​As I child I had a budgerigar called Dinsdale. Dinsdale was a pretty happy bird who would cheerfully run round my desk, leaving special presents on my homework (!) and hop back into his cage on command. He loved hitting the bell that was attached to a little round mirror and pecking hard at his cuttlefish.  Of course, he didn’t really have a whole cuttlefish in the cage with him, just the hard, bony bit.

Cuttlefish bones aren’t actually bones at all, they are a special kind of shell.  And while we are at it, cuttlefish aren’t fish either. Cuttlefish are one of the Cephalopods and they have their own family name Sepiidae. The early ancestors had a shell for protection and existed before the first fish had evolved. Modern cuttlefish don’t have an external shell but rely on camouflage for evading predators. 

The common cuttlefish (Sepia officinalis) also produces a brown ink, which can be harvested from their ink sacs.  Most of us would recognise the colour of the ink from old fashioned brown sepia photos, and that’s how the ink got its name. The chemistry and biosynthesis of ink in cephalopods is fairly complex. It is a form of the common biological pigment melanin which is the same molecule responsible for your skin developing a tan after exposure to sunlight. Who’d have thought you and the cuttlefish would have so much in common? The eumelanin in the ink sacs is also found in fossils from the early Jurassic period around 200 million years ago.

For a soft bodied animal, it seems strange that there should be a good fossil record. The cuttlebone is generally well preserved. When the cuttlefish are alive the cuttlebone is a mix of chitin (a really large structural sugar molecule) and aragonite (one of the three forms of calcium carbonate). After the animal dies, the chitin will break down fairly readily but the aragonite persists. That means that it is possible to find fossilised remains of cuttlefish that are readily identifiable and from modern catches of cuttlefish, your budgie gets a calcium supplement.

The cuttlebone has a very specific function in the cuttlefish. It’s clearly not for defence – what use would an internal bone be? Cuttlefish have a short life span, maybe only 1-2 years and during that time they have a phenomenal growth rate (up to 10 kg) so for a cuttlefish conserving energy is critically important. The cuttlebone structure is full of holes and the cuttlefish can control liquid or gas into those spaces to effortlessly control its buoyancy.

The cuttlebone is a long oval structure made of around 100 chambers, with the chamber lying at the head end being the oldest, other layers are added as the cuttlefish grows. Lying along the cuttlebone is the siphuncle, which is a strand of tissue that connects all the small chambers.  In order to add water to the cuttlebone, the cuttlefish makes the blood in the siphuncle more salty by pumping salt out of the chamber. Water in the chamber is drawn out of the chamber and into the blood by osmosis and oxygen and carbon dioxide come out of solution and make up the volume in the chamber by diffusing from the siphuncle.  So it’s not true to say the siphuncle pumps the water….more that it pumps the salt and that causes the water to be drawn out. Siphuncles rarely get preserved in fossil records but you can usually see the notches in the cuttlebone where the siphuncle used to be.
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Removing water from the chambers of the cuttlebone reduces the overall density and causes the cuttlebone to float. Cuttlefish aim to maintain neutral buoyancy and will swim up or down with the minimum of effort. In addition to this, the cuttlefish can control whether the chambers towards the head or the tail end are water filled or gas filled, making it’s journey from depth towards the surface even easier as it adjusts its trim. Next time you’re diving, be more cuttlefish, perfect buoyancy and perfect trim.
 

​At the end of 2016, diver training agencies including BSAC launched Sea Survival training courses developed in conjunction with the RNLI. Despite the common view of the RNLI as being the provision of lifeboats and crew, there is much more that they are involved in from a safety at sea perspective, with particular foci on fishing industry accidents, Swim Safe training courses and safety advice for all water users. It’s astounding how many fishermen don’t wear lifejackets, especially local pot-boat skippers who often work alone.
To try to educate fishermen the RNLI brought a dozen of them from around the UK down to their training base in Poole. All the skippers had previously attended the mandatory Personal Survival Techniques course (and it’s predecessors) which are run in swimming pools around the country. The RNLI trainers asked about lifejackets and got the usual story, the fishermen had them but rarely wore them. The general feedback was that as strong swimmers they were confident that should they fall in the sea, they would be able to swim back to their boat, climb up the tyres on the side and self-rescue. Interestingly, qualified divers and anyone who swam in the sea was excluded from the test group. Repeated attendance at sea survival training had led each fisherman to conclude that their lifejackets weren’t necessary.  The RNLI sought to challenge that belief.
The night before the training course, the trainers opened the doors around the training pool to let out the heat. Overnight the water temperature dropped to 15 degrees.  If you are a diver around the British Isle I am sure there are days where you dream of 15 degree water! At the first attempt the fishermen were asked to wear their normal deck attire and jump in to deep water to simulate falling off their boat. With no life jackets on, the impact of cold water shock was immediate. None of the 12 fishermen lasted longer than 5 minutes before a rescuer intervened. Post dip interviews revealed their shock and surprise at how debilitating the cold water was, definitely nothing like their sea survival training course.
Cold water shock is an immediate short-lived response to immersion in water less than 15 degrees. Blood vessels at the skin contract rapidly, increasing blood pressure and the heart rate. An initial gasp for air can be followed by a breathing rate that is 6-10x higher than normal. It is likely that cold water shock accounts for most deaths when people have unexpectedly entered the water. If you are not wearing flotation during this phase, keeping your head above water becomes the biggest problem.  Over the next 10 minutes, cold incapacitation reduces blood supply to the muscles, making it difficult to swim or self-rescue. A crew member throwing a life ring to you during this time will be frustrated that you can’t actually hold onto it or kick towards the safety of the vessel.
The following day the exercise was repeated but this time with lifejackets being worn. The same cold water shock reaction was initiated, but the fishermen didn’t have to work so hard to keep their airway out of the water, the cold incapacitation stage took longer therefore improving their chances of getting back to the ladder on their boat. You can see the videos from this exercise on the RNLI website.
This started me thinking about why divers were excluded from the test group. I’ve realised I still brace myself for the cold water after decades of diving. OK, I’m wearing a drysuit and the cold water shock reaction is pretty much limited to my head and hands. But how many of us drop beneath the surface in anticipation of that brain freeze moment? As the blood vessels rapidly contract they stimulate the trigeminal nerve sending pain signals to your brain. It hurts for a few moments until you become acclimatised. The fishermen in the RNLI training exercise couldn’t get past that brain freeze feeling.
I think we sometimes underestimate the impact that cold water immersion has on new divers. I can recognise it enough now, but when I think back to learning in a wetsuit I can remember the feeling of panic, rising heart rate and accelerated breathing rate as I used to get into the water. Although we will all recognise increased air consumption by trainee divers, perhaps part of this is their reaction to cold water immersion? I’m sure that with experience comes the anticipation, the forced control of breathing rate for the first few seconds, but until our new divers have developed their response, maybe we should keep a close eye on them for those first couple of minutes? If your trainees are hoofing through their air and their buoyancy is being disrupted by their rapid breathing rate, maybe it’s something to consider?

