The left half of this painting shows the range of structures nudibranchs use for sensing their environment. These earlike stalks are called rhinophores, suggesting their relationship to noses. Most rhinophores can be drawn back into the body when danger is sensed.
Their world is defined by chemistry: sea slugs are virtually blind, though some have patches of cells able to detect basic light and dark. Nudibranchs are basically blind to color perception as far as we can tell ….. all of this wild color is not for each other.
The right half of the painting represents the variety of cerata: worm, acorn, leaf, or blade like projections from many slugs’ backs. Cerata are used for oxygen exchange: species with cerata do not have gills on their butts, (affectionately called “butt lettuce”) and vice versa.
In species that willfully eject cerata under duress, some of those cerata wiggle on their own once ejected, adding to an attacker’s confusion, possibly allowing the slug to escape.
The more blade-shaped cerata function as housing units, hosting symbiotic photosynthesizing algae, called zooxanthellae. Host nudibranchs will orient their cerata perpendicularly to incoming sunlight, this maximizes their zooxanthellae’s production of sugars, which both host and hosted will consume.