Marine Toxins

by Lauren Siba, July 23, 2014

For those of us lucky enough to go scuba diving, it’s usually a pleasant experience – we drift along enjoying the pretty colours, fabulous patterns and interesting behaviour of fishes, corals and amazing invertebrates like octopuses and nudibranchs. But when you begin to look into the marine world more closely, it becomes apparent that full-on chemical warfare is being waged.

Many marine animals are toxic, either poisonous or venomous or both. What’s the difference? Poisonous animals are only dangerous if eaten, or, in some cases, if handled. Examples of poisonous critters include many nudibranchs, which advertise their toxins with bright colours and eye-catching patterns, and pufferfish, which can be deadly if ingested.

A colourful hydroid-eating nudibranch

Venomous creatures on the other hand inject their toxins into victims, for example by means of a beak, spines or fangs, and can use this venom either as a defence mechanism or as an offensive weapon in order to secure prey. The banded sea krait (a sea-snake) and stonefish are examples of highly venomous sea creatures.

Sea-snake at Gunung Api/Banda Sea

The author looking at a stonefish in the Komodo National Park.

Some animals are both poisonous and venomous – studies suggest that the blue-ringed octopus, well known for its deadly bite, may also contain the toxin TTX (tetrodotoxin) in their flesh in which case deep-fried blue-ring octopus may not be a good choice of menu item.

The blue-ringed octopus (Hapalochlaena)

Research into marine toxins is still in its infancy and there are more questions than answers. It is widely believed that the flamboyant cuttlefish, Metasepia pfefferi, is poisonous – it seems to make sense that they would be, as they flash bright colours, lack a protective shell and swim poorly. However there is no scientific proof of this theory and the jury is still out on whether this mesmerising critter is genuinely toxic or just fooling potential predators with its bright colours.

The flamboyant cuttlefish (Metasepia pfefferi)

To further complicate things, what is toxic to one animal might be harmless to another – one study on cuttlefish and squid toxins tested their effects on crabs and mice. Surprisingly, they concluded that three species of cuttlefish were lethal only to crabs but three species of squid were lethal to both mice and crabs!

The broadclub cuttlefish (Sepia latimanus)

The medical community is also looking into marine toxins as sources of new medicine. There are already anti-cancer drugs which were developed from toxins found in sea sponges, sea squirts (also known as ascidians or tunicates) and even from nudibranchs such as Jorunna funebris- the ‘panda nudibranch’. Chemists must then synthesise the drugs in a lab, since these toxins are naturally present in minute quantities; in the case of Yondelis, an anti-tumour drug, more than a ton of sea squirts would have to be harvested to produce just one gram of the drug!

Tunicate in Misool/Raja Ampat

Panda nudibranch (Jorunna funebris)

Another amazing medicine is a pain killer derived from the venomous cone shell, Conus magus. It is a much more potent pain killer than morphine, yet is not addictive. The cone shell hunts fish with a venom-tipped harpoon and the venom, a mix of more than 20 different toxins, anaesthetises and paralyses its prey. The medicine derived from these ‘conotoxins’ interferes with a particular pain receptor in the brain, and side effects appear to be minimal.

Marbled Coneshell (Conus marmoreus)


The marine world is both beautiful and potentially full of useful medicines, but there’s more going on than meets the eye, and learning more just convinces us of the wisdom of the old saying, “look but don’t touch”!

This article was researched at Lembeh Resort, unless specified otherwise all photos where taken in the Lembeh Strait while diving with Critters@Lembeh.

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