I want to dive all year round.  I’m not addicted to diving, of course.  I could give I up any time I want.  It’s my choice to have the kit and the training that allows me to be out whenever the conditions permit.
I started out my diving career in a wetsuit, a badly fitting, compressed, slightly smelly dive centre wetsuit.  It wasn’t described to me in those terms of course – it was a semi-dry suit, which is a term that has always puzzled me.  Semi-dry must by definition be semi-wet and how wet do you have to be before you are just wet?  Maybe my approach to life is too scientific but wet and dry are opposites and in my world ‘damp’ is still a form of wet.  So there I am in a wetsuit.  On my 9th dive, whilst on a diving course, I had my first hit of hypothermia.  It was a sunny day in Cornwall, but an old and poor fitting wetsuit was enough to make me cold in the water and then the wind chill back on the boat sapped my remaining core temperature.  I just remember feeling extraordinarily tired as I huddled down into the bottom of the RIB and passed out.  It took 3 or 4 days before I felt well again and the experience was enough to send me off to find out about the dangerous world of dry suit diving.

My Diving Office at the time thought that drysuit diving was best left to the experienced guys so his ruling was that only Sports Divers with at least 2 years diving could use drysuits.  (And just in case you were wondering, yes Nitrox was the devil’s gas.)  I went outside the club for my training and have never looked back.  The first training session wasn’t auspicious. Despite the 5am arrival at Stoney Cove we still didn’t manage to get into the water until after some other divers had been in and come out.  The water dripping off their kit froze on the path and my buddies and instructor slid elegantly down into the water.*  The instructor I was diving with provided a membrane suit without any undersuit, but I was reassured that hiking socks, tracksuit bottoms and a long sleeved t-shirt would be fine.  The sweatshirt I’d brought was a layer too far and would have “doubled the amount of lead” I was carrying, so it was left in the car.  Needless to say I was freezing during and after the dive.  Inversion drills aren’t funny when your feet come out of the suit boots but I was so cold that I didn’t notice until I tried to fin.  It seems funny now, but as an instructor I would be mortified if I had taught such a poor course.  Half the students in my group gave up diving totally shortly afterwards!

So now I am an instructor I am acutely aware of hypothermia.  We’re taught on instructor courses that your kit as an instructor should mirror your students.  All of my instructor team wear dry suits, so it’s natural that so do our students.  Besides, there’s no point trying to teach someone whose only thoughts revolve around wanting to be somewhere warmer.  Our students not only get drysuit training but proper undersuits and socks as well.  We explain the cooling properties of sweat on cotton t-shirts and frown on those who keep mentioning that their mate in [insert name of a.n.other UK diving location here] has just been learning in a wetsuit – and they said it was sooo much easier.  Less dives and less to remember.  And ultimately less diving too!

Around the Isle of Man the sea hits it’s coldest at the end of February, beginning of March.  Over 120 years of continuous data recording at the sadly closed Port Erin Marine Lab have shown this to be the case.  It’ll bottom out at around 4 degrees and dry suits become essential.    Every so often the devil inside me offers the newly qualified diver with the ‘mate who learned in a wetsuit in the Irish Sea’ the chance to do a wetsuit dive.  That is usually the end of the topic and within a couple of weeks they’ll be in the dive centre being measured up for their drysuit.  This ensures that they too have the kit for that year-round hope that the wind will stop blowing, the swell will subside and the vis will superb. They join me in the ranks of fevered weather watchers just looking for the gap in Atlantic low pressure systems. The winter storms have stripped the kelp off the rocks now and this breathes a whole new perspective into our favourite sites.  Maybe I am addicted to diving. Winter diving?  Bring it on!
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*I lied about the elegant part.