Monophyly of the porostomes

March 28, 2002
From: Terry Gosliner

Dear Bill,
I want to thank you for bringing up the discussion of the the recent paper that Angel and I published on the radula-less dorids. I think the forum is a great place to have some of these ideas aired and certainly appreciate your perspective. I know Angel has also sent you a response, but I also wanted to add some of my thoughts.

I think readers should know that this kind of discussion is what we systematic biologists live for and while it may be considered by some to sound adversarial, it is how our science progresses. Secondly, systematic biology or how we classify organisms has undergone some major changes over the past several decades that represent a major change in how we do our work. Not all of these new ideas are universally embraced and even those who use these techniques use them slightly differently and can reach different conclusions. One of the fundamental ideas of this new approach (called cladistics or phylogenetic systematics) is to recognize only monophyletic groups. These are groups that include a common ancestor and all of its descendants. These groups must share advanced characters as opposed to primitive ones. Many traditional taxonomic groups such as the Invertebrata, Prosobranchia and Cephalaspidea do not have advanced characteristics shared by all of their members. Therefore, cladists (people who embrace this methodology) do recognize these traditional taxa as valid groups of organisms.

That is the philosophy. How does that translate into practice. Well, our contention with regard to the radula-less dorids (porostomes: traditionally the Dendrodorididae and Phyllidiidae) is that the radula was lost only once in this group and represents a shared, advanced characteristic that all of these species share. Bill, suggests quite accurately, that it is important to compare different lines of evidence to reach these conclusions. Angel and I put together anatomical characteristics from all organ systems and compared them. These ultimately led us to the conclusions we reported. Since then, Angel has also produced a new set of data based on mitochondrial genes form many representatives of nudibranchs. These data were evaluated independently and also support the idea that the radula-less dorids are more closely related to each other than to any other group of nudibranchs. Bill, suggested that it would be good to have histological (cellular makeup of organs) characters to support the idea that the organs that we considered to be of the same evolutionary origin (homologous). Interestingly, Gilianne Brodie presented a paper and published an abstract at the World Congress of Malacology in Vienna last August that also supports the idea that radula-less dorids are a monophyletic group. Thus, we now have three independent lines of evidence to suggest that the phyllidids are more closely related to the dendrodorids than to any other groups of nudibranchs. Does this mean that the case is closed? Certainly not! Science must continually test hypotheses with new evidence.

Bill also suggested that we had based our synonomy of Phyllidia and Fryeria on the examination of Atlantic species. This was not the case. What we found is that species Fryeria shared an advanced characteristic, the possession of a ventral anus. But when you separate Fryeria from Phyllidia, the remaining species of Phyllidia no longer share any advanced characteristic. Thus they are no longer in a monophyletic group. Separating Fryeria as a genus means that all the Phyllidia species are left hanging in space. This is the same basis by which we could no longer separate the aeolids in Coryphella from Flabellina.
While systematic biologists can often reach different conclusions and those differences appear frustrating to people who just need to have a stable name for a species that interests them, it is how science works. We don't make changes in names just to be difficult or egotistical. What is important in modern systematics is that we explain what assumptions we made and how we got to that point. Laying all of our cards out on the table then permits others to start where we left off rather than starting from scratch each time. Systematics is a great science because evolution is like a criminal leaving us clues through the fossil record, through the way in which different evolutionary adaptations are expressed in body shape and arrangement of organs or the molecular sequences of the DNA molecule. Like any good mystery, some of the clues will lead us in the wrong direction, but hopefully good detectives will be able to work together to solve the mystery of how evolution unfolded.
Terry Gosliner

Gosliner, T., 2002 (Mar 28) Monophyly of the porostomes. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Thanks Terry,
I think it would be wrong to give our readers the impression that all taxonomists are as polite and civilised as we are in our taxonomic debates. In fact, like all human endeavours, there are taxonomists who are outrageous, egotistical, self-opinionated, bigoted etc etc, and most of us have stories about colleagues and events which make humorous anecdotes in conference 'happy-hours'.

To return to our polite discussion. Basically I will be happy with any conclusion that is reached on the monophyly of the Porostomata, the position of Fryeria bayi and the relationship of Phyllidia and Fryeria. Interestingly, the concept of the Porostomata as a monophyletic group is more than 100 years old, but seemed to go out of fashion in the 1970s ironically because people were interested in evolutionary relationships and felt that the similar suctorial foregut in the Dendrodorids and Phyllidiids were just as likely to have evolved independently as to have had a common origin. It will certainly be interesting to see the publications on Gilianne's histological work and Angel's mitochondrial DNA.

My mention of the Atlantic species was really about the Phyllidiopsis - Fryeria bayi question. I have a philosophical difficulty with the same character apparently popping up independently a number of times in closely related groups. It seems quite logical to remove F. bayi from Fryeria but to place it in Phyllidiopsis and to merge Fryeria with Phyllidia means that we believe that there is some gene in the family which allows the anus to switch from dorsal to ventral, even within a single species. I think we should be asking geneticists whether this is a likely scenario.

I don't quite understand your point that removing the Fryeria clade from Phyllidia would render Phyllidia paraphyletic. This doesn't necessarily follow from your paper where you say "it is possible that members of the genus Phyllidia with a ventral anus could form a monophyletic subclade." Are you suggesting that perhaps we should consider Fryeria to be a subgenus of Phyllidia?
Best wishes,
Bill Rudman

Rudman, W.B., 2002 (Mar 28). Comment on Monophyly of the porostomes by Terry Gosliner. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from


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