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)
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)
This genus was first described in 1807 by Gotthelf Fischer von Waldheim. He dubbed this group of organisms: “xeno-” meaning foreign and “-phora” meaning bearer, in reference to the shell that bears foreign bodies. The genus is one of three genera currently recognized in the family Xenophoridae. This family used to have a single genus, , with three subgenera: Xenophora, Onustus, and Stellaria. Included in a 1999 description of a new subgenus of , 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)
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)
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)
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)
The parental care of carrier shells has not been described.
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)
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)
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.
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)
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)
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)
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)
There are no known adverse effects of carrier shells on humans.
There is no record of the conservation status of carrier shells.
AJ Buttala (author), Colorado State University, Audrey Bowman (editor), Colorado State University.
the body of water between Africa, Europe, the southern ocean (above 60 degrees south latitude), and the western hemisphere. It is the second largest ocean in the world after the Pacific Ocean.
body of water between the southern ocean (above 60 degrees south latitude), Australia, Asia, and the western hemisphere. This is the world's largest ocean, covering about 28% of the world's surface.
Referring to an animal that lives on or near the bottom of a body of water. Also an aquatic biome consisting of the ocean bottom below the pelagic and coastal zones. Bottom habitats in the very deepest oceans (below 9000 m) are sometimes referred to as the abyssal zone. see also oceanic vent.
having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.
helps break down and decompose dead plants and/or animals
the nearshore aquatic habitats near a coast, or shoreline.
having a worldwide distribution. Found on all continents (except maybe Antarctica) and in all biogeographic provinces; or in all the major oceans (Atlantic, Indian, and Pacific.
having markings, coloration, shapes, or other features that cause an animal to be camouflaged in its natural environment; being difficult to see or otherwise detect.
animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature
union of egg and spermatozoan
fertilization takes place within the female's body
the area in which the animal is naturally found, the region in which it is endemic.
mainly lives in oceans, seas, or other bodies of salt water.
reproduction that includes combining the genetic contribution of two individuals, a male and a female
that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).
the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.
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