Octopuses are truly tenta-cool. We asked a marine biologist to explain why.

About the Octopus

Octopuses are soft-bodied animals famous for their eight tactile arms, big eyes and intelligent behaviour. Together with squid, cuttlefish and the chambered nautiluses they are grouped into an animal class called Cephalopoda. The word is derived from Greek and literally means head-foot, which relates to the fact that their arms are directly attached to their head.

Scientists have identified over 300 octopus species and they can be found in every ocean of the world. Most octopuses live on the seafloor in dens under rocks or in crevices. With a few exceptions, they prefer a solitary lifestyle and only interact with other octopuses when it’s time to mate. But even then males like to keep their distance, as females can sometimes snack on their mates in the process.

Octopuses show lots of interesting features: they have a funnel for jet propulsion, adaptive skin for camouflage, an ink sack to fend off predators, three hearts and pulsating veins, big eyes, that are very similar to our human eyes, an elaborate brain, and prehensile arms with some mind of their own. Considering that their closest relatives are clams and snails, it is not surprising that many people jokingly refer to them as aliens. Scientists are still trying to fully understand how cephalopods evolved into these intelligent and agile creatures. However, studying their fossils has provided us with some clues about cephalopod evolution.

Today we believe that cephalopods derived from a group of small, bottom-dwelling mollusks (soft bodied animals) called monoplacophorans, that looked very similar to limpets and lived about 530 million years ago (mya). Over time their shells increased in size and protected the early cephalopods from predators. In addition, their buoyancy allowed them to swim freely through the water column and hunt for prey. For a very long time cephalopods dominated the world’s oceans. The arrival of modern fishes and mammals around 66 mya, however, marked the end of shelled cephalopods. Their large and slow bodies were a welcome snack for the agile and fast new predators. Nautiloids were the only shelled cephalopod to survive this mass extinction as they adjusted to a life in the deeper seas. The evolutionary pressure from fish and mammals forced cephalopods to adapt their bodyplan to a similarly agile and predatory lifestyle. Over time cephalopods internalised, reduced or (in case of octopuses) completely lost their shell. This did make them faster and more flexible but also rendered them more vulnerable to predators. In order to protect themselves from turning into a tasty snack, octopuses therefore evolved fascinating defence mechanisms and novel features...

The actual body of the octopus - the mantle - is a large, muscular bulge, which contains all vital
organs, including their gills (and yes, it sort of looks like their nose). When the octopus breathes in, the strong mantle musculature pumps water into the mantle cavity. The water then gets expelled through a structure called the funnel. Although octopuses prefer to crawl, this mechanism is used during jet propulsion, which helps the octopus to remove itself from a scene as fast as possible.

The octopus skin can change both texture and colour in order to match their environment. Even though octopuses are themselves most likely colour-blind (there is a chance that they perceive colour slightly differently than us), they use three different types of skin cells (chromatophores, iridiophores and leucophores) to blend into any background within a blink of an eye. Some octopuses take this camouflage to the next level and adapt their body to look like a completely different (and more dangerous) animal species all together. The mimic octopus for instance can reproduce the shape of up to 15 different animals including a venomous sea snake and the spikey lionfish.

A very common way for a cephalopod to get out of an iffy situation is by ejecting ink. This liquid contains melanin (which provides the colour), a mucus (which provides the 3D texture) and an irritant called tyrosinase (which disrupts smell and taste). When threatened, the octopus will eject ink through the funnel, which distracts the predator and buys the animal time to flee the scene. Apart from defence, cephalopods also use ink as a way to communicate with other cephalopods (as a warning) and to hunt. Interestingly, the octopus is not immune to its own ink and will die if exposed to it for an extended period of time (for instance in a fish tank).

With increased agility came the need for better oxygen supply. Like all mollusks, octopuses have blue copper based blood (haemocyanin) rather than red iron based blood (haemoglobin) like humans. Unfortunately, octopus blood binds oxygen less efficiently than human blood. To overcome these deficiencies and to support their high oxygen demands, octopuses have evolved a completely closed circulatory system. This is powered by three hearts, one regular (systemic) heart, which pumps the blood through the body, and two auxiliary (branchial) hearts, which pump the blood through the gills. In addition, in areas furthest away from the systemic heart (e.g. arms), pulsating veins help to keep the blood pressure up.

With such high demand for processing information, it is not surprising that the cephalopod brain is relatively large for a mollusk. Like in all cephalopods, the octopus brain is divided into multiple lobes, each of which has a different function and processes information from a different region within the animal (e.g. skin and eyes). The central brain is situated between the octopus eyes and has a doughnut-like shape, as their oesophagus runs right through its centre. And yes, that means octopuses swallow their food through their brain. In order to not damage the brain in the process, their parrot like beak (the only hard structure in an octopus body) and a specialised tongue (radula) process their food into small particles.

The octopus body contains over 500 Million neurons, about the same amount as a dog’s brain. In comparison, a sea slug has about 18 000 neurons and a human around 86 billion. However, over two thirds of those neurons actually reside within their arms. Researchers have found, that the octopus arms can work somewhat independently from the central brain by creating reflex loops to create coordinated movement. This means that octopuses can move their arms independently, without waiting for the brain to instruct them to do so. However, interaction with the central brain is still necessary to help interpret the information acquired by the arms.

