Giant Squid - (Architeuthis dux)
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All squid have more than one way to get around. One way is to cruise at slow speeds using 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, then other muscles contract, forcing water out through the funnel, which causes the mantle to return to its original size.
“Squid expand the mantle by contracting sets of muscles”??? 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 skin. The mantle then behaves like a syringe, with water drawn or, or more accurately, pushed in.
The jet of water closes the flaps around 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.
The other way squid move is with great bursts of speed and acceleration. They do this by using the muscles in the mantle to make dashes through the water. Basically, they suck more water into the mantle and jet it out more forcefully. For squid, this is like running a 100-meter dash. They can swim at full speed for a short distance before they are worn out. They usually use these quick bursts to evade a speedy predator such as a tuna, shark, or wahoo. Some squid may even leave the water and glide above the surface like flying fish when fast predators chase them. Unfortunately for the squid, this makes them available to squid-eating birds!
By examining the anatomy of giant squid specimens and comparing it to better-known species, scientists can determine how giant squid probably move. It seems that Architeuthis has less well-developed musculature for bursts of speed than some shallow-water squid known to be fast. Nonetheless, the giant squid obviously swims fast enough to be a survivor in its natural deep-sea habitat.
Growth and reproduction
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 reach adult size. On the contrary, cephalopods actually grow very 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. Evidence from statoliths (solid calcium carbonate granules) found in the statocyst, the organ responsible for equilibrium (balance and stability), suggests that giant squid live no more than five years. This means that the growth rate of giant squid is extremely rapid. It is one thing to be a fast-growing squid that reaches maturity at 12 inches (30 cm), and quite another to grow to nearly 60 feet (18 m) within a few years! To grow at such a rate, giant squid have to be in areas where there is an abundant supply of food; they would need to eat enormous quantities on a continuous basis. They must be feeding machines!
Unlike most fish, squid have a different strategy for reproduction. In association with a short life span, giant squid reproduce once, releasing millions of eggs, then die.
Scientists do not know whether giant squid live in groups, or what the size and makeup of those groups might be. They may be solitary hunters. How Architeuthis individuals interact with each other is unknown. This is one of the many aspects of giant squid behavior that intrigues people and is an inspiring factor in the desire to continue to study them.
Only single specimens of Architeuthis are caught in fishing nets at any one time. Furthermore, so few juveniles have ever been captured that we have very little information about growth or social habits. The individual squids observed by the Japanese scientists in 2004 and 2006 were alone. However, others may have been nearby but unseen in the blackness of the deep sea. Whether or not giant squid are solitary, gregarious, or occur in schools only as juveniles, are questions yet to be answered.
Every aspect of Architeuthis anatomy and physiology is a form of adaptation. Foremost is the animal’s size. Is there any reason why Architeuthis grows so much larger than other squid? Perhaps the most obvious 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 and large sharks.
Animal size is associated with the abundance and nutritional qualities of its food items. Larger animals must eat more food than smaller ones. As a rule, the rate of metabolism decreases as animals increase in size. This means that, per kilogram, larger animals eat less than smaller ones, even though they must eat more by quantity.
Architeuthis have the largest eyes in the animal kingdom—at up to 10 inches (25 cm) in diameter; they are the size of a human head! There are vertebrates larger than Architeuthis, but none has eyes of comparable size. 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 that both groups of animals see in a similar way, but their visual systems (eyes) evolved separately in each group. If we compare human eyes to squid eyes, we find remarkable similarity in the anatomy. Squid and people both have eyes with single lenses, pupils, irises, and retinas. We can only imagine the light gathering capabilities of the giant squid’s enormous retina!
Far longer than the arms, the tentacles seem impossibly long to manage with enough accuracy to snag prey at distances over 33 feet (10 m). They may use quick bursts of speed like other squid, or they may sneak up on their prey with stealthy movements. Without direct observations on feeding behavior, scientists assume that they behave like other squid. The still photographs taken in the sea off Japan showed a giant squid attacking a bag of bait in a similar pattern known for shallow-water squid. There is still much to be learned about these fascinating deep-sea giants!
For more information about the giant squid specimens in the Sant Ocean Hall, see: http://www.mnh.si.edu/exhibits/ocean_hall/squid.html
Web and multimedia support courtesy: Dana Dolan
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