Nudibranch Predation Experiment

July 15, 2003
From: Rosemary Romero

I am a student at the University of California, Santa Cruz working on my senior thesis. One aspect of my project concerns the sequestering of nematocysts as a form of defense in the aeolid Phidiana crassicornis. I have raised some slugs (collected in Monterey, CA) on mussel tissue for a period of five weeks and have noticed a lack of nematocysts in cerata when examined nder a compound scope. I recently dropped a slug into the tank of a juvenile Copper Rockfish to test any detection of this lack of nematocysts. The fish engulfed the slug within seconds of introduction and in about the same instant spat the slug out. Being a pilot experiment I only repeated the treatment with one other fish and observed the same outcome. When feeding the fish their usual food, ΒΌ in pieces of smelt, a half an hour later both fish were suspicious of the smelt. The first collided with the smelt, backed away, and ate it. The second fish followed the smelt to the bottom of the tank and ate it about 30 seconds after it had landed. I originally hoped that the fish would eat the unarmed (nematocyst free) slugs and spit out armed slugs (slugs containing nematocysts). I am now pondering what it is about the unarmed slugs that make them unappetizing to the fish and my original question of why aeolids expend the energy to sequester nematocysts in the first place. If any one has any ideas on the subject I would greatly appreciate the input.

Rosemary Romero

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

Rudman, W.B., 2003 (Jul 15). Comment on Nudibranch Predation Experiment by Rosemary Romero. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from


Aeolid cnidosac

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