Squid (Mollusca)
By Seth Johnson

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(Figure 1)

An artist's depiction of a giant squid attacking a sperm whale
An artist's depiction of a giant squid attacking a sperm whale

(MDS) [1]

1. Classification/Diagnostic characteristics
Squids are in the kingdom animalia, the phylum cephalopods, and the order teuthida. Under that, the order breaks into subgroups such as the giant squid: architeuthis (this is a genus name, which is subsequently split into even more species). Simply put, squids are a diverse group of invertebrates with hundreds of different species all over the world. Squids can be identified by the following common characteristics:
  • Squids have three body components: a foot, a visceral mass, and a mantle.
  • Squids are cephalopods, meaning their “feet” have become tentacles topped with a head full of complex sensory organs.
  • The “visceral mass” contains their heart and their digestive, excretory, and reproductive organs.
  • The mantle is tissue covering the visceral mass. In squids, this mantle is modified to regulate water content by the use of excurrent siphons- tubes through which water flows.
  • Squids use a giant axon that can be up to one millimeter in diameter.
  • Squids have internal shells and ink sacs.
  • Squids have big eyes to find out the tiny amounts of sunlight in the sea to stay there for time being.(15)(NC)

Video of squid anatomy. (6)(BB-V)

2. Relationship to humans
From a phenological standpoint, squids are quite different than humans. In fact, they only share a common Kingdom: Animalia. Squids provide a unique opportunity for scientists to study the nature of neurons close up. Because they use a giant axon to send signals fast to their sensory “cortex,” humans can more easily study the biology of neurons and the nervous system.

Squid is also a common delicacy. As a result of this, a lot of money goes into funding for nets and boats to catch them. And humans need not be careful of squids; while their ink is deadly to sea creatures, it is too weak to harm humans. It is actually used as food coloring in most restaurants.(Shwetha) (18)

Squids have one similar relationship to humans which is between the squid and the human eyes. They are similar because the eyes of both work by refracting light through a lens to see, but squids are always colorblind. Squid's eyes are created from their skin during growth, while human eyes are derived as an outgrowth of our brain. Squids are most often eaten by humans as calamari. The popular calamari makes squids a great target for hunting. (19) (PS)

3. Habitat and niche
Different types of squid spend their time in almost every part of saltwater oceans. They are found in all seven oceans of the world.\

Squids tend to prefer salt water due to the fact that there are so many species found in the major oceans out there. Yet there are plenty of freshwater locations where they survive as well. All of the species of giant squid though are to be found in saltwater oceans only. They are also found very near the bottom of the ocean floor where they can be undetected most of the time." The North Atlantic is home to many more species than other habitat locations. Most of the time squid will be found at least 1,000 feet below the surface. The water is cooler there and they can be alone. It is the younger squid that tend to be at the surface as well as some of the different species. "(14) (MC)

Although they are found in all seven oceans of the world, more species live in the North Atlantic than any other habitat. The majority of the time, squids will below depths of 1,000 feet. This is because in these depths the water is cool and they can be alone. The squids that tend to closer to the surface are usually the younger squids. (8) (WSS)

Squid are able to adapt to a changing environment. If their food source moves, the squid will move with them. They move as their own habitat is destroyed, which is why it is believed squid have been able to survive on the earth for so long. It is believed most squid prefer salt water, but some squid are found in freshwater locations as well. (8) (MDS)

4. Predator avoidance
Squid’s large axons allow quick reactions in response to threats. In addition, the excurrent siphons in squids are tubes capable of controlling water content. This, in turn, allows squids to move quickly by jet propulsion.
Squid also have the ability to change color to match their environment. On their skin they have chromatophores which are turned on and off by signals from nerves and muscles. When they chromatophores are on, the squid can match their surroundings of any color. When they are off, the skin appears white. (LC) 4

Squids can also escape predators by releasing clouds of ink. When frightened by a predator, a squid may open their ink sacs, releasing ink that diffuses into a dark cloud in the water. These clouds are thought to mask the squid's escape or trick the predator into attacking the cloud of ink instead of the squid. However, a squid's ink may serve additional purposes that have not yet been identified. [2] (FZ)

