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.
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.
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!
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.
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?
Over a year ago when we set up our marine tank, complete with filters and chiller, I envisaged that it would be mainly the rock pool animals that would survive. My inexperience at maintaining a marine tank would be compensated by the relatively hardy nature of rock pool creatures. Any critter that has evolved to tolerate massive variations in temperature and salinity (rainwater run off dilutes the salt in rock pools, but evaporation makes them saltier) was probably going to survive my attempts to keep it alive. As time has passed, we have become more adventurous about the creatures that we have attempted to keep, but the latest inhabitant of our tank is another step up…
We have a shark egg case. With a live lesser spotted catshark. And we can see it growing!
We see catsharks quite often on dives, in fact they’re probably the most common shark. Many non-divers are stunned when you tell them that there are 21 species of shark that are native to British waters, and probably another 19 or so migratory species too. The film Jaws was released in 1975 and over the past 40 years has had a massive negative impact on the general public’s perceptions about sharks. Organisations like the Shark trust have been battling this perception ever since. Even my own children make jokes about Jaws like sharks, when they have never seen the films and have had the privilege of diving and snorkelling with sharks. And now we have a live shark in the dive centre!
We have no idea if the shark in our tank is male or female. It’s currently less than 2cm long and inside it’s 7cm long protective egg case. At this stage there is more yolk than embryo shark. Shining a torch through the egg case allows us to see the embryo wriggling around inside the case. The hunt has been on for a gender-neutral name, and we’ve settled on Charlie the Catshark. We are much more used to seeing mermaids purses wash up on beaches than finding one alive and still growing, and managing to get it into an environment that we can observe its development. We don’t have any dog whelks in our tank. They are voracious predators and you can bet that anything capable of drilling into a Periwinkle shell for a meal would have no trouble getting through the mermaids purse to eat Charlie.
Whilst we watch Charlie grow we will see the yolk sac diminish in size. It’s easy to understand that the yolk must contain ‘nutrients’ to allow this growth; amino acids to make proteins for muscle and cartilage, and fatty acids which are vital to make cell membranes and to metabolise for energy. Shark egg yolk can contain over 50% fat which is roughly similar to chicken egg yolk. Most of the fatty acids are unsaturated fats which is important to keep them mobile in the low temperature of the sea. There are no carbohydrates. It’s hard to get carbs into an efficient storage molecule in such a limited space. There are other substances in the yolk vital to Charlie’s development especially hormones which will drive growth and sexual characteristic development.
We are going to have a long time to wait. It will take 9-10 months before Charlie chews his/her way out of the egg case. At that point, Charlie will be around 15cm long and will be heading back to the sea before he/she has a chance to eat everything else in the tank. My money is on the shrimp being the first casualties. But really a marine tank in a dive centre is no place for a young shark to grow up. For now, Charlie is safe inside the egg sac and the other inhabitants in our tank are safe from Charlie.
We have a rare chance to capture the attention of local children. We often have young visitors into the dive centre to hold a hermit crab, small urchin or starfish, but now we can show them a shark and talk about conservation. We are hoping to catch them before the cultural references to sharks start to become ingrained. The latest viral hit song ‘Baby Shark’ will help too. A combination of an ear-wormy irritating song that kids adore plus the chance to watch Charlie grow. This is a hearts and mind battle we have a good chance of winning. Do do do do do (a quick online search required if you haven’t seen/heard it yet – and my apologies!).
In May 2017, a female Orca (known as Lulu) was found washed up dead on the shoreline on the Isle of Tiree in Scotland. Lulu was one of the last remaining members of Britain’s only resident Orca population, which now only comprises 8 individuals, 4 males and 4 females. This west coast community had been monitored by researchers for over 25 years.
Orcas are very social animals with the Orca mother at the centre of the pod. Her children, including adult sons, stay together throughout their lives. Female Orcas start reproducing around 13 years of age and during her lifetime a female will have 4 to 6 calves and stop reproducing around 40 years of age. Pregnancy lasts about 17 months and is followed by a period of mothering in which other members of the pod will assist with babysitting duties.
Different organisms have evolved to have different reproductive strategies. At one end of the spectrum there are organisms which produce a huge number of offspring, in some cases the parent then dies or never reproduces again. This Big Bang reproduction is a good strategy if the organism has a high probability of dying early. If the chances of surviving are low, then it makes sense to invest your energy into a Big Bang reproduction when conditions are right and thus ensure your genes carry on into the next generation. Many fish and some insects employ this strategy including salmon.
