Venom in the sea – the marine animals boaters and swimmers should know

International marine biologist and journalist Giovanna Fasanelli takes a fascinating look at the subject of ‘venom in the sea’ and considers both the role it plays and why, as boating people, we should be interested …

One bite from any of the four species of blue-ringed octopuses found around the Indo-Pacific will result in a highly distressed call for medical support. The surprising thing about these killers is their size: they are all small, so at 10-20 centimetres the empty shells lying in tidal pools and on reefs are attractive hiding places. Furthermore, unless they are flashing their blue rings as a warning signal or when excited, their camouflage renders them practically invisible.

Their venom contains one of the most potent natural toxins known: tetrodotoxin (TTX). Fascinatingly, the manufacture of TTX is outsourced to the symbiotic bacteria living in the octopus’s salivary glands. This exceptional neurotoxin disrupts the transmission of signals from nerves to muscles by blocking the movement of sodium across neural membranes. Just 1 milligram of TTX is all that it takes to end a human life, and there is currently no antidote available. Once again, paralysis is swift, and victims often succumb to respiratory failure. Ironically, it is TTX’s paralysing effect that researchers are studying as a meaningful analgesic, especially for those suffering extreme cancer-related pain. It has even been shown to assist recovering heroin addicts by dampening cue-induced drug cravings and associated anxiety.

This may come as a no-brainer to many, but if a sea snake is encountered on your holiday, by all means enjoy the wonderful sighting, but you should definitely resist the urge to touch it.

Despite this being an obvious rule, the number of people that break it is an embarrassment (artisanal and commercial fishermen aside). While most species have reputations for being very docile and small-fanged, and even if bitten the likelihood of envenomation is indeed very low, these snakes possess some of the deadliest venom of any on the planet. It really is a case of ‘which way would you rather be wrong?’ Receiving medical support and an antivenom can take hours, especially from remote tropical islands. Once again, a potent neurotoxin serves these predators by immediately subduing panicking fish, eels and crustacean prey that could otherwise easily escape.

Grab those thick-soled water shoes if you plan on fishing or frolicking in the warm shallow waters of reefs, tidal pools, estuaries or river mouths anywhere in the balmy Indian and Pacific oceans, for there are creatures whose primary defence against attack is spines, some of which you do not want close to your soft, spongy skin! All five members of the supremely camouflaged stonefish genus Synanceia are highly weaponised, with a pair of venom sacs positioned just below each of their 13 dorsal spines. Venom is discharged along ducts in the spines when the thick protective sheath covering them is punctured by applied pressure – the kind a clodhopping human hand or foot would cause. There will be no mistaking the situation as the list of ensuing symptoms is bewilderingly comprehensive: unbearable pain, swelling and necrosis at the wound site, secondary infection, prolonged kidney pain – and in more serious cases without swift medical assistance, convulsions, shock, respiratory and cardiovascular failure, paralysis and, potentially, eventually death … So watch your step!

Often when a creature appears strikingly beautiful, there is a hidden message: stay away or be punished! The spines of the ineffably beautiful fire urchin will deliver a painful, though not deadly, sting. However, it is the infamous flower urchins of the genus Toxopneustes that one needs to keep a wary eye open for. The potent potion delivered by the petal-like pedicellariae can not only cause debilitating pain but can increase the risk of drowning due to muscular paralysis, disorientation and breathing difficulties, which may set in while underwater. Notwithstanding, they are popular among aquarists and are still considered a delicious delicacy worth the risk!

The unifying feature of all group members is the presence of countless microscopic, venom-laden stinging cells known as ‘cnidocytes’ or ‘nematocysts’ that deliver their lethal serum into a victim’s skin via a spring-loaded harpoon. The venom’s toxicity to humans ranges from undetectable to ‘call an ambulance NOW’ and ‘who knows CPR?!’ The issue with Chironex fleckeri in particular is its enormous size and the number and length of its tentacles. The bell of these silent hunters can grow to the size of a basketball and from each of its four corners trail up to fifteen 3-metre-long tentacles that harbour over half a million nematocysts per square centimetre. The chances of surviving a comprehensive envenomation are slim as the venom is one of the most lethal and fast-acting currently known, resulting in excruciating pain and swift respiratory and cardiac arrest. Although an antivenom has existed (this time using hyperimmunised sheep) since the 1970s, there is a lot of controversy surrounding its efficacy, and much of the way the venom works in the human body is yet to be understood. Just think about the challenges researchers face in ‘milking’ a box jelly!

There is even scope for putting spider toxins to work in the development of eco-friendly insecticides. This area of scientific research is thrilling, but none of this excitement may be enough to encourage you into the water after reading all this. However, I am here to tell you that after thousands of hours spent underwater in the tropics, and following countless encounters with many of these same venomous creatures (dangerous jellyfish aside), I have never needed to dial 911. I do, however, always stick to these simple common-sense rules. And if you are still really worried about being envenomated by, well, anything at all, follow Rule number 4: Don’t holiday in Australia!

Main image: Blue-ringed octopus
Copyright iStock / Subaqueosshutterbug