Home alone: mollusks build their shells from sea water

box turtle on the move.  Any minute now....
photo: Jonathan Zander, Wikimedia Commons

In warm summer months, many people travel across continents in mobile homes (a.k.a. campers or caravans), so they can visit new places and outdoor spaces with the convenience of always having their familiar nightly shelter nearby.

In nature, many critters just grow their own homes and carry them along. Not just turtles, but also snails and myriad species of shellfish carry their supportive structure on the outside.

Shells are actually the exoskeletons of various types of invertebrates (animals with no backbone), including snails, conch, clams, and oysters. Shells protect the soft mushy critter inside and give it structural support.

So where are the animals in all those shells we see on the beach?


Shells are part of the animal's body, so if you find an empty shell on the beach, the animal has died and the rest of its body has been eaten or decomposed. If it is a really pretty, highly valued shell, a person has probably killed the animal to get the shell.

A veined rapa whelk with the animal inside. This species is native to Asia
(but is now a problematic predatory invasive in the Chesapeake Bay in the US!)
photo: George Chernilevsky,Wikimedia Commons

As the animal grows, it must add new material onto the outer edge of the shell to expand it, but where and how do they find materials that becomes so hard and protective?

They get them from sea water -- very convenient.

DIY Home building: Underwater version

Each little clam, mussel, conch, or other mollusk uses the calcium and bicarbonate molecules found in sea water to produce the hard calcium carbonate substance that forms their shell. Shell production requires both calcium (Ca) and carbonate (CO3).

The ability of these animals to calcify, or produce the hard calcium carbonate structures we call shells has helped them survive for millions of years. It's a successful strategy -- yes, we humans can pry the shells open, but only using high heat or a knife, which birds, fish, and other predators lack.

This video from Minute Earth on shell-building and calcium carbonate explains this amazing DIY home building process:

As the video explains, the many millions of these critters that depend on a certain range of water chemistry face great uncertainty in the future. As we add carbon dioxide (CO2, not CO3) to the air, and thus the oceans, and the dissolved CO2 forms carbonic acid, the water becomes more acidic. The production of carbonic acid, instead of carbonate, makes shell formation possible at increasingly shallower depths, and potentially not at all.

Scientists already see thinner shells in oysters and other creatures. Have a look at this animated view of the calcification process and why adding so much CO2 to air/oceans is likely to hinder shell development.

A gorgeous Maxima clam, a small giant clam, needs CaCO3 to build its shell
photo: RevolverOcelot, Wikimedia Commons

Not only shellfish, but also corals, need the calcium to make their skeletons, so even if they don't die, they are weak, won't grow, and are more vulnerable -- how can a clam survive without a strong, hard shell to protect it from hungry fish, shorebirds, starfish, or sea otters long enough to reproduce?