Sea Base Alpha: Giant Squid

Whether living or extinct, on land or at sea, in literature or in life, large animals have long fascinated people. The largest animals have been known and hunted since prehistory: whales, walruses, elephants, rhinos, hippos, giraffes, and large fishes. In fact, much of the large species of fauna went extinct in Australia, New Guinea, Asia, Madagascar and the Americas, quite possibly due to overhunting or climate change.

However, one large animal has gone almost unnoticed or certainly unobserved in its habitat. That animal is the giant squid. Although these animals have been found in the nets of commercial fishermen, in the stomachs of sperm whales, and washed ashore on different continents, no scientific information has been gathered by direct observations of live giant squid.

Giant squid (Architeuthis) are the largest invertebrates on Earth, reaching lengths of 20 m, and are one of the biggest predators surviving in the deep sea. But if they are so big, why don't we know more about giant squid? Since they are so deep, how do we know anything at all? Search for Giant Squid begins with the myths that surround this most elusive creature, then takes us through the steady accumulation of knowledge that brings us to our present understanding of these magnificent cephalopods. Dr. Clyde Roper is on a quest to find these giants and record their behavior by direct observation.

Whether floating at sea or washed ashore, reports of dying or dead giant squid appear in documented records reaching back to the 16th century. No doubt these fabulous creatures have been scattered around the world's beaches for as long as giant squid have existed. In 1857, Jappetus Steenstrup, an eminent Danish biologist, pieced together from stories, ancient drawings, and one beak the fact that these kraken were nothing more or less than squid, giant squid. Steenstrup gave these giant squid the scientific name, Architeuthis, Greek for ruling squid. Professor Addison E. Verrill of Yale University was able to confirm Steenstrup's assignment by examining two kraken that ended up as flotsam along the Newfoundland coast in 1873.

More than 100 Architeuthis individuals have since been found on beaches and coastlines in both hemispheres of the world ocean. This number of specimens, though substantial, has not contributed nearly as much detailed information as scientists would like. Foremost is the fact that many of those specimens were destroyed and never preserved. Once dead, giant squid decompose rapidly. They are frequently fed upon by other marine organisms. Their eyes, skin and internal organs usually are the first to disappear. Tentacles and arms are commonly missing parts. Stomachs are nearly always empty, and when remnants of food are found they are so fragmented that the identities of the prey species are difficult to detect.

Sperm whales stranded on beaches and caught by whaling ships bear circular scars inflicted by the powerful suckers of giant squid. The size of the scars has led some to estimate the size of squid eaten by the whales. Scars as large as 20 cm were reported and some believed it would take a 75 meter long squid to bear such a sucker! Scientists later noticed no fresh scars were in excess of about 5 cm, and certainly no giant squid examined have suckers larger than 5.2 cm.

Perhaps people's imaginations increased the size of the sucker scars. Or it may be that the scars expand as sperm whales grow. Whales also get infected with a species of ringworm, a fungus that grows in a circular pattern, so it is possible for ringworm scars to have been mistaken for old sucker scars.

The current scientific conclusion is that adult giant squid suckers are between 2-5 cm in diameter, whether on the arms or tentacles. There are no credible reports of larger suckers.

The general areas where giant squid occur and a very rough idea of their abundance can be estimated from the stomach contents of sperm whales. Soft tissues of squid are quickly digested by enzymes in the large mammals' stomachs. But the squid's hard beaks, which are made from a carbohydrate known as chitin, prove indigestible. Sperm whales have a way of ridding themselves of the thousands of pointy squid beaks that accumulate in their digestive tracts. They coat the beaks with a slick, waxy substance known as ambergris, then pass the mass of beaks and ambergris through the digestive tract.

As commercial fishing operations drop nets to ever greater depths, they more frequently bring up giant squid. These specimens generally are in better shape since the entrapped squid are likely to be healthy specimens, freshly caught, though dead or in the throes of death from trauma. While many of these giants are not saved for examination by scientists, some of those that are have enough remains in their stomachs that certain species of fish or other squid can be identified. During the 1990s, the greatest numbers of trawled squid have come from the deep waters of New Zealand.

Although giant squid are equipped with eight arms up to to 3 meters in length, likely none of them are used to grab prey. The two much longer tentacles actually make the catch. Tentacles may reach a length of 10-12 meters. At the business end, away from the head, the tentacles expand to resemble clubs. Both arms and tentacular clubs are equipped with suckers, which are sub-spherical cups lined with sharp, finely serrated rings of chitin. Suckers do exactly what you think they do: they use powerful suction to hold onto prey while the rings of chitin sink into the hapless animal. The arms have suckers arranged in rows from the base near the mantle to the tips, whereas the tentacles are smooth from the base to near the club where there are small suckers

Suckers cover only the inner surface of the arms and tentacles, with two rows on the arms and four on the clubs. Although giant squid tentacles are very, very long compared to the arms and mantle, and even proportionately longer than the tentacles of most other squid, they have the same function, snatching prey at a distance. In the case of the giant squid, that may be a snatch more than 10 meters (30 feet) from the animal's eye!

Squid have more than one way to get around. One way is to cruise at slow speeds using the paired fins at the rear of the mantle, combined with gentle, rhythmic pulses of water pushed out of the mantle cavity through the funnel. Squid expand the mantle cavity by contracting sets of muscles within the mantle, water fills the expanded space, the muscles relax, and the elastic mantle then snaps back to a smaller size, jetting water out through the funnel. The jet of water closes the flaps on either side of the squid's head so water can exit only through the funnel. This rhythmic flow of water is also the way in which squid breathe. As water passes in and out, the gills are refreshed with oxygen.

