The mysterious deep ocean has always been a fertile source of tales of fearsome giant animals. Some of the innumerable accounts of seafarers' experiences with strange, gigantic sea creatures are undoubtedly based on misinterpretation of things such as tree trunks, whales, shallow-living sharks and other large fish swimming or floating at the surface. In this session we will learn that other accounts are certainly founded on truth, because real deep-sea giants do exist.

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Not a sea-serpent but a relatively small specimen of an oarfish proudly held by members of staff of the Natural History Museum, London, in the 1970s. With its elongated body and snake-like movements in the water, an oarfish could easily be mistaken for a sea serpent of mythical proportions.

	Octopuses
	Apart from the ten-armed squids, the Cepahalopoda also include the eight-armed Octopoda or octopuses. Most octopuses live on the sea floor but one unusual deep sea group, the cirrate octopods, spend much of their time in mid-water. Their name comes from the small outgrowths on their arms, called cirri, which are thought to be sense organs for detecting other animals in their pitch black world. The cirrates have a pair of fins which stick out and presumably help them to swim. Several of them have their eight arms connected by thin webs. With their arms outstretched and these webs forming an umbrella-like structure they can remain suspended in the water for long periods. Small crustaceans and other planktonic animals swim into their outstretched arms and become enmeshed in a sticky mucus. Cirrate octopods do not have good vision but recently, one species, Stauroteuthis syrtensis, was found to emit blue-green bioluminescence from the suckers on its arms. It seems likely that some similar species may do the same. Whether the octopus can see their own light or not, it may be important in attracting prey animals. In contrast to the visually impaired cirrates, one deep sea octopus has relatively enormous eyes and a remarkable array of light organs all over its body. Vampyroteuthis infernalis (meaning 'the vampire squid from hell') grows to no more than about 20cm long and therefore probably feeds on fairly small animals in the mesopelagic zone (400-1,000m deep) where it lives. Although the oxygen levels are low here, Vampyroteuthis is very efficient at removing it from the water, so it can get around quite rapidly in search of food or to escape a predator. It also has an extra pair of 'arms' tucked into pockets outside the ring of main arms. These are unique structures which are extended one at a time into long, fine filaments, rather like fishing lines, to detect prey which are then grabbed by the other arms. So Vampyroteuthis seems to hover taxonomically, somewhere between the eight-armed octopods and the ten-armed squids, yet another strange mystery from the depths of the ocean.

The oarfish, or king of the herrings, Regalecus glesne, is an obvious candidate for sea-serpent sightings. With a flattened, snakelike body up to almost 10 metres long, a bright-red dorsal fin and crest of long fin-rays on the top of its head, this strange and harmless fish is seen occasionally at the surface and may even be washed on to the shore. Because it is encountered so rarely, very little is known about the oarfish's life style, though its stomach contents have revealed that it feeds mainly on planktonic animals. Because it is so large, it probably needs to live where its food is abundant, so it is likely that it does not live deeper than a few hundred metres.

Squid
Many myths of monsters must be based on sightings of squid, particularly the giant squid Architeuthis dux, which lives deep in the ocean.

Squid are members of the mollusc phylum and, along with the octopuses, belong to a group called the Cephalopoda, meaning 'head-footed', because in place of the flat 'foot' on which a snail crawls, cephalopods have a ring of long arms surrounding the mouth. A squid's mouth has a pair of powerful horny jaws forming a beak. They are mainly fast-swimming, mid-water animals whose ten arms--and in particular two extra-long ones, the tentacles--are used to grab hold of food organisms and sometimes of one another.

Squid bodies are generally cigar- or torpedo-shaped, often with a pair of fins at the tail end, away from the mouth and arms. Instead of an external shell like snails or mussels, squid bodies are stiffened by an internal skeletal rod, the pen, made of fairly pliable material more like cartilage than bone. They swim by jet-propulsion, sucking water into the mantle cavity, a space near the mouth, and shooting it out through a tube called the siphon by strong contractions of the mantle's muscular wall. By altering the direction of the outlet of the siphon they can move either tail first or arms and mouth first, as they must do when catching food.

There are several hundred different squid species in the oceans, ranging in size from 2cm in length to the largest animals without backbones on Earth. Many of the shallow-water forms are tough-bodied, fierce, fast-moving hunters ranging up to half a metre or so in length. Most deep-sea squid, on the other hand, are at the small end of the size range and have rather soft and jelly-like bodies. Rather than actively hunting, they probably mostly wait for suitable food animals to bump into them. There are, however, a number of big deep-sea squid, which, although probably not as fast as their shallow-water cousins, make up for this in their awesome size. The biggest of all, up to at least 15m long from the tip of the tentacles to the end of the tail and weighing a tonne or so, is Architeuthis dux, the giant squid.

Because it lives in the deep sea, and can presumably move out of the way of slow-moving nets, Architeuthis is hardly ever caught by scientists or fishermen. Most of what we know about the species has been gleaned either from its remains in the stomachs of sperm whales or from dead or dying specimens floating at the surface or washed up on to beaches. As a result, the anatomy of Architeuthis is very well known but almost everything written about the behaviour of giant squids is guesswork. It is not known exactly where Architeuthis lives, except that it is presumably at fairly deep levels. It has the largest eyes of any animal on Earth, up to about 30 centimetres across, the size of a dinner plate. This suggests that it hunts its food where the natural light level is very low, perhaps in the lower part of the mesopelagic zone. It is possible that the eyes are particularly good at seeing bioluminescence, in which case they could live in much deeper and darker waters, although unlike many other squid, Architeuthis does not produce any light itself.

So what does a monster squid feed on? It seems likely that the giant squid is not an active predator, but hovers in mid-water waiting for some unsuspecting potential food, maybe another squid or a large fish, to swim within reach of its tentacles. A big Architeuthis may need 50 kilogrammes or more of food each day, so it certainly qualifies as a major predator of the deep sea, but not as the diabolical killer and attacker of ships and sailors that mythology would suggest.

Deep sea 'Jaws'

Natural History Museum

Giant squid harpooned by the crew of the French corvette Alecton off the Canary Islands in 1861.

The biggest known deep-sea fish is the Greenland shark, Somniosus microcephalus, which grows to over 7m in length and has been photographed at a depth of 2,200m. But this shark is not exclusively a deep-sea species. It also occurs close to the surface, where, because of its eating habits--they are attracted to offal thrown overboard from fishing boats--it is also known as the sleeper shark.

It seems unlikely that in the food-poor abyss a variety of powerful predators would be able to survive but it would be a mistake to assume that we already know all the deep-sea giants. As recently as 1976, American scientists working in the Pacific hauled aboard a shark 4.5m long, which was quite different from any other known shark family. It was named Megachasma pelagios meaning 'swimming big-mouth', partly because it had swallowed a large cargo chute that had been lowered from the ship as a sea anchor. Although the sea was about 1,500m deep where Megachasma was caught, it had been swimming at only about 150m down. Nevertheless, the fact that such a large and relatively shallow-living fish could remain totally unknown until so recently suggests that the deep ocean still holds many surprises for us.