At the other end of the spectrum are the organisms that have a single or only a few offspring but at multiple times in their lives. Humans along with other large mammals use this strategy, known as iteparous reproduction. These organisms have a low probability of dying early, so it is expected that the offspring will have a good chance of survival to maturity. Any species that takes years to reach breeding maturity and has a limited number of offspring is vulnerable to any factor that impacts on breeding success.
In between these two extremes are organisms like coral, who breed en masse at limited times, generating lots of offspring but only when the moon is in the right phase. These organisms have a constant risk of dying throughout their lives and so a constant reproduction strategy is a good idea, balanced against the resources needed for reproduction to happen.
Researchers are certain that there have been no new calves born into Lulu’s pod in the last 25 years. The entire pod is ageing and it is likely that during our lifetimes there will be no resident Orcas in British waters. A post mortem on Lulu has provided the clues that man-made polychlorinated biphenyls (PCBs) are likely to be the culprit. There has been concern over the persistence and biological effects of PCBs for many years. Scientists have established that physiological effects can be measured with around 30 milligrammes of PCBs per kg of body tissue. Lulu’s level was 975mg per kg. Furthermore there was not evidence that Lulu had ever been reproductively active or ever had a calf.
PCBs are a group of chemicals that were originally made from 1929 until they were banned in 1979, by which time it is estimated around 700,000 tonnes had been produced. They are chemically stable, non flammable, have high melting points and are poor electrical conductors. For many years PCBs were used as insulators in transformers and capacitors and cable insulation, plasticizers in paints and plastic, and pigments and dyes. Does anyone remember the carbonless copy paper? For the younger readers, you’ll need to look up carbon copy (and then you can understand why we cc people on a email) and understand what a revolution it was when carbonless copy paper arrived.
PCBs aren’t very soluble in water, but they are easily soluble in organic solvents and in body fat. They accumulate through food chains and long lived species will end up with high levels. PCBs are still being released into the environment today from poorly maintained waste sites, leaks from electrical transformers and incinerators using a temperature too low. By the 1960s PCBs could be found at trace levels in people and animals around the world, not just in heavily populated areas, but in the Arctic too. The health effects are widespread; PCBs cause cancer, reduce the function of the thyroid and the immune system and cause neurological deficits. Importantly for survival of Orca, PCBs reduce conception rates in females and sperm count in males and reduce the birth weight of calves.
Lulu’s death was caused by becoming entangled in fishing ropes not by PCBs. However, you don’t often hear about Orcas becoming entangled. They are very intelligent, nimble and aware creatures. It’s highly likely that the PCBs in Lulu’s body impaired her ability to navigate a common hazard, but to be brutally honest the PCBs had killed her chances of reproducing years ago. Her body may have washed up in 2017 but her pod have been dying for years. Humans changed the chances of survival and sadly there is no hope of saving them now.
Ever since the footage from Blue Planet II hit our screens, there has been a growing awareness of the devastating impact of plastic on the marine environment. Anyone who has taken part in any of the Great British Beach Cleans organised by Marine Conservation Society volunteers will have already know much of the debris collected every year is plastic, from fish crates to drinks bottles, rope to the cotton bud sticks with cigarette butts thrown in for good measure. Whilst the MCS encourage volunteers to collect every item in a 100 metre stretch for their survey work, its certain that there are some plastic particles that are just too small to be picked up. These are the microplastics.
Microplastics are particles less than 5mm in size, ranging down to 5μm. That’s the Greek letter mu which represents micrometers, one million times smaller than a metre, a thousand times smaller than a millimetre. This size range is why these particles are called microplastics. Microplastics are not as easy to count. A collaboration between the University of Portsmouth and a charity called Just One Ocean is running a scheme to encourage people to survey their beaches for microplastics. There’s a carefully designed citizen science method to allow data from around the UK shores to be submitted and collated. We don’t fully understand the scale of the problem yet, but scientists from a number of disciplines are expressing concerns that it’s much larger than anyone suspected.
Microplastics have many sources. Large plastic pieces tumbling around in the marine environment will gradually break down into smaller and smaller pieces. Washing synthetic clothes in a washing machine releases microfibres into the waste water. If you run a tumble dryer you will know that you have to clear out the lint from it. That is just the larger fibres that get trapped. Smaller particles will enter the air blown out from the dryer. Microplastics have been found in the air, soil and water across the world. Since the 1960s, plastic particles were deliberately put into toothpastes and body care products as microbeads. They helped scrub the plaque off your teeth or acted as an abrasive layer to remove dead skin in facial scrubs. Those plastic beads were useful for about 2 minutes (because we all brush for the recommended time don’t we?) and when you rinsed and spat, those plastic beads were set free on the world. And the chances are they will exist long after you have gone.
