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Predators of Cephalopods: Part III<< Cephalopod Articles | By Steven BenjaminsIn many marine environments, bony fish are thought to be the most important predators to cephalopods. Almost all kinds of medium-sized to large predatory fish have been implicated. One such fish is the strange-looking Lancetfish Alepisaurus. This fish can reach up to 215 cm in length, with a maximum weight of only 4.5 kg. It dwells in deep waters far from shore and feeds on a variety of squids (Roper and Sweeney, 1975; Roper and Vecchione, 1993). The different species of both Pacific and Atlantic salmon spend most of their adult lives at sea; here they prey on many different kinds of cephalopods, including octopuses and squid (Nesis, 1998). Members of the cod family are especially abundant in the north Atlantic. Here, species such as cod (Gadus morhua), hake (Merlucciussp.), haddock (Phenacogrammus aeglefinus), as well as deepwater genadiers (Macrourus sp.) are known to prey on cephalopods. This includes sepiolids such as Rossia and Sepiola, octopuses (Eledone, Bathypolypus) and small to medium-sized pelagic squids (Illex) (Grieg, 1930; Macalaster, 1976; Nesis, 1998; Royer et al., 1998). Herring (like other filter feeders) do not usually directly target adult cephalopods for food, but will go after hatchling squids (or octopus paralarvae). Octopuses are particularly preyed upon by eels in all shapes and sizes. From conger eels in the cold waters of the north atlantic to moray eels and snake eels in the warm waters of the tropics, these fish make many an unsuspecting octopus meet an unfortunate end. The relationship between eels and octopuses has been known at least since Aristotle mentioned it in his work Historia Animalum in 330 BC. Octopuses are thought to be especially vulnerable to eels for a number of reasons: firstly because the eels can squeeze through holes almost as well as the octopuses can, and secondly because they appear to hunt mostly by smell, rather than sight. Against smell, all the intricate colour-changes of the octopus are in effective. One way to counter this threat is to foul up the detectors, and this seems to be one of the functions for the sometimes enormous amounts of ink that octopuses can produce. A number of experiments in the sixties by MacGinitie and MacGinitie (1968) pointed in this direction, when they put an octopus in a tank that already contained a moray. When the moray closed in for the first time, the octopus squirted a large cloud of ink directly in its face, at the same time jetting away. Subsequently, the moray was unable to locate the octopus, even when its head was practically on top of it! These results have been explained as evidence that some component in the ink temporarily 'blocks' the moray's sense of smell. Still, this aspect of cephalopod biology needs a lot of further investigation! Many of the large oceanic fish are important predators of cephalopods. For example, scombrids (a group of fast-swimming fish including mackerel and tuna) often make a living by hunting down schools of squid. Tuna are renowned for their swimming capabilities and these probably enable them to not only lunge at squid but to actually overtake them once they have jetted away. The same line of reasoning applies to the billfishes such as marlins and sailfishes. These fishes are known to consume large amounts of oceanic squids (such as Thysanoteuthis and Ommastrephes, both of which are strong swimmers), as well as Loligo closer to land, and the pelagic octopus Argonauta (Hanlon and Messenger, 1996). Swordfishes (Xiphias gladius) prey on a variety of cephalopods. Most of these are oceangoing squids such as Todarodes, Illex and Thysanoteuthis, but there is also a record of the bottom-dwelling Bathypolypus arcticus (Scott and Tibbo, 1968). In the case of the squids, the swordfish apparently slash their way through large schools, before turning round and swallowing the remains (Scott and Tibbo, 1968). Decapitated heads and slashed mantles have been found in the stomachs of swordfishes, and it has been assumed that this is what most swordfish actually feed upon. Perch-like fishes (Perciformes) are considered to be the most modern, or recently evolved, group of fishes. Many of the fishes found on and around coral reefs belong to this group, and many of these feed on cephalopods in one form or another. Threats vary from groupers preying on ovipositing (egg-laying) Sepioteuthis females in the Caribbean to barracudas and jacks hunting for octopuses in the Pacific (R. Hanlon, pers. comm., 1999, on O. cyanea), to Nautiluses being 'eaten out of their shell' by triggerfishes (Saunders et al., 1987). It starts when cephalopods are very young, when coral reef fish such as butterflyfish (Chaetodontidae) and Moorish Idols (Zanclidae) prey on cuttlefish eggs (Gutsal, 1989), and wrasses do the same for squid eggs in colder climes (personal observation). The list of perciform cephalopod predators probably includes all but the smallest and/or most ferociously herbivorous species. Reptiles are nowadays not an important group of marine vertebrates when numbers are concerned. The situation was very different, however, in the Mesozoic. For those of you who have seen the BBC-documentary Walking with Dinosaurs, remember Episode 4? All those gorgeous plesiosaurs, ichthyosaurs and pliosaurs? Many of those guys preyed on cephalopods, in the form of the hard-shelled ammonites and nautiloids, as well as the fast-swimming belemnites. Several species of ichthyosaurs fed on those; some specimens have been found with large amounts of belemnite hooks and beaks in their fossilized stomachs (Dr. N. Monks, pers. comm.). They probably performed the same predatory role as large whales do today. They had adapted to this diet by losing their teeth (see Ophthalmosaurus from the series). In some ammonite fossils there are marks of teeth from contemporary predators, giving direct evidence that cephalopods played an important part in their diet. The best known are the Placenticeras specimens from the Late Cretaceous of North America which have bite marks made by mosasaurs (Dr. N. Monks, pers. comm.). Nowadays the only documented case of a marine reptile preying on cephalopods seems to be a sea turtle which was housed in a public aquarium, and which at one point consumed a Nautilus with whom it shared its tank (Ward,1987). Many seabirds make a living by feeding on cephalopods. This is especially true in the Antarctic, where so many different birds are present. Penguins, for example, feed on a variety of squid genera such as Gonatus and Illex (see also Piatkowski and Pütz, 1994). King penguins have been reported to capture as many as 90 squids, each weighing 159-200 g, per dive (Kooyman et al., 1982). These birds, which can dive down to considerable depths, can forage both during day and night. Other birds, such as the members of the tubenose family (Procellariiformes; petrels, shearwaters and albatrosses) are often forced to forage at night when the squids they are after come closer to the surface (Clarke, 1983; Nigmatullin and Arkhipkin, 1998; Thompson, 1994). These birds are not well equipped for deep dives, but birds such as gannets are. They can dive-bomb from high in the air on top of the squids before the squids know what is happening (Montevecchi and Myers, 1997). Most of the cephalopods hunted by albatrosses and their relatives are bioluminescent; for example, the Great-winged Petrel (Pterodroma macroptera) derives 25% of its diet from the consumption of Spirulaspirula (Imber, 1973). Presumably the light organs make it easier for the birds to see the cephalopods. This poses some interesting questions on why Spirula retains the (upward-pointing) light organ in the first place, because there is evidently a strong evolutionary incentive to get rid of it ('lightless' Spirula would probably be a lot more difficult for a bird to spot). Exactly how this works on an evolutionary basis is as yet unclear, it can only serve as a testimony to our ignorance.... Although some birds will feed off remains left behind by sperm whales, or by scavenging dead squids, most of the squids found in their stomachs are 'self-caught'. Other cephalopod hunters are found among the auks of the Northern Hemisphere, in particular the Thick-billed Murre (Uria lomvia). 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