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The great Southern Ocean which swirls in a clockwise circle around Antarctica is home to many of the Earth’s largest animals. Blue whales come here to gorge on vast schools of krill. Among the icebergs and the towering waves, southern elephant seals (the largest member of the order Carnivora) fight duels to build their harems, and highly intelligent killer whales hunt together in pods. There are populations of sperm whales living in the Southern ocean as well and these leviathans dive to the cold floor of the world hunting for the world’s largest mollusk, a huge cephalopod which can only be found in the Southern Ocean. In fact this bizarre creature, the colossal squid (Mesonychoteuthis hamiltoni) is also the world’s largest invertebrate. Also known as the Antarctic squid or the giant cranch squid, the colossal squid lives in the abyssal depths. Unlike other squid, the colossal squid does not have tentacles–its powerful arms are studded with sharp hooks (much like the long-extinct belemnites). Some of these hooks swivel while others have three barbs in the manner of a fish spear.
The measurements of the colossal squid are staggering. Its eye alone (the largest of any known creature) measures 27 centimetres (11 in). A fully grown adult squid is estimated to be 12–15 metres (39–49ft) long. Although giant squid have longer tentacles, the colossal squid a long stout mantles and are thus much more massive. Their upper weight limits are unknown but are well over 500 kg (+1000 lbs).
The colossal squid is believed to be an ambush predator, which lurks in the depths waiting for chaetognatha, other squid, and benthic fish (such as the Patagonian toothfish) to pounce upon. It is hypothesized that they have a slow metabolism and do no need great reserves of food (unlike the energetic endothermic sperm whales which prey on them). The colossal squid are believed to be sexually dimorphic—the females become much larger than the males.
There is a reason that so much of this article is couched in ambivalent language such as “estimated”, “believed” and “probably”: colossal squid live in an environment where humankind can barely venture. The colossal squid are fast enough and clever enough to usually evade our nets, lines, and traps (although fishermen trying to catch Patagonian toothfish hooked a 450 kg (990 lb) specimen which was about 39 feet (13 m) long). Additionally our submarines and submersible robots are too slow and noticeable too stalk the squid in the abyssal depths. Other ocean creatures do not suffer from the same problem. Juvenile colossal squid are eaten by beaked whales, elephant seals, sharks, toothfish, and even albatrosses, however the adult squid are so large that only massive sleeper sharks and giant sperm whales can threaten them. Sperm whales are often covered with scars from their battles with the giants but the whales easily have the upper hand. Sperm whale stomachs have been found filled with hooks and beaks (which coincidentally were much larger than those found on the largest squid specimens recovered by humans to date).