The word "alien" usually evokes a quaint, quintessential mental image of a humanoid-like extraterrestrial with an over-sized head, anorexic body, and spindly limbs.
It's a fairly mundane vision, and doesn't hold a candle to the appearance of countless creatures that have roamed our own planet, both extinct and still living. Arguably, the most fascinating of these creatures is the octopus. There are a number of reasons to state this, but let's start by looking at the octopus body structure, which is rather unique (1).
The octopus circulatory system is something of a marvel, with three hearts, blue blood, and contractile vessels. There is a systemic heart, which pumps blood throughout the body, and two branchial hearts, which pump blood through a pair of gills. Rather than hemoglobin, which makes our blood red, the octopus utilizes the copper-rich protein hemocyanin to transport oxygen, which is superior to hemoglobin under cold, low-oxygen conditions, and makes its blood blue. Unlike our vessels, which remain relatively static while the heart contracts, much of the octopus venous system contracts, acting in concert as a singular, divergent pump.
Then there is the octopus respiratory system, which does more than just breathe, it also moves the animal, and does not rely solely on the gills to do so, but utilizes the skin as well. Using its respiratory muscles, the octopus can draw a large volume of water into its respiratory chamber, then eject it through a siphon which allows it to propel its body quickly through the depths of the ocean. Furthermore, although the octopus does rely on its gills to absorb most of its oxygen, it can directly absorb nearly half (40%) of its total oxygen requirements through its thin skin while it is in a state of rest.
Truly alien...the Giant Pacific octopus.
Beyond its cardiorespiratory systems, there is the octopus nervous system. Despite the fact that it shows the highest brain-to-body mass ratio of all invertebrates (not to mention many vertebrates), two-thirds of an octopus's neurons are located in its eight limbs such that each limb is neurologically autonomous, able to make complicated movements even when there is no input from the brain. Thus, the octopus nervous system is distributed throughout its brain and eight limbs. There is also an armada of sensory receptors that heighten what an octopus can detect in its environment - it has excellent eyesight, and uses statocysts to detect the orientation of its body in the water at all times. Moreover, although it has an outstanding sense of touch, each octopus limb contains chemoreceptors, meaning the octopus not only feels what it touches - it tastes it too.
Yet an elaborate body structure is just the beginning of the wonder that we call the octopus.
The octopus possesses a fascinating skill-set of behaviours that humans can only dream of, or perhaps portray in the form of fictional superhero characters.
Octopuses possess the power of regeneration. When one of its eight limbs is severed, it is replaced. When attacked, some octopuses can even perform limb autotomy, in which a limb is purposefully self-detached from the body in a sacrificial manner. The neurologically autonomous severed limb remains sensitive to stimuli and continues to move in a semi-purposeful fashion, distracting any potential predators away from the octopus itself.
Octopuses also possess the power of contortion. Most of their bodies consist of soft tissue, with no bones, which allows them to lengthen, contract, and contort themselves in seemingly impossible ways. Since an octopus has no joints, it can bend or rotate any part of its body in any direction. Even the larger species of octopus can alter their bodies to squeeze through tiny gaps as small as 2-3 cm (1 inch) in diameter; there are many incredible stories of octopuses escaping through a small hole or crack in a supposedly secure chamber.
Octopus contorting itself through a small opening (2).
Moreover, octopuses possess the power of camouflage. Their skin contains specialized cells that can be altered in colour, opacity, or reflectivity so as to imitate the features of the immediate environment. An octopus can make itself look like a collection of sand, a piece of algae, or just another rock - one well-described octopus behaviour is the moving rock trick whereby the creature makes itself look like a rock rolling along the ocean floor, matching its velocity to that of the water current, which permits it to move in plain sight of a potential predator.
Despite these wondrous behaviours, there is a species of octopus that takes it one step further still.
The Greatest Of All Mimics
The mimic octopus (Thaumoctopus mimicus) possesses an incredible, almost outrageous skill-set of abilities that confer upon it a unique status among all creatures, even amongst its fellow octopuses (3).
The mimic octopus can imitate not just its environment, but shape-shift itself to imitate other creatures as well. In fact, it can selectively imitate up to 15 other animal species; when faced with a particular situation, the mimic octopus selects the most advantageous species for the moment, and alters its appearance to "become" that animal, which allows it to completely deceive other creatures.
The variety of animals that may be imitated is somewhat astounding. For ease of transport, a mimic octopus can make itself look like a wandering flatfish. To ward off potential predators, a mimic octopus can alter itself to appear as a dangerous lion fish, holding its limbs out in a radial fashion to imitate the spines of the fish, or it may imitate a venomous sea snake, hiding six of its limbs while holding the remaining two limbs parallel to each other. When it is more relaxed, a mimic octopus can also make itself look like a jellyfish, inflating its body and trailing its arms behind it, or a sessile sponge, a harmless tube worm, or colonial tunicate. Furthermore, a mimic octopus can employ its shape-shifting abilities in an aggressive manner, such as altering its form to look like a crab, pretending to be a potential mate to another crab, which allows it to catch the creature off-guard - and consume it.
A mimic octopus pretending to be a flatfish (top), a lionfish (middle), and a sea snake (bottom) (4).