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Meet the shell collector of the sea: carrier shells

By Staff | Mar 6, 2009

One of the most unique behaviors in mollusks is displayed by carrier shells. Many species are found worldwide, with a few present in the Caribbean and Florida. If a shell collector turned into a shell it’s likely he would become a carrier shell. It builds it its own shell by cementing whole shells, pieces of shells, or rocks to its upper surface. The materials chosen by the snail reflect their surroundings. Other marine creatures like sea urchins adorn themselves with shells so they can blend into the background of their environment, but unlike carrier shells, sea urchins can discard their “decorations” at any time.

The first few times I found a carrier shell it was immediately tossed back into the water, with the belief it was simply a random accumulation of material. Then one day I witnessed one of these curious masses moving in a tidal pool. I was certain that I was seeing things but after careful examination it became apparent that this moving pile of shells was actually concealing a living shell. It reminds me of playing hide-and-seek with my Grandchildren. It’s always so much fun to hide somewhere quite obvious, shout boo, and watch their sense of amazement that they were unable to extract me from the environment. If this creature had a sense of humor I suspect he’d be having quite a laugh as shellers are puzzled by his hide-and-seek appearance.

A logical explanation for this shell-cementing ability could be that this collection of shells serves as camouflage. Some animals find their prey by touch. All of the carrier shell’s attachments might confuse them. Shells with heavy attachments tend to be top

heavy. By strategically placing their collection of attachments they are provided with a broader base making them less likely to tip over, exposing their vulnerable underside to predators. Also, attachments would be of some use in protecting them against drilling gastropods. One theory is that since the shells added extend outward, creating a snowshoe type effect, they may prevent the shell from sinking into the mud. Carrier shells found in different areas of the world have unique methods of decorating their shells; using rocks, live coral, and sponges.

The carrier shell’s building methods are intriguing. It’s amazing to think that these small creatures can actually select just the right building materials needed, materials that are the perfect size, shape, and texture. Attaching these building materials is not as simple as it seems. The carrier shell turns the new material over, twisting it until it is in the exact position required. It uses its head and proboscis to lift the new piece into place, and its foot to raise and lower its shell at the same time. Up to an hour and a half is sometimes spent putting a new piece in place. Gaps are filled with pieces of sand and debris. Its head and proboscis is used to gently rock back and forth to check the security of the newest piece of armor. Then the mollusk remains stationary to assure a tight bond. With larger attachments this could take over ten hours.

Initially, carrier shells grow at the top or apex of the shell, fastening small pieces of shells with his special glue. As the shell grows, new wardrobe additions are added to the perimeter of the shell. Several Pacific species display incredible patterns. They add mostly whole shells and larger shell pieces to construct the outside trimming layer. The edging material is placed with a downward slope. Mature shells add a final border of shells. Single valves of bivalves and gastropod shells are the construction materials of choice. The shells used in this final layer are roughly the same size and are arranged to form a scalloping effect, protruding from the outer edge of the shell itself.

Florida hosts two deep water species and a shallow water species, the Atlantic carrier shell (Xenophora conchyliophora). It is one of the smallest carrier shells, ranging from one to three inches in diameter. It’s found from North Carolina to the Caribbean. The thin shell is cone shaped and wider than it is high with construction resembling a spiral staircase. The sticky substance secreted by the snail not only holds the camouflage material in place but also reinforces the thin shell.

The animal itself is a deep maroon red. In order to move, a large muscular foot thrusts the operculum (trap door) into the mud, flipping it forward. The foot can extend to a

height equal to that of the shell and lift up to three times the weight of the shell.

Xenophora conchyliophora snails chow down on microscopic algae, grazing in the pattern of a patchwork quilt, leaving no trail for a predator to follow. Only after it depletes the food in one area does it move to a new area.

The scientific name for this species originates from Greek words meaning “bearing strangers, bearing shells” an apropos handle for the shell collector of the sea.