Unionida

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Unionida
Common pond clam (Anodonta anatina)

Common pond clam ( Anodonta anatina )

Systematics
Sub-stem : Shell molluscs (Conchifera)
Class : Mussels (Bivalvia)
Subclass : Autolamellibranchiata
Subclass : Heteroconchia
Superordinate : Palaeoheterodonta
Order : Unionida
Scientific name
Unionida
Stoliczka , 1871

The Unionida , also called freshwater mussels or naiads , are an order of mussels . They belong to the superorder Palaeoheterodonta . The first representatives are known from the Middle Devonian . The Unionida are probably the sister group of the Trigoniida . About 700 recent species are currently included in this group. In addition, there is a previously unknown number of fossil species.

Characteristic

The Unionida order essentially comprises the so-called "freshwater mussels". However, other groups of mussels have also penetrated into the freshwater, so that the reverse conclusion that all “freshwater mussels” belong to the Unionida is incorrect. The housings are usually single-sided. The shell consists of a thick layer of mother-of-pearl, a thinner prismatic layer overlaid by a thick layer of the organic periostracum. The castle is heterodont, schizodont or largely reduced. The ligament is usually external. Almost all forms have more or less distinct isomyar sphincters. The surface line is integripalliat.

Systematics

The classification of the order Unionida is not yet uniform. While some malacologists still advocate a division into one or two superfamilies (plus two exclusively fossil groups), others only list the families. The phylogeny within the Unionida order is currently still very much in flux.

Phylogeny

The phylogeny of the recent Unionida groups was recently examined by Graf & Cummings (2006) who found the greatest likelihood of the following cladogram:

 Palaeoheterodonta  
  Unionida  
  Unionoidea  

 Unionidae


   

 Margaritiferidae



  Etherioidea  

 Hyriidae


  NN  

 Etheriidae


  NN  

 Mycetopodidae


   

 Iridinidae






   

 Trigoniida



Unionoidea and Etherioidea likely form sister groups. Within the Unionoidea, in turn, Unionidae and Margaritiferidae form sister groups. Within the Etherioidea, Hyriidae and an as yet unnamed taxon consisting of Etheriidae (freshwater oysters), Mycetopodidae and Iridinidae face each other.

literature

  • Michael Amler, Rudolf Fischer & Nicole Rogalla: mussels (Haeckel library, volume 5). Enke Verlag, Stuttgart 2000, ISBN 3-13-118391-8 .
  • Rüdiger Bieler & Paula M. Mikkelsen: Bivalvia. A look at the branches . In: Zoological Journal of the Linnean Society , Vol. 148 (2006), pp. 223-235, ISSN  0024-4082
  • Daniel L. Graf & Kevin S. Cummings: Palaeoheterodont diversity (Mollusca: Trigonioida + Unionoida). What we know and what we wish we knew about freshwater mussel evolution . In: Zoological Journal of the Linnean Society , Vol. 148 (2006), pp. 343-394, ISSN  0024-4082

annotation

  1. On the one hand the name is appropriate because all representatives of the Unioida live in fresh water, on the other hand it is somewhat misleading because other groups of mussels also occur in fresh water.
  2. This name is also used for the superfamily Unionoidea.