When walking on the beach do you ever wonder why shells have so many different shapes and colours? After all, as exoskeletons they all have the same job to do: protect the soft body within. Doing that though, they display an incredible variety of ridges and spirals, twists and frills, pigments and patterns. It turns out that this not only reflects the life cycles of molluscs as species, but their individual experiences as well.
In the shells of molluscs (chitons, mussels, oysters, limpets, and sea snails like dogwinkles and periwinkles) geo-matter and bio-matter meet: they have two mineralized outer layers (largely calcium carbonate), and an inner layer called “conchiolin”, which is made of chitin and protein. Chitin is analogous to cellulose in plants; both are derived from glucose and have the same strengthening function. Chitin is secreted from a “mantle” as the shellfish grows, forming a base on which the outer shell is built. The molluscs we see in tide pools don’t shed their shells like crustaceans do. Their shells grow continuously from the mantle, which contains neuro-sensory cells whose active synapses fire in response to environmental stimuli like temperature, sunlight, depth of water and tidal action. The operative word is “response”: according to the Proceedings of the National Academy of Sciences (U.S.) the manifestations of molluscs’ diverse shell structure and pigmentation depend on how they feel. Their nerve cells transmit “travelling waves of excitation and inhibition” which create the beautiful patterns and colours we love in shells. Who hasn’t collected a few favourites to range along their shelves at home? We are displaying not only an example of nature’s beauty, but also of nature’s blending of matter and energy, chemistry and sensation.
The individual life experiences of molluscs can be raucous and challenging. The beautiful ridged and banded spirals of dogwinkle clothe an animal which is preyed on by starfish and crabs, and in turn eats mussels, barnacles, limpets and fellow snails. It’s a dogwinkle-eats-dogwinkle world down there in the inter-tidal zone.
The life of an oyster is even more soap-operatic. For them, gender is differentiated differently than for mammals: they begin life as males and end it as females. In between, adults release either eggs (50 to 200 million in a spawning) through gills via the mantle or sperm from the opposite end of the shell, into the surrounding water where fertilization takes place. Larvae develop in stages and eventually attach themselves to surfaces like rock or bigger oysters, and become sedentary adult filter feeders. If left un-harvested Pacific coast oysters can live up to 30 years, but they are prey to an array of viruses, bacteria and fungi (which can make them dangerous for human consumption).
The hard, rough surface of an oyster shell is very different from the smooth curves of zigzag-patterned snails and clams, and apparently that’s due to their different experience of environment-driven neuronal triggering and oscillations. Scientists have measured these inputs mathematically (see A. Boetinnger, “The Neural Origins of Shell Structure” if you want to pursue this), but more interesting to the beachcomber is the subjective-experience aspect. Next time you walk on the beach among shellfish, you can wonder how they’re feeling today. Excited or inhibited? Fresh or polluted? Cool enough, or is that tide-pool water getting a bit too warm?
We measure ocean characteristics with scientific instruments, but shellfish measure it through their own sense organs. For them, it’s personal. We all know that dogs and cats are people too – but oysters? Well, no. But who knew that even they have each a life story?
Barbara Julien is a local writer and nature enthusiast.