Reticulidia halgerda & R. fungia togetherter

Dear Mary Jane,
We know very little about behaviour in nudibranchs. I don't know of any study that demonstrates the presence of pheromones or equivalent chemical attractants being produced to attract mates. It is possible that many can sense a chemical in the mucus trail of other animals, but this has not been demonstrated except in some case of predators stalking prey, such as Navanax inermis. One phenomenon we often notice when collecting nudibranchs of the same species and keeping them in small containers is that they quickly end up mating. I personally don't think this is because they are sex crazed but rather because they only recognise their own kind when in actual contact. In the natural environment their chances of meeting would be quite slim so it would be pretty important to mate every time contact was made.

In this case I suspect that R. halgerda treated R. fungia as part of the substrate, crawling over it simply because it was in the way. But that is only my guess, which is no better than your suggestion at this stage.
Best wishes,
Bill Rudman.