Sunday spinelessness – live-bearing land snails

By David Winter 29/04/2012

People seemed to like the idea of a marsupial land snail, so today I thought I’d go one step further, and introduce you to land snails that give birth to live young. 
I was lucky enough to spend a little time in Vanuatu a while ago, and, although I was really there to relax and see in a new year, I couldn’t travel that far and not spend a little of my time looking for snails. As it turns out the island on which we stayed  is heavily modified, and there is not much natural habitat left for native land snail species. In fact, the only really interesting snails I found were living on the side of our host’s house. I collected a few of those snails, transported them to the fridge in our lab and forgot about them for the best part of year.
More recently it dawned on me that these snails would be useful for a project I am working on, so I grabbed them from the fridge, set them up under the microscope ready to dissect away a tissue sample for genetic work and saw this:

Embryos developing inside the shell of their mother. 
We sometimes think of live-bearing as being a trait that sets the mammalian branch of the tree of life apart from other animals, but that’s wrong. Most of the major groups of animals have some species that give birth to live young – there are live-bearing frogs, snakes, lizards, insects, fish, crustaceans and star fish. In fact, the only large group without live-bearing species that I can think of is birds (and, it seems, dinosaurs, a group that contains birds). Most land snails lay a clutch of many eggs, each containing a single-celled zygote which is left to develop on its own. A few species, like theses ones, have evolved a different reproductive strategy: producing fewer eggs than their relatives, but retaining those eggs within their shell before giving birth to much more developed young.

This behaviour seems to be particular common in snails that live in rocky outcrops, and those that live in the tropics, especially the Pacific. I’m not sure about what species the snail depicted above fall into – but they are from the sub-family Microcystinae, which is one of the dominant groups of land snails in the Pacific and is made up entirely of live-bearing species. The large evolutionary radiations that used to live in Hawai’i and the Society Islands were also all live-bearers.

So why give birth to live young? It is easy to see why live-bearing is an advantage to snails living in rocky habitats with few places to deposit eggs. It’s less clear why the Pacific is full of live-bearers. It has been suggested that tropical weather can lead to unpredictable patterns of boom and bust – with snails that can hold on to and grow their offspring in the bad times and release them “ready to go” when conditions are better having an advantage over egg-layers. As far as I know no one has ever come up with a way of testing that idea, so the reasons for the prevalence of live-bearers in the Pacific remains an open question.