Bulla occidentalis A. Adams, 1850, Bulla striata; B. occidentalis; B. umbil

[Marlo]

Read post Aug 5, 2008 at bottom first. All three of these shells are now considered to be a single species - Bulla occidentalis.

Bulla occidentalis A. Adams, 1850 - the western Atlantic species
Bulla striata Bruguičre, 1792 - the eastern Atlantic species
Bulla umbilicata Röding, 1798 - synonym to B. occidentalis.

The shells of the genus Bulla Linné, 1758 have been described as ovate (having an outline like a longitudinal section of an egg with the basal end broader) and compactly involute (having whorls that obscure the axis or other volutions). The shell is smooth with an umbilicated apex (depressed like a navel), often described as a sunken spire. There is no operculum.

Abbott (1974) presented only one species of Bulla as commonly found in North America; namely, Bulla striata Bruguičre, 1792. He also indicated that the Caribbean form may take the subspecies name B. umbilicata Röding, 1798 (synonym: B. occidentalis A. Adams, 1850). Malacolog’s Database of Western Atlantic Marine Mollusca treats both of the latter species as synonyms of Bulla striata. However, there has been much uncertainty about the exact taxonomy of these species with many current workers restricting Bulla striata to the eastern Atlantic, considering the western Atlantic species to be B. occidentalis, and giving B. umbilicata full species status. K. M. de Jong and H. E. Coomans in Marine Gastropods from Curacao, Aruba and Bonaire, 1988, assigned B. occidentalis to their shells and tentatively treated B. umbilicata as a separate species. Most recently Colin Redfern in Bahamian Seashells: A Thousand Species from Abaco, Bahamas, 2001, adopted this same approach.

August 5, 2008: I worked on this presentation during May/June 2008 with the knowledge that a group in the United Kingdom was working on a complete revision to Bullidae and would soon publish their results. Since I had provided Florida material to support their work I was looking forward to a definitive resolution of the species status of our Florida shells. I proceeded with the below presentation knowing it might soon become obsolete. Well, the revision was published in July: Systematic revision of the living species of Bullidae (Mollusca: Gastropoda: Cephalaspidea), with a molecular phylogenetic analysis, Manuel António E. Malaquias and David G. Reid, Zoological Journal of the Linnean Society, Volume 153, Number 3, July 2008 , pp. 453-543(91). The authors found that of all the various names ascribed to the B. striata group in the western and eastern Atlantic only two species could be validated - B. striata in the eastern Atlantic and B. occidentalis in the western Atlantic. They concluded as follows:

“The study of material from the entire distribution range of the B. striata group, including shells and anatomy, together with a molecular phylogenetic hypothesis, has revealed the existence of two species, B. striata and B. occidentalis, in the eastern and western Atlantic, respectively. Nevertheless, only the molecular characters and geographical distribution clearly separate these species, whereas the high intraspecific and interpopulational variability of shells, and similarity of anatomy on both sides of the Atlantic, would likely have led to recognition of only a single species.” (emphasis added)

So, the below presentation is obsolete in that B. umbilicata has been demonstrated not to be a separate species from B. occidentalis. But, the treatment of B. occidentalis as a separate species from B. striata is confirmed. I am leaving the presentation unchanged as it still provides interesting illustrations of the variable B. occidentalis and will also serve as an example to novice collectors of the problems with identification and how species naming is resolved (up to the limits of current technology and technique).


I recently did a quick photo comparison of the questionable Bulla I’ve collected along eastern Florida and sent it to Colin Redfern and Harry Lee for comment. Here is one of the comparisons:



Colin tentatively assigned the names indicated above. However, he qualified his opinion by commenting, “Note, though, that there's still work to be done on this complex, and I use the name umbilicata with caution.” Harry, upon his review of the shells, commented as follows:

“I've struggled with these western Atlantic Bulla for years, and I have never been completely satisfied with a reconciliation of the literature with the conchology I directly observe."

