How to Build A Snail

Raphaela Osterauer, Leonie Marschner, Oliver Betz, Matthias Gerberding, Banthita Sawasdee, Peter Cloetens, Nadine Haus, Bernd Sures, Rita Triebskorn, Heinz-R. Köhler. Turning snails into slugs: induced body plan changes and formation of an internal shell. Evolution & Development, 2010; 12 (5): 474

Osterauer et al. recently published a paper in the journal Evolution and Development where they were able to radically change the bodies of adult snails ( Marisa cornuarietis ) by exposing embryos to the metal platinum.

Snails, along with slugs, clams, octopi, and squid, are part of the group Mollusca (unsure of where the molluscs fit into the animal tree? Check the reference phylogeny).  Molluscs share a basic bodyplan, although it has been highly modified in different groups.

Snails, for example, develop their shells through a bizarre developmental process called torsion.  During torsion, multiple parts of the body are rotated 180 degrees.  By the end, the anus and gills are sitting above the head of the animal, the nervous system has turned into a figure 8, and the right and left hand sides of the animals have become assymetrical:

While scientists don’t entirely understand why torsion occurs, it is related to the growth of the external shell and the muscles that attach to it.  In other species of molluscs, such as cephalopods (octopi, squid, cuttlefish), nudibranchs (sea hares), and pulmonate slugs, torsion never occurs.  Instead, the shell grows inside of the animal, creating a cone-shaped internal shell. The cuttlefish bone, which is often sold in pet stores as a calcium supplement for birds, is an example of this internal shell.

While trying to study the effects of certain toxic metals on the snail M. cornuarietis, Osterauer et al. found that exposure to platinum led some snails to loose their shells.  In this study, the scientists discovered that when M. cornuarietis embryos are exposed to a concentration of 164.4 milligrams of platinum per liter during the fourth and fifth days of development, the mantle tissue stays inside of the body (at this level of platinum, 100% of animals tested had this result).  In a normal snail, the mantle tissue moves to the outside of the animal, and is responsible for secreting the chemicals that harden into a shell.  In the image below, the edge of the mantle tissue is stained black and pointed out with an arrow; notice the difference between the normal snail embryo on the left and the platinum-exposed embryo on the right:

So instead of forming a normal shell, the shell developed within the body, like one might find in a squid or slug. These shells developed cone-like shapes that are reminiscent of those found in mollusks that lack external shells. Below are photos of adult shell-less snails, with images of their internal shells superimposed to show size and location:

Without normal shell-formation, the gills and mantle tissue stay in the posterior end of the animal like in other molluscs.  The gut, however, still goes through torsion, with the anus moving into the anterior region.

The fact that they recovered similar results from a second, distantly related snail, Planorbarius corneus, increases the possibility that the authors have discovered an important aspect of snail torsion, as opposed to an anomaly restricted to Marisa cornuarietis.

Here are the points I think are important in this paper:

1) This study does NOT suggest that platinum was responsible for the evolution of snails into slugs and cephalopods (or vice versa).

Because platinum is not having any effect on the DNA of the snails, they cannot pass these changes on to their offspring.  If a change is not heritable then it cannot be important to evolution in the long term. 

2) What this work does suggest is that a small change in development plays a huge role in the design of a snail.

The most important point of this paper is that one change, the position of the mantle tissue, affected many attributes of the snail, including the shape of the shell, the position of the shell and the position of the gills.  Instead of each of these characteristics having to evolve independently in the evolution of snails into slugs and cephalopods (or vice versa, the evolutionary history of the molluscs is still not well known), one change in development could affect all of these attributes at once.

While platinum exposure almost certainly was not responsible for this evolutionary change, the platinum is probably disrupting a gene network that is important to snail torsion.  If scientists can determine which gene(s) are involved in this change, they might discover how natural mutations in snail DNA led to the same results as the platinum exposure. Hopefully this work will be followed up on by these researchers.  In fact, Osterauer et al. already have an educated guess that the platinum might be disrupting a Calcium-based positioning system, although the authors did not find any significant difference in calcium uptake between experimental and control animals.