​Stefano Lorenzini was an Italian physician born in Florence in 1652. One of his teachers at the medical school in Pisa was Francesco Redi who as well as being a physician spent time experimenting to prove that spontaneous generation (life just appears) was a myth, mostly with maggots and flies. As part of his work Redi, like all scientists of a time without cameras, made incredibly detailed anatomical drawings. Redi inspired Lorenzini, who in 1678 published the work that he is best remembered for, an in-depth study on the anatomy and physiology of sharks. This work contained the discovery of the Ampullae of Lorenzini, the special electromagnetic sense organs found in sharks and rays. But although Lorenzini described and drew the Ampullae, it would take nearly 300 years before modern science could explain how sharks had an additional sensory organ not seen in most other animals.

The Ampullae of Lorenzini form a network of jelly filled canals that open in small pores through the surface of the skin which can be seen as dark spots on the skin. At the back end of the canal is a collection of receptor cells that have a unique structure and allow the shark to sense electric and magnetic fields. The gel in the canals is keratan sulfate which has one of the highest known conductivity of any known biological material (about 1.8 milliSiemens/cm). The ampullae are clustered and each cluster connects to a different part of the skin, so the shark can get detailed directional information from this network.

The cells that actually detect the electrical field have a very special structure. There is a single layer of excitable cells closely packed together. The membrane of these cells facing into the canal is packed with voltage dependent calcium channel proteins. The rest of the canal lining has a very high resistance, which means that any voltage difference is effectively concentrated on the electroreceptor cells. A change in the electric field in the jelly allows these proteins to open and calcium floods into the cells. This causes the other side of the cell (away from the canal) to release neurotransmitter signalling to the shark.

Sharks have been shown to be more sensitive to electric fields than any other animal, possibly as low as 5 nanoVolts/cm – that’s 5/1,000,000,000 of a volt detected in a 1cm long ampulla. All living creatures produce an electric field by muscle contractions and as a result of the internal body chemistry. So being able to detect weak electrical stimuli probably allows sharks to hunt prey animals, including those buried in sand. Basking sharks have been observed swerving around jellyfish whilst feeding, after all the last thing you’d want clogging up your gill rakers would be lots of jellyfish and their stinging nematocysts.
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It’s likely that the Ampullae of Lorenzini also allow sharks to sense magnetic fields.  Any moving conductor will induce an electric field, so seawater moving on ocean currents, with the Earth’s magnetic field will produce electric fields well within the range that sharks can detect. Behavioural studies have shown that magnetic fields can change the behaviour of sharks. The ability to sense and orientate along magnetic field lines may help explain some of the large distance migrations that have been found using shark tagging experiments. Great white sharks have been shown to migrate over 2000 miles in open ocean to feeding grounds in the Pacific Ocean. Whale sharks have been tracked on journeys over 4800 miles. That is an astonishing figure considering that they don’t have sat nav guiding the way! So next time you’re up close and personal with any shark or ray, have a look for the black dots around their snout and remember that 340 years ago an Italian doctor and fish fanatic spotted the same thing.

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.

​Comb jellies are a common sight around British waters, particularly the sea gooseberry (Pleurobrachia bachei). With a clear body and measuring only a couple of centimetres long, it’s easy to overlook these beautiful little animals apart from one thing, stunning rows of rainbow coloured hairs running along their body. With no skeleton and composed of 99% water, you’d be forgiven for thinking that these were very simple creatures, but actually they have some pretty neat stuff going on.

Firstly, let’s start with where the comb jellies fit in, they are not the same phylum as jellyfish. Jellyfish (Cnidaria) have complex lifecycles and stinging cells. Comb jellies (Ctenophora) have a simple life cycle and make glue to catch prey. The problem for scientists trying to study the evolution of the Ctenophores is that they don’t make very good fossils so the records are patchy at best.  Genetic studies indicate that Ctenophores are much older in origin than other animals, and predate the bilaterian animals (those with bilateral symmetry) by millions of years.

