13 January 2011

Where and how did slugs evolve?

Slugs are snails that changed their body plans and lost their external shells during evolution. I don't yet know the answer to the question posed in the title, but the late Alan Solem, in his 1974 book The Shell Makers: Introducing Mollusks, claimed that slugs evolved "under conditions of plentiful moisture and scarcity of calcium".

I don't find Solem's meager arguments in support of his hypothesis very convincing. Many species of slugs are native to areas where moisture is not plentiful, but calcium is abundant. For example, slugs live in western Turkey where the summers, under the influence of the Mediterranean climate, are bone dry, while limestone rocks provide an unlimited supply of calcium.

I am inclined to believe that if slugs can and do survive in places where moisture is not always plentiful, then the plentifulness of water couldn't have been a critical factor during their evolution.

The question boils down to this: if species A evolves into species B as a direct result of the influence of the environmental condition X, can species B continue to survive at the same location (and without undergoing further evolution) if the environmental condition X ceases to exist?


Anonymous said...

From my extensive field experiences in England, I've always thought it was quite clear that land slugs, by virtue of not having a shell, can occupy a whole range of habitats that similar-sized snails with shells cannot exploit at all. Even quite large slugs can fit under stone slabs that are lying flat on the ground, or under bark on dead tree trunks in woodland, and other similar habitats where living space is very compressed indeed, but where residual moisture lurks, and where the slugs are well-protected from most species of predators when resting. To me, all these new microhabitats that slugs could move into seem like reason enough for shell-less-ness to have had survival value.

Susan J. Hewitt

The King of Sweden said...

I've heard that slugs(shell-less-ness) have evolved numerous times over their evolutionary history. "Slugs belong to several different lineages that also include snails with shells. The shell-less condition has arisen many times independently during the evolutionary past, and thus the category "slug" is emphatically a polyphyletic one. The various groups of land slugs are not closely related, despite a superficial similarity in the overall body form."

therousedbear said...

Lack of moisture might be a factor encouraging slug evolution if slugs can find the microhabitats (i.e., cracks within rocks) that have the most moisture.