Cuthona or Trinchesia?

Dear Marli,

The use and misuse of the names Cuthona, Trinchesia, Catriona and many others for species of the Tergipedidae has had a very tortured history. I have discussed earlier the continued use of Trinchesia for some tergipedids by some workers on the West Coast of North America [message #20011]. As I mentioned there until the family is studied in more depth, using all available characters, I think we should use Cuthona for them all.

I haven't seen the book you refer to but I assume there is no discussion on any changes to accepted nomenclature found in the book. I therefore assume they have based their decision on Michael Miller's (2004) proposal in his reappraisal of New Zealand tergipedids. Considering that Michael was the one who helped clarify the nomenclatural mess the family was in 30 years ago (Miller, 1977) it is rather ironic that his latest proposal is likely to throw us back into an era of confusion.

To keep a rather confusing nomenclatural problem as simple as I can, Greg Brown (1980) showed that Precuthona peachii, a tergipedid which had been placed alone in the genus Precuthona Odhner, 1929, was just a large form of Cuthona nana. The genus Precuthona had been proposed on the basis of some secondary branching to some of  the digestive gland ducts to the cerata.  Miller agrees with Odhner in thinking that the branching of the ducts is sufficiently distinct to keep Cuthona nana (= Precuthona peachii) separate from all the other species we at present place in Cuthona. However this leads to a complicated nomenclatural problem because Cuthona nana is the type species of the genus Cuthona [that is the species on which the name Cuthona is based]. If we accept Miller's argument that the branching of the ducts is important, then all the other species placed in Cuthona need to be moved to another genus and Trinchesia is the next oldest available name. 

As I have said before we need a more exhaustive examination of all the available characters before we can sensibly start subdividing these species. As Miller showed (1977) there were at least eight genus names in use for species we now place in Cuthona, plus many other generic names from other families ehich had been misused for species of the family. Whenever anyone thought they had a new character that didn't fit the existing definitions then a new genus name was proposed. I found a similar nonsensical situation in the genus Phyllodesmium where every different arrangement of the digestive gland ducts had been given a new genus name [Rudman, 1981, 1991]. In the case of Cuthona nana the 'branching' in large animals seems nothing more than a staggering of some cerata to allow more to fit in each row - rather than a single, line the cerata are staggered so they can nest closer together. I personally can't see the phylogenetic significance of that - I do not want to return to a system where an animal which doesn't fit a definition exactly is excluded from that taxon. Surely we should be looking at the value of the definition. Whatever I think of this character, I can't agree to significant nomenclatural changes like this which are based on a single character.

Which brings me to my final point. If Debelius and Kuiter have not explained why they have accepted this significant change to existing nomenclatural practice, then they have done us all a disservice. As I have said often before, nomenclature and taxonomy advances in small steps. It requires research to be conducted and published, and like all fields of scince it requires any new hypotheses to be tested and accepted by other scientists. It is vital to realise that the latest published name or taxonomic decision is not correct just because it is the most recent. If we had a system where the most recent work was accepted just because it was the most recent we would reduce out science to a meaningless race where research meant nothing and publication meant everything.

Michael Miller may be right, but I don't think the case for using Trinchesia has been justified as yet. I suspect Cuthona is a polyphyletic group, but until we have a comprehensive revision, tinkering like this just leads to further confusion.

• Brown, G.H. (1980) The British species of the aeolidiacean family Tergipedidae (Gastropoda: Opisthobranchia) with a discussion of the genera. Zoological Journal of the Linnean Society 69: 225-255
• Miller, M. C. (1977) Aeolid nudibranchs (Gastropoda: Opisthobranchia) of the family Tergipedidae from New Zealand waters. Zoological Journal of the Linnean Society 60: 197-222.
• Miller, M. C. (2004) An appraisal of the identity of the New Zealand species of the aeolid family Tergipedidae (Gastropoda: Opisthobranchia). Journal of Natural History 38: 1183-1192.
• Rudman, W.B. (1981) The anatomy and biology of alcyonarian-feeding aeolid opisthobranch molluscs and their development of symbiosis with zooxanthellae. Zoological Journal of the Linnean Society 72: 219-262.
• Rudman, W.B. (1991). Further studies on the taxonomy and biology of the octocoral-feeding genus Phyllodesmium Ehrenberg, 1831 (Nudibranchia: Aeolidoidea). Journal of Molluscan Studies, 57: 167-203.

Best wishes,
Bill Rudman