There are 2 major cell layers, the external epidermis and the internal gastrodermis. Between these layers is the mesoglea, a jelly like layer.  The Ctneophores have muscles running through them and the outer epidermis contains a basic nervous system known as the nerve net. Comb jellies are named for their unique feature, plates of giant fused cilia (small hair-like cells) which run in eight rows up and down their bodies.  These cilia are used to propel the comb jelly through the water.
Many comb jellies have a single pair of tentacles and often these tentacles are branches and give the illusion of many tentacles.  The branched tentacles are used to catch prey, but unlike the toxins from jellyfish, comb jellies use glue.  Special cells called colloblasts respond to touch by firing a spiral filament and releasing sticky glue. Once the prey is stuck the comb jelly reels in the tentacles and brings the food to its mouth. Most comb jellies are carnivorous and will eat anything. Until recently it was believed that comb jellies spat out the indigestible waste particles, but it now seems like they release them through pores in the rear end – which is probably part of the evolutionary puzzle in explaining when animals developed anuses.

What about the beautiful rainbow coloured pulses of light? Whilst some comb jellies can produce light by a special chemical reaction to give photoluminescence, our sea gooseberry can’t. The rainbow colouring is caused by refraction of light through the hair like cilia cells. Light travels at a constant speed in a vacuum, but when light encounters a material that is more dense, it slows and its pathway bends. The different wavelengths in visible light bend at slightly different angles.  This means that the cilia act like the glass prism you played with in physics classes as a schoolchild, splitting the light into separate wavelengths. The refractive index (how much the light will bend is almost the same for the comb jelly tissue as it is for the salty sea water. The hair like cilia amplify the refractive effect creating an iridescent pulse that is mesmerising to watch. The effect works across the visible and UV light wavelengths, meaning a fluorescent torch will also give a great image.
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Lastly, there’s a few things you should know about comb jelly reproduction. Comb jellies are hermaphrodites (both sexes in one individual) and can spawn eggs and sperm freely into the sea, through their mouths. They do this on a continuous basis unless they are starved of food, when they will shrink down and stop breeding. But feed them up again and they will start spawning again. Rainbow coloured hair, sticky glue fishing nets and continual reproduction – what’s not to admire?

We’ve had a rather enjoyable club trip away during the winter.  A chance to escape the grey, wet and cold and get a bit of sunshine. It helped that the trip was planned as part of an Advanced Diver exped, as that meant I didn’t actually have to do much to smooth the whole trip along. Our AD candidate had picked a BSAC centre to go diving with, assuming this would mean that our reasonable levels of experience (including First Class Divers and instructors) would be recognised and we would be treated accordingly. The dive plan was submitted by email and when we arrived the owner of the dive centre went to quite some lengths to tell us he was going to follow our plan exactly.

Day 1 wasn’t so bad, apart from a really strange conversation about compasses, and how if you look at your compass and it says 90 degrees then you are in fact swimming West. We tried to explore this assertion, after all it would mean rethinking the entire way we brief for dive sites, skipper boats and well, just generally everything. How could we have all got this far and been 180 degrees out? We asked if this was caused by having a compass with a sighting window, maybe looking at it from the top, but nope.  We were assured that 90 degrees was West. End of discussion, let’s go diving.  We took our own compass bearings on the shore (safe bearing was North). Our check out dive took a strange turn when instead of the 15 metre bimble we’d been briefed for we ended up at a 32 metre wreck. During the surface interval, we were assured that this dive had been in the plan, but even the AD candidate looked puzzled and it was her plan! Whilst on the surface we asked about the time for the second dive. It was a mandatory hour away, but who knows why it’s an hour. A couple of us tried to look at our plan functions to gauge when we’d have enough dive time for the next dive but we were stopped because “decompression is only a theory anyway”. We were doing an hour on the surface and then we’d be going back in.

By the next day, things took another turn for the bizarre. The first dive brief of the day explained that the dive 1 would be out from the harbour and turn right, and the second dive at the same site would be out and turn left. The first dive was pretty good, shoals of fish and a couple of angel sharks. We came back to the shore, had a bite to eat and swapped cylinders. Whilst we were standing in the sun to warm up a bit, someone asked about the topography for the second dive. At this point the story changed, we were told that there was nothing to the left apart from barren rocks, our dive guide couldn’t imagine why anyone would want to turn left, or who had suggested we should and we would be going right again. The second dive to exactly the same site was disappointing.  The tide had changed, the shoals of fish had moved on and we resorted to a little bit of wombling to clean up the reef.

