Smooth or Spiny?

by Peggy Williams

The sculpture of shells has a lot to do with the way the animal lives. "Sculpture" means the outside profile of a shell - whether it has spines, ridges, is smooth, etc.

Animals that burrow in sand need smooth shells to help slide into the sand. Spines and bumps would get in the way. Many of these shells are bullet-shaped as well. Think of an Olive shell: long, smooth, and spineless. Olives burrow into the sand in search of bivalves (which are also buried) to eat and for protection. Some members of the olive family live in the surf and flip in and out of the sand as the waves wash over them. They have to be able to burrow especially quickly, lest they are washed far into the depths where they don't want to be.

To help even more, the olives secrete a thick mucous that oils their way underground. This helps keep the shell smooth, without any encrustations, and so does the fact that, when moving, the animal completely covers the shell with its mantle. If you pick up live olives and put them in your bucket you will find, in the end, a bucket full of slime!

Many of the bivalves, too, are smooth of shell, for example, Coquinas, and Tellins. They, too, burrow and need to burrow quickly when dislodged by waves.

Other bivalves that bury themselves go so deep that they are seldom dislodged, and their

shells help them both to burrow and to stay put once underground. Angel Wings are a prime example: their ridged shell can be rocked back and forth to help excavate the mud around them, and, once settled, the same

ridges help keep the shell in place. Pen shells, too, have spines to help anchor them, and they also secrete a byssus, or holdfast, to help keep them in place.

Spiny shells, in contrast, almost never bury in the sand. Murexes are mostly surface dwellers, and their spines help in various ways. They are efficient protection from predation, making the shell much larger than needed for living space with efficient use of a small amount of shell material. The spines also attract algae, barnacles, and other settling organisms that aid in camouflage. Those murexes that do occasionally bury (during low tide, for instance) have a long, slim siphonal canal and a relatively smooth body whorl.


Thorny Oysters are bivalves that "stay put" by cementing themselves onto the surface of rocks and corals. The spines are not mere decoration (though we value them for that) - they are protection from predators and gatherers of camouflage.

slender colonies of soft corals known as "Gorgonians". The corals have a semi-hard outer shell which may be colored yellow, orange or purple, while the coral animal stretches out a corona of white tentacles. The shells and base color of the mantle of the simnias that live on these corals are colored exactly like their hosts, and the mantle of the mollusk mimics the coral polyps with splashes of white. The shell itself is slender and fits exactly onto the "blade" of the gorgonian colony so it is nearly impossible to see.

The incredible variety of shapes is one of the things that most attracts us to the shells of mollusks; and yet, it's not due to any attempt at aesthetics. Such things are products of natural selection for survival in a variety of marine habitats. What a wonderful world!

Most shells that live in the intertidal zone on rocks have a large mouth. This allows for more area of the foot to hang tight to the rock, keeping the animal in place during surges of the tide. Good examples are the "Wide-Mouthed Purple Shell" and limpets, which have no visible early whorls at all.

Some shells slide into and out of holes in rock and coral. The cowries' smooth, spineless shells help them do so.

Shells that live in unique habitats are also

especially adapted. The Simnias live on long,

Article and pictures copyrighted by Peggy Williams. They may be used in Shell Club publications with attribution to Peggy Williams, www.Shelltrips.com.