Animal Diversity Web

Diversity

Members of the genus Xenophora are marine snails with a habit of attaching foreign objects, such as shells, pebbles, and coral, to their own shells. It is this unusual practice that provides their common name, the "Carrier Shells." The earliest carrier shells appear in the fossil record during the Cretaceous period, though most of their fossil diversity appears in the Cenozoic era. Carrier shells dwell on continental shelves and enjoy a benthic lifestyle, feasting on algae. There are currently 15 extant species recognized within the genus Xenophora, with a wide distribution across the shallow waters of the globe. There have been many studies looking at the possible reasons for the group's decorative habits, and though camouflage has been the most popular theory, there are still some alternative theories. In addition to the unanswered questions regarding their shells, there is much to be learned about the reproduction, anatomy, and natural history of these organisms. (Berg, 1975; "Devonian Carrier Shells (Euomphalidae) from North America and Germany", 1973; Ponder, 1983; "Xenophora Fischer von Waldheim, 1807", 2019)

Geographic Range

Carrier shells have a wide distribution across the continental shelves of most of the Earths' landmasses. Many species within the genus can be found in the Indo-Pacific, though they are generally limited to the shallow waters closer to shore rather than the open ocean. Carrier shells are also distributed along the Western coast of Africa, the Gulf of Mexico, and the Mediterranean Sea. Their southernmost distribution belongs to Xenophora peroniana peroniana at the 40S parallel along the Bass Strait of Australia. Xenophora crispa has the northernmost range of the genus, extending into the Bay of Biscay off the Northern coast of Spain. (Crippa, et al., 2020; Ponder, 1983)

• Biogeographic Regions
• indian ocean
  • native
• atlantic ocean
  • native
• pacific ocean
  • native
• mediterranean sea
  • native

• Other Geographic Terms
• cosmopolitan

Habitat

Carrier shells are benthic animals that live on the sea floor of continental shelves, generally existing at a depth ranging from 0 to 450 meters. Given their proclivity for shallow reefs and subtidal algal plans, carrier shells are almost completely limited to coastal areas. However, Xenophora pallidula, Xenophora japonica, and Xenophora crispa have all been recorded living past depths of 1000 meters on continental slopes in the Aphotic zone. (Berg, 1975; Feinstein and Cairns, 1998; Ponder, 1983)

• Habitat Regions
• temperate
• tropical
• saltwater or marine

• Aquatic Biomes
• benthic
• coastal

Systematic and Taxonomic History

This genus was first described in 1807 by Gotthelf Fischer von Waldheim. He dubbed this group of organisms Xenophora: “xeno-” meaning foreign and “-phora” meaning bearer, in reference to the shell that bears foreign bodies. The genus Xenophora is one of three genera currently recognized in the family Xenophoridae. This family used to have a single genus, Xenophora, with three subgenera: Xenophora, Onustus, and Stellaria. Included in a 1999 description of a new subgenus of Xenophora, the authors stated that the differences between these three subgenera were large enough to advocate genus status for them all. Since then, it has been commonplace to recognize these three rankings as separate genera within Xenophoridae.

A 2021 study of the mitochondrial genomes of snails within the superfamily Stromboidea placed the family Xenophoridae within this group. (Fischer von Waldheim, 1807; Irwin, et al., 2021; Kreipl, et al., 1999; Nielsen and DeVries, 2002; Ponder, 1983; "Xenophora Fischer von Waldheim, 1807", 2019)

• Synonyms

  • Phorus Montfort, 1810
  • Xenophorus

• Synapomorphies

  • More than 1/3 of the dorsal surface of the shell covered in foreign objects
  • Shell with ventral side of peripheral flange non-porcellanous
  • Shell with narrow, simple peripheral flange

Physical Description

The shells of this genus are orbicular, have a flatly conical spire, and a narrow peripheral flange that separates the visible upper surface from the flat or concave base. The shells are wider than the are tall. Some species are larger than others, but they generally range is diameter from 20 to 50 millimeters. Their shells are white, but certain species may have subtle rose, brown, or yellow tints, especially towards the upper part of the shell. The defining visual trait of carrier shells are the shells, pebbles, and other debris that are attached in spiral sequence along the shell whorls. Foreign matter usually covers more than one third of the shells' upper surface, though some species carry more than others.

