The snails that are native to Hawaii are typical of many other organisms that evolved here. Their ancestors arrived by floating on logs or hitching rides on/in birds, with arriving founder animals surviving few and far between. After the survivors were able to reproduce reliably for a time, the organisms radiated outwards, found niches, possibly died but maybe stuck around, and in the process of adapting to a wetter or drier valley or ridge lost some traits that might have been physically expensive to keep up. Things like making chemicals that give them a bad taste, making pigments for warning coloration or camouflage are metabolically expensive adaptations and usually don’t develop without outside pressure. What happened in the case of the Hawaiian tree snails is that they lost or never developed the things that make mainland snails really successful and prolific- they don’t lay hundreds or even tens of eggs; they don’t eat copious amounts of greenery, they don’t leave large, icky slime trails.
These snails live in trees. They graze on a microscopic fungal community that grows on the leaf surfaces. Since the tree snails evolved essentially without predators (there is still debate about whether native birds ate them, if so, it was likely opportunistic eating and not something that targeted them specifically) they didn’t need to have many offspring to sustain their populations. The snails are long lived (10-15 years), they aren’t reproductively mature until they are 4-6 years old, and when they finally do start having baby snails, they have only one, born alive with a shell, as many as 4 offspring per year. These (and many other) snails can store genetic material from sexual encounters, and so can have multiple offspring over time from one meeting; they are hermaphroditic but self-fertilization has been observed only once and I think this report is a little unreliable.
One thing we snail biologists wonder about is if the snails evolved in concert with some native plants- were they assisting the plants, increasing or improving photosynthetic rates by grazing the fungal colonies? If so, it could be that at this point the snails are already evolutionarily extinct, no longer filling their role in the native forest in which they evolved. One thing we do know is that the snails occurred in high densities. Even though they weren’t reproducing in great volume, the long life and apparent availability of food led to densities so high they were given the Hawaiian name Kahuli- the singing snails, possibly because of the sound of the wind clacking their shells together? That’s the only explanation for the name that I have come up with; I’ve spent a lot of time with the the snails and have never heard them sing, nor has anyone else I know.
Round about the late 1800s shell collecting was all the rage in Europe and Hawaii was chock full of missionaries from Europe and America. And boy did the missionaries collect shells. The Bishop Museum here houses a lot of the collections from that time period. Millions of shells. Cabinets full. Drawers full. A giant snail graveyard. One collector recalls in his diary an afternoon picnic where he and others collected over 4,000 snails from a small park setting in a nearby valley. I picture the picnickers, walking home with (servants carrying) their buckets of snails, filling the buckets with water so the snails drown, leaving the dead snails laid out in the sun so they dry up. And what a stink the decaying bodies of four thousand snails would give off. I hope the picnickers were more than mildly inconvenienced by the stench.