Protaeolidiella sp. from Korea

Dear Dong Bum,
Thanks for these photos and the ones in your other message. As I have discussed, in reponse to an earlier message, I can't find any good characters to separate Protaeolidiella atra and Protaeolidiella juliae. I can't find anatomical differences, and the only constant external difference is the presence or absence of a dorsal median white band. Also the egg ribbon is identical in all forms. 'Typically' both 'species' are black with white tipped cerata. In some cases the cerata seem very short and other times they appear much longer. Of interest though are the brown animals you have photographed here. They have very short cerata in comparison with the body length. However if we look at the brown animal in Yasuhiro Shirai's photo we see it has very long cerata in proportion to the body length.

So it seems both the black and brown forms can have short or long cerata. And certainly the black form can either have a white stripe or not. What may be happening is that the animals are matching the hydroid colony they are on. Your animals here are on a brown stalked colony with very sparse polyps. Its body shape and colour are very cryptic on this colony. If you compare this with black forms with long cerata they are often on hydroid colonies which are black, with many short branches forming a very tangled network. The black long-cerata form is certainly very cryptic on this type of colony.

I don't know if there is more than one species of the hydroid, or whether it is one species with different growth forms. Unfortunately I can't see any simple way of determining whether we have one or two species of Protaeolidiella present. Another possibly confusing factor is that the present two names apply to black animals. Anyone considering brown to be an important character, would need to seriously contemplate naming a third species - which would certainly not help to clarify the situation.
Best wishes,
Bill Rudman