The Colossal Squid: Natures Largest Invertebrate

The Colossal Squid (Mesonychoteuthis hamiltoni) has been known to science since 1925 from the remains found within a sperm whales digestive tract in the Falkland Islands. Since then there has been very few whole specimens found to be larger than juveniles. However no complete adult specimens where discovered until 2003. The specimen in 2003 was captured on a long lining fishing vessel unintentionally. This specimen was dissected at Auckland’s University of Technology with Dr Steve O’Shea leading the examination. This specimen had a mantle length of 2.5m and a total length of 5.4m and even with a large amount of damage it still weighed about 300kg. After this examination in 2003 Dr O’Shea stated that this species could grow to be as large as 500kg, but this statement came with some doubt in the scientific community until 2007. This was when a 495kg specimen was caught again on a long lining fishing vessel. This specimen was frozen for 14 months to give planning time for the examination with the help of preserving alcoholic solutions which caused an estimated 22% shrinkage (mainly in the tentacles and arms) in the specimen due to the large amount of dehydration. This may be why the measurements of this specimen where smaller than the 2003 specimen even though it weighs so much more. The mantle measurement was 2.5m and the total length was 4.2m. This is the largest invertebrate by mass ever discovered, however very little about the behaviours of this animal is known, we cannot even draw a diagram of the internal organs yet.

The Body

The main part of the squid’s body is the mantle that encases all of its organs. This mantle is made up of muscle and skin, the specimen’s mantle examined in 2008 was 2.5 metres long and had a diameter of 98.2cm.  The mantle has a reddish-pink colour due to the pigment containing cells found in the skin called chromatophores. It is believed that the colour of the mantle changes as the muscle contracts but the reasons for this are just speculative, one theory is that it could be used to confuse predators (the only predator that the colossal squid has is the sperm whale, Physeter macrocephalus). Inside the top of the mantle is a rigid structure made up of chitin known as the ‘gladius’ which gives the mantle support.  The tail fin of the squid is made up purely of muscle. When compared to smaller squid species the colossal squid’s tail is relatively much larger. This is probably due to enable it to quickly swim forward in bursts to ambush its prey. There is a space between the organs and the mantle that allows water flow. This allows the oxygenated water to pass over the gills which allows

The Beak

Fig. 2 Close up of the colossal squid's beak surrounded by muscular tissue

The beak is to cut its prey into small chunks before it reaches its digestive tract. This is vital because its oesophagus passes through the centre of its donut shaped brain, so if food isn’t cut small enough brain damage could occur. The size of the beak can be used to estimate the size of the squid without any other data. The beak measurement used is the straight cutting section of the lower beak known as the lower rostral length (LDL). The 495kg specimen in 2008 had a LDL of 42.5mm. Whilst waiting for that specimen to thaw a much smaller 160kg specimen’s beak was measured at 40mm showing that small changes in a beak’s LDL constitutes in very large differences in squid size. This is put into perspective when the largest beak ever discovered had an LDL of 49mm (this was found inside a sperm whale) which means for all we know colossal squid could grow much larger than 500 kg.

The Arms and Tentacles

Fig. 3 Close up of rotating hooks on colossal squid's tentacles

Like the giant squid and other squid species the Mesonychoteuthis hamiltoni has eight arms andtwo tentacles. However unlike other squid species the twotentacles have a very useful physiological trait. The club-end shaped tentacles have rotating hooks. These hooks can rotate over 360 degrees and is believed to better clamp on to prey (its main prey is the Antarctic tooth fish (Dissostichus mawsoni)) It is also believed that these can be used in defence to fight off sperm whales (which may explain why sperm whales have so many cut like scars) . The arms also have many serrated suckers and hooks which help grapple prey which is not an unusual characteristic in other squid species.

The Eye

The colossal squid has the largest known eye in the entire animal kingdom and possibly the largest ever. The eye can span 20 to 30cm across and its lens is as large as an orange. It also has photophores which are light producing organs behind the eyes to act as torches to allow them to see in the very deep parts of the ocean with zero visibility. The eyes are located at the front of the head which is linked with being a predatory characteristic. This is because it gives a binocular vision which enables it to judge distances more effectively and therefore hunt prey with more deadly accuracy.

Fig. 4 Showing relative size of colossal squid

The Mystery Continues…….

A lot is still not known about Mesonychoteuthis hamiltoni likehow it reproduces and how a male adult differs in anatomy to the female as no large male specimens has been captured. It has never been filmed or photographed or seen in the wild so how it behaves is still very much sceptical. As well as the simplest questions like just how large can it grow? And how many are there? we still dont have answers.


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12 years ago

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