Colossal squid —
the inside story

We don't know a lot about the internal anatomy of the colossal squid — the inside story is still being worked out. At present no one can draw an accurate diagram of the layout of the insides of the colossal squid.

The scientists wanted to keep the colossal squid intact to put on display, so they could not dissect it to look at the internal organs. However they were able to dissect a smaller, damaged colossal squid. This, and use their knowledge of the internal anatomy of other squid, tells us something about the squid's internal anatomy.

The team also used an endoscope to look inside the thawed colossal squid. The endoscope was inserted into the eyeball, giving the animal-vision scientists a good look at the largest animal eye in the world.

When the endoscope was inserted into the mantle, the team could get a close-up view of the eggs — confirming that the specimen is a female, and that the eggs are immature.


Squid use oxygen from seawater for respiration. The seawater enters the mantle through the opening near the head, and passes over the gills. Oxygen diffuses from the water into the blood, and is transported to the gill (or branchial) hearts by a network of many blood vessels.

The colossal squid has two large gills, each with 20 to 80 gill filaments on either side, which hang down into the mantle. In the dissected smaller colossal squid the gills are striped with lines of dark pigment.


Squid have three hearts: two branchial hearts and one systemic heart.

The branchial hearts pump blood to the gills, where oxygen is taken up. Blood then flows to the systemic heart, where it is pumped to the rest of the body. The systemic heart is made of three chambers: a lower ventricle and two upper auricles.


Squid blood is blue, not red as in humans. This is because squid blood contains a copper-containing compound called haemocyanin. In humans the blood is red and contains the iron compound haemoglobin.


The oesophagus of the colossal squid leads from the beak into the stomach and caecum, where food is digested. It's only about 10 mm in diameter and passes through the middle of the squid's doughnut-shaped brain. The colossal squid has to cut up food into small chunks so it can pass through the narrow oesophagus.

Digestive system — stomach and caecum

The squid's stomach is a small, shiny white sac that connects to the stomach pouch or caecum. Digestion of food begins in the stomach. The caecum also performs some digestion and is the primary site of absorption of nutrients. Enzymes from the liver and pancreas help digestion.

Waste passes into the intestine, a narrow tube adjacent to the stomach pouch, then empties into the rectum. The end of the digestive system is the anus, from which waste exits into the funnel.


Squid and octopus have an intricate nervous system, more complex than other molluscs, and invertebrates in general.

The squid brain is enclosed in a cartilaginous head capsule and includes two large optic lobes. These indicate that vision is very important to squid. Up to 80 per cent of the brain is devoted to processing visual information.

The brain is shaped liked a doughnut and surrounds the narrow oesophagus. It is very small in comparison with the overall size of the body — a 300 kilogramme colossal squid has a brain weighing less than 100 grammes!


Squid can tell how they are positioned in the water. This information is provided by two statoliths located within the squid's brain. Each statolith is a small calcareous structure which sits within a chamber called a statocyst. As the little bone moves around within the chamber according to gravity, the squid can work out which way is up, in the dark.

Ink sac

All squid have a sac of ink inside the mantle. The ink is a dark liquid and is expelled through the funnel. If the squid meets a predator, it shoots out a cloud of ink, which hides the squid so it can escape.

No one has ever seen a colossal squid producing ink so we can't be sure what the ink looks like or how the squid uses it. As there is no light down at 1,000 metres in the ocean, dark ink would be useless! It is possible that the colossal squid has luminescent ink.

Gladius, or pen

All molluscs have a shell. The colossal squid has an internal shell called the gladius. The gladius is a rigid internal structure that supports the squid's body and runs through the upper part of the mantle, between the paired tail fin. It is made of chitin — a tough, protective, and semi-transparent substance, which is primarily a nitrogen-containing polysaccharide. The gladius is easy to remove when dissecting a squid, and looks like a long piece of plastic.

Image 01
Chris Paulin holds the gladius removed from the smaller colossal squid.
Image 02
Members of the squid team examine the partially-dissected smaller colossal squid while the large specimen in the tank (to the right) thaws.
Image 03
Dissection of a smaller colossal squid reveals one of the pigmented leaf-like gills, lying alongside the oviduct and the darker brown stomach.
Image 04
The delicate, leaf-like gill of the squid has a large surface area, which helps absorption of oxygen as water
flows over the gill.
Image 05
While the large squid thaws, dissection of a smaller colossal squid reveals part of the female reproductive organs, to the left of the striped gill.
Image 06
Dr Tsunemi Kubodera and Dr Kat Bolstad examine the internal organs of a smaller colossal squid, as they wait for the large squid to thaw.
Image 07
While the team wait for the large colossal squid to thaw, they dissect a giant squid and open up the stomach.
Image 08
While the team wait for the large colossal squid to thaw they dissect a giant squid. The small doughnut-shape brain surrounds the narrow oesphagus.
Image 09
The team dissect a giant squid while they wait for the large colossal squid to thaw. The greyish organ to the right of the centre is one of the branchial hearts, lying alongside the white ovarian tissue.
Image 10
A dissected arrow squid, showing the ink sac inside the mantle.
Image 11
Ink from a dissected arrow squid can be used to write with!
Image 12
A juvenile colossal squid showing ink released inside the mantle.

Ross Sea, 2008
Photograph by Darren Stevens. Courtesy of NIWA

Specimens and data collected and made available through the NZ International Polar Year-Census of Antarctic Marine Life Project.
Image 13
Cross-section through the statolith of a giant squid (Architeuthis dux). The rings are formed each day so counting the rings gives an estimate of the age of the squid in days.

Image courtesy of Discovery Channel
Discovery Channel report: O'Shea et al. (2000).
- a study of giant squid biology, development and age in New Zealand waters.
Image 14
A statolith of a giant squid (Architeuthis dux). It is about 2.3mm long.

Image courtesy of Discovery Channel
Discovery Channel report: O'Shea et al. (2000).
- a study of giant squid biology, development and age in New Zealand waters.
Dr Steve O'Shea points out the gills of the smaller, dissected colossal squid. He explains that the dark colour is pigment, not blood, and that squid blood is blue.
Dr Steve O'Shea pulls back the mantle of the smaller, dissected colossal squid specimen and reveals an 'enormous, exquisite gill'.
Dr Steve O'Shea dissects a giant squid (Architeuthis dux) and shows how the oesophagus (throat) passes through the brain.
Dr Steve O'Shea and Dr Tsunemi Kubodera find the oesophagus (throat) of the smaller, dissected colossal squid specimen, 'with something in it'.
Dr Steve O'Shea and Dr Tsunemi Kubodera examine the colossal squid in the tank. Dr Kubodera finds the caecum within the mantle and puts his hand through it.