By now we had established that we were becoming the victims of gaslighting. Gaslighting is a form of persistent manipulation and brainwashing that causes the victim to doubt her or himself, and ultimately lose her or his own sense of perception, identity, and self-worth. The term is derived from the 1938 stage play Gas Light, in which a husband tries to convince his wife that she’s insane by causing her to question herself and her reality, flickering the gas lights as he does. Despite having been around for over 75 years, the term has recently resurfaced with Trump’s presidency in the USA.
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We had the classic signs; blatant lies, denial of what had been previously agreed and attempts to manipulate and divide opinion the group. Once we’d identified that this sort of behaviour was in play, everyone’s guard was up. Fortunately, we’ve all know each other for a long time and dived together for many years. Each night our dinner and beer debrief covered very little about the dives and much more about the bizarre antics. The carefully written up dive plan that was the point of the trip had disappeared and was never mentioned again. Next time we’re all going to put our basic qualification and not mention any further diving experience. We think it will be easier all round. 

Before 1920, having tanned skin was associated with working outdoors and indicated that you carried out manual labour. The societal ideal was pale skin which showed that you were rich enough to stay indoors, although the pursuit of pale skin often involved powders containing lead and mercury, which weren’t exactly great choices from a health perspective.  The Western Europe attitudes started to change in the 1920s when Coco Chanel started to popularise the idea of a tanned skin, and this trend accelerated through the explosion in cheap air travel from the 1960s, so that having a tan showed you could afford to take the time off work and lounge around catching some sun’s rays.

The concept of desirable skin colour varies around the world. Whites try to tan, whilst across Asia the sale of skin lightening creams and lotions is at an all time high. Your natural skin tone is determined by genetics and that is directly correlated to how much UV radiation you are exposed to. The higher the level of UV radiation, the darker the tone of indigenous skin. As some populations of humans moved to more northern latitudes there was a shift in genetics. With less UV radiation causing damage, there was a positive selection of individuals with lighter skin, who could synthesise more vitamin D. As societies changed from hunting to agriculture, there was a need to maximise the vitamin D synthesis to make up for the loss from the diet. About 40,000 years ago the mutations for pale skin emerged in both the East Asian and Western European populations.

The most important pigment in the skin is called Melanin. Specialised cells call Melanocytes make the pigment and pack it into the Keratinoctyes that make up a layer in the skin.  Melanin is an important molecule as it controls the amount of UV radiation that can penetrate the skin. Some UV radiation is needed for vitamin D synthesis, but too much is harmful. UV light is divided into 3 different wavelengths of light, UV-A, UV-B and UV-C.  UV-C and some UV-B are absorbed by the ozone layer in the atmosphere, which is a very good thing as without this absorption UV levels would be dangerously high (and quite possibly life on earth wouldn’t exist!). Some UV-B is absorbed by the epidermis (the upper layer of the skin but UV-A can penetrate further into the skin and interacts with the cells in the dermis. UV-B causes an increase in melanin production and UV-A causes the melanin molecules to change and become darker.

However, tanning is not the only consequence of UV exposure. Although no-one knew it during the 1960s, too much exposure to sunlight is the cause of 90% of all skin cancers, eye damage, immune system suppression and all the signs of ageing associated skin damage. As this message started to become understood, the tide has turned against tanning salons, and anyone out in the sun was urged to wear a hat, cover up exposed skin and slap on the sun cream….and then we hit another snag. In March 2018, Hawaii announced that it was bringing in laws to ban sun creams containing oxybenzone, which is the most widely used UV absorbing molecule in all sun creams in use today.

