18 April 2005

Slug's shell: a flimsy evidence for evolution

Deroceras reticulatum is a European slug that has been introduced into the U.S. It is now common in gardens, abandoned farm fields and parks in the eastern states. This individual was 50 mm long. The hump with the breathing pore behind the head is the mantle.

Hidden inside the slug Deroceras reticulatum is a clue that betrays the slug's roots; a clue that exposes the identities of its ancestors. The clue is a shell buried under the slug's mantle. But unlike the familiar spiraling shells of snails, this one is only a thin, fragile membrane of calcium carbonate that is barely noticeable. It hardly deserves to be called a shell.

But what an evidence it is. It reveals wonderfully that slugs evolved from shell-bearing snails. The slug's shell is a vestige of a distant past. It is a perfect example for Darwin's1 exquisite logic: "Rudimentary organs may be compared with the letters in a word, still retained in the spelling, but become useless in the pronunciation, but which serve as a clue in seeking for its derivation. On the view of descent with modification, we may conclude that the existence of organs in a rudimentary, imperfect, and useless condition, or quite aborted, far from presenting a strange difficulty, as they assuredly do on the ordinary doctrine of creation, might even have been anticipated, and can be accounted for by the laws of inheritance."

The shell of Deroceras reticulatum revealed under its mantle. The concentric growth lines are vaguely visible on the shell.

1. Darwin, C. 1859. On the Origin of Species by Means of Natural Selection. Full text


Turkey Villas said...

Seems convincing at first but then why would an animal with defence (shell) 'evolve' into a more vulnerable form?


Actually, land snail shells don't offer much of a defense against snails' predators. Animals that eat snails (many rodents, birds, reptiles, insects & even other snails) simply crush, break or drill thru snails' shells. Shells are more useful against drying. But slugs have overcome that problem by evolving a thick mucus that coats their bodies & also by evolving behavioral traits. For example, when the weather is dry they hide under rocks or bury deep into the soil. In comparison, under the same conditions snails can survive out in the open, because their shells retard water loss.

Even if shells were useful defense, they could still be lost, if snails evolved something else to replace them with. Slugs thick mucus also protects them to some extend against predators.

Also, the loss of a shell has many advantages that counteract the disadvantages. Building a shell is both energy, material & time consuming. By not building a shell, slugs can divert their resources to reproduction instead.

Eve said...

but don't snail shells sometimes have a protective value?
aren't there any predators thatare able to eat slugs and not snails (the snails being protected by their shell in this case)???

Anonymous said...

Yes, the shells of landsnails do have some protective value against predators, but not much. As Aydin says it mostly protects the animal against drying out. When you think about the fact that most pulmonates have thin shells and also cannot close off the aperture of their shell (not having an operculum) that leaves them pretty vulnerable to being eaten.

Snails with shells can be out in drying weather. Slugs mostly only come out after rain or at night.

Anonymous said...

Don't forget that the shells make snails so much easier to pick up :-)

This Machine Kills Communists said...

A major reason that slugs, like their distant relatives the octopi, probably lost their shell is that it allows them to easily move in and out of tight spaces that other animals can not. This provides both a defense and foraging advantage.