Mollusc Anatomy: Aquarium Snails and Bivalves
Aquarium snails have a calcium-carbonate shell, a muscular foot and a rasping radula, and breathe by gill or lung. Learn the anatomy and why soft acidic water harms shells.
Aquarium molluscs are mostly gastropods (snails such as nerites, mystery snails, ramshorn and Malaysian trumpet snails), with freshwater bivalves (clams and mussels) kept less often. They share a common body plan built around a shell, a muscular foot and a mantle, and their biology explains their famous appetite for algae and their need for calcium.
The snail body plan
- Shell: a one-piece, usually coiled shell of calcium carbonate secreted by the mantle; where calcium is scarce, shells grow thin and translucent
- Foot: a large muscular ventral foot used to creep over surfaces
- Mantle: the tissue that secretes and lines the shell and encloses the mantle cavity
- Head: bears one or two pairs of sensory tentacles with eyes
- Operculum: in many species a 'trapdoor' that seals the shell opening when the snail withdraws
The radula
Snails feed using a radula, a toothed chitinous ribbon that works like a rasping tongue. Herbivorous species drag it across surfaces to scrape off algae and biofilm, which is why grazing snails are valued for cleaning glass and decor.
Breathing: gill vs lung
Respiration differs by group. Many aquatic snails breathe with a gill (ctenidium) inside the mantle cavity, while pulmonate snails such as ramshorn and pond snails have a lung and must periodically come to the surface to gulp air. This is why some tank snails are seen at the surface, and most molluscs carry the copper-based pigment haemocyanin in their blood (the ramshorn family is an exception, using haemoglobin).
Bivalves in brief
Freshwater bivalves (clams and mussels) have two hinged shells (valves), a burrowing foot, and no radula. Instead they are filter feeders, straining plankton from the water through their gills. Many freshwater mussels have a parasitic larval stage called a glochidium that attaches to a fish before settling, which makes them difficult to keep.
Why water chemistry matters for shells
Because the shell is calcium carbonate, snails and bivalves need adequate calcium (reflected in general hardness, GH) to build and maintain it. In soft, low-GH or acidic water the shell is not properly mineralised and acidic conditions actively dissolve it, causing pitting and erosion at the tip and edges. Keeping GH and pH up (for example with calcium-rich substrate or supplements) protects the shell.