Snail eggs on a stalk of grass.
A typical trochophore|
Image: Daniel J. Jackson
|Hermaphroditic snails mating (Garden Snail, Helix aspersa) Image: Carla Isabel Ribeiro|
The young snail within the egg has a shell and the same basic form as an adult. The shell is transparent initially, but becomes opaque soon after hatching, when the snail starts eating food containing calcium. Thereafter, the snail looks like a small version of an adult. From the beginning, a snail adds layers to the rim of its shell so that it grows in a coil, round and round, larger and larger.
But most sea snails differ from land snails in that they have a larval stage, known as a trochophore (see figure right), that differs greatly in appearance from that of the mature animal (Margulis et al. 2011, p. 188), just as a maggot looks very different from a fly. Trocophores are planktonic and use their cilia to swim.
After reaching maturity, both female and hermaphrodite snails lay eggs (see upper image at right). In those snails where separate sexes exist, males fertilize the eggs produced by females. Hermaphrodites usually mate with each other, but self-fertilization can occur.
However, in many hermaphroditic snails, for example the Garden Snail, Helix aspersa (see image), the spermatozoa develop so much earlier than the ova that they are absent when fertilization needs to take place. Under such circumstances mating with another individual must take place if reproduction. Hermaphrodites of this kind are called protandrous.
Some snails are even parthenogenetic, that is, their eggs develop without fertilization.
At any rate, by one or another of the methods listed above, the eggs are produced. The life cycle of a snail then begins again.
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