Once upon a time, Para-amino benzoic acid (PABA) was widely used in suncreams.  Patented in the 1940s PABA was the first molecule to be used absorb UV-B in sun creams, but it fell out of favour as it stained clothes and caused allergic reactions. Then came Oxybenzone as the next generation molecule with the ability to absorb UV-A and UV-B. Oxybenzone isn’t just used in sun cream, but as a UV protection for a wide range of plastics too.  Hawaii have recognised a study that showed that tiny amounts (microgrammes per litre) of oxybenzone cause coral larvae to stop moving around and prevented them from developing a hard skeleton. To understand the concept of just how toxic this is, that’s lethal levels at half a teaspoon in an Olympic sized swimming pool.
It’s time for a switch away from the UV absorbing molecules like PABA (still in use as the derivative padimate O) and oxybenzone. Perhaps we need to return to the mineral reflective suncreams with their chalky finish. Have a look at the label on your suncream. There are several alternatives hitting the market now, although for some of them the claims can be hard to verify. Perhaps the safest option for us and the environment would be to take the lesson from Victorian society and just cover up? Stop putting any plastic solutions into the sea including the lotions you slather on.

​In the depths of winter, there are two major factors that reduce diving time, low pressure weather systems and snot. As the air becomes colder and drier, the cells lining the nasal cavity have to work quite hard to warm and moisten the air that we breathe in. The cells producing the mucus are called goblet cells (which is a reference to their shape, not an instruction for what to do with the mucus). The mucus itself is a mix of proteins which contribute to the protective role in a number of ways; enzymes that can attack bacterial cell walls, antibodies to bind to pathogens and lactoferrin to mop up any free iron.
But the real star of the snot show is Mucin, a group of large proteins with lots of sugar molecules bound to the central regions of the molecule. These sugars are important as they allow the Mucins to have gel-like properties with an amazing water holding capacity. Aggregations of Mucin molecules are secreted by the cells lining the airways (and digestive tract too) and the sugar coating helps them to resist digestion. Over 20 human Mucin genes have been identified and the proteins that they produce help bind pathogens together, and are one of the reasons why you will make more snot when combatting a nasal infection.
It’s not just humans and other mammals that can make Mucin, a similar group of proteins is found in the most humble gastropods. We are all familiar with snail trails. (I’m sure that was my Nan’s phrase for a small child with streams of nasal mucus running down their top lip!) Snails move using a combination of their muscular foot and a lubricating slime. Now here’s where it starts to get strange, mollusc slime is a non-Newtonian fluid. It doesn’t follow the normal rules that govern viscosity in fluids, but rather changes as stress is applied to it. This explains why the same mucus can be used to allow snails to move and to bind to a surface. As the wave of contraction from the muscular foot of the snail acts on the sticky slime, the slime changes to become a free-flowing liquid. When the pressure is removed, the slime becomes gel-like again, allowing snails to lodge in overhangs and defy gravity.
For marine snails, it’s slightly harder to see the need for a lubricant, but it turns out that the slime trail for some species has even more functions. It’s a big commitment for some species to make a slime trail, estimated at up to 60% of their total energy use. Periwinkles will sniff out and follow fresh trails made by other molluscs to reduce this energy requirement. Mucus trails bind microalgae from the water when they are fresh and so they can be an excellent food source. Yep, that’s right, eating the algae from someone else’s snot trail is a good thing for Periwinkles, but please don’t try this at home!
Limpets are grazing feeders who return to their ‘home-scar’ on the rock every time the tide goes out. For them, the mucus trail is their route to find the carefully etched out rock into which their shell can clamp down to protect them from predators, sealed with a mucus layer to prevent them drying out. Not so much “Follow the yellow brick road” as “Follow the limpet snot trail” to get home. With the right conditions, you can see limpet snot trails on rocks as the tide falls.
For some molluscs, their slime trail is also important for mating. Chemical signals indicate the sex of the snail, allowing prospective mates to find and copulate. Male periwinkles can track down a female by following chemical markers in the slime. But the females of one species of periwinkle (Littorina saxatilis) turns off this signal to avoid mating. L.saxatilis live in dense colonies and like other periwinkles will mate up to 20 times a day throughout the year. This seems like a strange strategy for any species to survive, the general rule being that males mate as often as possible, whereas females try to be selective about mates.
Why would female L.saxatilis try to avoid mating? Males mount onto their mate and crawl around to the lip of the shell. This means that the female is then bearing the load of adhering both parties to the rock, and remember that our slime is non-Newtonian, more stress makes it flow. Having a male periwinkle on your back will double the stress and can result in both parties being swept off the safety of the rockline. For females, mating will increase their chances of being predated upon. So the female L.saxatilis turns off the sex signal in her slime. Males will still follow the slime trails, but it’s a 50:50 chance that they could be trying to mate with another male at the end of the journey.
Since Ancient Greece snail slime has been used in cosmetics. It contains high levels of hyaluronan which is a major component of the proteins that support our cells. It is freely available as a cosmetic claimed to promote the formation of collagen and help to improve skin structure. More seriously, hyaluronan is gaining popularity as a biomaterial scaffold which is helping the next generation of bioengineers to promote the formation of blood vessels in tissue engineering. Something to ponder when you are relegated to shore cover as you are too snotty to dive… 

