By Judy Mann (SAAMBR Conservation Strategist)
With jet propulsion resulting in rapid bursts of speed and magical camouflage abilities, cephalopods make use of fascinating defence mechanisms that deserve a closer look. In keeping with the theme, Judy Mann highlights the unusual ability of cephalopods to eject ink to deter predators.
When we think of molluscs, we generally think of snails or slugs, of slimy animals with or without shells or of ocean delicacies like mussels and oysters or calamari. But, how often do we stop to think about these animals before they land up on our plates or disappear in front of us when diving?
This article unveils some of the more unusual members of the mollusc family – the octopus, the squid and the cuttlefish. In keeping with the black-and-white theme of this edition, we will look at how these animals defend themselves by using ink as well as other defences.
Octopuses, squid and cuttlefishes belong to a special group of molluscs called cephalopods. But, unlike many other molluscs, cephalopods do not have shells. This means that they need to use other ways to defend themselves from predators and this is why their ability to eject ink is so useful.
The ink is a chemical secretion which is produced by and released from a muscular bladder called the ink sac. It is expelled from this sac into the exhalent siphons or funnel. It is generally a deep blue-black in colour and contains, amongst other compounds, melanin and dopamine. When a cephalopod is frightened or being chased by a predator, it is able to release ink which forms a large greyish-black cloud in the water. This temporarily confuses the predator, giving the cephalopod a chance to make a quick getaway. The smokescreen does not disperse quickly as it contains a compound which becomes viscous in water.
Image by Bruna van Sean
There have been very few scientific studies on cephalopod ink, but it is known that the cephalopods can release ink at any depth and that there are different types of ink release, ranging from ink ropes and smokescreens to diffuse puffs and mantle fills. While most ink releases are used for defensive purposes, the ink puffs are also thought to be used as a form of intra-specific communication. Interestingly, one of the current research studies is investigating the role of one of the compounds in squid ink in cancer and other therapies, while other scientists are investigating the use of the melanin in squid ink to produce printer ink.
Another way for cephalopods to avoid being eaten is to hide and they are well known for their ability to camouflage themselves. They have chromatophores in their skin which are black, yellow, red and orange pigments. These pigments are contained in minute sacs, the opening and closing of which they can control. This means that the animal decides when to appear darker or lighter. Some species can even change the texture of their skin and this helps them to blend into their surroundings perfectly. Some squid have luminescent organs which enable them to glow in the dark depths of the ocean, possibly as a means of communication or to attract prey.
The cephalopod’s third defence mechanism is to quickly escape through jet propulsion. Like other molluscs, these animals breathe by using gills which are found in the mantle cavity. Water is pumped into and out of this gill cavity through a funnel. The mobile funnel can also be used to propel the animal through the water. This jet propulsion is very effective and can result in rapid bursts of speed.
Image by Els van den Borre
The cephalopods are thought to be some of the most intelligent of all invertebrates and have a well-developed nervous system. The eye of the octopus is complicated and they can see very well. The learning ability of the octopus is well known and this has been extensively studied. There have even been research studies on play behaviour in octopuses. Some octopuses have been known to learn how to remove food from a closed bottle and will often learn to recognise individual people. However, whether or not an octopus can predict a soccer result is a matter of opinion.
Cephalopods range in size from the tiny pygmy octopus, which only weighs 30g, to the giant pacific octopus which weighs in at a whopping 50kg. Giant squid have been recorded reaching over 150kg!