In Japan it takes 3 or more years or rigorous training to become a Fugu chef and the pass rate for the exam is less than 40%. Fugu is a dish prepared from pufferfish, which is potentially lethal due to the presence of Tetrodotoxin (TTX). Budding chefs learn how to carefully remove the most toxic parts of the fish without contaminating the meat. Even though the liver of the fish is considered by some to be the tastiest part, its also the most poisonous organ. With such high stakes, restaurants serving Fugu are controlled by law, but there are around 20-40 incidents a year in Japan, with a fatality rate of around 7%.  Most of the fatalities aren’t associated with restaurants, but with home preparations. Never has the phrase “I’m dying to eat” been more apt.

The symptoms of ingesting a lethal dose of TTX include dizziness, exhaustion, headache, nausea and difficulty breathing. The victim remains conscious but cannot speak or move. Breathing stops and asphyxiation follows.  There is no known antidote. Treatment consists of emptying the stomach, giving the victim activated charcoal to absorb the TTX and putting them on life support until the poison has worn off. You can add in whatever prayers you feel like at that point. If the patient survives the first 24 hours then recovery over a few days is common.

So, what on earth is the pufferfish doing with this potent neurotoxin? It turns out that in the marine environment pufferfish aren’t alone. TTX is also found in blue ringed octopus, Astropecten star fish, Xanthid crabs and some newts and toads too. But despite its widespread appearance in the animal kingdom, none of these creatures are actually synthesising the TTX. The toxin is produced by bacteria that infect or live in the animal, and the animal concerned uses the TTX as a defensive biotoxin, and sometimes also as a venom eg octopus and arrow worms (Chaetognaths).
TTX acts by blocking sodium ion channels. Nerve cells have to pump sodium ions outside of the cell in order to be ready to transmit a signal. Sodium ions can’t cross cell membranes, so a special protein gateway exists in the nerve cell membrane that helps move the sodium out. Block this protein and the nerve cells will lose their ability to send a signal. This prevents the nerve from being able to send messages to muscles, resulting in reduced muscle movement. There are slightly different sodium ion channel proteins in the nerves controlling skeletal muscle and those controlling cardiac muscle. Victims of TTX poisoning will experience progressive paralysis throughout their body, whilst remaining conscious until shortly before death. A truly awful way to go.

TTX turns out to be remarkably toxic. The lethal dose of cyanide is 8.5mg per kg of bodyweight, but it would take only 4% of that amount of TTX to be lethal. TTX can be absorbed orally, by ingestion, by injection or through abraded skin. As the TTX levels are due to bacteria, there is a seasonal, geographical and species variation. It’s no wonder that the Fugu chefs need years of training. Poisonings from TTX are almost exclusively associated with the consumption of pufferfish from Indo-Pacific regions, but this is mainly because pufferfish just aren’t eaten as commonly elsewhere. In New Zealand in 2009 several dogs died after eating grey side gilled seaslugs (Pleurobranchus maculate) found washed up on beaches. The slugs were found to have ingested high levels of TTX.
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Naturally occurring TTX has been used in traditional Chinese medicine for over 4000 years to treat convulsive disease. In 1774 Captain James Cook recorded that his crew ate locally caught tropical fish and fed the left overs to the pigs kept on board. The crew had mild symptoms (numbness and shortness of breath) but by the morning all the pigs were dead. The crew had a very lucky escape, although its not known if they had pork for lunch the next day!

​One of the hardest skills for some divers as their diving career progresses is learning to use a compass. Once you’ve mastered the technical aspects of making sure it’s moving freely and not locked off inside the casing, the biggest hurdle is trust. You need to gain a Jedi like perspective as you accept Obi-Wan’s guidance to “Use the Force”. And generally, that’s fine, until there’s a nearby wreck and your compass stops being attracted to the earth’s magnetic field and starts interacting with the ship’s magnetic field instead.

When iron hulled ships were introduced, the effect of the metal hull on steering compasses was first observed. During construction the metal in the ship adopts the magnetic field of the dockyard used for construction. In modern construction methods, the high currents used for welding the steel plates together create magnetic dipoles in the steel, thus magnetising the ship…

During the American Civil War, mines were developed that were activated by contact. The target ship hits the horn on the mine. The soft metal of the horn buckled under impact, smashing a glass ampoule with battery acid inside. The acid electrolyte dropped into the waiting battery, energizing it and heating a platinum wire inside the mercury fulminate detonator. Boom.

At the start of WW2, the Germans developed a new magnetic trigger for mines. For a while the British were stumped as to how these mines worked. But in November 1939 a German mine was dropped from an aircraft and landed on mudflats in the Thames estuary at low tide. The mine was disarmed and taken to Portsmouth, where the mechanism was examined.  A magnetic needle which was pulled by the target ship’s magnetic field completed the circuit and fired the mine.  Later sophisticated versions would use a counter that didn’t fire for the first few ships to pass.

Establishing how the mine worked held the secret to protecting vessels, you just need to wipe out the magnetic signal. Magnetic field strength is measured in units named after Carl Guass, so the process of removing the magnetic signature is known as degaussing. Remove the magnetic field from the ship and it can safely pass over the mines without triggering an explosion.

There were two ways of cancelling out the ship’s magnetic field.  The permanent one was to put thick bands of electrical wire around the length of the vessel, known as coiling. Passing an electrical current through these cables generated an electromagnetic field that cancelled out the ship’s own field, thus rendering the ship invisible to the mine mechanism. Royal Navy Commander Charles Goodeve oversaw this system, and it even allowed for the polarity to be reversed when ships were in the southern hemisphere so that the ship appeared to have the same magnetic field as the natural background.  But this equipment was expensive and difficult to install.

Measuring a ships natural magnetic field was a complex business. A series of magnetometers are anchored to the seabed about 5 metres apart for a 150 metre run. Each magnetometer wa connected to a fluxmeter on the shore. The ship passed over the magnetometers and the readings from the fluxmeter were used to create the ship’s signature.  From this starting point the number of turns of the degaussing cable could be increased or decreased, or the current altered until the signature was minimised.
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A second quicker method was to wipe the hull of the ship, with a current carrying cable running a pulse at about 2000 amps.  The large cable was dragged down the sides of the ship in a process known as deperming.  This wasn’t a permanent solution though, as the ship travelled through the Earth’s magnetic field it slowly became magnetised again. This started in late 1939 and helped protect many of the vessels that carried out the evacuation from Dunkirk.  In a 4 day marathon session prior to the evacuation over 400 ships were ‘wiped’ in this way, though there are concerns that some of the ‘wiping’ may have not been as effective as hoped.  The Isle of Man vessel Mona’s Queen was lost to a magnetic mine on 29th May 1940 just outside Dunkirk harbour, and stories persist to this day among the relatives of the crew that the ship wasn’t properly protected.