These colorful critters are carnivorous sea slugs. They are favorites of photographers because they come in many beautiful colors. It is this coloring that warns predators that they are not a tasty meal. Many recycle toxins they eat from their prey.
The name nudibranch translates into “naked gills.” Their secondary gills are on the outside of their bodies and appear as feathery, lacelike tentacles on their backsides. Most also carry a pair of horn-like tentacles or “rhinophores” at the front, that are used primarily as sensory organs.
Nudibranchs are hermaphroditic, and thus ave a set of reproductive organs for both sexes, but they cannot fertilize themselves. They typically deposit their eggs within a gelatinous spiral.