The Evolution of Molluscs


Shellfish are often on plates in our homes and in restaurants. Many of these are bivalves – clams, oysters, mussels, abalone, and scallops. Think of how difficult it is to get to the tasty meat of some of these animals. Over millions of years, the bivalves evolved formidable defenses in their hard shells

Bivalves are molluscs, a large phylum (group) of animals consisting of over 70,000 species. Many species you might not imagine are related are: they’re all molluscs. If you go tide pooling, you might see lots of molluscs, depending on where you live. Chitons and snails are molluscs that reside in tide pools, as do those shellfish – clams, oysters, mussels, and abalone. Much more flamboyant are the sea slugs, shell-less snails (nudibranchs) that live in the low intertidal and also deeper. You might see them snorkeling or scuba diving. If you do, take only pictures of these flashy molluscs.

If you snorkel or scuba dive, you might be lucky enough to see a cephalopod: the squids, octopus and cuttlefish. Cephalopod means “foot-on-head.” These are molluscs that evolved the ultimate defense of becoming fast swimmers and masters of disguise.

Molluscs first appeared in the Cambrian period over 500 million years ago. They’ve evolved, adapted and survived. But, today molluscs face threats from our actions in the form of climate change. As greenhouse gas emissions have increased, the oceans have been absorbing tons of carbon dioxide. But, all that carbon dioxide has changed ocean chemistry – it’s becoming more acidic. As shelled-molluscs grow, they need calcium carbonate to build their shells but the ocean is becoming more and more corrosive, dissolving their shells. This isn’t a problem in the future, it’s happening in some places right now.

To read a blog I wrote about ocean acidification and oysters see:

To see a video about the molluscs, watch Molluscs: the Survival Game on the Shape of Life: website:

To see some molluscs in action, watch these short videos:

This video shows the basic body plan of molluscs:





About Author

My passion for the natural world began as a child on the rocky shores of Maine, and in streams and woods in elementary school. My connection to all of nature continued with a path to a M.S. in animal behavior and ecology. As a resident and interpreter of the central California coast, I try get out on the ocean and into the tidepools and mudflats as often as I can. I’ve researched the natural history, oceanography, marine life and ecosystems, and human history of this coast for decades. As one of the early employees of the Monterey Bay Aquarium, I committed to interpreting ocean life and habitats to the general public. Since those early days, I’ve been involved in the research, interpretive planning and writing of exhibitions for aquariums and natural history museums. I helped to create and continue to develop content and write for the Shape of Life.

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