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?
Any diver who has been to Scapa Flow to visit the remains of the World War I German naval fleet will know the story of the 21st June 1919 when 74 ships were scuttled. Due to some heroic efforts only 52 ships actually hit the seabed. Initially, the British Admiralty were determined to leave the German fleet on the seabed and let them rust. But the wrecks were a considerable hazard to local vessels, with several being grounded on up-turned hulls. By 1922 the demand for scrap metal had increased and the Admiralty started selling off the wrecks for salvage, £250 for a destroyer and a mere £1000 for a whole battleship (of course 100 years ago that was the equivalent of £54,000 but that still seems cheap).
Over the next 8 years Ernest Cox developed some incredible techniques to lift huge battleships from the seabed within Scapa Flow. The wrecks were salvaged for fixtures and fittings with artefacts being recovered in good condition including bottles of wine, musical instruments, and the metal ships were broken up for scrap sales. By 1930, the price of scrap metal had crashed leaving the whole operation in danger of financial ruin and by 1933 Cox sold out to Metal Industries Group and they lifted the last of the battleships in 1939 as World War II loomed. Just 3 battleships and 4 cruisers remained. These are the wrecks that divers now visit, but they all bear the scars of the salvage work carried out by Nundy Ltd and then from 1970 by Dougall Campbell as Scapa Flow Salvage Company. Explosives were used to blow holes into engine rooms to get the non-ferrous metals and to open up the hulls to recover the valuable torpedo tubes. Campbell also realised that the armour belts of 14 inch thick steel with a high content of nickel and chrome were easy to recover and valuable enough to be worth the effort. [Dougall freely shared his amazing knowledge to a number of Scapa Flow projects in advance of the centenary, and sadly passed away on July 26th 2018].
Sensibilities have changed regarding wrecks and their salvage. HMS Vanguard is now considered a war grave after she was lost with 845 men following an explosion in her magazine in July 1917, but by the 1950s and then again in 1970s, she was salvaged for her propellers, condensers, torpedo tubes, armour plating and Weir pumps. It wasn’t until 2002 that the wreck became a Controlled Site and diving was restricted, but for 85 years she was stripped of any useful metals ie the ones that would command a good price on the scrap market.
Early salvage operations were driven by the demand for scrap metal. In post WWI industrial development was being held back by lack of metal. But post WWII there was a new driver for recovering steel produced before WWI, lack of background radiation in the metal itself. As the nuclear and space races took hold on the 1950s and medical advancements in 1960s, there was a growing demand for steel that had very low levels of background radiation. Atomic testing in the 1950s released Cobalt-60 into the atmosphere. Steel production involves pumping air (or oxygen) through molten pig iron, which reacts with impurities creating oxides that can be removed as slag. Atmospheric atomic tests released atmospheric radiation which peaked in 1963, so any air or oxygen used to produce steel since that time introduced low levels of background radiation into the metal.
Ordinarily, this isn’t an issue. You wouldn’t notice or be affected by a trace in your cutlery or the spice rack in your kitchen or the panels in your car. But it matters if you are trying to build sensitive instruments to detect radiation. Geiger counters, body scanners and space equipment can all be affected by the low level of radiation introduced during the manufacturing process. That makes the steel armour plating from the pre-atomic era significantly more valuable than would otherwise be the case. Our latter found sensibilities to protect the wrecks as war graves wasn’t apparent decades ago. In Scapa Flow the salvage has stopped, but internationally wrecks are disappearing in their entirety. Happily, the half-life of Co-60 is short, only 5.26 years. Since the moratorium on atmospheric testing in 1963, levels of atmospheric radiation have been rapidly dropping, with only occasional contributions from Chernobyl, Windscale, Three Mile Island. This means that production of low background radiation steel is now possible, combined with computerised correction for background levels. However a drop in the market value of pre-atomic steel is unlikely to save wrecks around the world, particularly in Asia where even the former pride of the British Navy HMS Prince of Wales and HMS Repulse have been targeted by metal scavengers.
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.