BIVALVED GASTROPODS

This post is not in error! Bivalved gastropods have existed in the geologic past and still do today. Nearly all gastropods have either a single valve (can be internal) or have no valve all, but this post concerns two gastropod genera that HAVE TWO EXTERNAL VALVES. Both of these gastropods are “living fossils,” and they are comprised of the genera Berthlinia and Julia (both belong to family Juliidae). Both are opisthobranch (sacoglossan = “sap suckers”) snails with sluglike bodies, which like the valves, are green. This is because these gastropods live in clumps of green algae, graze on the algae, and incorporate the photosynthetic algal cells into their own body tissues for energy storage. This process is called kleptoplasty. Thus, the bodies of these two gastropods are also green. After death, however, the color of these translucent shells can easily fade. The larvae of both genera are planktonic (= floaters), therefore, both genera are geographically widespread. They are restricted, today like they were in the geologic past, to tropical waters.

BERTHLINIA:

A Recent Berthlinia limax (Kawaguti and Baba, 1959), showing mostly its left valve, 3 mm length, locality not known.

Berthlinia is found today in Japan, Australia, California, Hawaii, Jamaica, and the Indian Ocean region. The hinge of Berthlinia has no teeth, but faint corrugations are present. The left valve has a small spiral nucleus, which projects noticeably outward from the top of this valve. This nucleus (protoconch), which is the initial part of the left valve, is white and pearly. The animal living within the two valves of Berthlinia is sluglike and also green in color.  

Genus Berthlinia has a discontinuous geologic range of early Eocene (Ypresian Stage) to recent (Le Renard and others, 1996). Most of the Eocene specimens are found in Europe, especially the Paris Basin of France. The type species of this genus is of middle Eocene age and from France. 

There are a few other fossil genera/species of bivalve gastropods: most of them are Eocene in age or Miocene-Pliocene in age (Le Renard and others, 1996, table 1).

JULIA:

A living Julia exquisita Gould, 1862, showing mostly the exterior of its right valve, about 3 mm length, locality not known.

 

Exterior and interior views of a left valve (2.7 mm length) of living J. exquisita from Wailea Beach Hawaii. Gift from Bert Draper (deceased), March, 1987.

 Exterior and interior views of the opposing right valve from the same specimen of J. exquisita, immediately illustrated above.

Julia exquisita is found today in the Indo-West Pacific, including Hawaii, the Seychelles Islands (in the Indian Ocean), and the Kermadec Islands (off New Zealand) (Kay, 1979).

Genus Julia has a discontinuous geologic range of Oligocene?, Miocene, and early Pliocene (Le Renard and others, 1996) to recent. These specimens are found in Europe. 

References Cited:

Kay, E. A. 1979. Hawaiian marine shells. Reef and shore fauna of Hawaii: Section 4. Mollusca. Bishop Museum Press, Honolulu, Hawaii. 653 pp.

Kawaguti, S. and K. Baba. 1959. A preliminary note on a two-valved sacoglossan gastropod, Tamanovulva limax, n. gen., n. sp., from Tamano, Japan. Biological Journal, Okayama University, 5(3/4):177–184.

Keen, A.M. and A.G. Smith. 1961. West American species of the bivalved gastropod genus Berthelinia. Proceedings of the California Academy of Sciences, Fourth Series, 30(2):47–66.

Le Renard, J. and others. 1996. On Candinia (Sacoglossa: Juliidae), a new fossil genus of bivalve gastropods. Journal of Paleontology 70(2):230–235.