Squid, octopuses, and cuttlefish are all in
the class of animals called “cephalopods”.
These animals can squirt ink to distract their
predators and run away. Squid have a muscular sac
of ink under their bellies that contains mostly
pure melanin. Melanin is the same thing that
gives our skin its color! It also contains some
stuff that can stop the predator's smell and taste
for a little while so that they can’t find the
squid after it escapes. When threatened by a
predator, they release a large cloud of thick ink
and flee. Also, squid also have the ability
to change to a darker color while inking,
making them even more invisible. Cool!
ScienceLine, this is a response by many
cephalopods (one of those answers claims "all",
but the set does not include Nautilidae or
Cirrina) to a perceived predation attempt.
Although the "smokescreen" cloud-type inking is
perhaps most familiar, some squid can alter the
amount, direction, and speed of the ink to produce
different effects (
scientific articles here and [Section 5]
here; if paywalled, also
here) . For example, some produce what are
pseudomorph clouds. These are believed to be
attempts at creating shapes which look like
cephalopods which might confuse the predator.
The ink produced by squid is not the same as
the ink used for writing with pens and
Cephalopod ink is a mixture of two
secretions, one providing the coloring and
the other being a sort of thick mucus [Section
The colored ink portion is darkened by
melanin (the same compounds that gives color
to the skin, hair, and eyes of humans), while also
containing various other proteins and enzymes. On
the other hand,
pen and print inks are composed of a colorant
and a carrier fluid (and a number of other
ingredients which modify the drying, permanence,
wetting characteristics, etc., of the ink); the
fluid evaporates after writing/printing, leaving
only the colorant.
Colorants used in inks span a range of organic
and inorganic compounds, from simple suspensions
of carbon (to make black) to complex compounds
triphenylmethanes (blues and purples).