(Sphaerium corneum) is a river dweller. It possesses a brownish lens-shaped shell with yellowish edging and regular yellow stripes. The mollusc body is yellow-gray. The ends of respiratory and digestive systems grow together and extrude out the shell. They look like a pair of reddish tubes. The caviar develops inside the mollusk in special pouches at the inner surface of the gills. The clam burrows in sand mixed with mud at the tank's bottom but in the lack of oxygen in the water it can lift in to upper water layers.
Slugs and Snails
The animals of this group are more active than bivalves. They do not burrow in the substrate but crawl on the plants and the tank bottom and the walls. The species that live in the aquarium usually have a spiral shell and at the abdominal body side carry a muscular foot, which they use to move. At the front end of body (a head) they have two pairs of palps. At the base of the second pair there are the eyes. Some of Gastropoda have no gills but possess lungs instead. Many freshwater species are hermaphroditic. The fertilization is internal. Mollusks lay eggs on the lower surface of water plants leaves and other objects. The embryo develops inside the egg and the young snail hatches when is completely developed.
In the world aquarium literature of the past years it is mentioned that many new species of Gastropoda for tank keeping have appeared, but still the most popular kinds are as follows (except for that which occur in the tank occasionally).
Bladder snails - Physa fontinalis & Physa acuta - inhabit sludge at bottoms of bogs and ponds. These are small snails (less than 0.4 in.), have a yellowish-brown left-coiled shells. The foot is black-blue. The snail can excrete a sticky thread and attach one of its ends to the aquatic plant leaf at the water surface and another - to a pebble or a plant base. The mollusc uses this thread to lift and to go down. The bladder snails reproduce very quickly (they lay bar-like clutches) and can be useful in the tank since they clean the plants and eat the uneaten food. But they can do harm in a spawning tank. Moving along the fish eggs they destroy the egg membrane and this may cause death of the developing fish embryos.
One more often habitant in the tank is another bladder snail - an Australian species Physastra proteus. Not long ago some scientist referred it to Isidora (Isidorella) or Bulinus species. This snail has a red, very ornamental shell and plays the same role in the aquaria as other bladder snails.
Great Ramshorn snail (Planorbis corneus, or Planorbarius corneus) - a freshwater mollusk that lives in ponds and bogs. Its shell is dark-brown, flattened to the sides. The foot is flat and wide.
In spring the mollusk lays the eggs in the form of jelly-like clutches on the water plants and the tank walls. In six weeks the young snails hatch. This mollusk is very useful since it cleans the tank walls and eats up the uneaten food.
Its red variety (Planorbis corneus var. rubra) easily intercrosses with the ordinary great ramshorn and the first generation is dark-brown. It needs the higher temperature of water (72-75 'F).
Great pond snail (Lymnaea stagnalis) - this large light-bodied mollusk with a green-gray shell lives in ponds and lakes. The snail lays eggs on the lower side of plants leaves and various objects. The clutch has the form of a long bar. Young snails hatch on the 20-40th day. They grow very fast and are very gluttonous. Adult snail feeds on plants, fish eggs and even the fry. Therefore it is not recommended to settle this animal in spawning and planted tanks.
Translated by Tatiana Karpova (Moscow)
(MSU, Biology faculture, Dep. zoology and ecology).