Brad Pryde captured an image of mating nudibranchs whilst diving in Tulamben, Bali in November 2010. He entered the image into the Ocean Art Photo competition 2010 in the compact camera behavior category where it received third prize and as a result of which it was viewed by Dr. Richard Willan, Curator of Molluscs at the Museum and Art Gallery of the Northern Territory, Australia. He determined that the image shows reciprocal mating between two different species, the first time that this has been recorded. He goes on to say:

“Chromodorid nudibranchs of the genus Risbecia are notoriously promiscuous and this behaviour seems to be unique to this genus only. This promiscuity is usually exhibited with individuals of the same species where a much larger animal will mate with a much smaller one – this behaviour is quite well documented now, as is their ‘trailing’ behaviour. Brad’s photo clearly shows reciprocal mating (yes; it is reciprocal because the vas deferens of both individuals can be seen within the enlarged genital apertures) of an individual of Risbecia tryoni (above) and an individual of Risbecia pulchella (below). So certainly, these species have never heard of sex as a species-isolating mechanism!”