For many animals that live at the edge of the water, the rigid designations "terrestrial" and "aquatic" are meaningless. There are many animals that are equally comfortable both on land and in water and others that obligatorily spend their juvenile stages in water and adult lives on land. Take, dragonflies, for example, whose larvae grow in freshwater. Are they terrestrial or aquatic insects?
I have written about another such animal, Littoraria angulifera, the snails in the picture above. The first time I saw these snails in a mangrove forest in Florida, I didn’t know what species they were and assumed they were some terrestrial tree snails, for which Florida is famous. About a year later I returned to the same location with a more knowledgeable friend who correctly identified them. However, one cannot find L. angulifera in standard references for land snails like Pilsbry's Land Mollusca of North America, because they are considered to be "marine" snails. Imagine a marine snail that doesn’t like to get wet!
"Adult L. angulifera were never found submerged but always occurred above the water line. During low tide periods, snails might migrate down [a concrete seawall] into the encrusted zone to feed, but as they became wet with the rising tide, they moved up and remained out of the water.1"
To become completely liberated from the sea, an animal must fulfill three requirements. First, it must be able to obtain oxygen from the air. Second, it must be able to prevent excessive water loss from its body. Third, it must be able to reproduce on land. Successful reproduction on land, in turn, requires that 3 conditions be satisfied: 1. unless the animals are parthenogenetic or can reproduce asexually, either a mechanism must evolve to safely transfer sperm from a male to a female or the animals must become selfing hermaphrodites (hermaphrodites that fertilize their eggs with their own sperm); 2. the free-swimming veliger stage of the mollusk embryo must be suppressed; 3. the embryo must be protected from drying either inside a hard-shelled egg or by being deposited in a damp location or by remaining inside the mother’s body until fully developed.
Note that sperm transfer doesn’t necessarily involve mating. For example, in some species of pseudoscorpions, which are terrestrial animals, sperm is transferred from a male to a female in a little packet, a spermatophore, without the 2 sexes ever coming together. A male leaves its spermatophore in a suitable location and a female, if she happens to chance upon one, takes up the sperm directly from the spermatophore2.
Littoraria angulifera and its relatives (family Littorinidae) satisfy the first 2 requirements: they can breathe air and their shells, which evolved in the oceans and were definitely a preadaptation for terrestrial life, protect them from drying when they are out of the water. However, they remain marine snails, at least in biologists' eyes, because when the time comes, the adults must return to the sea to spawn. In Florida this takes place during warmer months of the year (May to October) when the adults mate and the females release their larvae into the sea. The planktonic larvae first develop in the sea and then settle at the shore. As the snails get older, they move away from the edge of the sea as if they are repelled by their ancestral home.
Development of zonation in Littoraria angulifera. As the snails get older from November to May, they move away from the edge of the sea. Compiled from Gallagher & Reid, 1979.
Had the littorinids' ancestors evolved a way to modify the veliger larva and at the same time evolved some sort of protective egg, they would have paved the way towards full terrestriality.
Nevertheless, the littorinids' life style is just another way to exploit what the environment offers and is as good as any other way of surviving. They live on land and use the opportunities provided by the sea to propagate. And when the conditions are right, they can reach enormous numbers as I wrote about in this post.
1. Gallagher, S.B. & Reid, G.K. 1979. Population dynamics and zonation in the periwinkle snail, Littorina angulifera, of the Tampa Bay, Florida, region. Nautilus 94:162-178.
2. Peter Weygoldt. 1969. The Biology of Pseudoscorpions. Harvard University Press.