The sea slug (or Nudibranch), Flabellina lineata is a very specialised feeder and only preys on Tubularia hydroids - here on Tubularia indivisa. It climbs the stem and consumes their flower-like polyp heads. It is quite common to find whole areas of Tubularia with no polyps remaining due to the activities of this animal which is very common in some years. Also see next photograph.
St. Abbs Marine Reserve, North Sea