Why do nudibranchs like algae so much?

Dear Andrea,
The short answer is I don't think they do like algae that much and I am sure they don't eat it. You certainly see nudibranchs on algae - perhaps because they stand out against the rather uniform background colour of the plant - but seeing them there doesn't mean that most of them are on algae. All it measn is that most of the ones you are seeing are on algae. prehaps most of the population are well camouflaged on their food, or in crevices etc etc.

Why are they on algae at all? Both cheromodorids and phyllidiids are sponge feeders so I presume that if you find them on algae its because they have crawled there. Although I love them dearly, I must admit that they are creatures of little brain. They search for their food by sensing chemicals in the water. Unless they have a good scent to follow I presume their movement is rather undirected, so if they crawl up the stalk of a seaweed the chances are quite high that they will stay on the plant for some time. On the other hand there are many nudibranchs which feed on hydroids and bryozoans that grow on algae. These nudibranchs, including some aeolids, dendronotoideans and polycerids, are therefore commonly found on algae. Althopugh they may seem to be grazing on the algae they are in fact grazing on some epiphytic animal colony growing on the algae.
Best wishes,
Bill Rudman