>>They are usually placed in the Muteloidea<<

There has been a lot of confusion about the proper family names for these and related taxa.  Although Muteloidea is probably the most popular (e.g., Treatise on Invertebrate Paleontology), Etherioidea turns out to be the oldest available family name and thus the proper superfamily.  I think Iridinidae is also prior to Mutelidae as well.

Kabat, A.R. 1997. Correct family names for the freshwater “Muteloid” bivalves (Unionoida: Etherioidea). Occasional Papers on Mollusks, Museum of Comparative Zoology, Harvard University 5(72):379-392.

Another problem has been the question of whether the oyster-like forms of South America, Africa, and India are close relatives or independently derived.  Hoeh and Bogan (2000) found, based on molecular data, that all three appear to represent independent adaptations to the cementing habit, as previously suggested based on anatomy.  There is also a cementing corbiculid in Lake Posos in Indonesia.

Hoeh, W.R., and A.E. Bogan. 2000. On becoming cemented: evolutionary relationships among the genera in the freshwater bivalve family Etheriidae (Bivalvia: Unionoida). pp. 159-168 (in) E.M. Harper, J.D. Taylor, and J.A. Crane (eds.). The Evolutionary Biology of the Bivalvia. Geological Society Special Publication No. 177. 494 pp.

Bogan, A., and P. Bouchet. 1998. Cementation in the freshwater bivalve family Corbiculidae (Mollusca: Bivalvia): a new genus and species from Lake Posos, Indonesia. Hydrobiologia 389:131-139.


>Is the shell protected by a periostracum?<

Photos I've seen suggest that the periostracum is often eroded, as with other freshwater taxa.

> Are the etheriids protecting themselves from acidic water or from predators? What kinds of predators eat these bivalves?<

Hoeh and Bogan speculate on this a bit.  One possibility is that freshwater crabs are a major predator.  They may also be an adaptation to fast currents, though this seems unlikely for the lake-dwelling corbiculid.

    Dr. David Campbell
    Old Seashells
    University of Alabama
    Biodiversity & Systematics
    Dept. Biological Sciences
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