Giant and Colossal squids have been known to be the prey of many sperm whales in the antarctic Although one of these conflicts has never actually been witnessed and a living Giant squid has only recently been caught on camera scientists have been able to determine that they occur by searching the contents of dead sperm whales stomach. If the squid is not capable of avoiding the whale it is capable of fighting back with small razor sharp appendages on its suckers and with its enormous beak. It is not normally capable to killing the whale but is believed to be able to harm the whale enough to escape.(16) (DA)

5. Nutrient acquisition
Because of their enhanced mobility, squids are viable predators. They capture prey with tentacles. They have large eyes that are adept at sensing even the smallest images under water in low light conditions. The sensory head is closely linked to the tentacles used to trap prey. Squids and other cephalopods also have a radula, which is a feeding structure modified for scraping algae from rocks.

6. Reproduction and life cycle
Squids reproduce sexually and can produce thousands of eggs at once. Most cephalopods have extensive courtships, involving males that change colors in order to attract the females.

Squids are a part of the r-strategist group, meaning they have high reproduction rates, development rates and mortality rates. Because of the high mortality rates, the average lifespan of a squid is one to two years, in which the squid generally mates only one time. After the mating session, a female squid may lay up to thousands of eggs. Only a fraction of those will survive until the hatching stage. The baby squids survive on their own because their mother leaves her eggs upon laying them in and dies shortly afterwards.[3] (AY)

Male squid have special long arms called spermatophores which hold "packets" of sperm which attach themselves to or penetrate a female squid's mantle during the squid's 15 second mating session. Some squid prefer to carry their eggs in "clutches" to guard them rather than laying them. (9, 10) (MDS)

7. Growth and development
Like all multicellular organisms, squids follow normal development paths starting from a zygote and developing into a mature animal.
Squid eggs which are released in 11 lb groups of about 100,000 eggs hatch in about 2 weeks. The squid hatchlings fully mature in about 3-5 years, however some species of squid have been known to not live longer than 12-18 months. The larvae that hatch go through three stages of life, the larval, juvenile and finally the squid stage. (9) (MDS)

8. Integument
Squids are one of the few types of cephalopods (a subgroup of mollusks) that retain an inner shell for protection. This system is enclosed within the mantle.
A cross sectional diagram of a squid
A cross sectional diagram of a squid


9. Movement
Squid’s mantles are modified to regulate water content by the use of excurrent siphons- tubes through which water flows. This, in turn, allows squids to move quickly by jet propulsion. The “feet” of a squid have been modified into arms and tentacles, also used in movement.
Squids swim at about 23 mph by both pulsing water out of the mantle cavity and through the funnel, and by using the paired fins at the rear of the mantle. They are not as fast as their predators and therefore must use a variety of protection ( turing colors to blend in and spraying ink) to survive. Pumping of water through the mantle to move is also the way they breathe. [4] (SM)

The mantle cavity is a significant part of the anatomy of mollusks: it is the dorsal body wall which covers the visceral mass and usually extends in the form of flaps beyond the visceral mass. When the mantle is contracted, these spaces closed. Water then is ejected forcibly through a siphon, propelling the squid through the water. Many squids are powerful swimmers, capable of capturing fish and other aquatic animals with ease. (HSC)

10. Sensing the environment
The squid “head” contains complex sensory organs. For example, they have eyes that are adept at sensing images under water. Moreover, the squid uses a giant axon along with other neurons to transport electrical signals to the sensory cortex.

The squid has arguably the most developed nervous system of all the invertebrates. The squid can focus each eye separately, sending nerve impulses through an optic nerve to the brain where the information is processed.
The squid nervous system also has a feature that allows it to detect and orient itself in the gravitational field. The structured that allow this are called statocytes, which contain a chalky fluid. Statocytes come in pairs and are embedded in the brain of the squid, sending signals telling the squid that its orientation in 3D space is. (11) (AA)

11. Gas exchange
Almost al mollusks have gills in their mantle cavity used for gas exchange. Cilia on the gills beat, which creates a current. There are many blood vessels on the gill tissue, which take up oxygen from the passing water and release carbon dioxide.
Unlike other mollusks, squids have no countercurrent system. The water flow in squids is caused by the constant filling and emptying of the mantle cavity that occurs during jet propulsion movement. Gas exchange occurs during the slow filling of the mantle, rather than on the rapid emptying.[5] (TM)

12. Waste removal
Aquatic invertebrates such as squid are ammonoletic, meaning that do not have to convert ammonia in order to get rid of it. Ammonia is a nitrogenous base, and is soluble in water so relatively easy for cephalopods to excrete.

