steals the nematocysts (stinging cells) from these dangerous creatures.

The blue dragon loads these stolen weapons into its fins, and an errant swimmer can find themselves hit with Portuguese man-o'-war venom... which is potentially fatal!

Geography Cone Snail

Do you think of snails as predators? The geography cone snail (so called because its shell looks like an old map) is probably the deadliest of these harpoon-wielding mollusks (and yes, there have been confirmed human fatalities - despite it being small enough to fit in a teacup.)

The creature hunts fish, but the speed disparity between a cone snail and a swimming fish means it needs to paralyze a victim almost instantly. It begins by releasing a cloud of insulin into the water to drug potential prey, then closes and jabs with a harpoon to inject a fast-acting paralytic venom.

There's no antidote to a geography cone sting, so treatment simply involves trying to keep the patient alive until the venom passes from their system. Curiously, components of the venom may be useful as potent pain-killers. Researchers believe they may be up to 10,000 times as effective as morphine... without the addictive drawbacks!

Blue-Ringed (and Blue-Lined) Octopus

There are several species of blue-ringed octopus, none of which are particularly large. The biggest example ever caught had a mantle (body) length of 16cm, while the blue-lined (it still has blue rings like the picture above, but has blue lines on the body) octopus has a mantle only a few centimeters across and a total length of around 15cm.

Despite their diminutive size, these cephalopods are armed with an incredibly dangerous toxin provided by symbiotic bacteria living in their salivary glands. These micro-organisms provide the creature with a supply of tetrodotoxin, a powerful paralytic that rapidly impairs muscles... which is not a great thing when one's trying to swim!

Though these octopuses are not aggressive towards humans, they can bite if they feel threatened. The real danger here is that the bite is so small that many people don't even feel it (at least until they start to go numb.) There's no cure for the venom - the best one can do is keep the patient on life support until the effects wear off.

Irukandji Jellyfish

Clocking in at a maximum of 3.5cm across the bell and frequently only being a centimeter across, Irukandji jellyfish pack an almighty punch for their size. There's even a medical condition (Irukandji syndrome) named after the effect of their stings, especially those of Carukia barnesi.

These marine invertebrates are a group of box jellyfish, living in the waters off the coast of Australia and occasionally encountered by unfortunate swimmers. They're transparent, vaguely cube-shaped and have a tentacle protruding from each of their bell's four corners. The jelly uses these to hunt for other invertebrates and small fish, delivering a powerful but almost unnoticeable sting that develops into a life-threatening condition over 20-30 minutes.

So what about Irukandji syndrome? Victims experience pain, stomach cramps, vomiting, hypertension and more, with severe cases resulting in pulmonary edema, heart failure and cerebral hemorrhaging. Perhaps the most fascinating aspect of the syndrome is that victims experience a strong sense of impending doom, suggesting that the venom may be having some kind of psychological effect as well!

Thanks for reading - you might also like...

• Body-Snatching Barnacles and Skeleton Shrimp: 8 Curious Coastal Crustaceans
• Giant Jellyfish and Colossal Squid: Discover 6 Real Sea Monsters
• Body-Burrowing Hagfish and Spider-Tailed Vipers: 5 Natural Nightmares

Sources and Further Info:

• Guinness World Records: Most dangerous sea urchin
• Fire Coral
• Millepora species (Fire Coral) Sting: A Case Report and Review of Recommended Management
• What is the most venomous marine animal?
• Sea Wasp
• Blue dragon
• Toxic Snail Puts Fish in a Sugar Coma, Then Eats Them
• Geography Cone
• The blue-ringed octopus: small, vibrant and exceptionally deadly
• Irukandji jellyfish
• Irukandji