The Tryon’s Risbecia (Risbecia tryoni) nudibranch is a tropical species that belongs to the Chromodorididae family. This species is found throughout the waters of the tropical western Pacific Ocean and has been recorded from Malaysia, Japan, Vietnam, Solomon Islands, Indonesia, Vanuatu and along the east coast of Australia. In Australia it is found along the Great Barrier Reef and in to the waters of New South Wales at Julian Rocks in the Cape Byron Marine Park.
This species is very similar in appearance to several other nudibranch species including Chromodoris geminus, Chromodoris kuniei and Chromodoris leopardus. These species are all characterisied by large purple/brown spots that are generally ringed with white. Risbecia tryoni can be distinguished from the other species by its body shape and thin purple boder line around the mantle margin. Chromodoris leopardus and Chromodoris kuniei can be distinguished from Risbecia tryoni by their broad purple border, consisting of three slightly different coloured bands in Chromodoris kuniei, and by the leopard-like multiple spots forming a hollow square found on Chromodoris leopardus. Chromodoris geminus differs in having four colour bands around the mantle edge.
Risbecia tryoni is known for displaying ‘trailing behaviour’; this is where one animal appears to follow the mucous trail of the other until they actually come together and touch. Then the following animal keeps contact by touching the 'tail' of the leader and a large trail of nudibranchs can develop! It is possible that this is part of their mating behaviour.
The Risbecia tryoni nudibranch grows to a maximum length of approximately 10 cm.
