Giant squid

The giant squid (genus: Architeuthis) is a deep-ocean dwelling squid in the family Architeuthidae, represented by as many as eight species. Giant squid can grow to a tremendous size: recent estimates put the maximum size at 13 metres (43 ft) for females and 10 metres (33 ft) for males from caudal fin to the tip of the two long tentacles (second only to the colossal squid at an estimated 14 metres (46 ft), one of the largest living organisms). The mantle is about 2 metres (6.6 ft) long (more for females, less for males), and the length of the squid excluding its tentacles is about 5 metres (16 ft). There have been claims of specimens measuring 20 metres (66 ft) or more, but no animals of such size have been scientifically documented.

On September 30, 2004, researchers from the National Science Museum of Japan and the Ogasawara Whale Watching Association took the first images of a live giant squid in its natural habitat. Several of the 556 photographs were released a year later. The same team successfully filmed a live adult giant squid for the first time on December 4, 2006.

Morphology and anatomy

Like all squid, a giant squid has a mantle (torso), eight arms, and two longer tentacles (the longest known tentacles of any cephalopod). The arms and tentacles account for much of the squid's great length, making giant squid much lighter than their chief predators, sperm whales. Scientifically documented specimens have masses of hundreds, rather than thousands, of kilograms.

The inside surfaces of the arms and tentacles are lined with hundreds of sub-spherical suction cups, 2 to 5 centimetres (0.79 to 2.0 in) in diameter, each mounted on a stalk. The circumference of these suckers is lined with sharp, finely serrated rings of chitin. The perforation of these teeth and the suction of the cups serve to attach the squid to its prey. It is common to find circular scars from the suckers on or close to the head of sperm whales that have attacked giant squid. Each arm and tentacle is divided into three regions — carpus ("wrist"), manus ("hand") and dactylus ("finger"). The carpus has a dense cluster of cups, in six or seven irregular, transverse rows. The manus is broader, close to the end of the arm, and has enlarged suckers in two medial rows. The dactylus is the tip. The bases of all the arms and tentacles are arranged in a circle surrounding the animal's single parrot-like beak, as in other cephalopods.

Giant squid have small fins at the rear of the mantle used for locomotion. Like other cephalopods, giant squid are propelled by jet — by pushing water through its mantle cavity through the funnel, in gentle, rhythmic pulses. They can also move quickly by expanding the cavity to fill it with water, then contracting muscles to jet water through the funnel. Giant squid breathe using two large gills inside the mantle cavity. The circulatory system is closed, which is a distinct characteristic of cephalopods. Like other squid, they contain dark ink used to deter predators.

Giant squid have a sophisticated nervous system and complex brain, attracting great interest from scientists. They also have the largest eyes of any living creature except perhaps colossal squid — over 30 centimetres (1 ft) in diameter. Large eyes can better detect light (including bioluminescent light), which is scarce in deep water. It is thought the giant squid cannot see colour, but they can probably discern small differences in tone, which is important in the low-light conditions of the deep ocean.

Giant squid and some other large squid species maintain neutral buoyancy in seawater through an ammonium chloride solution which flows throughout their body and is lighter than seawater. This differs from the method of flotation used by fish, which involves a gas-filled swim bladder. The solution tastes somewhat like salmiakki and makes giant squid unattractive for general human consumption.

Like all cephalopods, giant squid have organs called statocysts to sense their orientation and motion in water. The age of a giant squid can be determined by "growth rings" in the statocyst's "statolith", similar to determining the age of a tree by counting its rings. Much of what is known about giant squid age is based on estimates of the growth rings and from undigested beaks found in the stomachs of sperm whales.

Size

The giant squid is the second largest mollusc and the second largest of all extant invertebrates. It is only exceeded in size by the Colossal Squid, Mesonychoteuthis hamiltoni, which may have a mantle nearly twice as long. Several extinct cephalopods, such as the Cretaceous vampyromorphid Tusoteuthis, the Cretaceous coleoid Yezoteuthis, and the Ordovician nautiloid Cameroceras may have grown even larger.

Giant squid size, particularly total length, has often been misreported and exaggerated. Reports of specimens reaching and even exceeding 20 metres (66 ft) in length are widespread, but no animals approaching this size have been scientifically documented. According to giant squid expert Dr. Steve O'Shea, such lengths were likely achieved by greatly stretching the two tentacles like elastic bands.

Based on the examination of 130 specimens and of beaks found inside sperm whales, giant squid's mantles are not known to exceed 2.25 metres (7.4 ft) in length.[11] Including the head and arms, but excluding the tentacles, the length very rarely exceeds 5 metres (16 ft). Maximum total length, when measured relaxed post mortem, is estimated at 13 metres (43 ft) for females and 10 metres (33 ft) for males from caudal fin to the tip of the two long tentacles. Giant squid exhibit reverse sexual dimorphism. Maximum weight is estimated at 275 kilograms (610 lb) for females and 150 kilograms (330 lb) for males.