​Seals are collectively known as pinnipeds, which means from the Latin pinna (fin) and pes (foot). This classification includes the walruses, eared seals and true seals. The Isle of Man and the rest of the British Isles are home to resident populations of Grey seals (Halichoerus grypus) and Common seals (Phoca vitulina). Common seals (also known as Harbour Seals) are found in both the North Atlantic and North Pacific.  About 35% of the European population of common seals lives in UK waters.  By contrast Grey seals are only found in the North Atlantic, Baltic and Barents Sea.  The entire world population of Grey seals is probably no more than 400,000 individuals, with about 40% of them living in UK waters. Although we tend to take seals for granted, we should perhaps appreciate how lucky we are to have them in the waters around us and see them so often.
There’s been a long understanding that the pinnipeds evolved from land based mammals. This concept in itself is a little strange, as the general gist of evolution is that our ancient ancestors left the watery environment for a life on land. But somewhere millions of years ago, some of the mammals returned to the sea to take advantage of the feeding opportunities that existed there. Whales and dolphins have definitely taken their return to the marine environment to the extreme and evolved to the point that they can no longer safely return to the land. When they do, the amazing guys from the British Divers Marine Life Rescue swoop in and work their hardest to throw them back into the briny again.  In the pinnipeds we have a group of species who spend their time mostly in the marine environment, returning to land only when necessary. On land seals are ungainly, slow and clumsy, which made them an easy target for hunting. In the water, they are agile hunters, capable of diving to about 200 metres for up to 15 minutes.
The clues to the pinnipeds evolutionary past are clear in a number of ways. Their forelimb has five webbed fingers, with claws that are used for grooming and fighting. This five fingered (pentadactyl) limb structure is a common evolutionary feature, linking many vertebrates including reptiles, birds, mammals and amphibians. Just let that sink in for a moment.  You can see the same bone structure in pretty much every group of animals with bones. The humerus at the top, an elbow where the radius and ulna join, a wrist connecting to fingers. It’s there in the bats wing (with elongated fine boned fingers and skin stretched over them), it’s there in frogs (although the ulna and radius have partly fused), and cats and dogs and tigers and crocodiles and in us..
In seals the flipper bones that would be the equivalent of your arm are shortened, so that what appears to be their armpit is in fact their elbow (front flipper) or ankle (hind flipper). Their metatarsals (fingers) are elongated compared to ours and the skin in between gives them something akin to swimming gloves. Close interaction with a seal will reveal that they can still bend their webbed fingers to grip and hold onto objects or, if you are lucky enough, onto you as you are diving. Their flippers are well adapted to propel them through the water. When swimming quickly, the hind flippers are used in a side to side motion, and the front flippers are held against the body.  If you have watched seals turn under water, you’ll know that they stick out a front flipper to perform sudden changes of direction. Cruising speed for seals is about 2 to 3 knots, but when hunting seals can move at an astounding 20 knots (that’s probably faster than most club ribs!).
Seals are part of the Caniformia (dog like) sub order of the Carnivora group of Mammals. In fact, most divers that have had encounters with seals will tend to describe them as being like big puppies. Despite this, there have been many studies suggesting that seals are in fact more closely related to bears than they are to dogs. Perhaps the fact that we are more likely to have encountered and interacted with dogs rather than bears gives rise to our misconception? Remember that Grey seals are the largest living carnivore in Britain, can grow up to 2.3 metres and weigh over 300kg and treat these amazing creatures with the respect they deserve.  When you get to shake hands with a seal next time, count his fingers and say hello to a very distant cousin.

