Sea pens Ptilosarcus gurneyi are reported to possess a substance ptilosarcone, with suspected chemical defense.  However, nothing is known about its function. Wratten et al. 1977 Tetrahedron Letters No. 18: 1559.

A sea pen flexes in the current 1X

Several diterpenes (gersemolide, rubifolide, and epilophodione) have been isolated from soft coral Gersemia rubiformis collected from islands off the northern tip of Vancouver Island, British Columbia.  Their function in defense, if any, is not known.  Williams et al. 1987 J Org Chem 52: 332.

NOTE  the soft coral commonly known as Gersemia rubiformis is apparently an undescribed specis of Alcyonium.  Fautin & Hand 2007 p.173 In, The Light and Smith manual Intertidal invertebrates from central California to Oregon (Carlton, ed.) Univ Calif Press, Berkeley

A nudibranch Flabellina trilineata crawls over a colony
of Alcyonium sp. (formerly Gersemia rubiformis),
apparently undeterred by its content of diterpenes. The
mouth of the nudibranch is facing into the photo, and
is screened from view 1.5X