We also find microplastics in food and bottled drinks. Filter feeding marine life picks up the particles from the environment. Mussels tested from 8 locations round the UK and samples bought from 8 UK supermarkets were all found to contain 70 microplastic particles per 100g of meat. Beer, honey and sea salt have also been shown to be good sources. Some studies have shown that over 80% of tap water samples contained microplastic and bottled water was even worse. One sample of bottled water had over 10,000 plastic particles in a litre. Science has a funny way of examining such issues, they’ve even looked at human faeces to gauge the levels that we are consuming. They found 20 microplastic particles for every 10g of stool sample.
So, what’s the big issue? Indestructible little bits of plastic that are so small you can’t see them with the naked eye? And they obviously pass through your intestine because they are too big to be absorbed. Why should anyone care? And to a certain extent we don’t fully know yet – and that may be the scariest aspect of this story.
When plastics are made, they aren’t a pure substance. Thousands of different chemical additives can be thrown into the recipe to manipulate the properties of the plastic. As large plastics breakdown, these additives are released. The other concern is that microplastics are very good at adsorbing (not absorbing) certain chemicals onto their surface. You might have heard of a few of these; PCBs, PAHs and DDT are on the detected list. We’re reasonably certain that probably 90% or more of the plastic microparticles that you’ve ingested have passed through your intestinal tract. But what scientists cannot quantify yet is whether they transferred damaging levels of toxins to you on the way. The data so far suggests possible disruption of your gut microbiome and inflammation. The consequences of long term repeated exposure to the toxic chemicals carried by the microplastics is still being assessed. It doesn’t bode well. We know from other species how devastating persistent organic pollutants can be.
There are estimated to have been 8.3 billion metric tonnes of plastic made since the 1960s, of which over 4.9 billion metric tonnes has been discarded into the environment. This is a problem that will not be easily solved and we are all at risk.
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…
July 1969 is best remembered in popular culture for the Apollo 11 moon landings and Neil Armstrong’s declaration of “..a giant leap for mankind.” But at the same moments as the world was transfixed by NASA’s reports, another equally challenging expedition was underway off the North American coastline. Six men on board the Ben Franklin drifted for 30 days in the currents of the Gulf Stream, using very little power apart from their floodlights.
The Gulf Stream project grew out of earlier bathyscaph explorations. Bathys means deep and skaphe means light boat. Bathyscaph expeditions were designed to explore the depths, to plummet to the sea bed, make observations and then return to the surface. During the 1950s successful dives were made into the Marianas Trench at a depth of over 10,000 metres.
The Ben Franklin was different, she was a mesoscaph, meso meaning middle, and her design reflected that idea. The Ben Franklin was designed to be a light vessel using modern steel and Plexiglas to ensure that in the event of any problem, she could always return to the surface. Whereas launching bathyscaph required considerable winch power and large support vessels, mesoscaphs were expected to operate in shallower waters, with minimal surface support.
It took Jacques Piccard, a Swiss oceanologist (and no relation to Jean-Luc) nearly a decade to design and build the Ben Franklin. Jacques father, Dr Auguste Piccard was himself an ocean explorer who designed bathyscaphs and it was his discussions with Jacques that initiated the mesoscaph project. The design was a cylindrical cabin capable of holding the crew, with 400kg of Lithium hydroxide in panels to absorb the carbon dioxide, over 1100 kg of silica gel to absorb the water and reduce humidity, 250 kg of liquid oxygen and enough dehydrated food to last for 30 days. Piccard orginally envisaged the interior being lined with mahogany, to emulate a fine yacht, but was overridden and the internal structure was clinical and white.
Setting out from Florida, there was no big fanfare or press launch. The initial plan was to see if they could last 3 days and then make a decision over whether to continue. So on 14th July 1969 the Ben Franklin was towed into position and at 20.34 the hatch was closed. The crew descended initially to the bottom of the sea and then rose slightly controlling their descent by ditching ballast weight or adding air to ballast tanks. Over the next 30 days they would drift for 2800 km at speeds between 0.2 and 3 knots. They experienced internal waves within the Gulf Stream that would cause their depth to oscillate by over 100 metres, although the rate of rise didn’t concern the crew. They established that there was perceptible daylight at 600 metres and conducted a series of acoustic measurements using explosions set off from two surface support vessels.
