The Colossal squid (Mesonychoteuthis hamiltoni), the world's largest invertebrate, is anything but a ferocious predator, according to research to be published in an upcoming issue of the Journal of the Marine Biological Association of the United Kingdom.
The Colossal squid is known from the circumantarctic Southern Ocean, where it reaches an estimated maximum size of 12–14 metres in length (slightly larger than the giant squid, Architeuthis, which is estimated at a maximum size of 10–13 metres).
This poorly known inhabitant of the deep sea was previously thought to be an aggressive hunter like its cousin, the giant squid (this was supposed on the basis of the Colossal squid possessing the largest eyes and beaks of any cephalopod and the presence of large, predatory fishes such as the Antarctic toothfish, Dissostichus mawsoni, in their guts).
Because the squid is so rarely captured, let alone observed in the wild, authors Rui Rosa and Brad Seibel used data regarding the metabolic rates of other deep-sea squids (most notably in the family Cranchiidae, to which the Colossal squid also belongs), and extrapolated this data to scale it up to the Colossal squid's size. They also took into account the relatively low temperatures of the Southern Ocean by applying a co-efficient to their calculations.
The estimated metabolic rate was found to be considerably lower than that of giant squids and other actively hunting deep-sea squids.
The authors also projected the daily energy consumption of the Colossal squid, and found it to be 300–600 times lower than the daily energy requirements of other similar-sized top predators (baleen and toothed whales) of the Southern Ocean.
Based on their results, the authors estimate that the Colossal squid is able to survive on as little as 30 grams of Antarctic toothfish per day, and that a 5 kg toothfish would provide sufficient nourishment for a 500 kg colossal squid for approximately 200 days.
As the data indicates that the Colossal squid is not a voracious predator capable of chasing prey at high speeds, the authors hypothesise that the Colossal squid is an ambush or sit-and-float predator that uses the hooks on its arms and tentacles to ensnare prey that unwittingly approach.
The authors also speculate that low temperatures may be the main cause of the Colossal squid's gigantism.
For more information, see the paper: Rosa, R and BA Seibel (2010) Slow pace of life of the Antarctic colossal squid. Journal of the Marine Biological Association of the United Kingdom, doi:10.1017/S0025315409991494