While the exact method used is a mystery, each Norris’ kelp snail (Norrisia norrisi), like most other marine snails spends the first weeks to months of its life passing through microscopic larval forms that bear scant resemblance to the final adult form. Only once the last larval form receives the necessary cues signaling it to settle to the bottom, does it begin to develop into something that we would recognize as a snail.
But even at this point the once and future snail is microscopic, as is the tiny shell that has accompanied it since it began to form as an embryo. As it develops, this initial shell is replaced by a shell of calcium carbonate, or limestone. The problem is, limestone is a mineral that does not grow, so how does the shell of our microscopic snail accommodate the snail as it grows. One clue can be seen in today’s photos.
Look closely at the shell and you will clearly see the spiral shape of the shell as you trace it from the large opening from which the snail’s fleshy body orange body emerges, all the way back down to what appears to a be a single point in the center. Way down there is where the snail’s original shell can be found. After noticing the main spiral, look for the parallel curved lines that run across the direction of the main spiral. You can find these lines all the way out to edge of the shell opening. And it is these lines that provide us the clue we need to understand how a snail’s shell grows.
The limestone of the shell is deposited by that part of the snail’s body that is in contact with the inner surface of the shell at the opening. This tissue, called the mantle, grows as the snail grows, and as it does, it lays down a new, slightly large rim at the current opening. Thus, each of the curved lines represents the location of the opening of the shell when the snail was smaller.
This tricky bit of chemistry and biology allows the snail to make an inanimate piece of mineral, its shell, grow.
This photo was taken at Santa Cruz in the northern Channel Islands on the California coast with a Canon EF100 mm f/2.8 Macro USM lens on a Canon EOS 5D Mk III in an Ikelite underwater housing. Lighting was provided by an Ikelite DS161 strobe set to eTTL exposure mode. The exposure was set to 1/200 sec at f/6.7 and ISO 400