On board for the trip was an observer from NASA, whose primary function was to look at the interaction between the crew. 50 years ago NASA was already considering the International Space Station and longer voyages to Mars. The Gulf Stream project provided an ideal test bed to study sleep patterns which deteriorated after the first 3 weeks and eating arrangements, as crew members chose to eat alone rather than together. Bacterial contamination, firstly of the cold water supply and then of the walls and floors would have posed a serious threat to the crew had the experiment continued beyond 30 days. Even an increase in the cleaning regime failed to stem the growth of bacteria.
Communications between the Ben Franklin and the surface support vessel the Privateer take place using an acoustic telephone, and the recordings made by the NASA observer show that the stress levels of the crew rose enormously at times when communications were restricted by weather or surface vessels deviating their course due to other ships. On 24th July 1969 as the crew of Apollo 11 splashdown into the ocean, the crew of the Ben Franklin are experiencing a temperature of 14 degrees and tumultuous seas as the internal waves catch the vessel. It wasn’t until 14th August that they surfaced and at 8.09 the hatch was opened. They were due east of Baltimore and the US Coastguard vessel Cook Inlet was there to rendezvous with them. A week later the Ben Franklin had been towed into New York for a hero’s welcome. A remarkable tale of human endeavour had come to an end, but in a world transfixed by the moon it was barely noticed.
Scallops (Pecten maximus) are a national concern on the Isle of Man. We have some of the most protected scallop populations in the British Isles. Licensed boats can only fish during daylight hours in certain areas of the sea and not during the summer months when the scallops are breeding. The catch is landed into harbours around the island; creamy, pink shells in 25kg bags loaded onto pallets for the forklift truck to move them into wagons.
If you glance down into the harbour its usually possible to spot the white inside of a few discarded shells shining on the seabed below. These shells eventually wash across the bay and onto the beaches, but they don’t always arrive in the same colour as when they were discarded. Many of the shells are stained dark brown or black, colours we never see during dive surveys of scallops.
Shells are mostly made of calcium carbonate which is white in colour, mixed in with about 2% of protein. As molluscs develop they absorb minerals from their environment and secrete calcium carbonate from their mantle to create their shell. The protein makes the shell very strong, but lightweight and resistant to dissolving in water. Shells are self-repairing, and the mollusc can secrete more shell material as needed for repair or growth. Naturally occurring colour and patterns in shells is as a result of mineral ions incorporated into the shell structure. But that doesn’t explain the post-mortem colouration of the scallop shells.
Shallow burial of shells causes iron oxides to form in the tiny pits on the surface of the shell and causes brown staining. The black colour is usually due to microscopic crystals of iron sulphide. These crystals form in the absence of free oxygen which can occur if shells become buried deeper in mud or sand.
Although my local harbour is sheltered, it doesn’t provide the deep mud conditions required to blacken shells, but there is a much more common cause. Burial under just a few centimetres of seaweed rotting on the beach will provide suitable anoxic conditions for sulphide formation. Hence blackened shells on the beach is a relatively quick process occurring under mounds of kelp and wrack.
There are some mollusc species that live well buried into deeper sediment. The Ocean Quahog (Arctica islandica) is a subtidal species of clam that is renowned for it’s longevity. Some individuals have been recorded at over 500 years old. The shells of Quahogs have dark black colouration, but they have a long time to absorb the necessary pigment. Whilst the shell is buried in the sediment, a siphon to the water provides for food and oxygen to the creature below.
Naturally acquired pigment probably strengthens the shell. Colour patterns often align with spiral or axial sculpture. Instead of producing and transporting a thicker shell, it might be more energy efficient for molluscs to make pigments. Pigments impede propagation of a crack in the shell. The structural explanation also works for colour inside of shells. A good example is Mercenaria mercenaria (the quahog or cherrystone clam). The purple inside the shell, hidden when the animal is alive, lies along the edges of the shell, just where predator whelks are likely to attack. Strangely young Merceneria don’t make the purple pigment. Their shells are too thin to resist attack anyway, so they concentrate their efforts on growing a thicker shell and surviving to when their pigment strengthened shell is going to ensure a long life.
There are lots of other reasons for shells to have different colours. A favourite project for marine science students is to send them to look for colour variation in Flat Periwinkles (Littorina obtusata). In this case pigment is used for camouflage, allowing the winkles to hide amongst the bladder wrack on the shore. Pigments may serve as a warning to possible predators, or the pigmentation pattern may provide a template for future growth of the shell. But there doesn’t have to be a reason for pigmentation in all cases. Oxygenated mammalian blood is red, not for any evolutionary reason, but because that’s the chemistry of the situation.
Michelle has been scuba diving for nearly 30 years. Drawing on her science background she tackles some bits of marine science. and sometimes has a sideways glance at the people and events that she encounters in the diving world.