Cephalopods have a last-ditch tactic for escaping predators – a sudden excretion of ink to confuse and evade. Karen Boswarva delves deeper
Reef octopus in the act of inking
Little squirts
Cephalopods have a last-ditch tactic for escaping predators – a sudden excretion of ink to confuse and evade. Karen Boswarva delves deeper
Have you ever just been finning along, enjoying your dive then suddenly you spot something in the corner of your mask, but by the time you( and your camera) turn towards it all you see is a puff of black smoke? If the answer is‘ yes’ then you my friend have been inked!
Ink is a rare phenomenon in the marine realm. Very few species are known to
Dutch wildlife artist Esther van Hulson used the reconstituted ink from a 95-million-year-old fossil octopus( Keuppia levante) to draw this portrait of the extinct species, displayed alongside the fossil at Oslo’ s Natural History Museum
OCEANSCIENCE
Bigfin reef squid
possess it. In fact, it’ s only a small subset of molluscs, the cephalopods and sea hares( Aplysia spp.) that do.
Let’ s start with the cephalopods, in particular squid, cuttlefish, and octopus. They all belong to a subclass of cephalopods called Coleoidea. All orders within Coleoidea( around 800 species) have something in common – they all have ink
sacs and produce ink. Granted, over time some species have lost them, these are typically a suborder of deep-sea dwelling and shallow water species called Cirrata that prefer the dark.
It’ s becoming apparent that cephalopods use their ink for a whole host of purposes. Ink is used as a defence mechanism in combination with‘ jetting’ off and changing colour. The clouds of ink act like a screen of smoke, which confuses would-be predators and enables a hasty escape. Cuttlefish coat their eggs in ink to act as a protective barrier against microbial infection. Also, the ink of some species is unpalatable to predatory fish.
The male cuttlefish of the species Sepia andreana uses ink during his elaborate courtship display. During the climactic moment he’ ll release a diffuse backdrop of ink, extend his sexually dimorphic arms, and whilst morphing his body pattern to a pale colour in front of the inky backdrop he’ ll proceeds to mate. Sounds quite a performance.
You may now be thinking what is this ink? Where does it come from?
Well, this is equally fascinating! Ink is composed of the secretions from two glands – the ink sack containing the ink gland, and a mucus gland called the funnel organ. The ink gland produces melanin, the pigment that gives ink its dark colour. We know it as the pigment that determines the colour of our skin, eyes, and hair, and protects us from UV radiation. An ink sac contains 1g of melanin, which constitutes 15 % of the total weight of ink; proteins make up an additional 5-8 %. Ink also contains high levels of cadmium copper and lead, and macromolecules called peptidoglycans, which have been studied for their medicinal cancer fighting qualities.
The release of ink and mucus is controlled by muscular walls and a pair of sphincters that are believed to be controlled independently via different neural pathways. This process enables the release of differing amounts of secretion from each gland, resulting in inks that take on different forms. Forms include
20