What are cuttlebones for?
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.
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.
While reviewing some marine survey videos lately I found myself mesmerised by watching an octopus move across my survey area, settling and changing colours before moving again and changing yet again. The colour change was so rapid and the octopus completes three quick changes before scooting off out of the video. It got me to thinking that colour changing is a pretty cool skill to have. The most us puny humans can manage is to develop a tan, producing melanin in the skin to try and prevent the damage that UV radiation can cause.
Colour in cephalopods (octopus and squid) depends on 4 different types of cells. The first layer of colour controlling organs in the skin are the chromatophores. Each chromatophore consists of a small balloon like sac filled with pigment. Each sac is connected to around 20 muscles, and each muscle is controlled by 2-6 nerves linked directly to the brain of the octopus. The octopus can stretch the balloon-like sac and allow the pigment to cover a large surface area, so we get to see the black, brown, orange, red or yellow colour just under the surface of the skin. When the muscles around the sac relax, it shrinks and the colour is hidden. Chromatophore sacs are individually controlled so the cephalopod can control which colours are displayed and where, hence the patterns seen in cuttlefish. Deep water cephalopods have very few chromatophores as colour isn’t much use in an environment with little light.
The next layer of colour organs under the chromatophores are the iridophores. Iridescence is the property of luminous colours that change depending on the angle they are viewed from. Iridophores are the key to how cephalopods create the metallic green, blue, silver and gold colours. Iridophores work by reflecting light from stacks of very thin cells. It’s not certain how iridophores are controlled, but they are slower to respond than chromatophores so it’s unlikely to be controlled by nerves but more possibly by hormones.
Then there’s the leucophores. These are cells that scatter full spectrum light, so they appear white. In fact, they will reflect any light that is shone on them, and the light doesn’t change with the angle that you view at. It’s thought that having leucophores underlying the chromatophores increases the intensity of the colours that we observe. Leucophores also help with the cephalopods ability to colour match because they reflect the surrounding light.
Cephalopods have 3 types of specialised colour creating organs in their skin to mimic their background for camouflage and communicate. The cephalopod eye is remarkably similar to a vertebrate eye consisting of an iris, lens and photoreceptor cells. The similarity is often cited as an example of convergent evolution, both vertebrates and cephalopods need to observe their environment and they have solved how to do this in a similar way. But there is a critical difference, cephalopods are colourblind, so their eyes only see in black and white. How on earth does that make sense? An animal with the ability to make a myriad of colours, metallic sheens and mesmerising patterns can’t actually see in colour?
The explanation for this apparent contradiction is that the cephalopod eyes have wide pupils in a strange variety of shapes, U-shaped, W-shaped or dumbbell shaped. When light passes through the wide pupil, the lens in the eye acts as a prism and splits the light into different colours, a large pupil allows for more splitting, known as chromatic aberration. Cephalopods use their wide pupils to create the maximum chromatic aberration and focus on these different wavelengths by changing the depth of their eye ball (altering the distance between the lens and the retina). So, cephalopods can detect colour, not by using special proteins embedded in cells in the retina (like we do) but by changing whether the light focusses on the retina at all. They find it easy to focus distinguishing between bright and dark colours, so that probably explains why display patterns are usually colour separated by black bars.
But if a cephalopod can’t really see so well, how on earth do they mimic their environment? The secret to this lies in the presence of opsin (light detecting protein) in the skin. Its thought that its possible for some cephalopods to sense how much ambient light is present across their periphery and adjust their skin colour and brightness accordingly. To camouflage yourself, you don’t have to be a perfect match for your surroundings, you just have to match it slightly more than your predator can distinguish.
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.