This aeolid nudibranch is so small it is virtually invisible.  Most of the animals photographed here are around 5 mm.  Their eggs will often stand out against the Obelia longissima hydroids they are laid upon (or close to).  Look closely (perhaps with the macro lens of a camera) and you may see the Eubranchus rupium itself.   In a few of the photos below the eggs are visible within the transparent mantle of the animal and/or they are being laid.  The common name of Green Balloon Nudibranch is often used to refer to Eubranchus olivaceus, an aeolid nudibranch that looks almost identical.  There seems to be some dispute as to whether they are one and the same animal, but Karin Fletcher has confirmed that the “eubranchus which have been sequenced from WA, BC and AK have aligned more closely with the [circumboreal]  E. rupium than with E. olivaceus.”  

When I first saw this tiny nudibranch I thought I may have found the Doto columbiana, but a quick look at the rhinophores dismissed that possibility entirely. The White-crusted Aeolid or Trinchesia albocrusta is another virtually invisible aeolid that can easily be mistaken at first glance for a Eubranchus rupium.