Re: Petalifera ramosa with probable eggs

Dear Leanne & David Atkinson,

Thanks for these photos. The upper photo showing the egg ribbon on a blade of Halophila [see middle right close-up] certainly lends support to the idea that it is its egg ribbon. Your question about how they lay the egg ribbon raises an interesting question. Firstly I need to explain a little anatomy. In Petalifera the typical sea hare parapodia [see mantle cavity page] are partly fused together but there is a dorsal gap that can open a bit. You can see the gap in the upper photo in my message recording an animal from Lifou [#3687].

Now to mating and egg laying: Sea Hares, like all sea slugs, are hermaphrodites, but the penis is quite separate from the female openings. The penis is contained in a sac on the right side of the head while the female openings are situated at the front end of the parapodial cavity. Have a look at the drawing of the parapodial cavity of Stylocheilus [#7872], which is quite similar. I haven't got the reproductive openings labelled in that drawing but the black spot just in front of the parapodial cavity is the common genital opening and the curved line running forward to the right rhinophore represents an open groove down which the sperm is moved to the penis. So during mating the 'male' partner has to put the right side of its head near the common genital opening for copulation to occur.

How is the egg ribbon deposited? The egg ribbon is extruded from the common genital opening but I don't know how it is manipulated and attached in such a tight arrangement. In Sea Hares the tangled spaghetti-like tangle suggests that not much manipulation takes place but with Petalifera I suspect that like many nudibranchs, the ribbon is manipulated by the foot. If in your photo the egg ribbon is in the process of being deposited it would suggest that the head may wave back and forward to produce the zig-zag pattern. In Beeman's paper on Phyllaplysia taylori, which I mentioned in Ron Greer's message, the egg string is extruded from the common genital opening and then travels down the external sperm groove to the right side of the head where it is held by the lips and oral lappets and pressed to the substrate. The animal then moves its head from side to side to fasten the ribbon in the characteristic pattern. I think we can assume that Petalifera does exactly the same, but it would be worth adding it to your 'things to do' list, just to be sure.

Thanks for the question - if you hadn't asked I wouldn't have checked Robert Beeman's paper and found how they do it.

Best wishes,
Bill Rudman