by Mark Carnall, Collections Manager in the Life Collections
When giving tours of the invertebrate collections at the Museum, I don’t have much time to cover the considerable diversity of invertebrate animals. When it comes to molluscs (the group including snails, bivalves, squid, octopuses, chitons etc.), which perhaps most people aren’t too excited about, I try to inspire, enthuse and engage with this diverse group by pulling out some of the more weird and wonderful species from the group.
Xenophorids, or Carrier Shells, are up there on the list of weird and wonderful molluscs. Xenophoridae is a small family of around 30 species of marine snails that live on sandy and muddy sea floors in subtropical and tropical seas. So far so snail.
What makes them interesting is that these animals attach objects they encounter to the outside of their shells. The scientific and common name of the group is derived from this behaviour xenos and phoros from Greek translates to “foreign carriers” or “carrier shells”.
Carriers have been found with shells, shell fragments, pebbles, fossils, corals and even shark teeth attached. Predictably human detritus such as coins, bottlecaps and glass and metal fragments have also been found. In this way, they are one of nature’s collectors. Shells preserved in museum collections give us some direct evidence of habitats, environments and preferences of different individuals and species, from the objects affixed to them.
The mechanisms and evolution of this behaviour are still not well understood and few of the described species have been studied in depth. In species where the process of attachment has been observed, objects are painstakingly manoeuvred to an attachment site by the tentacles, proboscis and foot, the shell surface is then cleaned and the object cemented in place. Generally, objects are deliberately cemented to follow the lines of the shell maintaining an overall cone-shaped shell.
There are a number of competing or complementary explanations for why carrier shells attach objects to themselves. This behaviour in molluscs is not unique to Xenophorids, a number of fossils species are known, and some living species in other groups attach things to their shells, but not to the same extent as found in this group.
Perhaps most obviously, this behaviour has been suggested as a form of camouflaging, breaking up the outline of the shell against a sandy sea floor. In some species, long objects attached to the outer whorl of the shell raise it up like stilts, and it’s been suggested that this help to break up a continuous scent trail.
Other suggestions to explain this behaviour include; added armour; predator deterrent; prevention of shells being flipped over and damage to core tissues from shell-borers. It may even increase the surface area in contact with substrate, preventing it from sinking into soft sands or muds.
Xenophorids aren’t the only animals to ‘collect’ objects from their surroundings. Some of the better known examples are; Bower Birds, which discriminatively collect coloured objects to decorate their bowers; a number of insect larvae and marine worms, which incorporate objects into cases, shells and tubes, and Hermit Crabs, famed for collecting shells in which to live.