Mating and egg laying
General
From April through the northern summer, the number of snails copulating increases due to the higher temperature and humidity, which enhance the possibility of oviposition. The pulmonate snails are hermaphroditic. Although they have both male and female reproductive organs, able to produce both spermatozoa and ova, they must mate with another snail of the same species before they lay eggs. Some snails may act as males one season and as females the next. Other snails play both roles at once and fertilize each other simultaneously. The age of sexual maturity is variable depending on species of snail, ranging from as little as 6 weeks to 5 years. Adverse environmental conditions may delay the onset of sexual maturity in some snails. When the snails is large enough and mature enough, mating occurs in the late spring or early summer after several hours of courtship. The courtship may last anywhere between two and twelve hours. H. aspersa snails stab a calcite spine, known as a "love dart", at their partner. The love dart is coated with a mucus that contains a chemical that enables more than twice as many sperm to survive inside the recipient. A few days after mating, the eggs are laid in the soil.Sometimes there is a second mating in summer. (In tropical climates, mating may occur several times a year. In some climates, snails mate around October and may mate a second time 2 weeks later.) After mating, the snail can store sperm received for up to a year, but it usually lays eggs within a few weeks. The eggs are usually 4–6 mm in diameter. Snails are sometimes uninterested in mating with another snail of the same species that originated from a considerable distance away. For example, a H. aspersa from southern France may reject a H. aspersa from northern France.
There have been hybridizations of snails; although these do not occur commonly in the wild, in captivity they can be coaxed into doing so.
Parthenogenesis has also been noted in certain species.
Helix aspersa
Garden snails bury their eggs in a nest in shallow topsoil primarily while the weather is warm and damp, usually 5 to 10 cm down, digging with their foot. H. aspersaeggs are white, spherical, about 3 mm in diameter and are laid 5 days to 3 weeks after mating. (Data varies widely due to differences in climate and regional variations in the snails' habitats.) H. aspersa lays an average of 85 eggs. Data varies from 30 to over 120 eggs, but high figures may be from when more than one snail lays eggs in the same nest.
In warm, damp climates, H. aspersa may lay eggs as often as five times from February through October, depending on the weather and region. Mating and egg-laying begin when there are at least 10 hours of daylight and continue until days begin to get shorter. In the United States, longer hours of sunlight that occur when temperatures are still too cold will affect this schedule, but increasing hours of daylight still stimulate egg laying. If warm enough, the eggs hatch in about 2 weeks, or in 4 weeks if cooler. It takes the baby snails several more days to break out of the sealed nest and climb to the surface. In a climate similar to southern California's, H. aspersa matures in about 2 years. In central Italy, H. aspersa hatches and emerges from the soil almost exclusively in the autumn. If well fed and not overcrowded, those snails that hatch at the start of the season will reach adult size and form a lip at the edge of their shell by the following June. If you manipulate the environment to get more early hatchlings, the size and number of snails that mature the following year will increase. In South Africa, some H. aspersa mature in 10 months, and under ideal conditions in a laboratory, some have matured in 6 to 8 months. Most of H. aspersa's reproductive activity takes place in the second year of its life.
The snail's shell develops while it is still an embryo; it is, however, very weak, and needs an immediate supply of calcium. Newly hatched snails obtain this by eating the egg from which they hatched. The cannibalization by baby snails of other eggs, even unhatched ones, has been recorded. Promptly after they are finished ingesting their egg casings, they crawl upwards through the small tunnel in order to digest the egg. At this stage, the young are almost completely transparent and colorless. Their shell is usually slightly smaller than the egg they hatched from, but their length when out of their shell is slightly greater than the egg diameter. After a few weeks, the snails will begin to show their first tinge of color, usually slightly blue, before they turn their adult color. Roughly three months after they have hatched, they will look like miniature versions of their mature kin.
They will continue to grow, usually for two to three years, until they reach adult size, although there have been confirmed recordings of snails growing amazingly fast - becoming even bigger than their parents in little more than a month. Irrespective of their rate of growth, however, it will still take at least 1 year before they are sexually mature.