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Page last updated
3 August 2015

Mussel of the Month

The August 2015 Mussel of the Month is Contradens contradens. Contradens is a genus of six species, widespread in tropical Southeast Asia and the Sunda Islands.

Contradens contradens
USNM 85185. Java. (type of Unio contradens Lea 1838)

Our current concept of Contradens as a genus largely dates from Haas (1969), and as such, this is a genus hanging low for revision. We suspect that the type species, C. contradens, is “lumped” — that is, that the taxon represents multiple biological/phylogenetic species. Our evidence? Several separate species recognized by Haas (1969) were lumped by Brandt (1974) into a single species, Uniandra contradens,* with multiple geographically isolated subspecies. To you, future gentle-reviser, we offer caution in trying to determine which (Haas or Brandt) was correct. Our guess is that probably neither got it right.

It turns out that Contradens contradens is particularly interesting among freshwater mussels because the species has asymmetrical glochidia. That is, on one larval valve there is a conspicuous marginal appendage (i.e., a hook). If you've never heard of asymmetrical glochidia, it's because they have been largely overlooked. However, they have been described for various SE Asian genera. In a brand new paper by Pfeiffer & Graf (2015), the meager literature on the subject was reviewed, and the authors performed a comprehensive phylogenetic analysis to test the monophyly of the asymmetrical-glochidium-bearing species. The results are somewhat complicated, but the gist is that Contradens is part of a clade of mussels (at least Contradens + Trapezoideus) with asymmetrical glochidia, within the Rectidentinae.

If you are interested in reviewing the history of larval evolution in the Unionidae or examining the most comprehensive phylogeny of the Unionoida to-date, you can download the early online edition of Pfeiffer & Graf (2015) here. This research was done as a part of John Pfeiffer’s graduate work at the University of Alabama. Nice job, John! And, Roll Tide!

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* There has been some disagreement as to whether Contradens or Uniandra is the correct name for this genus. The use of Uniandra as senior to Contradens seems to date from Brandt (1974). From where we are sitting, this is just clearly an error. Contradens was first mentioned in the captions to plates 18-22 of Haas (1911) in the Systematisches Conchylien-Cabinet without description. The genus was formally described in a subsequent volume Haas (1913b: 173), but AFTER it was described in Haas (1913a). Uniandra was introduced as a subgenus of Ensidens by Haas (1913b ). From the dates of the Systematisches Conchylien-Cabinet and the Nachrichtsblatt der Deutschen Malakozoologischen Gesellschaft, it is clear that Haas (1913a) was published before (1913b), and thus Contradens has priority.** In fact, it would seem that Haas’s (1911) figure captions satisfy ICZN Article 12.2.5, and the name Contradens would be available from that date.

** Even if one (somehow) wasn’t satisfied with these dates and insisted that both Contradens and Uniandra date from the same work, Modell (1964), acting as First Reviser (ICZN Art. 24.2), recognized Contradens as the senior synonym. Starobogatov (1970) also considered Uniandra junior to Contradens.

Classification:

Phylum Mollusca
Class Bivalvia
Subclass Palaeoheterodonta
Order Unionoida

Family UNIONIDAE Rafinesque, 1820
Subfamily RECTIDENTINAE Modell, 1942

Genus Contradens Haas, 1911

Species Contradens contradens (Lea, 1838)

To find out more about Contradens and asymmetrical glochidia, check out:
  • Brandt, R.A.M. 1974. The non-marine aquatic Mollusca of Thailand. Archiv für Molluskenkunde 105: 1-423.
  • Haas, F. 1911. Die Unioniden. [in] H.C. Küster, Systematisches Conchylien-Cabinet von Martini und Chemnitz 9 (pt. 2, h. 43): 41-64, pls., 12a-17.
  • Haas, F. 1913a. Neue Najadengattungen. Nachrichtsblatt der Deutschen Malakozoologischen Gesellschaft 45: 33-38.
  • Haas, F. 1913b. Die Unioniden. [in] H.C. Küster, Systematisches Conchylien-Cabinet von Martini und Chemnitz 9 (pt. 2, h. 47): 137-160, pl. 36-41.
  • Haas, F. 1969. Superfamilia Unionacea. Das Tierreich 88: 1-663.
  • Modell, H. 1964. Das natürliche System der Najaden. 3. Archiv für Molluskenkunde 93: 71-126.
  • Pfeiffer, J.M. & D.L. Graf. 2015. Evolution of bilaterally asymmetrical larvae in freshwater mussels (Bivalvia: Unionoida: Unionidae). Zoological Journal of the Linnean Society.
  • Starobogatov, Ya.I. 1970. Fauna Mollyuskov i Zoogeograficheskoe Raionirovanie Kontinental’nykh Vodoemov Zemnogo Shara [Fauna of molluscs and zoogeographic division of continental waterbodies of the globe]. Nauka, Leningrad. 372 pp.
 
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