Cephalopod Predators: Part IV

<< Cephalopod Articles | By Steven Benjamins

Among mammals, most predators of cephalopods are obviously marine. However, they do not have to be restricted to the marine habitat. A number of species of mustelids (martens, stoats, badgers and their allies) regularly forage in marine habitats along the coast, and have been noted to hunt for benthic octopuses. Examples include the American Mink (Mustela vison) which has been known to eat young Octopus dofleini (Hartwick, 1983), the Cape Clawless otter (Aonyx capensis) from South Africa, preying on Octopus granulatus (Verwoerd, 1987) and the Eurasian otter (Lutralutra), which sometimes will take Eledone or Octopus (Beja, 1991). Sea otters (Enhydra lutris), which obviously are marine, will also prey on juvenile specimens of Octopus dofleini (Kenyon, 1975). Apparently they will go as far as to search for and forcibly open aluminum cans in which octopuses might be hiding (McCleneghan and Ames, 1976)! Almost all other (groups of) mammalian predators of cephalopods are exclusively marine. Of these, whales probably prey on cephalopods more than anybody else. Sperm whales (Physeter macrocephalus) in particular seem to feed on little else but squid (Clarke et al., 1998). In fact, when they were still hunted, sperm whale stomachs provided the easiest means of access to deepwater cephalopod material! They are capable of very deep dives (down to 1100 m for up to an hour) and evidently encounter a great number of edible species of cephalopods along the way. They are of course well known for their consumption of the largest squids of all, the members of the genus Architeuthis (Ellis, 1998). Although many fanciful accounts of such a struggle deep down have been published, it is unlikely that fights such as these are very even. Architeuthis is not very muscularly built for a squid of its size, and it might therefore not be able to put up much of a fight once it has been grabbed by the sperm whale. Exactly how sperm whales capture their prey is still a matter of some debate. They do not usually possess functional teeth in their upper jaw, and in some cases the lower jaw is set at an angle, bent or broken, presumably severely inhibiting its usefulness to hold onto something. Yet these individuals do not seem to be lacking in their diet. In recent years, the idea has been proposed that sperm whales actually 'stun' their prey by emitting 'sonic booms', perhaps by somehow using their large spermaceti organ in the front of their head as an amplifier. But nobody has ever been able to test this, although some attempts have been made.

Many other oceanic toothed whales also feed on cephalopods. Among these are the Pygmy and Dwarf sperm whales, short- and longfinned pilot whales, false killer whales, narwhals, nearly all known species of dolphins, and all but one of the enigmatic beaked whales of which little is known (e.g. Desportes and Mouritsen, 1988; Pauly et al., 1995; Santos and Haimovici, 1998). Stomach analysis of the latter invariably reveals a fondness for cephalopods—a food source to which these creatures have become well adapted; nearly all species have lost (most of) their functional teeth. Unfortunately, beaked whales are far less well known than sperm whales, so their energetic requirements are also still a mystery. Pilot whales and Risso's dolphins also seem to live solely off cephalopods. A school of Common Dolphins (Delphinus delphis) was observed while hunting Illex illecebrosus (Major, 1986). The dolphins proceeded by aligning themselves and swimming right into the school of squid, trying to break it up and to isolate individuals: a common tactic for vertebrate predators. Apart from this sighting, little is known about the specific methods of pursuit of cephalopods by whales and dolphins.

Although not as well know as sperm whales in this respect, several baleen whales also prey on cephalopods. Fin whales (Balaenoptera physalus) have been found with oceanic squids (genus Morotheuthis) in their stomachs (Clarke, 1983). Being fast swimmers, they might conceivably be capable of overtaking a school of squid and swallowing most of it in one gulp! (Other baleen whales might also prey on such squids, but I am not aware of any definite accounts. Anybody else?)

Different species of seals and sea lions also depend on cephalopods for their nutrition. Gonatus fabricii, a common squid in cold northern Atlantic waters, falls prey to hooded seals (Cystophora cristata; Nesis,1965). Remains of squids, such as the oceanic squid Taningiadanae have been found in the stomachs of Northern Elephant Seals (Mirounga angustirostris; Roper & Vecchione,1993); it has also been known to prey on benthic cephalopods such as deepwater octopuses. California sea lions (Zalophus californianus) hunt down schools of Loligo opalescens (an inshore species of squid). In Alaska, Octopus dofleini is preyed upon by both seals and sea lions. Even Hawaiian monk seals (Monachus schauinslandi) hunt for benthic octopuses during breeding season. They can constitute 25% of their diet during this period (DeLong et al., 1984).

Finally, humans can also be said to be cephalopod predators, at least from the cephalopods' point of view. In recent years, commercial fisheries for cephalopods have grown rapidly in many parts of the world, in some places even bringing in larger catches than 'conventional' fisheries operations for fish (Rathjen and Voss, 1983). The Japanese fishing fleet is well known in this respect (the total annual Japanese catch, mostly the squid Todarodes, over 1996 exceeded 559,000 metric tonnes! (Source: FAO)) Over the years, many different techniques of capturing cephalopods have been developed; most of these have been aimed at different types of squids. A complete description would probably be best served in a separate essay (does anybody out there feel like a contribution?), but examples include gill nets, trawl nets, and jigs (hook-like devices), the latter combined with bright lights. For some unknown reason, squids are strongly attracted to bright lights at night, when they are close to the surface. Whatever the reason, many fisheries make use of this odd habit, bringing in large quantities of squids each year. This fishing method has the added advantage that there is no bycatch of protected or otherwise unwanted species.

One last and very specialized form of predation includes the different kinds of parasites found on, or in, cephalopods. Although this is of course a very diverse assemblage of unrelated species, they have been grouped here for convenience's sake. Most of them have a wormlike appearance. Nematodes (known as roundworms) or their larval stages have been recovered from squids such as Loligo, Illex and Dosidicus. Cestodes (tapeworms) and their larvae have been found in Loligo opalescens, Illex illecebrosus, and Octopus cyanea. Trematodes (also known as flukes) have been shown to occur in squids of the genus Dosidicus (Nesis, 1983; O'Dor, 1983; Summers, 1983; Van Heukelem, 1983). Our knowledge in this field is still rather limited. For instance, we do not know whether these parasites actually harm their host to the point of dying, failure to reproduce, or some other way. In several cases (e.g. some of the cestodes) it is clear that the cephalopod serves only as an intermediate host, the final "goal" of the parasite being sharks or rays. Anybody out there have some more information?

Acknowledgments

Most of the material used for this essay was supplied by the inimitable Dr. James B. Wood, author/editor of the CephBase database and The Cephalopod Page! In addition, I would like to thank Dr. Neale Monks for his welcome advice and contributions on predation on extinct cephalopods.

References

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