The heads of the snails, which can be quite colorful in certain species, bear two long sensory tentacles, with their eyes located at the base of these extensions. The mouth is located between the paired lobes of the snout, housing a small pair of jaws and the radula. The snout lies above the propodium, which has a rounded anterior edge and is connected to the mucous gland. The operculum, which acts as a closed door when the snail retracts into its shell, is hard and thick. This structure is large in comparison to the relatively small foot.

The mantle cavity of carrier shells is taken up mostly by a large ctenidium composed of long, narrow filaments used for respiration. The rectum also takes up a large amount of space in the mantle cavity and opens into the anus at the mantle edge. The mantle cavity also houses a sensory osphradium, a mucous producing hypobranchial gland, and glandular pad that aids in the disposal of feces. (Morton, 1958; Ponder, 1983; Reeve, 1843)

• Other Physical Features
• ectothermic
• bilateral symmetry

Development

Investigation of a Xenophora neozelanica specimen has led to the measurement of their small eggs, the largest being 0.18 millimeters. Nothing is known for certain about the larval development of carrier shells or the nature of their spawn. The anatomy of the protoconch, or embryonic shell, strongly suggests that carrier shells go through a planktonic larval stage. (Ponder, 1983)

Reproduction

The ways in which carrier shells find and attract mates have not yet been reported in the literature.

Though little is known about their reproduction, the genital anatomy of carrier shells offers some insight into their reproductive habits. Like most marine snails, members of this genus do have separate sexes. It is likely that the penis of the male is inserted into the muscular vaginal opening of the female, where it will release sperm into the posterior part of the vaginal tube to fertilize the eggs. The vaginal opening is in the posterior part of the female's mantle cavity, which is unusual amongst mesogastropods. This feature may account for the large penis size in members of this genus. (Ponder, 1983)

• Key Reproductive Features
• sexual
• fertilization
  • internal

The parental care of carrier shells has not been described.

Lifespan/Longevity

Little is known about the lifespan of carrier shells in the wild. Living carrier shells dredged off the coast of New Zealand survived in aquariums for more than 4 months. As these specimens aged, their bodies changed from a white coloration to a bright orange. ("Devonian Carrier Shells (Euomphalidae) from North America and Germany", 1973)

Behavior

Carrier shells are most known for their habit of attaching foreign material to their own shells. Carrier shells have been found adorned in a wide range of materials, including bivalve and gastropod shells, coral both living and dead, echinoderm spines, sponges, pebbles, sand grains, and even shark teeth. These materials are meticulously positioned on the mantle edge by the snail, always with a downward slope in relation to the mantle. New shell material is then secreted by the mantle to attach the object, a process requiring the snail to keep still for over 10 hours at a time. In the species that dwell in the photic zone, this practice is most likely undertaken to increase the shell's visual camouflage with the substrate it resides on. This theory likely does not hold up in aphotic-dwelling species like Xenophora japonica and Xenophora pallidula, where attachment of objects for visual camouflage would be an energy intensive and nonadaptive activity. These two species remain highly decorated despite having no use for disguising their physical appearance. In these cases, attachment of foreign objects is likely used for olfactory and tactile concealment.

These snails spend most of their time only making contact with the substrate via the peripheral flange of their shell and the the foreign matter that hangs over this structure. They keep their foot lifted away from the substrate, making contact with the surface only when moving. The locomotion of carrier shells has been described as a "one-legged stomp" or a "leaping motion." With the plantar surface of the foot on the substrate, the muscular column will extend the shell upwards. The shell is then thrust forward until it falls. Carrier shells spend most of their time motionless, but when they do move, their motion is sudden and discontinuous. This aversion to touching their foot to the substrate is suggested to be a way of concealing the chemical trail of the organism. A study of wild Xenophora conchyliophora revealed that these animals move, on average, 233.5 centimeters in a 24-hour period.

When tipped over, carrier shells right themselves by burying their propodium into the substratum and then contracting the footstalk, which slowly pulls the shell into its proper position. This process exposes the soft parts of the snail's body, so it is carried out cautiously. If there is any movement detected within 2 meters as the snail tries to crawl its propodium into the sand, it will retract back into its shell. Once the propodium is firmly in the substratum though, it will continue to try and right itself even if disturbances are nearby.

