Aeolidiella faustina or A. drusilla?

Dear Glenys,
Thanks for your comments. Both A. faustina and A. drusilla were described from single preserved specimens collected at Cook Sreait, New Zealand (Bergh, 1900). Since then, Bergh (1904) reported A. faustina from northern Tasmania and Bob Burn (1962) has reported it as very common under stones from a number of sites in Victoria. As far as I know, there is no record of A. drusilla being found since the original descritption. On this basis I was happy to follow Bergh and Burn in identifying the Bass Strait Aeolidiella as A. faustina. I realised there were differences in radular morphology but sometimes inaccuracies do occur in drawings of radular teeth.

I have copied the relevant radula drawings from Bergh alongside.

UPPER RIGHT: Drawings from Bergh, 1900: Fig 39 - Aeolidiella faustina; Fig 43 - Aeolidiella drusilla. Radula from holotypes collected in Cook Strait, New Zealand.
LOWER RIGHT: Bergh, 1904, Pl. 1. Figs 30,31. Radula of specimen identified by Bergh as A. faustina, from Ulverstone, nthn Tasmania. [mislabelled on plate captions as Aeolidiella pacifica.]

If we accept these drawings as accurate, then it looks unlikely that Bergh's Tasmanian specimen (Fig. 30-1) is the same as the New Zealand A. faustina (Fig 39). One other complication is that we have no information on the colour and shape of the living animals. Compare these drawings with SEM radula photos I have just posted of the animal from Tasmania.

So where does that leave us? The SEM photos of the Tasmanian specimen certainly looks most like Bergh's A. drusilla (Fig 43). What we really need to know is something about the New Zealand species. Powell (1979) publishes a colour painting of an animal from Auckland, NZ which looks very similar to the Tasmanian photo, but Powell, on the advice of Michael C. Miller identifies it as A. faustina. In Powell's monograph, Aeolidiella drusilla is not listed as part of the NZ fauna but no reasons are given for this omission.

On the strength of the shape of the radula teeth I guess this animal is most likely to be A. drusilla which would suggest that Bergh's and Burn's records of A. faustina from southern Australia are incorrect, as is Powell's from Auckland. I'll change the name on the Forum page to Aeolidiella cf. drusilla but I suspect this will not be the last word we hear on the topic.

References:
• Bergh, L.S.R. (1900) Ergebnisse einer Reise nach dem Pacific (Schauinsland 1896-1897). Die Opisthobranchier. . 13(3), 207-246, Pls.19-21.
• Bergh, L.S.R (1904) Malacologische Untersuchungen. In: C.G. Semper, Reisen im Archipel der Philippinen, Wissenschaftliche Resultate. Band 9. Lief 1.: 1-56,Pls.1-4.
• Burn, R.F. (1962) Descriptions of Victorian nudibranchiate mollusca, with a comprehensive review of the Eolidacea. Memoirs of the National Museum of Victoria, 25: 95-128.
• Powell, A.W.B. (1979) New Zealand Mollusca. Marine Land and Freshwater Shells. Collins, Auckland. 500 pages.

Best wishes
Bill Rudman