To touch or not to touch nudibranchs is very different from the question of "to touch or not to touch" OTHER critters. If we stick to nudibranchs then the answer is not all black and white. Most nudis can be handled with no harm to the diver or the nudi. For example, a nudi on a sand bottom can be moved slightly to allow a photo if moving it provides easier access so that the camera does not hit or bump something else. The caveat is that the nudi can NOT be placed on anything that hurts it and it must be put back exactly where it is found. Some nudibranchs abhor sponges or coral. The diver assumes a massive responsibility to know what he is doing and some divers are not capable of such fine control of their motions to grasp it as delicately as a tiger lifts its baby by the neck. Very few nudis can sting, so it is not the diver we need worry about, but the effect of moving the critter. Most divers are wise to simply look and not risk harming the natural array of things underwater. Also, since most nudibranchs move a lot, I have found that if I wait a little they will eventually get in to a nicer pose on their own. A slight tap in front of them can turn them if they are moving rapidly away from their preferred location. The answer, of course, is do only what you know to be safe, and avoid all else!
