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The Mountains of Antarctica as seen from the Southern Ocean

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 Colossal Squid (Mesonychoteuthis hamiltoni)

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.

A rare specimen of colossal squid

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).

An artist's depiction of a belemnite

This blog has already traveled back 400+ million years to the Ordovician, the era when great mollusks ruled earth’s oceans. The Ordovician ended in ice as Gondwanaland drifted into the Southern Polar regions—a tectonic shift which brought massive terminal cooling to the great reef systems of the time (and also fundamentally changed Earth’s climate and atmosphere), but the cephalopods were hardly done for.  They continued to evolve and adapt to the world’s ever changing oceans.  Today we pick up the cephalopod story hundreds of millions of years later during the Mesozoic era—the time of dinosaurs.

Artist's depiction of a belemnite school

The reefs and oceans of the Jurassic and Cretaceous were filled with nautiloid cephalopods—ram shelled descendants of the shelled tentacled monstrosities  from the Ordovician—but a new cephalopod had also evolved and filled up the shallow limestone seas in giant teaming schools.  These were the belemnites which lacked an external shell and superficially resembled squid.

Belemnites were distinct from today’s squids for several reasons.  Not only did they possess hard internal shells/skeletons composed of calcium carbonate but they also lacked the pair of specialized hunting tentacles present in modern cuttlefish and squid.  Instead the belemnites hunted with ten arms covered in tiny wicked hooks.

An Amzaingly well preserved fossil belemnite from the 155 million year old Jurassic Solnhofen limestones in Germany (notice the details of the animal's soft anatomy)

Belemnites fed on ostracods, crustaceans, and fish.  In turn they made up a sizeable portion of diet for, well, the sizeable predators of the time.  Fossils of plesiosaurs, pliosaurs, and giant sharks have been discovered with stomachs full of hooks or rostra. It is also thought that the dolphin-like ichthyosaurs survived largely on belemnites.  After feeding and digesting the mollusks, the ichthyosaurs probably vomited out the indigestible hooks and rostra of belemnites much in the manner that sperm whales expel the hooks of giant squid!

The bullet shaped rostra of belemnites have survived in vast numbers and are one of the most characteristic of all Mesozoic fossils.  These strange tapered cones weathered out of soft chalks nearly intact and proved extremely puzzling to people of past generations.  Numerous magical common names and magical folk beliefs grew out of the conical rocks. The English called the fossils “thunderbolts” and believed they were the physical leftovers from lightning strikes.  The ancient Scandinavians thought that belemnite rostra were candles dropped by gnomes, elves, and dwarves on the occasions they traveled from their realms through this world.  The ancient Chinese called them “sword stones” and believed they were imbued with ancient healing magic.

An opalized fossil of a belemnite rostrum

The end of belemnites was even more astonishing than these myths.  The creatures had short lives—which involved a larval phase drifting amidst the microscopic plankton.  The immense extraterrestrial bolide which struck the Earth at the end of the Cretaceous ended the dinosaurs and also finished off the belemnites. The little larvae were unable to survive the massive planktonic die-off which accompanied the long dark winter following the strike.  Fortunately other cephalopods proved hardier–and the most intelligent mollusks continued to change and adapt right up until today.

An artist's depiction of the Chickzalub bolide impact

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