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Predators of Cephalopods<< Cephalopod Articles | By Steven Benjamins
When you watch an octopus moving across a reef in search of food, its colours and patterning changing as it moves from one surrounding to the next, feeling for crabs with its arms, you admire its capabilities as a predator. The same when you observe a cuttlefish stalking a shrimp, its tentacles poised to strike out at just the right moment. In both cases, these are active, ferocious predators. We have a soft spot for predators. When we see a predator chasing after an intended prey, many people will cheer the predator on. We want them to be successful. Few people are so enthusiastic when it comes to cephalopods becoming other animals' dinners (except, of course, if you happen to like calamari or related dishes). While you admire the octopus on the reef, a barracuda might suddenly swim into view and make off with the octopus clenched securely between its sharp teeth. Cuttlefish can fall prey to a whole host of bony fish and sharks, not to mention seals and dolphins. And oceanic squids of all size classes must brave the onslaught of pods of voracious sperm whales. One of the reasons cephalopods are so popular as food for so many creatures must be the ease with which they can digest them. Unlike most of their molluscan relatives such as snails and clams, nearly all cephalopods have lost their external shell and thus expose their protein-rich bodies to the elements without much protection.
This article is intended to give an overview of the different kinds of animals that routinely dine on cephalopods. Not for the squeamish.
Cephalopod habitats include the entire length, breadth and depth of the world's oceans, but at the same time exclude all bodies of fresh water, not to mention dry land (although several coastal species of octopus may be capable of surviving outside of water for a while, as many aquarists have discovered). Modern-day cephalopods are rarely, if ever, top predators in their environment. The external shell that protected their ancestors to some degree (and still serves as protection for Nautilus) has been incorporated internally or, in some cases (such as octopuses), totally done away with. Most modern-day cephalopods must therefore rely on other means to avoid being eaten. In general, they are hunted (and eaten) by predators that are fast-swimming and hunt by sight. This includes sharks, bony fishes, marine mammals and seabirds, as well as other cephalopods. Most of the cephalopods' defenses are therefore visual in nature. Many predators also employ other senses when hunting, such as smell or a lateral line system. Cephalopods have risen up to the challenge and have developed a lateral line analogue of their own, with which they can detect minute pressure waves in the surrounding water (Budelmann and Blechmann, 1988). The very first survival trait for cephalopods against any predator is: keeping a constant watch on their surroundings. Often the keen senses of cephs will enable them to notice an approaching predator long before it comes close enough to do real damage. Once they have spotted a predator, the first reaction of many bottom-living species such as cuttlefish and octopuses is 'trying to look like the surroundings' by changing their colour and even the texture of their skin. As most of you will know, some cephalopods are very good at this!
Squids that live in midwater have fewer options, but they also employ colour changes, for instance in countershading when the squid adjusts its colour to the ambient light environment. By doing this, the squid effectively takes on the same brightness as the surrounding water when seen from above or below. Many species of squid that live in deeper waters use bioluminescence to a accomplish the same goal. Histiotheuthis, for example, has a great number of light organs on its ventral (belly)side. These organs emit a weak blue light, which mimics the blue downwelling light from the surface and make the squid next to invisible for predators beneath it. This appears to be a common adaptation (Young, 1977). Of course, every so often a perceptive predator sees (or smells) its way through these defenses. The most common reaction for any cephalopod then is to make a swift break for it. This is usually achieved by squirting a jet of water through the funnel, with the result that the cephalopod rapidly disappears in the opposite direction. In addition to this, many cephalopods go through rapid colour changes while jetting, making it quite difficult (for humans at least) to keep a continuous eye on the animal. As if that were not enough, nearly all cephalopods (except Nautilus) possess some kind of inking capability. Inking seems to be used primarily to confuse a predator, either by blocking its view of the animal, or by acting as a decoy which is then attacked instead of the real animal. There may also be some more insidious use of ink as a 'chemical weapon' (see below).
One of the main reasons we know anything at all about the threats cephalopods face from other animals is because of their beaks. One of the few hard parts in cephalopods' bodies, their beaks are made of chitin, a substance which is remarkably resistant to the digestive juices in most animals' guts. It turns out that it is possible to identify remains of cephalopods down to the genus or even species level by studying details in the beak morphology. Cephalopods are swallowed whole by most predators, so that a detailed study of the beaks found in the stomach of a predator will give valuable information about the different genera or species of cephalopods that the predator preyed upon. The relative abundance of different kinds of beaks can then tell us something about those species' importance in the predators' diet. As we shall see, many animals are far more efficient in catching cephalopods than we are (Hanlon and Messenger, 1996).
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