The most common gastrobods in the northern Baltic include two species from the genus Hydrobia –H. ulvae and H. ventrosa, the mud snail Potamopyrgus antipodarium, and the faucet snail (Bithynia tentaculata).
Gastropods and snails form an extensive and diverse group. Species able to thrive in local conditions in the northern Baltic include the mud snails Hydrobia ulvae and Hydrobia ventrosa, New Zealand mud snail (Potamopyrgus antipodarium), river nerite (Theodoxus fluviatilis), valve snails of the genus Valvata, pond snails of the genus Lymnaea, bladder snail (Physa fontinalis), ram’s horn snails (Bathyomphalus contortus and Anisus vortex), faucet snail (Bithynia tentaculata). Three sea slugs can also be found in Finnish coastal waters: Limapontia capitata, Alderia modesta and Tenellia adspersa.Most gastropods and snails are grazers that feed on algae (e.g. Lymnaea, Physa fontinalis, and Limapontia capitata) or microbes (Hydrobia sp.); but others (including Tenellia adspersa) also feed on polyps, and the great pond snail (Lymnaea stagnalis) feeds on dead fish, as well as algae, plants and polyps.
Gastropod species have varying reproductive mechanisms. River nerites have internal fertilization, with females laying about 20 eggs on a hard surface, and just one offspring surviving by eating its siblings. Female Hydrobia mud snails lay about 300 eggs on a hard surface, from which tiny snails develop without going through a larval stage. The New Zealand mud snails found as an alien species in the Baltic are all females, and they reproduce asexually, with tiny snails forming from their mother without undergoing any metamorphosis, and then starting to graze independently almost immediately.
Unlike most gastropods, sea snails have no protective shells of calcium carbonate, and instead they often warn predators of their toxicity using bright colours, or foul-tasting excretions. In spite of their protective shells, gastropods are preyed on by fish including cyprinids.
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