But let's back up. We said "squid expand the mantle by contracting sets of muscles." But how is it possible to make something bigger by making something (muscles) shorter? Muscles get shorter when they contract. Squid mantles are thick. Muscles within the mantle actually compress the thickness without collapsing the outer skin. The mantle then behaves like a syringe, and water is drawn or, more accurately, pushed in.

The second way squid move is with great bursts of speed and acceleration, which they do by recruiting other muscles to make dashes through the water. Essentially, they get more water into the mantle and they push it out more forcefully. For squid, this is like running the 100 m dash; they can swim at full speed only for short periods of time before they get worn out. But then that is usually all the time they need to evade a predator such as a tuna, shark or wahoo. Some squid may even leave the water and glide like flying fish when they are being chased by fast predators. Unfortunately for squid, this makes them available to squid-eating birds!

Examining how other squid move helps us to understand how giant squid probably move. Are the fins relatively large or small? Do the muscles of the mantle appear strong or weak? Are the internal flaps that close in order to direct water to the funnel efficient and tight-locking? With each of these features, it seems that Architeuthis has less well-developed anatomy for bursts of speed than squid that we know to be fast.

How long does it take to become a giant? Like people, large mammals such as elephants, whales, and even gorillas take many years to grow large, mature, and reproduce. Most larger fish also require many years to develop to adult size. So should we assume a 35-60 foot long giant squid is as old as Methuselah? On the contrary, everything we know about cephalopods tells us they grow quickly and die after a short life. Short lives and high rates of reproduction are their hallmarks. The giant octopus, a sizeable animal at 1/20th the weight of a mature giant squid, lives just 2-3 years. From evidence of statoliths, pea-sized concretions in the organ responsible for equilibrium, we can tell giant squid also live to be no more than five years of age.

This means that the growth rate of giant squid is phenomenally rapid. It's one thing to be a fast-growing squid that reaches maturity at 30 centimeters, and quite another to grow from a few millimeters to nearly 20 meters within a few years! If you have the genes to grow at such a rate, you need lots and lots of food. Not only would giant squid have to be in areas where there is an abundant supply of food, they would need to eat large quantities, regularly. They must be feeding machines! Unlike most fish, squid have a very limited opportunity to reproduce. If things go wrong one year, such as poor health, the wrong environmental conditions, or even not finding a mate, squid cannot wait until the following year to reproduce. Compensating for a short life-span, squid release massive quantities of eggs.

Since scientists have never observed giant squid alive in their natural habitat, we cannot say for sure whether they live in groups, or what the size and makeup of those groups might be. They may in fact be solitary hunters. There is no way to know for sure how Architeuthis individuals interact with each other. This is one of the aspects of giant squid behavior that intrigues people, and is certainly a motivating factor for seeing them alive for the first time in the deep sea.

The Architeuthis that have been caught in fishing nets are always singles; two or more giant squid have never been caught together. In addtion, so few juvenile giant squid have ever been recovered that we have precious little information on their social habits. Whether or not Architeuthis are solitary animals, gregarious, or school only as juveniles are questions yet to be answered.

We can think of nearly every aspect of Architeuthis anatomy and physiology in terms of adaptations. Foremost is the animal's size. Is there a reason why Architeuthis dux grows so much larger than other squid? Animal size generally is closely related to the size and abundance of food items as well as the nutritional value of the food eaten. Larger animals must eat more food than smaller ones. As a rule, the rate of metabolism decreases as animals increase in size. What this means is that per kilogram, larger animals eat less than smaller ones, even though they must eat more food. We call this an inverse relationship.

The cost of being large is the greater quantity of food required to sustain an animal. For some animals, one benefit to being large is the ability to feed on bigger items of food. Still, it is not yet clear how the monstrous size of Architeuthis is an adaptation to a specific diet. Does it feed primarily on large, not giant, squid? Does it feed on fish in a size range that is too large for other squid? A trip into the realm of Architeuthis may unravel the answers to these questions. Another very important aspect of large body size and fast growth rate is that giant squid are able to outgrow rapidly the large number of predators to which they are initially exposed. Even at half-size, giant squid will have outgrown all potential predators except sperm whales.

Architeuthis is equipped with the largest eyes in the animal kingdom. A number of vertebrate animals are larger than Architeuthis, yet none have eyes of comparable size. At 25 cm in diameter, the eyes of Architeuthis are the size of a human head! Cephalopods are visual animals and are distinguished among invertebrates for their highly complex visual system.

The visual systems of cephalopods and vertebrates are an example of convergent evolution. This means both groups of animals see alike, but their ability to see evolved separately in each group. In fact, if we compare squid eyes to our own, we find remarkable similarity in the anatomy. Squid and people both have eyes with single lenses, pupils, irises and retinas. We can only guess at the light sensing capabilities of an eye with such an enormous retina.

Far longer than the arms, the tentacles seem impossibly long to manage with enough accuracy to snag prey at distances in excess of 10 meters. Are we to assume they are like other squid, propelling sucker-laden tentacles with quick bursts of speed, not unlike the tongue flick of a frog? Or do they use them with stealth, sneaking the tentacles within close range of their prey? Without any direct observations of their feeding, it is best to assume they behave like other squid by snatching prey with quick bursts of tentacular speed.