Paul Shank's 1969 study of a captive Xenophora conchyliophora revealed an odd practice of burying feces. Using its snout and propodium, the snail would dig a hole in the sand into which its waste would be deposited. The excrement was then completely covered with sand. Large stores of fecal matter are found in carrier shell rectums, suggesting that waste is deposited infrequently. This process, like the locomotion and ornamentation practices of carrier shells, is likely a means of eluding detection. (Berg, 1975; "Devonian Carrier Shells (Euomphalidae) from North America and Germany", 1973; Feinstein and Cairns, 1998; Morton, 1958; Ponder, 1983; Shank, 1969)

Communication and Perception

There is no data regarding how carrier shells communicate with individuals within or outside of their species. The extensive record of carrier shells disguising themselves indicates their desire to remain undetected.

Food Habits

Carrier shells are algae-eaters, grazing on both microscopic algae and fine, filamentous algae of the seafloor. An investigation into the gut contents of several carrier shells found high levels of silt and miscellaneous detritus. Carrier shells tend to feed in discrete patches, instead of leaving one continuous trail. This is a result of their leaping style of locomotion and possibly a method of curtailing the efforts of predators to track them. The foreign material attached to these snails increases the shell's diameter, acting like stilts to create a canopy for the organism to feed under. When feeding, the snail will often keep its foot suspended above the substrate and only use its proboscis to feed on material that is under the protective cover of the shell. This is likely another behavioral adaptation intended to protect against predation. Notably, Xenophora neozelanica has been observed using its extensible proboscis to feed outside the margin of shell. Like all gastropods, carrier shells have a radula used to rasp and break apart food. Carrier shells have been observed feeding during both the day and night. (Berg, 1975; "Devonian Carrier Shells (Euomphalidae) from North America and Germany", 1973; Ponder, 1983; Shank, 1969)

Predation

Many of the carrier shell's key attributes seem to be methods of avoiding predation. The attachment of foreign objects to the shell is believed to be a method of camouflaging the snail from both visual and olfactory detection. The carrier shell's discontinuous manner of locomotion and reluctance to rest its foot on substrates have been suggested as methods of eliminating chemical cues that could be picked up on by predators.

Little is known about which organisms predate on carrier shells in the wild, but one lab study found that predatory starfish and gastropods did attempt to feed on a Xenophora neozelanica specimen. Interestingly, this individual did not recede into its shell while these predators were atop them, but continued to feed on algae. ("Devonian Carrier Shells (Euomphalidae) from North America and Germany", 1973; Shank, 1969)

• Anti-predator Adaptations
• cryptic

Ecosystem Roles

The shells of these organisms have been used as homes for hermit crabs. Carrier shells can also serve as the substrate for corals, sponges, and bryozoans to grow on. The agglutinating habits of carrier shells applies not only to dead matter, but to living organisms as well. In a 1998 study of corals attached to carrier shells, 24 of the affixed corals were alive upon the host snail's collection. This relationship can be considered mutualistic, as the carrier shell receives a new piece of material to add to its camouflage efforts, and the organism being attached receives a substrate to grow on. (Berg, 1975; Feinstein and Cairns, 1998)

• Ecosystem Impact
• biodegradation

Economic Importance for Humans: Positive

The collections of shells upon carrier shells have made these creatures popular collectibles for shell collectors.

Carrier shells can be useful for researchers looking to learn more about the organisms that live in unexplored environments, like the deep-sea benthos. Each carrier shell exists in a limited range, so the collection of objects affixed to their shell can provide a sample of what mollusks, corals, or other life exists in that area. Noah Feinstein and Stephen Cairns did just that in a study published in 1998. They were able to find two undescribed species of coral and expand the geographic range of 29 coral species by looking at the matter attached to carrier shells. (Feinstein and Cairns, 1998)

• Positive Impacts
• body parts are source of valuable material
• research and education

Economic Importance for Humans: Negative

There are no known adverse effects of carrier shells on humans.

Conservation Status

There is no record of the conservation status of carrier shells.

• IUCN Red List [Link]

  Not Evaluated

Contributors

AJ Buttala (author), Colorado State University, Audrey Bowman (editor), Colorado State University.

References

U.S. Department of the Interior. Devonian Carrier Shells (Euomphalidae) from North America and Germany. 824. Washington: United States Government Printing Office. 1973. Accessed February 06, 2022 at https://pubs.usgs.gov/pp/0824/report.pdf.

