Re: Pleurobranchus peroni from N. Sulawesi

December 15, 2006
From: Ron Velarde

Concerning message #18662:

Hi Bill,
You wrote " The internal shell in this species is transparent and only thinly calcified. It usually stops growing once it reaches about 7 or 8 mm in length, and in animals larger than about 6 cm it is often absent." Is this typical of the various species of Pleurobranchus?

Pleurobranchus and Pleurobranchaea are closely related genera and I thought one of the major differences was whether a shell was present (Pleurobranchus) or absent (Pleurobranchaea ). So in the description above, if the shell is absent, wouldn't that make this a species of Pleurobranchaea?

If possible, it would be nice if you could post a list of references that review the aspect of the shell in opisthobranch taxonomy.

Thank you for all you do for the forum!

Ron Velarde, 2006 (Dec 15) Re: Pleurobranchus peroni from N. Sulawesi. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Dear Ron,

Some months ago I almost finished a number of Fact Sheets on the 'notaspids' but unfortunately there are only 24 hours in the day. I have had a quick look at them and they are so close to finishing that your message may in fact be the catalyst I need to do so. They are like the set I have done for the 'anaspids' [see General Topics list] and cover the higher taxonomic levels and some basic anatomical features.

Concerning your specific queries, the shell in at least some species of Pleurobranchus certainly seems to reach a limited size and seems to disappear as animals reach a large size. It is possible that the apparent disappearance in large animals - such as 300 mm long P. forskalii - could be from it dissolving or breaking into small fragments in preserved animals, but I remember once trying to find shells in 15 cm long P. forskalii by rotting them in sea water - changing the water regularly so the water would not become too acidic - and ending up with jaw plates and radula but no sign of shells.

Pleurobranchaea is shell-less, but the best way to distinguish it externally is from the very reduced mantle skirt and the position of the rhinophores which are placed far apart on either side of the head [see P. maculata ]. In other pleurobranchs, the rhinophores are close together in a median position.

The shell in opisthobranchs is an enormous story. I will look out some references. While it is useful in some groups, in others it is often an impediment to identification. For example, in some cases, such as Retusa, Cylichna and Tornatina, it is often impossible to know from the shell even which family the species belongs to. Many shelled opisthobranchs were named by conchologists in the 1800s with a very 'conchocentric' view of the world, who felt their job was done by describing dead shells - often very poorly. Without photographs, the line drawings are often useless for identfications, and if the type is lost, we are lost when it comes to interpreting the name. Even when living animals are available, researchers can be blinded by the shell into making incorrect identifications.

A perfect example from the east coast of Nth America concerns some species of Acteocina. Wells & Wells (1964) described what they thought were the shell and radular differences between Acteocina candei and Retusa canaliculata and the direct development of R. canaliculata. Franz later described a planktonic larval stage for Acteocina canaliculata which led to the assumption that A. canaliculata exhibited peocilogeny [variable development strategies]. I won't go into all the following confusion but it was finally resolved by Paula Mikkelsen who showed on anatomical grounds that Wells & Wells' 'Retusa canaliculata' which was of course never a Retusa because it had a radula, was a third species of Acteocina. In this case the shells all turned out to be from one genus but in many cases its not as simple.

Another problem is internal shells. Sometimes we can't successfully identify a species from the outside alone. For example, while revising the Philinidae in New Zealand (Rudman, 1970) I discovered that when Suter described Philine auriformis he did not choose a holotype from his type series of specimens. In this type series of specimens I found two further species, both undescribed, from two genera, Philine powelli and Melanochlamys lorrainae. In this case the shells were quite different, but because they were reduced and internal, they were unnoticed.

I should stop before this becomes a book in itself - but I promise to try and get the 'notaspid' Fact Sheets posted before the end of the year.

  • Franz, D.R. (1971) Development and Metamorphosis of the Gastropod Acteocina canaliculata (Say).  Transactions of the American Microscopical Society, Vol. 90: 174-182.
  • Mikkelsen, P.S. and P.M. Mikkelsen. (1984) Comparison of Acteocina canaliculata (Say, 1826), A. candei (d'Orbigny, 1841), and A. atrata spec. nov. (Gastropoda: Cephalaspidea). The Veliger, 27(2): 164-192.
  • Rudman,W.B. (1970) A revision of the genus Philine in New Zealand with descriptions of two new species (Gastropoda, Opisthobranchia). J. Mal. Soc. Aust., 2(1): 23-34.
  • Wells, H.W. & Wells, M.J. (1962) The distinction between Acteocina candei and Retusa canaliculata. The Nautilus, 75: 87-93.

Best wishes,
Bill Rudman

Rudman, W.B., 2006 (Dec 15). Comment on Re: Pleurobranchus peroni from N. Sulawesi by Ron Velarde. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

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