Actinocyclus in the Red Sea?

Note added 4 August 2004: This is Actinocyclus verrucosus.

Erwin,
Your animal is almost certainly Actinocyclus japonicus, and if it is only 1.5cm long it is a juvenile because it can grow to about 12cm long. It is very hard and leathery to touch and the little "bubble-like" circlet of gills is very characteristic of Actinocyclus and the related genus Hallaxa. I often describe them as "goblet" shaped because of their resemblance to a wine glass. Some authors consider Actinocyclus and Hallaxa to belong in a distinct family Actinocyclidae. Apart from their goblet-shaped gill circlet their radular teeth bear a remarkable resemblance to those of some chromodorids. This is possibly a functional convergence as like, chromodorids, they tend to feed on sponges without spicules. I have found Actinocyclus and a number of species of Hallaxa on Dendroceratid sponges of the family Halisarcidae. These sponges form tough leathery, almost slimy layers over rocks or other hard surfaces and look more like a compound ascidian colony than a sponge, though they lack any obvious pores. Specimens of Actinocyclus japonicus range in colour and shape from a pale yellow with few spots and nodules to a very nodulose darker form. ... Bill Rudman