Aeolidiella glauca, and to a lesser extent, A. alderi are broader than A. sanguinea when all are at full extension. The tips of the propodial tentacles of A. glauca and A. sanguinea are often turned forwards but rarely, if ever, on A. alderi. Scale adjusted so comparison is of proportions on equally long images.

A. glauca and A. alderi from Menai Strait, Wales. 

A. sanguinea from Brittany, France, December 2018 © M. Meunier & L. Valence.