Pleurobranchus atlanticus and P. crossei

December 13, 2006
From: Bill Rudman

While answering Hugues Flodrops' message [#18785] on whether Pleurobranchus albiguttatus and P. atlanticus were synonyms, I had a look at my earlier  comments on Anne Dupont's message [#5301] where I discuss the identity of P. atlanticus in some detail, including commenting on earlier names for Caribbean pleurobranchs. I also looked at how Valdes et al. (2006) had treated the topic in their new Caribbean book, Caribbean Sea Slugs, and was somewhat surprised at what they have done. They accept two species, P. evelinae and P. crossei. In earlier messages I considered P. crossei [#5301] to be a synonym of P. areolatus, and P. evelinae [#10936] to be a synonym of P. atlanticus. It seems pointless to repeat the arguments here

Unfortunately they have apparently missed my comments, as they do not refer to them and don't mention the most distinctive external feature which is the series of slightly larger tubercles down each side of the dorsal midline which each have a white triangular mark on them.  This is the most distinctive feature on the Indo-West Pacific species P. albiguttatus and also in P. atlanticus. This is the reason I considered P. evelinae as a synonym of P. atlanticus.

In their book they consider P. evelinae as a distinct species, and yet you can clearly see the white triangles in their photo of it, although they don't mention them in the text. They also consider there is a second Caribbean species with the same colour pattern which they give the name P. crossei Vayssiere, 1897, and consider P. atlanticus to be a junior synonym. As I mention above, I suggested P. crossei was a synonym of another Caribbean species, P. areolatus, but any discussion is pointless because the species was based on a 22 mm long preserved specimen, for which the author had no colour information. As Vayssiere (1898) comments, "after a long sojourn in alcohol (from 1826 to 1896) the specimen has lost all traces of colour" - my translation. It is a colourless, unidentifiable juvenile specimen. I can't see any value in trying to identify it, and for the sake of nomenclatural stability should be treated as a nomen oblitum. Otherwise there is nothing to stop another group of authors re-identifying it as something else. They next list Pleurobranchopsis aurantiaca Verrill, 1900 as a synonym. This name has a complicated nomenclatural history. Thiele (1931) considered Pleurobranchopsis to be a subgenus of Pleurobranchus which meant that Pleurobranchus aurantiacus Verrill became a homonym of the Mediterranean Pleurobranchus aurantiacus Risso, 1818. To solve this homonymy Thiele introduced a new name Pleurobranchopsis verrilli Thiele, 1931, which Valdes et al also include in the synonymy of P. crossei.

Apart from this nomenclatural confusion there remains the question of just what Pleurobranchopsis aurantiaca Verrill is. Unfortunately the holotype is missing from the Yale Peabody Museum. Clark (1984) suggests the most likely identity is Berthella agassizii (Macfarland, 1909) which was also described from Bermuda. He notes:
"This species closely matches the description of Pleurobranchopsis aurantiaca Verrill, 1900, except that Verrill's description explicitly noted the absence of a shell. As noted here, the shell is highly transparent and easily overlooked in live animals, and this might explain Verrill's establishment of the shell-less genus Pleurobranchopsis. Willan (1978) reviews the history of Pleurobranchopsis and Gymnotoplax, which appear to be based on inaccurate description and damaged specimens. However, Verrill's description of P. aurantiaca lacks sufficient diagnostic characteristics to allow synonymy with B. agassizii in the absence of the holotype; thus P. aurantiaca and P. nivea must remain nomina dubia..... "

There seems little purpose to be gained by dredging through the literature to find old, badly described, names. That is not to say that old names, which were well described, should not be adopted, but reintroducing old uncertain names seems to be a recipe for nomenclatural instability, which will make no one happy. The earliest description I can find in the literature which illustrates or describes the triangular-shaped white marks, for a Caribbean pleurobranch, is Abbott's description of P. atlanticus. If anyone can find an earlier convincing description I would be happy to adopt it. And again I can see no reason why P. evelinae Thompson, 1977 should not be considered to be the same species.

• Abbott, R.T. (1949): A new Florida species of the Tectibranch genus Pleurobranchus. The Nautilus, 62(3): 73-78.
• Clark, K.B. (1984) New records and synonymies of Bermuda opisthobranchs (Gastropoda). The Nautilus, 98: 85-97.
• Mörch, O.A.L. (1863) Contributions à la Faune malacologique des Antilles danoises. Journal de Conchyliologie, 11: 21-43.
• Thiele, J. (1931) Handbuch der Systematischen Weichtierkunde. 2. Subclassis Opisthobranchia. pp. 377-461. Fischer: Jena. [p: 419]
• Thompson, T.E. (1977) Jamaican opisthobranch molluscs I. Journal of Molluscan Studies, 43(2): 93-139, pls.1-3.
• Valdes, A., Hamann, J., Behrens D.W. &  DuPont, A. (2006) Caribbean Sea Slugs
• Vayssière, A. (1897) Description de deux especes nouvelles de Pleurobranchides. Journal de Conchyliologie, 44: 353-356.
• Vayssière, A. (1898) Monographie de la famille des pleurobranchidés (premiére partie). Annales des Sciences Naturelles Paris, 8: 209-402.
• Verrill, A. E. (1900) The nudibranchs and naked tectibranchs of the Bermudas. Transactions of the Connecticut Academy of Arts & Sciences 10: 545-550, pl. 66.
• White, K.M. (1952) On a collection of molluscs from Dry Tortugas, Florida. Proceedings of the Malacological Society of London, 29: 106-120, Pl.6.
• Willan, R. C. (1978) An evaluation of the notaspidean genera Pleurobranchopsis Verrill and Gymnotoplax Pilsbry (Opisthobranchia: Pleurobranchinae). Journal of Conchology 29: 337-344.

Bill Rudman

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