Photo of Mexichromis porterae

Dear Aidan,
Thanks very much for thinking of us. These animals have their reproductive organs on the right side of the body just behind the rhinophores, so from the way your two animals are aligned I suspect they are not actually mating. But I suspect their position is either pre- or postnuptial.

Another interesting thing in your photo is the bluish grey patch below the two nudibranchs. McDonald & Nybakken (1978) reported that this species feeds on the sponge Dysidea amblia, and this patch could well be a dysideid. I am sure one of out west coast colleagues can correct me if I am wrong. Some dysideids are often partially covered by sand grains or have them incorporated in their outer layers, which can make it very difficult to know if a chromodorid is feeding on a sponge or just crawling over a patch of sand. My guess is that these two animals are together because of the food sponge. That is not to say they won't mate - but to be totally unromantic, the initial attraction is the food, rather than the beauty of the nudibranchs.

• McDonald, G.R. &  Nybakken, J.W. 1978. Additional notes on the food of some California nudibranchs with a summary of known food habits of California species. The Veliger, 21(1): 110-118.

Best wishes,
Bill Rudman