Split accepted by WoRMS: http://www.marinespecies.org/aphia.php?p=taxdetails&id=890436. The full text of the original paper is at https://www.researchgate.net/publication/308354922_Changing_spots_Pseudocryptic_speciation_in_the_North_Pacific_dorid_nudibranch_Diaulula_sandiegensis_Cooper_1863_Gastropoda_Heterobranchia

These two new concepts can be separated from photos. As the authors of the paper note,

the diagnostic trait separating the two morphs is not the shape of the spots, but rather the presence or absence of spots on the mantle margin: all of the ringed morphs (even those with solid spots) can be distinguished from the spotted morph by the absence of spots on the mantle margin

Here are some iNat photos that demonstrate the difference (click through for observations and photo credits):

D. odonoghuei

D. sandiegensis (sensu stricto)

Their ranges overlap, but not entirely:

D. odonoghuei is restricted to intertidal and bay habitats from Fort Bragg, California northwards, whereas D. sandiegensis is found throughout the entire region and is more common in subtidal habitats