This is a fairly large nudibranch and quite beautiful to watch as it moves.  I have seen individuals as large as 20 cm, but apparently they can get to 27 cm or 270 mm or 11 inches!! Though they are primarily subtidal, I have found them on the underside of leaf kelp and on docks, so do look closely when snorkeling or moving boats.  The burgundy/red dendroids against the white background of their bodies makes for some interesting photography. There is a rumour that these nudibranchs protect their eggs from predators. Difficult to comment on that, but if a Six-rayed Sea Star can protect its young, why not a nudibranch?