Dear Charlie,
If you go to the Aeolid cnidosac and cerata Fact Sheets you will find quite a few links to more information including a page of relevant references.

Best wishes,
Bill Rudman

Dear Rosemary,
You have learnt the first lesson in research - animals will do everything possible to ruin a beautiful theory. I guess you've checked out some of the relevant references listed on the Forum. Also have a look at the other messages on this page. Many nudibranchs, including aeolids, have dermal secretory cells which produce 'toxic' or 'distasteful' molecules, presumably for defence. In tergipedids such as Cuthona, they are called 'osmiophil gland cells' and are usually scattered throughout the ceratal wall. The name 'osmophil' comes from the stains they take up in histological sections. In some aeolids, like the coral feeders, these glands cells have replaced the role of the nematocysts. I don't think much work has been done on the question, but I suspect that in other aeolids, with prominent batteries of nematocysts in their cnidosacs, the role of 'skin glands' has been overlooked. If this is true, then what your fish are reacting to are secretory cells. I guess the first thing to do is to check out the histology of the ceratal wall and see whether such glands are present.

Concerning the function of the cnidosacs, and your question about why the aeolid spends so much energy sequestering the nematocysts. Firstly, if we look at nematocysts as something inedible and indigestible in the aeolid's food, much like many of the antifeedant chemicals in the food of sponge feeders and soft-coral feeders, then the aeolid's prime need is to get rid of them, or at least package them up so they don't cause any harm to the slug. I have often wondered whether the cnidosac is primarily a 'rubbish dump' for nematocysts - a way to get them out of the digestive system. The other intersting point is that I am still to see convincing evidence that there is a permanent pore or exit at the tip of the cerata to allow controlled expulsion of nematocysts - in effect a permanent ceratal anus. What seems to happen is that the tip of the cerata breaks open to release the nematocysts in times of crisis. If this is so then we can look on the cnidosac as a recycling area, initially developed as a place to isolate indigestible, and dangerous, parts of the food. It is a fascinating topic and I would welcome any thoughts

Concerning whether you should call this animal Hermissenda crassicornis or Phidiana crassicornis. As I have
discussed earlier, because it is such a common and well known species in California, I continue to use Hermissenda until the evolutionary relationships of the Family Phidianidae are better understood.

Best wishes,
Bill Rudman

ILLUSTRATION: First known illustration showing cnidosac. Trinchese, 1881: Plate 15, fig 3. 'Sacco cnidoforo' in Facelina punctata A & H [= F. annulicornis.]

Dear Asa,
Most aeolids have sacs [cnidosacs] at the tips of their cerata in which they store the nematocysts. For defence they apparently squeeze them out of a pore at the tip of their cerata. The French biologist Georges Cuvier first mentioned the internal sacs at the tips of aeolid cerata 200 years ago (1805), but it was not for another 50 years (Wright, 1859) that someone suggested that the nematocysts in the sacs were most probably obtained directly from the cnidarian food of the aeolids. This was confirmed by Grosvenor (1903). I have copied alongside the first reasonable illustration of the cnidosac. It was published by Trinchese in 1881.

If you look at the Fact Sheet on cnidosacs, and the messages below yours on this page you will see that we still have much to learn about how the nematocysts are moved, undischarged, to the aeolid, then stored, and then discharged as part of the aeolid's defence system.

Have a look at the Cerata Page on the Forum, and my message on Aeolidiopsis harrietae for a good photos of the cnidosac. Also look at the amazing Glaucus atlanticus. It chooses the most powerful of the nematocysts from its prey, and so is one of the few nudibranchs which can make a visit to the beach a nasty experience. If you are interested in other aspects of sea slug biology click on the GENERAL TOPICS button at the top of each page for an index.

References:
• Cuvier, G.L.C. (1805) Memoire sur la Scyllee, l'Eolide et le Glaucus avec des additions au memoires sur la Tritonie. Annales de Museum National d'Histoire naturelle, Paris, 6: 416-436, Pl 61.
• Grosvenor, Gilbert Hovey. (1903) On the nematocysts of aeolids. Proceedings of the Royal Society of London, 72(486): 462-486.
• Trinchese, S. (1881) Aeolididae e famiglie affini del Porto di Genova. Part 2. Anatomia, Fisiologia, Embriologia delle Phyllobranchidae, Hermaeidae, Aeolididae, Proctonotidae, Dotonidae del Porto di Genova. Memorie della Classe di scienze fisiche, matematiche e naturali, serie 3, 11: 1-142, Pls 1-80.
• Wright, T. S. 1859. [for 1858]. On the cnidae or thread-cells of the Eolidae. Proceedings of the Royal Physical Society of Edinburgh, 2: 38-40.

Best wishes,
Bill Rudman

Thanks Chad,
Bill Rudman.

Dear Annie,
I'm sorry I don't know how long nematocysts are retained in an aeolid's cnidosac. I guess it would depend on whether the aeolid had to use the ones it has, how often it feeds, its size etc. If anyone knows of a published study on nematocyst retention I would be interested.

Have a look at Cesar Megina's message about feeding in Hermissenda crassicornis, which shows it eats a lot of different animals. If you want to see how long it takes to replace its nematocysts, you will need to be able to identify nematocysts from different anemones and hydroids, so that you will be able to see when a changeover occurs. One possibility would be to feed them on a compound ascidian, which of course does not have nematocysts, and see if the aeolid loses the nematocysts in its cnidosac, and how long it takes. You could also try removing some of the cerata from Hermissenda and let them regrow while feeding on an ascidian, so that you know they won't have access to new nematocysts, then move them to a cnidarian food - sea anemone or hydroid, and see how long it takes for nematocysts to appear in the cnidosac.

With such a variable diet, Hermissenda crassicornis seems quite a good animal to use for some simple experiments.

Concerning your second question. I guess this is unrelated to the nematocyst question. Feeding preference experiments are difficult to interpret unless you know something about the biology of the food and the feeder. Navanax may prefer Phidiana crassicornis in laboratory experiments, but I would be surprised if they meet up with each other in the wild very often. So a laboratory preference may have no significance in understanding the biology and natural history of either the food or the feeder. As Cesar Megina mentions with Hermissenda crassicornis, in different places, Hermissenda eats quite different things. Usually in nature animals don't have the luxury of choosing their 'preferred' food. Food availability and competition from others, usually removes much choice from their list of possibilities.

Which doesn't really answer your question. I guess if you have to do a choice experiment you will have to find an animal with a fairly wide diet and let it choose. But trying to make sense of the results may be difficult.

Best wishes,
Bill Rudman.

Dear Dave,
Here is a quick List of References. It's not that difficult for me to generate such a list but I must warn you that I can't guarantee that it is comprehensive. It will however give you a chance to get into the literature.

If you do come across any more relevant references, can you please let me know so I can add them to the list. I would also appreciate a copy of your paper when you finish - or perhaps you would like to prepare a summary page or two which could be added to the Forum.

Best wishes,
Bill Rudman.