A series of striking convergences can be found in the sea-hares (Aplysia), a group of gastropods and only remotely related to the cephalopods.  Not only do they emit ink clouds (the colour is derived from ingested red algae), but they also employ chemical cues that assist in defense.

Ink in cephalopods

The capacity to generate clouds of ink in many cephalopods is well-known, but the matter is somewhat more complex.  First, ink is used for a variety of functions, including not only the more obvious ones of smoke-screens and pseudomorphs, but also as defence. In the latter case one instance is where the squid fills its mantle cavity with ink which might help to reduce its visibility to predators that employ polarized light.  Even more remarkable is another squid which produces a rope-like ink-cloud that may mimic a colonial cnidarian (with stinging cells) known as a siphonophore.  However, it has been suspected for some time that the ink has a chemical activity, and in particular the molecules L-dopa and dopamine may assist as alarm substances and trigger an escape reaction.

Ink in Aplysia

Fine and large, but it now transpires that a series of striking convergences can be found in the sea-hare (Aplysia), a group of gastropods and only remotely related to the cephalopods.  Not only do they emit ink clouds (the colour is derived from ingested red algae), but they also employ chemical cues that assist in defense.  In these animals two glands are employed (and ones quite different from the ink gland of cephalopods), one to secrete the ink and the other a whitish, more viscous substance (opaline).  Because sea-hares are much more sluggish, it is likely the chemical cues are especially important, but these are convergent with the cephalopods and serve for defense against predators.  These include alarm cues (including mycosporine-like amino acids, which elsewhere are employed for ultra-violet protectors), but perhaps most remarkable are the chemicals that induce what is known as phagomimicry that in combination with tactile cues persuade the spiny lobster, one of the sea-hare’s main predators, that it is eating the gastropod whereas it is only consuming the ink.

Chemical mixing

Another convergence relating to ink production in Aplysia is also worth drawing attention to, in that one of the chemicals that has an anti-predator effect involves combining two separate molecules, an oxidase in the ink gland, and its substrate (L-lysine) in the other gland. Only when they mix can the chemical reaction occur. This arrangement recalls, of course, the reaction engendered in the Bombardier Beetle where mixing hydrogen peroxide and quinines together leads to an explosive reaction. In the sea-hare it is not violent and slow, but the principal is the same.