By Jody Bourton
Earth News reporter
Not so fierce after all?
The world's largest invertebrate is not a fast and voracious predator as previously thought, say scientists.
The colossal squid, a creature once linked to maritime myth and feared as a sea monster, is really a slow drifting animal that ambushes unwitting prey.
That conclusion was reached by studying the physiology and feeding habits of other deep sea species and scaling up to the colossal squid's huge size.
Rarely seen, the elusive ocean giant is thought to reach up to 15m long.
Researchers reveal their findings in the Journal of the Marine Biological Association of the United Kingdom.
The colossal squid is not a voracious predator capable of high-speed predator-prey interactions
Dr Rui Rosa, University of Lisbon, Lisbon, Portugal
The colossal squid (Mesonychoteuthis hamiltoni) is thought to roam in the deep waters of the Southern Ocean.
Rarely seen alive or dead, little is known about its way of life.
Estimates vary, but it is thought to measure 15m in length with a large dome-shaped mantle.
It also has long tentacles and arms containing swivelling hooks used to grapple and maul prey.
A recent specimen studied had eyes approximately 27cm (11in) across, believed to be the largest animal eye yet discovered.
Its large size and predatory nature fuelled the ancient myth of the underwater "kraken" seamonster and modern speculation that the colossal squid must be aggressive and fast, attributes that allow it to prey on fish and even give sperm whales a hard time.
Yet as the creature is seldom encountered let alone studied, there are no direct measurements of the colossal squid's behaviour.
So instead, the team used a set of routine metabolic rates for other deep-sea squid species and extrapolated the data to match the colossal squid's size.
They also factored into their calculations the cold temperature of the Southern Ocean the squid inhabits.
"The understanding of the mode of life of the colossal squid is a significant event in the ongoing investigation of enigmatic large animals of the ocean," says Dr Rui Rosa from the University of Lisbon in Portugal, who undertook the study with Dr Brad Seibel from the University of Rhode Island, Kingston, US.
The colossal squid grows heavier and probably longer than the giant squid
"In this paper we present the first estimates of the metabolic and energetic demands of this cold-water deep-sea giant."
The researchers calculations reveal the colossal squid survives on only a small amount of prey.
"Our findings demonstrate that the colossal squid has a daily energy consumption 300-fold to 600-fold lower than those of other similar-sized top predators of the Southern Ocean, such as baleen and toothed whales," says Dr Rosa.
Recent studies have previously identified chemical variants known as isotopes in colossal squid specimens which have revealed large fish, notably Antarctic toothfish as its main prey.
This study reveals a single 5kg Antarctic toothfish would provide enough nourishment for a 500kg colossal squid to survive for 200 days.
It is thought the cold temperature in which the squid lives affects its metabolic rate.
That reduces the amount of prey it must take in relative to animals such as whales that live higher in the water and generate their own body heat.
The result is the colossal squid is not the creature once thought.
Swiveling hooks on a tentacle of the colossal squid
"The colossal squid is not a voracious predator capable of high-speed predator-prey interactions," says Dr Rosa.
"It is rather, an ambush or sit-and-float predator that uses the hooks on its arms and tentacles to ensnare prey that unwittingly approach."
The team also suggest that the colossal squid's huge eyes do not help it hunt, but instead help it detect and avoid predators of its own in the deep dark waters where it lives.