Xenophoridae is a family of marine snail also known as the carrier snail. Unlike the browny-green snails one might find in their garden, each individual carrier snail is beautifully unique. Many of species in this clade carry around a collection of rocks, shells, and even other organisms like coral on their backs. These animals adorn themselves by cementing foreign material to their shells as they grow and the results are spectacular.
Xenophoridae are deposit feeders and are found on the continental shelves and slopes of tropical and temperate regions around the world. I was first introduced to this clade when I visited the Harvard Museum of Comparative Zoology earlier in June. On my return to the Peabody at Yale, I was eager to investigate the Xenophoridae specimens in our own collection, a selection of which as pictured below.
As the images show, the carrier snail’s collecting ranges from a hodgepodge of debris to intricately laid out designs. Pebbles, shells and coral are all common adornments, and you can almost imagine a snail crawling along the seafloor spying something it fancies and adding it to its personal collection. This behavior makes scientific name of these creatures particularly fitting, Xenophoridae comes from two ancient Greek base words, Xeno meaning “foreigner” and Phore meaning “bearing”. By decorating their shells with foreign material, these animals transform themselves from basic snail into glamorous mollusk.
Artists or hoarders? How do these snails adorn their shells?
As stunning as the outcome is, the question that begs to be asked is how do these snails decorate themselves? A paper published by Malacologist (a scientist who studies Mollusca) from New Zealand, Winston F. Ponder, helps explain this behavior. Using either their foot or proboscis, a snail will lift a new decoration up to a spot on its shoulder. Here, it will cement the object to it’s shell using a gooey substance which it excretes from its mantle. The mantle is a thin layer of tissue that lies between the main body and the internal shell. This tissue’s main function is to secrete a calcium carbonate protein matrix that helps grow the shell, but in the case of Xenophoridae it is also used to cement new decorations to the shell.
Carrier snails tend to select decorations proportional to their size, with younger snails sporting smaller adornments and selecting increasingly larger objects as they grow. The image below nicely depicts this trend , as the snail’s shell spirals outwards you can see how the decorations increase from smaller fragments at the centre to whole bivalves on the outer whorls.
Because the shell spirals outwards as it grows, an adult organism’s decoration is not only aesthetic but also a timeline of its life. There are a number of theories as to why carrier snails choose to adorn themselves, but regardless of whether the behavior is for camouflage, to defend themselves from predators or to physically support themselves, these snails do it with style.
Variety in Design
It is not just their well-suited name and curious behavior that makes these organisms interesting, but the samples of foreign material cemented to each shell can also reveal clues about the ecosystem and environment in which the snail was living.
The Peabody collection hosts a variety of Xenophoridae, some shells are adorned by diverse coral pieces, while others are almost bare or embossed with spikes. In a collection of organisms so varied in appearance, one species, the sun shaped Stellaria solaris specimen stood out to me.
This species was collected from the Indian Ocean, near Burma. The radiating spines of this species are suggested to be an adaptation for stability in the soft mud environment in which it lives. These radiating spines increase the shell’s surface area and project downwards to help to hold the snail up off the soft sea floor, similar to how a snow shoe can help prevent you from sinking into the snow.
In contrast to Stellaria solaris, the Western Atlantic Xenophora conchyliophora is an example of a more heavily adorned species.
These two specimens, from Eluthera Island in the Bahamas are heavily armored with coral. These heavy carbonate shells would be more of a hinderance than a help if Xenophora conchyliophora lived in the same soft sediment that Stellaria solaris does. The specimen on the left (YPM IZ 105064) was collected from Governor’s Harbour, Eluthera. This location boasts clear water, coralline sand and reef, the shell of Xenophora conchyliophora is a perfect illustration of it’s environment.
Carrier snails who decorate their shells with bivalves typically select dead shells and position them inner side upwards. However in some environments Xenophora have been known to attach live coral to themselves in such a way that the coral will continue to live long after attachment. It is almost as if the carrier snail wants a cnidarian friend for company wherever it goes, I guess one way to never be lonely is to glue someone to your back. This however is unlikely the reasoning, qualitative analyses suggest that Xenophora have particular preferences for the shape and positioning of their coral, making them less lonely hoarders and more fastidious artists.
While they do sometimes select to attach live coral, Xenophora specimens have also been found with sponges, hydroids and polychaetes living on their shells. This though, isn’t necessarily by choice, the camouflage resulting from self-decoration can be so good that other animals can be fooled into taking up residence.
The collections of any natural history museum will tell us that humans will collect things as a hobby, for decoration, for nostalgia or for science . Although, Xenophoridae are the collectors of the deep, their collections are more for the purpose of survival than for pleasure. Nonetheless, while alive each carrier snail acts as its own curator and transforms itself into a mobile curiosity cabinet, a calcium carbonate legacy that remains even after the organism dies. This unique behavior makes the Xenaphoridae stored at the Peabody Museum more than individual specimens, but individually curated mini-collections waiting to be further explored.