Thanks Brian,

Its a bit hard using external dorsal features to separate the two genera, because some like Armina juliana clearly 'break the rules' and have the mantle fused to the head. One feature which does seem to hold is that in species of Armina there are gills and secondary leaflets on the underside of the mantle which are absent in species of Dermatobranchus. Another 'rule of thumb' is that species of Dermatobranchus are usually small and live on hard substrates or on soft coral colonies while species of Armina are larger and often buried in soft sediments. Unfortunately there are many quite large species of Dermatobranchus to confuse the issue.

Food used to be quite a good character, with species of Armina feeding on pennatulaceans [sea pens] and species of Dermatobranchus feeding on alcyonaceans [soft corals]. Unfortunately Armina cygnea was reported feeding on the alcyonacean Dendronepthya very early in the history of the Forum [message #4] which broke that rule.

I originally placed this species as an Armina because of its size and soft sediment habitat, but as you, and Brian Francisco in an earlier message show [#17740], Armina sp. 7 also feeds on alcyonaceans.

I suspect it is a species of Dermatobranchus, but it might as well stay as it is until someone looks at its anatomy - or even a peek under its mantle skirt to see if it has gills.

Best wishes,
Bill Rudman

Dear Brian,
Thanks for this observation. We don't know enough about the biology of the arminids to know whether some species are exclusively feeding on sea pens and others on soft corals such as Dendronepthya.

Best wishes,
Bill Rudman

Dear Ron,
Thanks for any information. It is an interesting find - perhaps a stray and lonely larvae .... and lonely adult when it grew up ... that drifted down to Port Stephens from the tropical north.
Best wishes,
Bill Rudman

Thanks Ron,
Yes it looks like a species of Armina but not one I am familiar with. Many species have a basic colour pattern of black and white longitudinal lines and it is possibly a species which normally has a thin black line running down each ridge. But even so I don't recognise it. It will be interesting to know if the species has become a regular resident in Port Stephens or whetjer it was a chance settlement of a tropical larva.

Can you give me a rough date please (month & year perhaps?)
bebst wishes,
Bill Rudman