​Somewhere back in your very first diving course there will have been reference to air being compressible because it’s a gas, as opposed to water being non-compressible. When we start thinking about how molecules interact, it’s helpful to go back to school science. In solids the molecules are packed tightly together, they can vibrate but they aren’t free to move around much. In liquids the molecules can move but stay interacting with each other and in gases the molecules fly around freely bouncing off the walls of whatever contains them.
Thinking about gas molecules as random whizzing around bumping into other molecules and their container, it’s easy to understand why if we push more molecules into the container the number of collisions with the container wall will increase and the pressure goes up. This kinetic model of gases assumes that the gas particles themselves are very small (they are) and there’s a lot of space between these gas particles. That leaves lots of extra space for us to jam more molecules in there and compress the gas together. Hence gases are compressible.  This nicely explains Boyle’s Law (remember that?) but makes a number of assumptions which create a concept of an ‘Ideal gas’. Sadly, Ideal gases don’t exist and we have to deal with real gases. But at low pressures Boyle’s Law is fairly useful.
Liquids don’t behave in the same way because the molecules are already close together and interacting to a limited extent. Each molecule forms temporary associations with the other liquid molecules. If you heat up a solid to melt it into a liquid, you can measure the temperature increasing. As you get to the point where the solid it melts, the temperature will stabilise (even though you are still heating it). This is the point where the molecules are absorbing heat energy to give them the kinetic (movement) energy to move around. Once all the solid molecules have absorbed enough energy to melt into liquid, then you can see the temperature start to rise again as you carry on heating it. During this phase the molecules will move faster, but they still interact with each other. Keep heating and give them enough energy and they will manage to escape the interactions and form a gas.
Water is a bit of a strange liquid, because it’s molecules interact more than other liquids, and this gives it some strange properties. Water molecules form hydrogen bonds to other water molecules, and then they break these bonds and reform them with other nearby water molecules. Although these bonds are quite weak, there are lots of them. If it weren’t for hydrogen bonds then water wouldn’t be a liquid at all. When the other elements in the same family of the periodic table, like Sulfur and Tellurium, bind to hydrogen they form gases not liquids.
Water is most dense (the molecules are packed tighter together) at 4 degrees. At this temperature the hydrogen bonds are quite structured and pull the molecules tighter. As the temperature rises the bonds start to make and break more frequently and allow the water molecules to move a little further away from each other, so water becomes less dense. At temperatures below 4 degrees, hydrogen bonds don’t form so well and so as water cools to become a solid, it also becomes less dense. This explains why ice floats on liquid water (good news if you’re a polar bear) and why cold water sinks into the ocean (that’s  thermoclines).
So, these little fairly weak hydrogen bonds have a lot of influence and it’s them we are fighting against when we try to compress water. They have already done the job of pulling the oxygen dihydride molecules far closer together than we’d expect from the other elements in their group. It’s because of them that water isn’t a gas at room temperature.  And once we’ve got as far as forming a liquid, there isn’t much compression left to achieve. At 4km down in the ocean, water has a measurable compressibility of just 1.8%. It’s not quite true to say water isn’t compressible, it’s just not very compressible and for the depths that we will visit we can probably ignore the marginally increased density.
Resistance to compression for any substance can be described by the bulk modulus value. This is a measure of how much pressure must be applied to reduce the volume by 1%. For solids, these values are predictably very high, eg diamond is 443,000,000,000 Pascals and steel is 160,000,000,000 Pa. For ice I would need 2,000,000,000 Pa to compress it by 1%.  For liquids we would expect the values are lower and generally they are. Water bucks the trend though and needs 2,200,000,000 Pa. So, I actually need more pressure to compress water than I do to compress ice. Blooming pesky things those hydrogen bonds!

