It's not snot, it's bogie!

​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…