Ink sac
Encyclopedia
With the exception of nocturnal and very deep water cephalopod
s, all coeloids which dwell in light conditions have an ink sac, which can be used to expel a cloud of dark ink
to confuse predators. This sac is a muscular bag which originated as an extension of the hind gut. 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. 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.
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.
Cephalopod
A cephalopod is any member of the molluscan class Cephalopoda . These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, and a set of arms or tentacles modified from the primitive molluscan foot...
s, all coeloids which dwell in light conditions have an ink sac, which can be used to expel a cloud of dark ink
Cephalopod ink
Cephalopod ink is a dark pigment released into water by most species of cephalopod, usually as an escape mechanism. All cephalopods, with the exception of the Nautilidae and the species of octopus belonging to the suborder Cirrina, are able to release ink....
to confuse predators. This sac is a muscular bag which originated as an extension of the hind gut. It lies beneath the gut and opens into the anus, into which its contents – almost pure melanin
Melanin
Melanin is a pigment that is ubiquitous in nature, being found in most organisms . In animals melanin pigments are derivatives of the amino acid tyrosine. The most common form of biological melanin is eumelanin, a brown-black polymer of dihydroxyindole carboxylic acids, and their reduced forms...
– 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. The ejected cloud of melanin is bound by mucus
Mucus
In vertebrates, mucus is a slippery secretion produced by, and covering, mucous membranes. Mucous fluid is typically produced from mucous cells found in mucous glands. Mucous cells secrete products that are rich in glycoproteins and water. Mucous fluid may also originate from mixed glands, which...
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.
A general level of provocation is necessary to trigger an octopus
Octopus
The octopus is a cephalopod mollusc of the order Octopoda. Octopuses have two eyes and four pairs of arms, and like other cephalopods they are bilaterally symmetric. An octopus has a hard beak, with its mouth at the center point of the arms...
to release its ink, as it is biologically costly to produce. Some species can even use their ink to stun or numb their predators.