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.
One of the best things about running a dive centre is the diversity of the clients that walk through the door. One of the worst things about running a dive centre is the diversity of the clients that walk through the door. Our centre is multi-agency which means we can honestly offer the most suitable training to each person that comes to us. We’ve become experts at chatting to prospective divers about their interests, travel plans and diving aspirations and offering them considered and justified advice about their training.
But when people walk through the door the first thing they will say is “I want to do my PADI.” The marketing spend and brand awareness for PADI is huge and there’s the common misconception that only a PADI cert will be accepted elsewhere in the world.
Like many experienced divers, I’m entitled to carry a range of cards covering a number of different aspects of diving, but I pick which ones I take on trips with me. Somewhere in one of my filing cabinets is my set of cards but rarely do they see the light of day. I once had the misfortune to be on a Red Sea liveaboard with a guy who when asked to prove his diver grade slapped a huge wallet of cards on the table with the classic line “There’s 50 years of diving experience in there. Take your pick.” It was unnecessarily aggressive and made all the sweeter when, later in the trip, this same guy was the one who surfaced from the night dive to check where the boat was before dropping back to 12m to finish his dive. Obviously the 50 years of experience didn’t include basic navigation.
In truth, BSAC qualifications generally carry a large amount of kudos. We train in some really tidal, very murky conditions and our training includes a huge amount of dive and rescue planning that stands us in good stead wherever we pitch up in the world. On a particularly difficult pick up from the Rosalie Muller in the Red Sea it was our experienced divers who took over recovering the divers from the water, threw out grab lines and carefully timed their assistance with the pitching of the boat. No shouting. No fuss. They just stepped forward and stopped a difficult situation developing into one that would have ended up in the BSAC incident report.
The Isle of Man hosts motorsport events throughout the year, the TT in June, Southern 100 in July and the Manx Grand Prix in August. These events attract large numbers of foreign visitors carrying a variety of dive qualifications, some stating CMAS equivalence and several requiring translation. We’ve met and dived with some fantastic divers from around the globe, and we’ve had some very deluded people walk through the door. My favourite has to be the bloke who walked in and announced in a very heavy Eastern European accent “I vant to dive vith the sharks!” A little gentle questioning established that he had done a try dive, in a hotel swimming pool in Turkey, about 5 years ago. We carefully explained how much legal protection covers the basking sharks that come to pup and breed in Manx waters, that they avoid divers, that he wasn’t qualified and no, we couldn’t just hire him some kit. He left muttering and we breathed a collective sigh of relief.
We run two boats from our dive centre and when necessary have the services of a third boat. The skipper’s first question when I call him to book a group is to ask “Are they BSAC? That’s OK then.” He knows he will get divers who, thanks to the strong club system, will turn up as an organised team and just get on with it. Delayed SMBs aren’t a special course, they’re mainstream. Nitrox isn’t something just for multiple dives on a liveaboard, but for safety in everyday diving. And should anyone have any difficulty there will be someone throwing out a grab line and watching the pitch of the boat as they go to assist.
But of course I’m not claiming BSAC clubs are perfect. We’ve had the divers who got confused when they ran out of dishwasher tablets and put washing up liquid in the machine instead (Don’t try this at home without a mop and bucket handy). We’ve had the clubs that plead for discounts, usually for the ‘poor students’ and their trip turns out to be comprised of university staff and post-docs on sizeable bursaries. And we all look forward to hearing about the club politics as the week goes on.
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.