About three fourths of the world’s 100,000 species of mollusks are single-shelled gastropods. Gastropod means “stomach footed.” The “foot” is a disk used for creeping. On top of this a twisted hump, covered by mantle, which contains the digestive organs. Gastropods have a distinctive large head with two pairs of sense organs that look like horns or antennae and have eyes at their ends. They also have tentacles and a radula, ribbon-shaped tongue covered with rasping, microscopic teeth. Most have a shell which is attached to the body and completely covers it. The shape of the animal defines the shape of the shell.
Gastropods come in a wide variety of shapes, colors and sizes. Among the different kinds are: 1) limpets, who produce a pyramid-shaped shell from secretion released equally round the mantle; 2) snails, including whelks, turbans and conches, that create a spiral because the mantle secretes shell material faster one side than the other; and 3) cowries, which make a clenched-fist shell by secreting material from both ends of the mantle. Others create a twist turret by secreting material from only one side.
A typical gastropod has a spiral-shaped shell and moves by creeping along with a suckerlike foot. But not all gastropods fit this description. Some have no shell at all. Most gastropods have separate sexes. Some are hermaphrodites and some change sex during their lifetimes.
flamingo tongue snail “Gastropods are characterized by torsion, a process in which the mantle cavity is rotated counterclockwise up to 180 degrees until it faces forward and is positioned over the head. This occurs early in larval development and means that the digestive, reproductive and excretory systems all discharge through the opening in the shell.”
Gastropods grow in a downward spiral by secreting new material along the edge of their lip. This produces a series of spirals. Each wider than the one above, that when viewed from the front appear to be stacked like the layers of classic Mayan pyramids, but unlike the pyramids built by humans the ones made by gastropods are built from the top down rather than the base upward.
Gastropods feed in a number of different ways. There are browsers, grazers and hunters. Some use their radula to scrape algae. Others hunt with radula modified into weapons. Whelks use the radula to bore into the shells of other mollusks. Some gastropods can eat through rock by secreting acid. And others swallow fish with a tube-like proboscises.
Radula diagram The tongue-like radula can take may forms. In grazing species it is shaped like a file and used to scrape algae. In others its shaped like a drill. Cone snails have transformed it into a kind of barbed spear that shoots small glassy, poison harpoons.
Types of Gastropods
Types of gastropods include limpets ,winkles, topshells, whelks, turbans, conches, cowries, cone snails, terrestrial slugs, terrestrial snails, sea slugs and sea hares. The largest snail shell ever found, weighing 40 pounds and measuring 30.39 inches in length, was a trumpet conch found off of West Australia in 1979 by a Taiwanese fisherman.
Helmet shells are large and beautiful. They can be as large as 12 inches in length and are so named because they remind some of gladiator helmets. They live in sandy sediments and feed on other mollusks and sea urchins.
Murexes are a kind of snail that feed on other mollusks and barnacles by boring into their shells. Some look like snails with sharp spikes and are greatly sought after by collectors. Others secrete a purple substance that was prized in ancient times as a dye.
The violet sea snail is one of few gastropods found on the surface of the sea. It its kept afloat on a raft of made of mucus bubbles. It feeds on by-the-wind sailors and other floating hydriods.
Conches are widely eaten in the Caribbean and other places. The animals are usually removed by making a cut in the shell, where the animal attaches itself, causing it plop out in a blubbery glob. To keep the shell in pristine condition, some people tie a string around the foot of the animals and hang it from a tree. The animals is slowly pulled out. After a couple days it stinks to high heavan and, with some species, the stretched out animals is almost a meter in length.
Deadly Cone Snails
Deadly Cone snails have been described as "the deadliest creatures on the planet, for their size." Small creatures with beautiful shells found in the South Pacific and Indian Oceans, they inject venom with short barbs into their victim. The poison can cause paralysis an even death in fish and humans.
cone shell Rhizoconus mustelinus There are 500 species in the Conus genus. Their shells feature and endless variety of intricate patterns created from repeating triangles, strips, spots and cobblestones. They range in size from grape-size to pear-size and are among the slowest moving of all mollusks. They rely on their poison for both defense and to catch prey. Some cone snails are quite aggressive. They can actively reach out to sting prey or potential predators.
