The European and Mediterranean representative of the family Subulinidea (a relative of Achatinidae), Rumina decollata is a special snail in so many ways.
First of all, it is a voracious predator (although it will also feed on vegetables and decaying matter). Second, any other snail with a fractured shell apex would not, under most circumstances, survive. But to this disconcerting species, a broken shell is its trademark. And an inevitable part of its growth.
June 2010: Almost transparent in sunlight , a young Rumina decollata shows
the truncated shell its species owns its name to (decollata = de-tipped)
The same snail, in September 2010: none of its juvenile whorls are left
The decollate snail (Rumina decollata) is a common garden dweller in many parts of Europe, easily identified by its conic, light-brown-to-cream shell with slightly convex whorls, and, most of all, its truncated apex (tip).
In most terrestrial snails, growth is simply a process of addition: new whorls form around the columella and the shell size progressively increases. Rumina decollata however is a special exception to this: a juvenile may have about 8 whorls, but an adult just as few as 4. Where did the missing whorls go?
May 2010, juvenile Rumina decollata: note the shell tip still intact
The answer lies in the process of shell decollation (colloquially known as “de-tipping”), a progressive destruction or loss of early whorls that occurs along the growth of new ones. Beginning with the loss of the blunt tip of the intact, juvenile shell.
Tip loss in juvenile Rumina decollata
A calcareous seal: survival of the snail despite the apex loss is ensured thanks to a calcareous deposit that formed inside before the tip loss.
As the snail continues growing and new, much larger whorls form, changes occur in the old ones. The snail retreats in the new whorls, forms a new calcareous seal inside the shell and the old whorls, now fragile and empty, are chipped off.
Shell growth after tip loss (August 2010)
Decollation – inevitable? Some sources explain the tip loss in Rumina decollata as a consequence of grinding the adult shell against hard objects, but based on my observations in captivity, this is not the case.
Firstly, it does not occur in adulthood, but at an early stage of growth, as shown in the pictures. Secondly, while in the wild friction against rocks and hard surfaces may aid or accelerate the decollation process, the loss of old whorls for Rumina decollata is inevitable even in a trauma-free existence.
Adult shell: by now, all the early, juvenile whorls have been lost
To put at test the theory that decollation occurs simply as a consequence of friction, the specimen shown above – nicknamed Dedalus – was raised in soft-substratum terrarium and all precautions were taken to preserve its shell intact for as long as possible. In spite of that, old whorls progressively changed, appearing thinner and more fragile just before chipping off.
The impression I was given was that the snail was actually reabsorbing calcium from the old whorls (despite the extra calcium supplies available in the tank in the form of cuttlefish bones) before ridding of them.
Costly compromise? As far as my personal theory goes, these snails, with their long, uncomfortable shells have been at great disadvantage as predators, but have successfully compensated for their handicap thanks to decollation. A process, however, which would have never been possible without their “ability” to seal the shell before chipping of the apex and early whorls.