2019. "Xenophora Fischer von Waldheim, 1807" (On-line). World Register of Marine Species. Accessed February 12, 2022 at https://www.marinespecies.org/aphia.php?p=taxdetails&id=138664#links.

Berg, C. 1975. Behavior and Ecology of Conch (Superfamily Strombacea) on a Deep Subtidal Algal Plain. Bulletin of Marine Science, 25/3: 307-317. Accessed February 12, 2022 at https://www.ingentaconnect.com/content/umrsmas/bullmar/1975/00000025/00000003/art00001.

Crippa, G., G. Pasinetti, M. Dapiaggi. 2020. How did the carrier shell Xenophora crispa (Konig, 1825) build its shell? Evidence from the Recent and fossil record. Lethaia, 53/4: 439-451. Accessed February 04, 2022 at https://onlinelibrary.wiley.com/doi/full/10.1111/let.12367.

Feinstein, N., S. Cairns. 1998. Learning from the collector: A survey of azooxanthellate corals affixed by Xenophora (Gastropoda : Xenophoridae), with an analysis and discussion of attachment patterns. Nautilus, 112/3: 73-83. Accessed February 04, 2022 at file:///Users/ajbuttala/Downloads/Feinstein_Cairns_1998_Nautilus_pg_73-83.pdf.

Fischer von Waldheim, G. 1807. Muséum-Demidoff (mis en ordre systématique et décrit par G. Fischer) : Ou catalogue ... des curiosités de la Nature et de l'Art. données à l'Université impériale de Moscou par ... P. de Demidoff. Moscow: Moscou. Accessed April 19, 2022 at https://doi.org/10.5962/bhl.title.160942.

Irwin, A., E. Strong, Y. Kano, E. Harper, S. Williams. 2021. Eight new mitogenomes clarify the phylogenetic relationships of Stromboidea within the caenogastropod phylogenetic framework. Molecular Phylogenetics and Evolution, 158: e107081. Accessed April 03, 2022 at https://doi.org/10.1016/j.ympev.2021.107081.

Kreipl, K., A. Axel, K. Gijs. 1999. A new subgenus of the family Xenophoridae Philippi, 1853. Spixiana, 22/2: 179-180. Accessed April 03, 2022 at https://www.researchgate.net/profile/Gijs-Kronenberg/publication/247041941_A_new_subgenus_of_the_family_Xenophoridae_Philippi_1853_Mollusca_Gastropoda/links/552023f60cf2a2d9e14335c6/A-new-subgenus-of-the-family-Xenophoridae-Philippi-1853-Mollusca-Gastropoda.pdf.

Morton, J. 1958. The Adaptations and Relationships of the Xenophoridae (Mesogastropoda). Journal of Molluscan Studies, 33/3: 89-101. Accessed February 06, 2022 at https://academic.oup.com/mollus/article-abstract/33/3/89/982903.

Nielsen, S., T. DeVries. 2002. Tertiary Xenophoridae (Gastropoda) of western South America. Nautilus, 116/3: 71-78.

Oyen, C., K. Fountain, R. Portell, G. McClellan. 2000. OCCURRENCE OF PLIO-PLEISTOCENE PHOSPHATIZED MACRO-INVERTEBRATES FROM THE UPPER WEST FLORIDA SLOPE, EASTERN GULF OF MEXICO. Bulletin of the Florida Museum of Natural History, 42/5: 219-252. Accessed February 05, 2022 at https://www.floridamuseum.ufl.edu/wp-content/uploads/sites/35/2017/03/Vol-42-No-5.pdf.

Ponder, W. 1983. A revision of the Recent Xenophoridae of the world and of the Australian fossil species (Mollusca, Gastropoda). Australian Museum Memoir, 17: 1-126. Accessed February 04, 2022 at https://media.australian.museum/media/Uploads/Journals/17611/393_complete.pdf.

Reeve, B. 1843. Conchologia iconica, or, Illustrations of the shells of molluscous animals. London: Reeve, Brothers. Accessed February 05, 2022 at https://www.biodiversitylibrary.org/page/11119343#page/464/mode/2up.

Shank, P. 1969. The timorous carrier shell. New York Shell Club Notes, 151: 5-7. Accessed February 25, 2022 at https://www.biodiversitylibrary.org/item/301164#page/431/mode/1up.