Re: Melibe from Alaska

Dear William,
Although I have never seen it alive, from what I have read and seen in photographs, I can well understand why you and your family find Melibe leonina fascinating. Quite a few people have studied aspects of its biology and I have listed most of the background information on the M. leonina references page. I am not sure how familiar you are with using webpages so in case you aren't very familiar, if you click on any underlined word, which will probably also be in a different colour, you will open another page on that topic.

To answer your questions. The larvae of all nudibranchs, all molluscs in fact, are quite different from the adults. They are called veligers, and have a shell and a large bilobed ciliated flap (velum) on the head which is used for swimming and for catching the microscopic plants that veligers all feed on. If you go to the General Topics List and look at the pages listed under 'larvae, larval development etc' you will find some photos of veliger larvae and how they develop into slugs. Also have a look at some good photos of the larvae of Hypselodoris zebra. In general if you want to look for things on the Forum use the SEARCH button at the top and bottom of each page and type in the word you are interested in.

Now getting back to Melibe. Veliger larvae hatch from their eggs about 10 days after laying. Like all veligers, they have a shell, operculum, velum etc., and they swim in the plankton where they feed on microscopic plants (phytoplankton). After 30-59 days they are ready to metamorphose, which means they are ready to settle out of the plankton, change shape and become little crawling slugs. Most nudibranchs that have been studied, need a chemical scent in the water, usually from their food, before they begin to settle and metamorphose. In Melibe leonina however, a chemical stimulus does not seem to be necessary. Metamorphosis is a major event for all nudibranchs, because they not only have to change shape but they have to change their digestive system and how it works, because they are changing from eating plants to eating meat.

After settling on the bottom, the small slug begins to change shape. After 12-20 hours it drops its shell and it is rapidly growing its oral hood and the first pair of cerata. Within 2 and a half days the oral hood is big enough for it to catch its first meal, which because it is still very small, consists of microscopic one-celled ciliated animals.

Which brings me to your question about the tentacles on the oral hood. Melibe leonina, like all species of Melibe, uses its oral hood like a throw net in fishing, to catch its food. The fine tentacles along the edge are used to feel for prey. When prey are detected the net can rapidly close, drawing the food into its mouth. Melibe leonina eats small crustacea and other small invertebrate animals, but large and small Melibe use the hood quite differently. Larger animals (25mm or longer) feed by lifting the oral hood into the water column and waving it through the water, regularly closing it to eat any animals which have been caught. The more prey in the water, the more often Melibe will go through the feeding cycle of waving and closing the hood. Small Melibe, less than 25mm long, seem to feed quite differently. They hold the hood out horizontally, just above the bottom, and gradually move along feeling ,with the tentacles around the edge of the hood, for small prey. This is the way that other species of Melibe, such as M. engeli and M. viridis feed, even when they are much larger.

There are very few observations of animals attacking and eating M. leonina. The only one I can find reports some of them being eaten by the Kelp Crab Pugettia producta in Monterey Bay, California. (Ajeska & Nybakken). M. leonina is well-known for the characteristic citrus smell it produces when touched or when it is aggregating in large numbers. This secretion is produced by skin glands in the oral hood and cerata and consists of a weak acid and a scent. Some consider it to be defensive, and it has also been suggested that it is a sex pheromone, attracting animals together for mating, but no one really knows. I don't know of people eating it. I would suspect the secretions from its skin would make it rather distastteful.

What else can I say about it? It can swim by lateral flexion of the body, but swimming seems to be a second best way of life, probably as a result of being washed off the plants it normally lives on. I almost forgot your question about the cerata. I am not sure what their function is, but they certainly increase the surface area of skin so are important in respiration (breathing, gas exchange)and so could be thought of as gills. One other important function of the cerata is probably defence. When disturbed, Melibe can break off one or more of the cerata, which then wriggle around and exude their characteristic secretion and scent for some time. This behaviour, much like a lizard dropping its tail, is called autotomy. Have a look at the autotomy page for some other examples of sea slugs which use autotomy in their defence.

Have a look at the information and photos on other species M. engeli and M. viridis. I hope this will give you plenty of information for your project. Perhaps if you take any photos of your Alaskan animals you could send a copy to the Forum, or if you see them doing interesting things, you could let us know
Best wishes,
Bill Rudman