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 etc...in relation to "Sea Hares" .
Thank you for any assistance you can give.
Assaf Zvuloni
azvuloni@hotmail.com
Zvuloni, A., 1999 (Jun 23) Re: Sea Hares - attack and defence. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/968Dear 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.
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