Tryon's Hypselodoris Nudibranch

Hypselodoris tryoni has a brown body and white foot. The body and upper dorsum are covered in purple spots ringed with white or light-brown. The gills are typically translucent white or light-brown and the rhinophores are brown to dark-brown and covered in circular white lines. It has a thin blue or purple line on the mantle edge. There is some colour variation between individuals.
Hypselodoris tryoni is similar in appearance to Chromodoris kuniei and Chromodoris leopardus, although lacks the wide purple mantle edge line present in those species.
Sea slugs such as Hypselodoris tyroni, are known to exihibit trailing behaviour where up to four animals will follow one another very closely, often touching. It is not known what causes this behaviour.

Source: http://en.wikipedia.org/wiki/Hypselodoris_tryoni