Written by Rhodri Irranca
“An old trick well done is far better than a new trick with no effect.”– Harry Houdini.
Gargantuan ocean-dwelling beasts have been depicted globally in folklore since the dawn of human existence, such as the kraken described by Pontoppidan in his mid-18th century work “The Natural History of Norway”. Subsequent works of fiction have formed around these creatures through the writing of authors such as Tolkien, and on the big screen through Disney and the like. Contrary to this however, most cephalopods spend most of their time avoiding getting eaten themselves.
It is widely believed that predator avoidance was a huge driving force in the evolution of cephalopods from other molluscs. Octopus, squid, cuttlefish, and nautilus species now display a massive variety of extreme physiological and behavioural survival strategies, with the power to leave you in awe…
Size: …it matters!
Large or small, some cephalopods utilise size as a survival adaptation. The largest invertebrate in the world is the giant Humboldt squid (Dosidicus gigas), and the sheer size of the organism is impressive in itself. The larger females can measure 5-6m in length just including their mantle (torso) and arms, and almost double this when the two long tentacles are also taken into consideration. This extreme size instantly limits the number of predators capable of taking on D. gigas, and it is therefore an effective mechanism for avoiding predation… until a sperm whale comes along.
In contrast, the aptly named pygmy squids that occupy the opposite end of the size spectrum have evolved small size and translucency as a method of predator avoidance. Females of the southern pygmy squid (Idiosepius notoides) typically measure between just 10-18mm in length, and males often half of that. This extreme small size coupled with their tendency to occupy dense seagrass meadows means that they are immensely difficult to spot for most predators.
Dermal cartilaginous tubercles: minature armoured tanks!
Glass squids are unusual amongst cephalopods in that they are armour plated. Cranchia scabra is characterised by a mantle possessing complex dermal tubercles (scales) similar to that of a puffer fish, forming a rough and rigid body rather than the soft features typically associated with cephalopods. The tubercles are formed subdermally (underneath the skin) from hyaline cartilage, and grow outwards until the points breach the skins surface.
These can range in size from small ovoid nodules of approximately 20 micrometres (μm) across to large branching structures resembling a three-dimensional Maltese cross (~300μm across). They possess approximately 30 of these structures for every 1mm2 of mantle, and would certainly not make a nice mouthful for a hungry, unassuming predator.
Autotomy: Talk to the hand…’cause I’m off!
Now, plenty of us have used the phrase “I’d give my right arm to…”. Yet very few of us actually have to live up to that claim. However, a deep-water squid called Octopoteuthis deletron takes this statement far too literally, and would undoubtedly complete the phrase with “…not get eaten”. These incredible cephalopods are often observed with irregular, blunt-ended arms, which highly indicates autotomy. Autotomy is the spontaneous self-amputation of extremities when seized by a predator. This species has evolved the capability to autotomise (sever) all eight of its limbs at a number of different places along each, to reduce tissue loss in a process termed ‘economy of autotomy’. This detached limb then displays ‘attack autotomy’, where the arm writhes and wriggles and given the opportunity, wraps around the predator allowing time to escape.
Hijacking: I am the captain now!
Many of our favourite stories are those of adventurous sea-farers embarking on terrible endeavours across distant oceans, however, the true pirates of the seven seas are the commandeering Argonautoidea superfamily. Included are four families that all display close associations with jellyfishes. These interactions typically involve shelter, camouflage, or transportation of the octopod, often observed as stowaways under the bells of jellyfish. Like true scallywags, species such as the paper nautilus/argonaut (Argonauta argo) hijack the mantle of a jellyfish species and feed on the mesoglea (gelatinous tissue of the bell), whilst secondarily benefiting from the protection provided by the long stinging tentacles. The seven-arm octopus (Haliphron atlanticus) of the same family, has however never been observed interacting with jellies for this same reason, but has been observed actively moving and manipulating the mauve stinger jellyfish (Pelagia noctiluca) to utilise its tentacles as a form of defensive “weapon stealing” – a fatal blubbery cutlass.
Anoxic tolerance: work smarter not harder!
The vampire squid (Vampyroteuthis infernalis) is a sinister looking cephalopod and the only extant member of its order. They exist off the Californian coast between depths of 600 and 900m, a region characterised by low dissolved oxygen (O2) levels. These oxygen minimum zones (OMZ) support very little life, however the vampire has the lowest metabolic rate of any cephalopod species and can therefore thrive here exploiting the lower predator numbers resulting from the anoxic (low-O2) conditions. Furthermore, due to the high affinity of V. infernalis’ blood for oxygen, when being pursued by a predator they head directly for the areas with the very lowest O2 concentrations, often causing the animal in pursuit to asphyxiate (die due to air deprivation), pass out, and either resuscitate in more oxygenated water or die.
Masters of evasion!
Cephalopods are found in every ocean on earth, in a diverse range of extreme environmental conditions, exposed to a large variety of extreme biological pressures. One of the most important decisions in a cephalopod’s day is how to respond to an imminent attack, in time to avoid predation. Physical adaptations and behaviours that allowed for faster, easier, and more successful predator avoidance were subsequently always going to be extreme as well.
From military-grade spiky armoured plating, to commandeering the defensive capabilities of jellies, to self-mutilation in the interest of self-preservation – cephalopods really have had to evolve down some pretty specialised routes, resulting in a diverse class of animals that respond to extreme threats with extreme responses.
Who needs science-fiction when the science-fact is even more astounding?