Polybranchid Cyerce nigricans
This colorful sea slug is a herbivore that browses on algae. It has no need of camouflage or a protective shell, as it has two excellent alternative defense strategies. First, it can secrete distasteful mucus, by utilizing substances in the algae it feeds on and secreting them from small microscopic glands over the body. Second, its body is covered with petal-like outgrowths called cerata, spotted and striped above and spotted below, that can be shed if it is attacked by a predator, in the same way as a lizard sheds its tail. This ability to cast off body parts to distract predators is called autonomy.
The cerata are also used in respiration, their large collective surface area allowing efficient gas exchange with the surrounding water. The head carries two pairs of sensory organs—the oral tentacles near the mouth and, further back, the olfactory organs (rhinophores). These are retractile and subdivide as the polybranchid matures. They are used to assist in finding food and mates. There is some debate as to whether this sea slug is a separate species or is simply a color variation of a similar mollusk, Cyerce nigra.