"I believe you may have two different species here. If you follow Olsson and Harbison (1953: pl. 25, fig's. 1, 2), the shell on the shell on the R in your images is Bulla occidentalis A. Adams, 1850, and the one on the L is Bulla striata Bruguičre, 1789. Since it is more striate, I find it easy to apply that cognomen*. However, there are forces (e.g. deJ. & C., 1988 and Redfern, 2001) that would limit B. striata to the eastern Atlantic, in which case the next available name, B. umbilicata Röding, 1798, can be applied to the striate form with the wider, deeper apical pit."

"As Colin indicates in his text, the distinction you so clearly demonstrate isn't always that clear when one looks at a lot of shells.”

*a catch-all term for adopted (or commonly applied) names

The following presentation presents the two forms of Bulla I’ve found in Florida’s shallow waters. For identification I have utilized the approach adopted by Redfern that uses the names B. occidentalis and B. umbilicata so that a direct comparison can be made to the presentations in his book. The shells illustrated are small and of similar size so as to be as directly comparable as possible. Personally, I’m as uncertain as Harry and Colin, but tend toward considering our Florida shells to be a single species with regional population variations that are most pronounced in juvenile shells. I have not observed the same differences illustrated below in mature specimens from the same locales, although admittedly, I have not really paid close enough attention to this ubiquitous shell.

[Marlo]

Update:

I worked on the above presentation during May/June 2008 knowing a publication was in works that would most likely provide a definitive resolution of the species status of our Florida shells. I posted it knowing it might soon become obsolete. And, sooner than I thought, it has become obsolete. See Systematic revision of the living species of Bullidae (Mollusca: Gastropoda: Cephalaspidea), with a molecular phylogenetic analysis, Manuel António E. Malaquias and David G. Reid, Zoological Journal of the Linnean Society, Volume 153, Number 3, July 2008 , pp. 453-543(91). The authors found that of all the various names ascribed to the B. striata group in the western and eastern Atlantic only two species could be validated - B. striata in the eastern Atlantic and B. occidentalis in the western Atlantic. They concluded as follows:

“The study of material from the entire distribution range of the B. striata group, including shells and anatomy, together with a molecular phylogenetic hypothesis, has revealed the existence of two species, B. striata and B. occidentalis, in the eastern and western Atlantic, respectively. Nevertheless, only the molecular characters and geographical distribution clearly separate these species, whereas the high intraspecific and interpopulational variability of shells, and similarity of anatomy on both sides of the Atlantic, would likely have led to recognition of only a single species.”

So, the above presentation is obsolete in that B. umbilicata has been demonstrated not to be a separate species from B. occidentalis. But, the treatment of B. occidentalis as a separate species from B. striata is confirmed. I have left the presentation posted and unchanged as it still provides interesting illustrations of the variable B. occidentalis and will also serve as an example to novice collectors of the problems faced with identifications and how species naming is resolved (up to the limits of current technology and technique).

P.S. (3/29/09)

For those of you who might ask the question "Why is the name Bulla occidentalis A. Adams, 1850 given precedence over the earlier Bulla umbilicata Röding, 1798," as did Harry Lee, here is the reason and Harry's followup comments:

"Bulla umbilicata Röding, 1798: 15 (invalid name, ICZN, 1959: opinion 549; types untraceable)."

Harry's comments:

"Röding (1798) introduced the name B. umbilicata in a sale catalogue. This name was neglected for more than 100 years, and this catalogue was only considered valid for nomenclatural purposes after 1926 (ICZN, 1958: opinion 96). During the 20th century several authors (e.g. Rehder, 1962; Abbott, 1974; Redfern, 2001) resurrected the name for western Atlantic specimens, but the International Commission of Zoological Nomenclature [had] suppressed it (ICZN, 1959: opinion 549)..." "It seems that Harald, RTA, de Jong and Coomans, and Colin didn't know about the ICZN Ruling."