This is a tiny aeolid nudibranch that reaches a maximum of 20 mm or 3/4″.  They are likely not an animal that a snorkeler will be lucky enough to see — though their prey, the Pennaria disticha hydroid (Christmas Tree hydroid) is found in fairly shallow waters.  These three tenellia were found in North Kihei, Maui, in about 4 metres of water while cleaning the reef of fishing line that was wrapped around a piece of dying coral. In between the fishing line, twigs and other debris, “Christmas Tree Hydroids” had gathered and grown.  While trying to separate the, line I noticed the eggs of the animals and then the animals themselves.  They varied in size from 8 mm to 18 mm when elongated. For more information please see Cory Pittman and Pauline Fiene’s Sea Slugs of Hawaii site.