These two Hawaiian freshwater species show the effect of water currents on the evolution of shell morphology. Erinna newcombi (the two shells on the left, bottom and top views) lives in the turbulent water of falls. Its sibling species, E. aulacospira (on the right), lives at the edge of water falls in the spray zone. They are both related to a more widely distributed high-spired species that inhabits streams with more slowly moving water.
This limpet-shaped species evolved in the turbulent waters of large rivers in the western United States. Most other species of this family have shells with high, coiled spires. The streamlined shape of Lanx is an obvious advantage against getting swept away by strong currents.
Left to right, Physella acuta, Radix natalensis and Fossaria truncatula. These three specimens demonstrate the diagnostic differences in the shells of their two families. The Physidae (represented here by Physella acuta) have left-coiled (sinistral) shells, while the Lymnaeidae (represented here by Radix natalensis and Fossaria truncatula) have right-coiled (dextral) shells.
The shells of these two Michigan species show extremes in shell morphology in the family. Radix on the left is globose, while Acella on the right is more or less needle-shaped.
Radix auricularia, Detroit River, Michigan. This is an introduced snail in North America, imported from western Europe, probably from the British isles. Unlike many of the introduced mollusks, it is not known to be a pest.
In this dorsal view, the kidney (the light tan band showing through the shell) is readily evident. Also noticeable is the large air bubble in the lung. Although an aquatic snail, Radix species, like other members of the order Lymnophila, are mainly an air breather.
The two larger specimens shown here are preparing to mate. As are all members of the Lymnophila, each of these snails is a hermaphrodite. Both have a complete set of male and female reproductive structures. The snail on top will act as the male during the coming copulation. After copulation is completed, the two snails will change positions, and the previous female-acting snail will then become the male-acting partner, and vice versa.
Copulation. The top snail is inserting its preputium (with penis at the preputium's tip) into the female gonopore of the bottom snail. The female gonopore is at the posterior of the "neck" stalk. The male gonopore is behind the right tentacle. During copulation, the preputium enverts carrying the penis at it enverted tip. Turgor pressure forces the preputium and penis out; penial retractor muscles pull it back after copulation. The vas deferens can be seen inside the preputium.
Stagnicola elodes (Say). This is a common freshwater snail of northern North America. It exhibits some morphological variation over its wide range, so it has been named as a "new," and different, species a number of times, resulting in a considerable synonymy (two or more different names applied to the same species. Also, differing ecological conditions can affect the morphology, which has resulted in more synonyms.
Stagnicola elodes, from Michigan. This is a common freshwater pulmonate snail of the northern United States and southern Canada. Notice the fleshy mantle collar bordering the shell aperture. This is the past of the mantle that secretes the shell. Just inside the angle of the mantle collar can be seen the closed pneumostome.
Stagnicola reflexa is a form of S. elodes that is one year older than individuals of most populations. In the middle of the last whorl of S. "reflexa" a lighter colored transverse stripe (a varix), marks the size of the shell when it over-wintered. As snails increase in size, the mantle secretes additional shell material at the edge of the mantle bordering the shell aperture. Thus, the shell also increases in size.
Stagnicola emarginata is a common species in northern Michigan and southern Canada but S. petoskeyensis and S. contracta are each known from only one locality. The latter two species are on the state's list of endangered species. Stagnicola petoskeyensis already may be extinct.
This species is a common inhabitant in our larger lakes in the Great Lakes and southern Ontario region. These shells represent some of the intrapopulational shell variation found in Houghton Lake, Michigan.
Bulimnea megasoma and Lymnaea stagnalis, from Michigan. Several Bulimnea megasoma (upper left) and several Lymnaea stagnalis (upper right) were maintained for months in the same aquarium. After a while, numerous young appeared which seemed to be intermediate in shell characters between the two species. However, on electrophoretic analysis, the young were found to be all offspring of Bulimnea megasoma. Their change in appearance was due to the changed environment in which the offspring snails were born and matured. Such offspring would be called ecophenotypes.
2008/08/31 07:23:16.308 GMT-4