The octopus arm crown consists of 8 muscular arms surrounding their mouth. Each arm is made up by a 3D network of dense muscle fibre and a central nerve chord, which allow the arm to move into virtually any direction and even create what is called “pseudo joints”. Octopuses use their arms to explore, mate, hunt for prey and crawl or walk. Each arm has a single or double row of circular suckers (depending on the species), which the octopus uses to examine their environment. While the outer sucker surface creates a seal, muscle contraction in the sucker base reduces the pressure in an inner chamber and creates suction. This allows the octopus to hold an object tightly in place. Octopus suckers have inspired applications in both medicine and robotics. In addition to helping the octopus understand what their surroundings feels like, scientists have found specialised cells in these suckers, which help the octopus to smell and taste their environment. This means an octopus acquires a lot more information from touching an object than simple texture. And if this wasn’t enough, octopuses are capable of regenerating their arms. Some octopuses even shed a few of their arms on purpose when hunted by a predator. These sacrificed arms are meant to distract the predator while the rest of the octopus escapes.

Other modern cephalopods (cuttlefish and squid) have one pair of tentacles in addition to their 8 arms. These tentacles are used for prey capture only and seem to have been lost in the octopus lineage. The evolution of these arm crowns is particularly interesting to scientists, as they are what is called a “morphological novelty”. This means, that no other close relative within the mollusks have similar structures. Studying these new features helps us understand the mechanisms by which new adaptations appear in evolutionary time. Interestingly, so far it looks like cephalopod arms are defined by very similar genes and gene pathways to human limbs during early embryonic development. This doesn’t mean that our limbs are similar to the ones of cephalopods. Instead, scientists believe that a shared genetic program exists that defines where a limb appears in a body, which direction it grows to and where its front and back, and left and right will be, respectively.

Scientists observed that their quiet sleep, during which the octopus skin remains mostly white, is interrupted by a very short active sleep phase (up to 1 min) like the human REM phase. During this time the octopus skin changes colour and texture, their eyes perform flicker movements, their suckers contract and muscles twitch on the body. In humans the REM phase is when we experience our most vivid dreams – what would octopuses dream about (if they do)?

Like all cephalopods octopuses use their arms for mating. In a process that can take several hours, the male octopus transfers sperm packets into the females’ mantle cavity. The female can mate with multiple males and stores their sperm until she is ready to spawn. Most octopuses only lay eggs once in their life. They attach their eggs in long strings to the ceiling or walls of their dens and look after them until they hatch. The female takes this job so seriously, that it stops feeding and never leaves their site. She constantly cleans and ventilates them until the babies are ready to hatch. The birth of their young sadly marks the end of their life - chemical signals in their brain induce a series of reactions that causes them to die. The young hatchlings are then left to fend for themselves. Depending on the species, octopuses either hatch as miniature versions of their adults and start life on the seafloor or go through a stage, where they drift in the open sea. There they feed on planktonic larvae until they are big enough to settle down to the bottom of the seafloor.

Most octopuses live in coastal marine waters and spend much of their time hiding in holes, crevices and dens. Octopus dens can often be identified by a pile of empty crab or clam shells outside their dens, both of which are among their favourite foods. Other things these predators like to eat are small fish, snails, shrimp and even other octopuses. They can use their specialised tongue to drill holes into crustaceans and snails and inject paralysing saliva into the animal. This relaxes the prey’s muscle and almost instantly kills them. Octopuses themselves are an important food source for moray eels, dolphins, albatrosses, sperm whales, seals and even humans. In fact, the global demand of cephalopods as food sources is rising with the decline of fish populations.

The exact number of the world’s octopus population is not entirely known (scientists are still discovering new species of octopuses), but they are generally not regarded as under threat. However, they are known to be sensitive to pollutants in the sea and have reported to accumulate large amounts of trace metals and minerals through their food. The effects of rising temperatures in the sea on cephalopods is not entirely clear. A recent study shows, that their bodies are actually well equipped to adapt to rising temperatures and ocean acidification for a short amount of time. Whether this is true in the long term and which effect these changes have on their young will have to be determined.

The two most common octopus species found at the UK coastline are the small curled octopus (Eledone cirrhosa) and the much larger common octopus (Octopus vulgaris). The curled octopus lives from shallow coastal water down to depths of 300m on a variety of seabed types, including soft to rocky bottom. The common octopus mostly inhabits rocky areas of the Southern and Western UK coastline. For both of these species seagrass meadows around the UK waters represent important hunting grounds, where they hunt for their favourite food – the crab. These seagrass meadows are important marine ecosystems and the home to thousands of animal species. Due to their large biodiversity, they play an important role in supplying the world fisheries and thereby support the livelihoods of many communities. Additionally, they absorb greenhouse gases (they are responsible for sequestering 11% of carbon buried in the ocean!), produce large amounts of oxygen, clean coastal waters (by filtering pathogens, bacteria and pollution), and stabilise the sediment, which plays an important role for flood regulation. However, they are in steady decline worldwide. Several projects around the UK are working on restoring this important ecosystem in order to bring back the biodiversity that is needed for a healthy ocean. This will also benefit our favourite animal – the octopus.

Cephalopods are remarkable animals that managed to push the limitations that were set by their molluskan ancestors and reinvented themselves in many new ways. They lived on this planet even before dinosaurs existed and hopefully will survive what is coming in the future. There is still a lot left to learn about their evolution, ecology, biology and behaviour today and that is why we will have to work hard to protect them.

by Marie-Therese Noedl