Predators of Cephalopods: Part II

<< Cephalopod Articles | By Steven Benjamins

First of all, in most cephalopods' lives, there is a threat from other cephalopods. Large cephalopods frequently make a living by hunting for small ones. An extreme example of this was found in a specimen of the giant squid Architeuthis, which apparently had eaten an Eledone, a small benthic octopus (Lordan et al., 1998). Size differences between predator and prey are not usually this large, but many oceanic squids (including Architeuthis (Nixon, 1983))depend for their nourishment upon other squids. Most of them have no qualms at all when hunting smaller individuals of their own species! Cannibalism is fairly common among cephalopods. It occurs in all groups, from Nautilus to cuttlefish and octopuses. Females of the latter (for instance, Octopus briareus) have been known to prey on unwary males trying to mate with them (Hanlon, 1983). In captivity this behaviour regularly occurs in cases where too many octopuses are kept under crowded conditions (a good example is, again, Octopus briareus; some other species of octopus, such as Octopus bimaculoides, are far more tolerant of crowding (Hanlon and Forsythe, 1985)). Cuttlefishes in captivity show tolerance for each other only when they have been raised together. Most squids are only found in schools of roughly equal-sized individuals. An obvious exception to this is the Caribbean reef squid Sepioteuthis sepioidea, where schools consist of animals of varying sizes (Moynihan and Rodaniche, 1982).

A very specific type of cephalopod predation occurs in several Indo-Pacific octopuses. Around the world, octopuses hunt for snails by drilling a small hole in their shell and injecting a poison. Only in the Indo-Pacific, however, do the octopuses get the opportunity to hunt Nautiluses in the same way. Many Nautilus shells have a little hole in the foremost chamber, which would be occupied by the living animal, and where poison thus would be most effective (Saunders et al., 1991).

Other invertebrates are only a limited threat to cephalopods. When they are still safely inside their egg casing, few animals will touch them. For instance, even though the egg cases produced by Loligo opalescens off the coast of California can number in the tens of thousands at specific egg-laying sites, hardly anybody seems to make use of these tasty morsels. Only the starfish Patiria sp., and possibly the sea urchin Lytechinus sp., have been observed to feed on the eggs (MacGinitie and MacGinitie, 1949; Kato and Harwick, 1975). Of course, once the eggs hatch, the majority of squidlets (or planktonic octopus paralarvae, for that matter) will fall prey to a host of creatures specializing in feeding off plankton, such as jellyfish. In the fossil record, there is also some evidence of cephalopod predation; some ammonite fossils from the late Cretaceous show healed fractures in the shell that resemble cuts made by crustaceans, such as crabs and lobsters (Dr. N. Monks, pers. comm).

One of the first animals to prey on cephalopods, the sharks have had nearly 400 million years to perfect their body design. From humble beginnings, they arose to become one of the winning models of evolution; the body shape of a generalized shark is perfectly constructed for hunting. Sharks are top predators in many marine environments, and most species will eat cephalopods if given a chance. One of the most notorious is the Blue Shark (Prionace glauca). This is an oceanic species that is known to hunt for squids. The annual mating aggregations of squid species such as Loligo plei off the coast of California are a powerful attraction to these sharks. They swim directly into the enormous masses of mating squid, for whom safety might indeed lie in numbers. Nonetheless, many squids die because of the sharks. As they migrate closer to shore in other parts of the world, Blue Sharks also get the opportunity to prey on species living on the ocean floor or close to land such as Sepia (cuttlefish), Rossia (the bobtail squid), the small squid Alloteuthis and the octopus Eledone, to name just a few (Clarke and Stevens,1974).

Other sharks have less wide-ranging habits, particularly those that live near the bottom in the deepsea. Nonetheless, they succeed in preying on large numbers of cephalopods. One of these deepwater sharks (Centroscymnus) is known to take around 15% of its food intake as squids. It also occasionally consumes octopuses and oceanic sepiolids (Heteroteuthis)(Yano and Tanaka, 1984). Other deepwater sharks have been reported to prey on Nautilus (Ward, 1983).

Benthic sharks such as catsharks and dogfishes hunt for octopuses, cuttlefishes and sepiolids (see also Bergstrom and Summers, 1983, on Sepietta), but neither will pass up a squid or two when they can get them. A particularly interesting tactic is used by the pyjama shark (Poroderma africanum), which lives near the bottom in the waters around South Africa. It searches out groups of spawning squids Loligo vulgaris reynaudii and then hides itself between the egg masses. As soon as a female approaches to lay her eggs, the shark lunges forward and tries to grab her (Smale et al., 1995). Rays are also dangerous predators, primarily for cuttlefish in shallow waters (Sanchez and Villaneuva, 1991). Filter-feeders such as whale sharks and manta rays probably feed on planktonic larvae and paralarvae of different cephalopod species, although presumably they are not targeted specifically.

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