Re: Sea Hares - attack and defence

June 23, 1999
From: Assaf Zvuloni

To whom this may concern..
Can someone please explain to us WHAT IS OPALINE?
It's chemical composition relation to "Sea Hares" .
Thank you for any assistance you can give.

Assaf Zvuloni

Zvuloni, A., 1999 (Jun 23) Re: Sea Hares - attack and defence. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Dear Assaf,
This is an interesting question. Sea Hares can have two special glands opening into the mantle cavity - the PURPLE GLAND and the OPALINE GLAND. Some species of Aplysia including Aplysia depilans and Aplysia juliana do not produce purple ink, they just produce secretions from the Opaline Gland. The Opaline Gland opens on the floor of the mantle cavity, to the right of the genital opening. In some species it opens through a single pore while in others it opens through a cluster of small pores.

Until reading your question I had not noticed that American workers have started calling the secretion from the Opaline Gland opaline. My understanding is that the gland was called the Opaline Gland because of its colour and the milky white colour of its secretion. It is an adjective not a noun. It is not the Opaline Gland because it produces opaline! So the secretion from the Opaline Gland doesn't have a name other than 'Opaline Gland secretion'.

What are the nature and function of the Sea Hare secretions? Although both the Opaline Gland and Purple Gland secretions have been thought to have a defensive function, there seems to be conflicting evidence for that. See following references:

Tobach,E; Zaferes,A; Migenis-Lopez,L (1989): Aplysia ink and opaline: exploration of their relation to predation. Bull. Mar. Sci. 45(3), 664-670.

Nolen, T.G.; Johnson, P.M.; Kicklighter, C.E.; Capo, T. (1995): Ink secretion by the marine snail Aplysia californica enhances its ability to escape from a natural predator. J. Comp. Physiol. A 176, 239-254.

In my reading of the literature there is also no evidence that these secretions have active chemical molecules that could have some function in the Sea Hare's biology. See following reference:

Avila, C. (1995): Natural products of opisthobranch molluscs: a biological review. Oceanography and Marine Biology: an Annual Review 33, 487-559.

So basically I can't answer your question. No active molecules have been isolated from the Opaline Gland secretion, and no defensive function has been shown for it. Perhaps, as some authors have suggested, it is just a way of excreting a waste product.

I would be interested in knowing what your interest in opisthobranchs is? Are you a student or research worker. Are you studying Sea Hares, or are you perhaps a natural products chemist?

Best wishes,
Bill Rudman.

Rudman, W.B., 1999 (Jun 23). Comment on Re: Sea Hares - attack and defence by Assaf Zvuloni. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from


Ink glands

Related messages

  1. Is Sea Hare ink poisonous to fish?
    From: Nicole Chilson, January 15, 2007
  2. How to make dye from the Sea Hare ink.
    From: Ralph Bowen, August 3, 2006
  3. Dolabrifera dolabrifera - Dissection diagram for ink gland?
    From: Kathryn Clark, June 17, 2006
  4. Aplysia californica - ink
    From: Malvina Papanastasiou, March 23, 2006
  5. Separating Ink Components of Aplysia dactylometa?
    From: Scott Schwartz, March 6, 2006
  6. Re: Inking in Aplysia
    From: lorrie creach, October 24, 2005
  7. Pharmacokinetic peptides in Dolabella auricularia
    From: Sallie Dugan, July 29, 2004
  8. Sea Hare color purple
    From: Patricia, October 24, 2003
  9. Allergic to Sea Hare Ink?
    From: Martin Masters, August 13, 2003
  10. Do frightened Aplysia secrete ink involuntarily?
    From: Dr. Stephen Hoskins, September 24, 2002
  11. Re: Inking in Aplysia
    From: Edwin Cruz-Rivera, June 27, 2002
  12. Re: Inking in Aplysia
    From: Dr.Stephen R Hoskins, June 26, 2002
  13. Re: Effect of secretions on Aplysia
    From: Cathryn Clarke, June 26, 2002
  14. Effect of secretions on Aplysia
    From: Stephen Hoskins, June 22, 2002
  15. Ink Glands in Octopus and Sea Hares
    From: Nadine Schoene, March 21, 2002
  16. Sea Hares and Inking
    From: Melanie, March 22, 2001
  17. Sea Hare Self Defense
    From: Tina Paget, February 21, 2001
  18. Siphon Withdrawal Reflex in Sea Hares
    From: Steve Potts, December 15, 2000
  19. Tyrian Purple
    From: Mary Townsend, August 28, 2000
  20. Turtles eating Sea Hares
    From: Michael D. Miller, May 5, 2000
  21. What Preys on Sea Hares?
    From: Claire, March 7, 2000
  22. Slugs that squirt ink
    From: Orso Angulo Campillo, November 25, 1999
  23. A new review of Sea Hare defence mechanisms
    From: Bill Rudman., August 14, 1999
  24. Re: Opaline Gland
    From: Assaf Zvuloni , June 28, 1999
  25. Sea Hares - chemical defence
    From: Tom Nolen, February 22, 1999
  26. palatability of sea hares: personal experience
    From: Patrick Krug, Ph.D., September 9, 1998
  27. Re: Sea Hares - attack & defence
    From: Conxita Avila, May 19, 1998
  28. Re: Sea Hares - attack & defence
    From: P.M. Johnson, April 21, 1998
  29. Sea Hares - attack & defence
    From: P.M. Johnson, April 17, 1998

Show factsheet and all related messages