Dear Gary,

Every time I see one of these fascinating animals I get more confused trying to identify them to the genus level. There are really no external features to separate Scyllaea and Notobryon. Internally, Scyllaea differs in having a central tooth in the radula, while Notobryon does not. My feeling is that this is probably Notobryon wardi which was described from Queensland, Australia. It doesn't help that it was described from preserved animals so we have no idea of the colour or living shape of the original specimens.

At present Scyllaea, as represented by S. pelagica, can be identified externally by the dark brown band along the side of the body, just below the mantle edge. On those grounds I think I would rather identify your animal as Notobryon. Of course the only way to be sure is to look at its radula. Whatever it is, it is a wonderful find. One of the reasons we have trouble with this group is that Scyllaea pelagica appears to be the only species which is regularly encountered. This is because it lives on floating weeds and often washes ashore after storms. If it were not for the radular difference, I would suggest we consider Scyllaea and Notobryon to be one genus, but until we have many more specimens - preferably with photographs or drawings of them alive - we won't be able to get past the impasse created by Odhner's study of preserved animals many years ago.

• Odhner, N.H. (1936)  Nudibranchia Dendronotacea - A revision of the system. Mémoires du Musee Royal d'Histoire Naturelle de Belgique, series 2, fasc. 3, 1057-1128. (Pl. 1)

Best wishes,
Bill Rudman

Dear Ura,

Thanks for these photos. I am afraid the Scyllaeidae is in a confiused state at the moment and we will only be able to solve it when more material is available. I suspect your animal is a species of Notobryon but we would need to look at the radula to be sure.

I have included a small close-up to show the bushy secondary gills on the inside of the parapodia.

Best wishes,
Bill Rudman

Dear Guido,

What an interesting observation. I can't say I have heard of such an observation before but perhaps there is something in the amphipod literature - the natural sciences haven't been good at cross-referencing. What I have observed often is that hydroid-feeding nudibranchs tend to compete with caprellids on hydroid colonies. You seldom find both aeolids and caprellids together on the same colonies. I certainly don't bother looking for aeolids on hydroid colonies with caprellids. I always thought that the caprellids were eating the hydroids but I came across an interesting paper (Caine, 1998) which showed that caprellids are herbivores feeding on the algal film, diatoms etc, which cover the substrate. Caine found that the caprellid amphipod Paracaprella tenuis Mayer has a mutualistic relationship with the hydrozoan Bougainvillia rugosa Clark, and adult male caprellids would aggressively displace the nudibranch Tenellia pallida (Alder and Hancock) when itvwas attempting to eat the hydroid. He reports that when the nudibranch crawls within a body-length of the amphipod it is aggressively displaced by the caprellid. This means that hydroid polyps around the caprellid are protected from predation, and continued attacks often mean that the nudibranchs leave the whole colony. Caine hypothesises that through this behaviour the caprellid receives a suitable substratum and food in the form of attached diatoms and entrapped detritus.

I guess its difficult to compare the relationship of Notobryon and caprellids to that of Tenellia, which is similar in size to the caprellids. Notobryon is a hydroid feeder, but as caprellids are certainly found in other places than on hydroid colonies, its not that they are being advantaged by a free ride to the next hydroid colony. Perhaps being on a 'big' nudibranch affords some protection. I don't know, but I wonder if Notobryon feels itchy?. This relationship certainly deserves a serious study.

• Caine, E.A. (1998) First Case of Caprellid Amphipod-Hydrozoan Mutualism. Journal of Crustacean Biology, 18(2): 317-320

Best wishes
Bill Rudman

Thanks Ali,
We often find that these hydroid feeders often occur in large numbers or scattered in ones or twos. I think it probably reflects the haphazard nature of their life history. If by chance a 'swarm' of veliger larvae stay together in the plankton then they are likely to metamorphose and settle on the same colony of the animal they feed on. If on the otherhand they spend their time in the plankton alone, then they are most likely to settle out of the plankton alone to spend a solitary life on their food. It is possible that other veligers may later settle on the same colony but I suspect good luck stories aren't that common in nature.
Best wishes,
Bill Rudman

Dear Ali,
Yes this seems to be Notobryon. In this genus there are a few large finely branching gills, while in Scyllaea the gills are much shorter and more numerous, I don't think we know enough about the genus to say with any certainty whether there is one widepread species or a number of different species.
Best wishes,
Bill Rudman

Thanks Bernard,
Alicia's messages about scyllaeids from the east Pacific suggest that some of these animals may have green and brown colour forms or phases. I suspect we need quite a few more specimens to be collected before sensible comparative work can be done,
Best wishes,
Bill Rudman

Dear Ali,
It's quite normal to be confused with these scyllaeids. They all look very similar externally. The radular morphology is quite a good character, at least at the genus level, but we have little knowledge of colour variation, and just how many species there are at present.

I am pretty sure this is a species of Notobryon. With some hesitation I will identify it with the Indo-West Pacific species Notobryon wardi. One point of interest is illustrated in the close-up of one of your photos alongside. See the branching ducts in the dorsal lobes. They look as though they are branches of the digestive gland, but in this family the digestive gland forms 3 distinct unbranched lobes. These ducts are in fact branches of the kidney.

I think this is the first record of this genus from the eastern Pacific.
Best wishes,
Bill Rudman

Dear Erwin,

This is Notobryon wardi, which is a close relative of Scyllaea pelagica and Crosslandia viridis.

Best wishes,
Bill Rudman.

Dear Pauline,
I could say that when you come to the Forum you get the correct story, but having been guilty of typographical errors myself, I won't tempt fate. In this case, I am glad to say my spelling, (bijecurum), is how it is spelt in the original publication, which I cite at the top of this page.

I am curious about your interest. Have you found specimens that fit Baba's description? Any news, information or photos would be welcome,
Best wishes,
Bill Rudman.

Dear Ron,
This is quite an interesting find. It is related to the pelagic species Scyllaea pelagica which Anne Dupont has sent such great photos from Florida. It does not seem to be that common, although Thompson & Brown (1981) reported that it was commonly under intertidal boulders in False bay, South Africa. We don't know much about its biology other than it feeds on hydroids.

As we can see in the enlarged photo of the head, the rhinophore stalks are relatively huge, the small rhinophores nestling at the tip.

Best wishes,
Bill Rudman.