Thanks Nils,
It's possible it will grow into a more colourful species as it matures, because animals of this genus do tend to be brown when juvenile. However the pale coloured juvenile clearly has the markings of C. annulata so that is one species which has its distinctive colour pattern quite early in life. I think the brown animals are a different species. Apart from the colour differences, in this species the cerata seem more numerous, and they are cylindrical, whereas in the juvenile C. annulata they are fewer and quite flattened dorso-ventrally.

I would be interested in some information on their habits. Did you find them crawling at the surface or were they buried in the sand? If crawling on the surface were they only active at night? We know so little about these animals that any little bits of information are useful
Best wishes
Bill Rudman

Thanks Nerida,
No I haven't seen this colour form before, but from its size it is most probably a juvenile colour form. Most species of Cerberilla have a dark juvemile colour form which starts to lighten up and develop the species distinctive colours when they reach about 20mm in length. Because these infaunal sand-burrowers are so seldom found, we know very little about their geographic range or colour variability. I have included a closeup of some of the cerata to show the yellow streak on the cerata. These may be as big as it gets, but it could also be the start of a yellow ring which only become complete in adults.
best wishes,
Bill Rudman