Lymnaeidae

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This family of pulmonate freshwater snails is found world-wide, and contains many species. It is most diverse in northern North America.

Like other families in their order, lymnaeids have a single pair of sensory tentacles on the head, and an eye at the base of each tentacle. All species in the Lymnaeidae have flat, wide, triangular tentacles, and their heads are divided into two flat lateral lobes. Most lymnaeids have "right-handed" shells (when viewed from the opening, the shell spirals to the right) and are found in waters with moderate to high mineral content. Shells of lymnaeid species vary from long needle-like spirals to flat conical forms, but most are more rounded spirals (see photos and comments below for examples). Like all pulmonates, they have a space within their mantle, lined with vascularized membrane, that is used for gas exchange. Most species maintain a bubble of air in the cavity, and occasionally refresh it at the surface of the water. A few species fill the cavity with water, and live without ever approaching the surface.

Many lymnaeids have large simple teeth on their radula that they use to scrape food, and this type of tooth structure is associated with a diet heavy in filamentous algae. Compared to other families, lymnaeids tend to be more herbivorous, consuming more algae and less detritus and animal matter than is typical for other families, though there are many exceptions.

Lymnaeids are simultaneous hermaphrodites, and can self-fertilize, though they will exchange sperm with other individuals if they can. They lay eggs in long, sausage-shaped egg masses. The embryos complete their larval stages within the egg, and emerge as tiny shelled snails. Temperature strongly affects the pace of their life-cycle; warmer temperatures allow faster growth. Probably most species mature and reproduce in 9-15 months, but in colder waters they may take several years, and in particularly warm regions, may complete more than one generation a year.

As is true for most freshwater snails, lymnaeid species are most common in calcium-rich waters. Most live in the shallows, where temperatures are higher and there is more food. There are exceptions though: several species are known from deep waters of lakes.


Sources:

Burch, John B. and Younghun Jung. 1992. Freshwater snails of the University of Michigan Biological Station Area. Walkerana 6(15).

Brown, Kenneth M. 2001 Mollusca: Gastropoda in Thorp, J.H. and A. P. Covich, Ecology and Classification of North American Freshwater Invertebrates. Academic Press, New York

Grant, Douglas G. 2001. Pennak's Freshwater Invertebrates of the United States, Fourth Edition. John Wiley & Sons, Inc. NY


Presented below is a collection of images and comments from Dr. J.B. Burch, Curator Emeritus of Mollusks, University of Michigan Museum of Zoology.

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 they are aquatic snails, Radix species, like other members of the order Lymnophila, are mainly air breathers.

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.

Glossary

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

ectothermic

animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature