Palys draw sediment into thier tissues to make them more ridgid. That's why you'll see white specks in the stalk, and white lines in the disk sometimes. They also have a strong feeding response, they close quickly over food. Many don't seem to share a common "matt" with the others in the colony, growing in small groups making up a colony.
The other Zoas, short dense ones, feed more on detrius and dissolved nutrients. They don't have the same feeding response, and have no sediment in the tissues. They share a common "matt" and are all attached as a colony.
All are part of the Zoanthus group.
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