Squid have a muscular stomach found in the midpoint of the visceral mass. At this point, the bolus, a mass of food, moves into the caecum, which is a long white organ for digestion. The food then moves into the liver for absorption. After, the solid waste is passed out of the rectum. (5) (JLau)

13. Environmental physiology (temperature, water and salt regulation)
The excurrent siphons in the squid’s mantle help control the water content of the creature by expelling water through the tube-like siphons. This also allows the squid to quickly regulate salt content.

14. Internal circulation
All mollusks have blood vessels that do not form a closed circulatory system. Instead, blood and other bodily fluids end up in a sac called a hemocoel, in which fluid move around and deliver oxygen to the squid’s internal organs. Afterward, the fluids are taken up by the blood vessels. Just like in humans, blood that is in the blood vessels is moves by the heart.

Squid have a system of three hearts. These consist of two branchial hearts which pumps blood to and from the gills and one systemic heart which pumps blood around the rest of the body. The systemic heart is made of three chambers, a lower ventricle and two lower auricles. The heart system is surrounded by the renal sacs which are the most important part of the squids excretory system. The hearts have a faint green appearance. (13) (BH)

15. Chemical control (i.e. endocrine system)
Female squids release pheromones (chemical signals) on the eggs after they are laid, which attract male squids. As soon as male squids come in contact with the surface of the egg, a chemical signal is triggered which causes them to become extremely aggressive, probably as a means to protect any fertile females and go through sexual selection. (17) (DM)

Review Questions

1. How do squids change color? What are two instances in which squids will change their color? (SJ)
2. How are squids helpful in nervous system research? (E.S.S.)
3. What do the internal circulatory systems of humans and squids have in common? How are they different? (JLev)
4. Describe the life cycle of the squid. Consider reproduction and growth. (AWC)
5. What is unique about squids in relation to other Mollusca? (BS)
6. Why do squid prefer to live at depths of 1000 ft?

Works Cited:

Original Page from:
1. Hillis, David M., David Sadava, H. C. Heller, and Mary V. Price. Principles of Life. Sunderland: Sinauer Associates, 2012. Print.
2. Figure 1: Sadava, Heller, and Price. "Animal Origins and Diversity." Principles of Life. By Hillis. Sunderland: Sinauer Associates, 2012. 472. Web.

Edits Include:
3. http://www.squid-world.com/squid-reproduction.html
4. http://gilly.stanford.edu/neuroscience.html
5. "Bolus (digestion)." Wikipedia. Wikimedia Foundation, 12 Apr. 2012. Web. 13 Dec. 2012. <http://en.wikipedia.org/wiki/Bolus_(digestion)>.
6. http://www.youtube.com/watch?v=OueQ9kU36i0
7. __http://tolweb.org/treehouses/?treehouse_id=4225__
8. http://www.squid-world.com/squid-habitat.html
9. http://www.ehow.com/facts_5813188_life-cycle-squid.html
10. http://tolweb.org/treehouses/?treehouse_id=4225
11. http://w3.shorecrest.org/~Lisa_Peck/MarineBio/syllabus/ch7invertebrates/Invertwp/inv_class_of_06_wp/mark_squid/nervous.html
13. http://marine-francine.blogspot.com/2007/11/squid.html
14. http://www.squid-world.com/squid-habitat.html
16. http://www-v1.amnh.org/exhibitions/permanent/ocean/01_dioramas/n_spermwhale.php
17. http://news.discovery.com/animals/squid-aggression-pheromone-mating-110210.html
18. http://www.squid-world.com/squids-and-humans.html
19. http://www.squid-world.com/squids-and-humans.html
  1. ^ http://bluewaterjon.blogspot.com/2011/09/swimming-with-humboldt-squid-photos-and.html
  2. ^ http://www.ucmp.berkeley.edu/science/profiles/bush_0704.php
  3. ^ http://cronodon.com/BioTech/Cepahlopod.html
  4. ^

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