ISSUE 14 ARCHIVE - THE SEA DOC INVESTIGATES: THE CUTTLEFISH

Dr John Carlin

I am often accused of over-using the word amazing so I will try to refrain from using it too often in this article. But unfortunately cuttlefish are amazing! They are possibly
one of my favourite marine animals and can be found all over the world in tropical and temperate ocean waters, even here in the UK. The first fact which often surprises people is that cuttlefish are not fish but actually come from the mollusc family which comprise of things such as snails, octopuses and squid.

They can often be difficult to spot and are referred to sometimes as the ‘Chameleons of the Sea’. This is due to their impressive ability to rapidly change their skin colour, colour patterns and the intensity to blend in with their background. In fact cuttlefish can change their skin colour much faster and with more intricate patterns than the ‘landlubbing’ chameleon. They can even change their skin texture so that they match perfectly with their habitat so they can hide away from predators and can produce ink clouds like octopuses to escape. They can use their colour changing skin to signal to other cuttlefish by flashing messages in combination with certain swimming postures and gestures of their tentacles. Research has catalogued 31 full-body patterns and calculated a potential repertoire of nearly 300 combinations of full-body patterns, partial-body patterns, skin texture and body posture.

Their ability to change colour is due to some special skin cells called chromatophores. These cells have a sac of pigment and a large membrane which can be folded and retracted using muscle cells on the sides to contract and squash this elastic sac into a disc against the skin. There are four different types of chromatophores; xanthophores, erythropores, melanophores and iridophores. Xanthophores are responsible for the
yellow colours and are found closest to the surface of the skin. Red and orange colours are created by the erythropores and the black and brown colours are made by the melanophores. metallic blues, greens, golds and silver colours seen in a cuttlefish are created by the iridophores. Just like a painter combining primary colours to create new ones, a cuttlefish uses a combination of its chromatophores to produce different colours.

Often it is only their movement or their strange ‘w’ pupil eyes which catch a diver
or snorkeller’s attention. Despite their chameleon ability to blend in with their surroundings, many scientists believe that cuttlefish are in fact colour blind. But just
like my favourite pair of Oakleys, the eyes of a cuttlefish make use of polarisation, which enhances their perception of contrast. Their eyes are in fact some of the most developed in the animal kingdom and have excellent vision. In fact they are even better than our own as they do not have a blind spot since the optic nerve is positioned behind the retina and they reshape their entire eye rather than just the lens as we do to focus.

Even weirder, is that cuttlefish have different coloured blood. In mammals we have a compound called haemoglobin in our blood which is made up of iron and is responsible for the red colour that transports oxygen to the cells in our body. Cuttlefish use a different compound in their blood called haemocyanin to carry the oxygen which is made of copper instead of iron resulting in their blood being
a blue/green colour. Cuttlefish have to pump blood more rapidly round their bodies because haemocyanin is not as effective at carrying oxygen compared to haemoglobin. Another interesting fact is that cuttlefish have three hearts! They use two to pump blood to their gills (one heart for each set of gills) and one heart to pump to the rest of the body.

Cuttlefish contain an internal structure called the cuttlebone which is often found washed up on beaches once a cuttlefish has died and its soft body has decayed. The cuttlebone is made of a carbonate material called aragonite which is porous and is responsible for providing the cuttlefish with buoyancy. It regulates its buoyancy by changing the gas to liquid ratio in the chambered cuttlebone. Interestingly, although cuttlebone is normally fed to budgies now, it used to be added to polishing powders and toothpaste!

Although cuttlefish are related to octopuses they differ in a number of ways. One immediate difference is that whilst they have eight arms, they also have two tentacles. These tentacles tend to be longer than the arms and only have suckers on the tips. They use their arms and tentacles to secure their prey and during mating a male and female will intertwine arms to facilitate the passing of a sperm package. Females then lay about 200 fertilised eggs under ledges attaching them to bare rocks. After a cuttlefish hatches they feed on small shrimps, but as they grow they tend to prefer a diet of crabs and fish.

They use their camouflage to hunt and sneak up on their prey. When they get near enough, they open out their arms and shoot out their longer tentacles which grabs the prey and pulls it towards their beak to be eaten. The primary method of movement for a cuttlefish is by jet propulsion. This is a very energy- consuming way to travel compared to the tail propulsion used by fish, but cuttlefish can also move short distances in any direction by the rippling of a flap of muscle around the mantle.

There are thought to be over 120 different types of cuttlefish. They range in size from about 15-25 cm to the largest species (Sepia apama) which can grow up to 50 cm and weigh over 10.5 kg. They are mostly found in shallow water depths but have been known to go to depths of over 600 metres.

Cuttlefish are amongst the most intelligent of invertebrates and have one of the
largest brain-body size ratios. Despite their intelligence and incredible range of defence and attack mechanisms they are relatively short lived. The life span of a cuttlefish is about 18-24 months with females dying shortly after spawning. Their main predators are sharks, fish and dolphins (that for some strange reason only seem to eat the heads).

Unfortunately cuttlefish is also a dish enjoyed by humans and they are caught for food in the Mediterranean, East Asia and our own English Channel. We have also used their ink to produce a dye called sepia however artificial dyes have now mostly replaced these natural sepia dyes.

My last encounter with a cuttlefish was when I was working in Malta. We had just finished doing a dive on the wreck of the P29 (ask Auntie Toomer, I do enjoy diving on wrecks) where we had seen star gazers, rays, moray eels, nudibranchs, squid eggs and so much life we didn’t know where to look. We headed back to the harbour wall where my dive buddy (Craig Lees) spotted this little cuttlefish.
It was so well camouflaged that I couldn’t initially see what he was pointing at and it was only when it slowly began to swim towards us that I saw it.

The dive guide, Antonio, then slowly raised his index and middle fingers on his right hand and the cuttlefish raised its tentacles mimicking him. It slowly glided up to him and touched his raised fingers whilst changing its skin colour and spot pattern. Antonio encouraged us to do the same and the cuttlefish came up to each of us to say hello. It even swam in up to our faces and touched the glass of our masks with its tentacles. I am not sure if it was curious to see more of our faces or if it could see its own reflection.

Once whilst I was working in Indonesia we took a large mirror down on a dive and a large male cuttlefish got quite aggressive, demonstrating an angry territorial display by flashing his skin colour from black to white and back again. It looked like black clouds being blown across a sky on a windy and stormy day and he also proceeded to charge the mirror as he thought that another large male had moved in on his patch!

We spent fifteen minutes or so with the little cuttlefish in Malta before unfortunately it was time to leave but it reaffirmed my love of the amazing and weird creatures that cuttlefish are.