​As divers we all know about the difference between diving in fresh and seawater in terms of needing an extra bit of lead because seawater is just a little bit denser than freshwater. Density is affected by 3 things, salinity, temperature and pressure. At extreme depths the density of seawater increases due to the high pressures. Given that we are recreational divers, the increase in density due to depth isn’t something we need to worry about. There are two common ways of dealing with measuring the density, we could use the density figures in grammes per cm3 or specific gravity.  Specific gravity is a comparative scale, but it’s actually fairly easy to calculate actual density using the temperature and salinity for where we are diving (and there’s some great online software that allows you to plug in the relevant figures).
Salinity (how much salt in g is dissolved in 1kg of water) varies quite a bit around the world. To understand the factors affecting salinity you would need to look at how much freshwater run off enters the area. Freshwater is less salty and will have the effect of partially diluting the salt concentration. Local climate will influence how much water evaporates from the sea; in hotter conditions the concentration of salt rises. Lastly, we need to consider how much current circulates the water. One of the most saline seas is the Red Sea, with little freshwater, high temperature and confined circulation. Salinity levels at the northern end can be as high as 4.0% (much higher than the world average of 3.5%). By contrast, in the Baltic Sea, especially around the Gulf of Finland and the Gulf of Bothnia, salinity can be as low as 0.8%. There’s lots of freshwater running in, limited circulation of saltier water through the narrow channel to the North Sea and the lower temperatures reduce evaporation.  This sea water is so not salty that it could be safely drunk in a survival situation. In the UK we actually see a little variation between the west coast (3.5%) and the east coast (3.4%). Don’t try drinking this stuff for survival!
So salt levels can vary, and as they do, so will the density of the water that you need to displace. How often have you heard someone in a dive resort tell you that you might need a bit more lead here because the seawater is a bit more salty? This got me thinking, how much more salty would it need to be for me to notice,or are there other explanations?
Let’s start with the UK. The density of seawater on the Atlantic side at average temperature (assume 12oC) is 1.0267 g cm-3 whereas on the North Sea side, the density will be 1.0258 g cm-3. I know from experience that I haven’t noticed the difference. I do know that wearing twin 7’s in fresh water (1.000 g cm-3) means I don’t need any additional lead, but exactly the same setup in Manx seawater takes 4kg of extra lead. So, for each extra 0.0067g cm-3 I need another 1kg of lead to get my head under water. Taking this logic forward means that in the salty Northern Red Sea at the coldest part of the year (24o), when the density is 1.0275 g cm-3 I’ll be needing another 1kg to sink, which seems reasonable. But, and it’s a really big but, I don’t wear the same neoprene drysuit, Fourth Element undersuit and base layers in the Red Sea as I do in the UK. If I did, I’d need the extra lead and treatment for heatstroke!
The Red Sea is a bit of a salinity extreme; for most of the world the average salinity applies, which makes it pretty much the same as the UK.  I think it is much more likely that other factors come into play when we head off to other waters. Every time I drag my 5mm wetsuit out I struggle to remember just how much weight I had last time, and of course my suit will be compressing with use and losing buoyancy from new anyway. I err on the side of caution and take an extra lump of lead for the checkout dive. Let’s face it we would all rather be under the water having to control a little extra air in our BCDs than bobbing around like a cork on the surface while the dive master forces a smile and swims over to us with another block.
Most of us will use cylinders from the dive centre and there can be a huge variation in the weight of steel cylinders. A 12l cylinder can weight between 13 and 15 kg, and a 15l can vary from 16 to 19kg. Unless your dive centre has cylinders of different weights, they may not even appreciate that they have ‘lighter’ 12s. The situation is worse still if the cylinders are aluminium and are neutral in the water…then you’re really going to need some lead round your waist! Your confusion at needing more lead is easily explained by blaming it on the ‘salty water’ but perhaps there are more factors at play?