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= Ink Sac = An ink sac is an anatomical feature that is found in many cephalopod mollusks used to produce the defensive cephalopod ink. With the exception of nocturnal and very deep water cephalopods, all coeloids (squid, octopus and cuttlefish) which dwell in light conditions have an ink sac, which can be used to expel a cloud of dark ink in order to confuse predators.[1]

The ink sac is a muscular bag which originated as an extension of the hind gut; it is a modified hypobranchial gland.[2] It lies beneath the gut and opens into the anus, into which its contents – almost pure melanin – can be squirted; its proximity to the base of the funnel means that the ink can be distributed by ejected water as the cephalopod uses its jet propulsion.[1] The ejected cloud of melanin is bound by mucus particles, so it forms a lump approximately the size and shape of the cephalopod, fixing the predator's attention while the mollusc itself makes a hasty escape.[1]

A general level of provocation is necessary to trigger an octopus to release its ink, as it is biologically costly to produce. Some species can even use their ink to stun or numb their predators.[citation needed]

Functions
The ink sac is a defensive mechanism to deter predators. Defense can include jet repulsion, camouflage, and release of chemicals. The ink released by Cephalopods is rich in melanin, which gives it its dark colour, and is stored in the ink sac lumen. The release of ink is controlled by the muscular walls and sphincters of the hindgut near the anus.

There are many ways the ink can be secreted, it can be secreted as ink clouds, blobs, and/or ink ropes. The ink cloud releases minimal mucus and acts as a smoky screen that blocks out the vision of their predators. The release of ink from the hindgut allows the cephalopod to swim in the opposite direction, giving it time to hide and/or escape. Blobs of ink contains mucus that can mimic objects. This is known as pseudomorphs because the ink released resembles the appearance of a cephalopod. The releasing of a blob of ink gives the cephalopod time to escape. Ink released in the form of ink ropes is also a pseudomorph as it resembles elongated siphonophores, mimicking stinging animals.

Types of Animals
Many Cephalopods, a class of Mollusca, that live in the dark, low-light marine conditions have ink sacs. These ink sacs are produced and present at birth. There are 2 subclasses within the cephalopoda class: Coleoidea and Nautiloidea. All orders of the Coleoidea have ink sacs that produce ink, while Nautiloidea members do not.

Evolution of the Ink Sac
The development of the ink sac is an evolutionary advantage and disadvantage. The ink sac serves as a defense mechanism to block the vision of their predators or confuse them. The presence or absent of an ink sac maybe beneficial depending on the habitat the cephalopod lives in. Ink sac loss is associated with depth. It is a disadvantage for cephalopods to have an ink sac when they live in the dark waters as it displays no use and requires energy to maintain it. The use of the ink sac was to deter predators for many cephalopods in light conditions.

Ink Collection
Different methods of ink collection are performed to analyze the components in the ink.

One method of collection includes the usage of a syringe without any milking or squeezing of the ink sac. The collection of ink from the freshly killed cephalopod using a syringe yielded a mixture containing DOPA, dopamine, and taurine. There is very minimal manipulation to the ink sac in the syringe method, thus to allow a natural release of ink.

Milking is another method performed on a cephalopod within 24 hours after its death. The mixture will be collected by taking out the ink sac and severing the ink duct. This method will include a composition of damaged tissue along with the collected ink. The result of milking yielded all the chemicals above (DOPA, dopamine, and taurine), in addition to tyrosinase and epinephrine. The release of tyrosinase and epinephrine was due to disruption of the ink gland that would not be normally released by the cephalopod.

Collection of ink from the sea water immediately after an animal secreted ink will be very dilute and contaminated with other chemicals from the water. This method allows the animals to freely behave without any sort of human manipulation.

Homogenizing the ink sacs is another method of collection. The collection of the ink sac mixture yields ink, tissues of ink sac, and ink gland. This method is commonly used for collecting ink for human uses.

An advantage of collecting ink using the syringe method over the milking method is that it prevents tissue damage. It also prevents other chemicals released from the damaged tissue into the ink.

Evolution of Melanin
The oxidation of tyrosine yields melanin. Melanin is a pigment that gives skin, hair and eyes their colour. The production of melanin occurs inside Cephalopoda ink gland. The melanin gives cephalopods pigment in their skins and is also a photoprotective. Melanin in the skin of cephalopods can function as camouflage and has a role as a visual cue. Over evolutionary time, melanin evolved as a photoprotectant to a defensive mechanism in cephalopods.