No ship yard ever built a ship with the intention that it should end up on the seabed. By design ships are intended to keep the water on the outside (or in controlled areas like ballast tanks). From the earliest hollowed logs and coracles to the ocean going supertankers, the aim is to find materials that create a good airspace and allow the vessel, cargo and crew to stay on the surface. As construction methods have evolved, so have the materials that are used. For any vessel owner, the necessary maintenance to keep the water out is an ongoing and relentless battle.
Once a ship sinks, the process of decay inevitably starts. There are a number of parameters that affect how quickly a wreck will break up. A shallow wreck is exposed to the mechanical shearing forces of wave action and the remarkably destructive scouring of sand. Many wrecks become wrecks because the end up punctured on shorelines. In the battle between the rock and the metal hull, rocks often come out the winner. However, deeper wrecks evade the action of the weather and therefore will remain intact for longer.
Any biological material on a wreck which will decay very quickly. Body flesh is quickly scavenged by crustacea and fish. The hard matrix of the bones that are left behind is mostly hydroxyapatite (a mixture of calcium and phosphate) which is soluble in the sea and becomes more soluble at depth. This is why the deep ocean isn’t several metres deep in fish and whale bones, they dissolve.
The next most fragile structure on any ship wreck is the wooden components. Modern ships with chipboard partitions fare particularly badly once submerged, and can fall apart within a very short space of years. Wood is made up of cellulose and lignin molecules. Cellulose is the main part of the cell wall from the tree that the wood came from. Cellulose isn’t water soluble but the chains of cellulose are held together by hydrogen bonds which water helps to promote. Surrounding the cellulose chains are lignins. These are complex polymers that give wood rigidity and resist rotting. The levels and type of lignin vary between different species of tree. Teak, the beloved material for decks of many vessels, has a high lignin content, which helps it to resist degradation.
For the metal components of a wreck to decay there are several factors that will affect the rate of decay. Salt minerals dissolved in the ocean, particularly sodium chloride (the same stuff you sprinkle on your chips) is a major player. In salt water metals will corrode about 5 times faster than in fresh water. Salts break into charged ions which allow the conduction of electricity and metal ions from the ship will enter the water, gradually thinning the metal plates of the hull. Salinity levels can vary massively depending on the location. A nearby source of freshwater can reduce decay. Salinity is maximum at the surface and decreases down to 500 metres, although it rises again around 2500 metres down.
Oxygen levels in the water will also affect the rate of decay. Oxygen reacts with metals to produce metal oxides eg iron reacts to produce iron oxide ie rust. Metal oxides are weaker than the metal they derive from. So gradually the layer of metal turns to rust, which will thin the metal hull even further. Oxygen levels are at a maximum near the surface and decrease down to about 1000m, and then they increase with depth.
Finally, let’s consider temperature. A higher water temperature means all the water molecule are moving faster and at a molecular level, all these reactions occur more quickly. Deeper wrecks in colder waters have a slower decay pathway.
So for a shipwreck to survive, we require a well-constructed, high quality metal in thick sheets, sunk in fairly cold water, deep enough to avoid wave action and sand scouring, so somewhere sheltered would be ideal……welcome to the wrecks of Scapa Flow!
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.
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.