Nudibranchs (literally "naked gills") are basically snails which lack a shell. As such, they would make an easy meal for many fish. The nudibranchs (NBs) take care of this problem by being very poisonous, so no one eats them. Most of them advertise their poisonous nature by being very colorful. They are notoriously difficult to keep, because most of them live on a single species of prey, and most reef tanks are unable to supply enough of the particular prey species to keep a NB alive
So, when I discovered not only one, but three adult NBs in my tank I was quite surprised. Here is one of the adults. Note the feathery gills on its back.
And this [fuzzy] picture shows two NBs happily eating this coral. The orange spot on the left center is one of the NBs, and the white area in the lower middle is another NB. The coral is the hand shaped object in the central top, covered with small brown dots. By the time I discovered the NBs, they had eaten so much of this coral that it was only useful as NB food (in a different tank!). The NBs probably arrived on the same rock as the coral, and were hiding under the rock during the day, eating at night. It was only by chance that I saw the NB and realized what was going on. Since corals often get larger and smaller during the day, I was not surprised when the coral began to shrink.
Here is a clutch of NB eggs, laid under the rock. It turned out that these particular NBs will eat virtually any soft coral, so they were quickly banished to a separate 10 gallon tank.
Here we see one of the NBs right along one of the egg clutches.
Here is a close-up of a baby NB, perhaps the size of a grain of rice.
An adult NB. Note the translucent antennae (or whatever they are) in the lower left part of the creature.
As seen from above. Notice the same antennae, only this time towards the right end.
A shot with both the orange and white NB's together on a rock. The food coral is in the background.
Another shot, with the NB coming straight at you, the translucent antennae visible again.
From below, through the glass, with the gills clearly visible.