At least 30 people have died from cone snail attacks. The Conus geographus (geographic cone snail) of Australia is the most deadly cone snail. It contains a venom so powerful that victims quickly go into a coma and die within several hours. Visitors to the Queensland Museum can see a shell from a specimen that killed a man, causing only fatality on the Great Barrier Reef from a marine animal. The man was collecting shells. He picked up the cone snail and put it in his pocket and was fatally stung through his clothes.
One young man who picked up geographic cone shell thought it would make a nice gift for his girlfriend. He put the shell on his shoulder to show his friends. The animal stung him with its needle-like radula. He was dead a short time later. Despite the potency of their toxins, people eat them in the Philippines and Samoa as a delicacy.
Demand for cone snails for their shell. meat and toxins (for medical purposes) plus loss of habitat and global climate changes have caused a dramatic reduction in the numbers of cone snails. The problem is so severe that several species may threatened with extinction.
Cone Snail Anatomy and Poison
Conus eating a_fish Some cone snail toxins are among the most powerful produced in the animal kingdom. A single creature can carry enough poison to kill a dozen people. The poison is potent and fast-acting nerve agents for which there is known antidote or antivenin.. If it was otherwise cone snail prey could escape before the cone snail could catch it. Death can occur is less than 30 minutes.
Powerful toxins (lethal dose): 1) anthrax (0.0002); 2) geographic cone snail (0.004); 3) textrodoxotine in the blue ring octopus and puffer fish (0.008); 4) inland taipan snake (0.025); 5) eastern brown snake (0.036); 6) Dubois’s sea snake (0.044); 7) coastal taipan snake (0.105); 8) beaked sea snake (0.113); 9) western tiger snake (0.194); 10) mainland tiger snake (0.214); 11) common death adder (0.500). Lethal doses is defined as the amount in milligrams needed to kill 50 percent of the animals tested.
Cone snail on fire coral Cone snails have transformed their radula into a kind of gun that shoots small glassy, poison harpoons. The harpoons are excreted with enough force to penetrate a diver’s wetsuit. The venom contains an analgesic so the victim feels almost no pain.
Cone snails feed on other mollusks, worms and fish. They detect prey with siphons that take in water and shoot it over their gills. After shoot hit they then pull their prey slowly back to their shell and engulf it.
Cone snails poisons are known as conotoxins, which consist of chains of amino acids called peptides. Each species produces 50t o 100 different kinds. Often the way the peptides are combined and mixed varies each time the poison is injected.
Cone Snail Drugs
Some scientists believe that cone snails possess more potential medicines than any other organism. With 500 different species producing 50 to 100 chemicals each that adds up to 25,000 to 50,000 chemicals that could be used to make medicines.
Cone snails contain analgesics and compounds that hold the potential for treating cancer and nervous system conditions such as epilepsy and stroke. Prialt is a synthetic derivative of the toxin of the conus magnus , the magician’s cone, a cone snail which kills prey in its swamp habitat. It is an extremely powerful painkiller, up to 1,000 stronger than morphine but is non addictive. Baldomero Olivera from the University of Utah, who extracted the peptide from the snail told Smithsonian magazine, “I thought if these snails were so powerful that they could paralyze the nervous system, smaller doses of the compounds from the venoms might have beneficial effects....Cones snails are of exceptional interest because the molecules they make are very small and simple, easily reproducible.”
various Prosobranchia, including cowries Prialt is already valued in the hundreds of millions of dollars. It has proved to very promising treating people with chronic, serious pain. It works by interfering the with the transfer of signals between nerve cells in the spinal chord and other parts of the body. One man whose spine was crushed by sawmill accident told Smithsonian magazine that he suffered excruciating pain for years that even morphine didn’t help. He started taking Prialt and said the pain went away after three days. Scientists are studying Prialt to make treatments for epilepsy.
Of the tens of thousands of cone snail conotoxins out there only about 100 have been studied. Many have been found to bind to ion-channel receptions, holes in cell membranes that allow changes to take place that can cause or stop diseases.
Milking the venom from cone snails is no easy task because the animal can bend it radula to almost every direction to sting at from virtually any angle, The venom is extracted by a Brisbane-based biotechnology company called Xenome by dangling fish in front of the snail. After the snail strikes the fish and kills it the venom is extracted from the fish.
Cowrie shells are found in the Pacific and Indian Oceans and have shiny, patterned shells much sought after by collectors. When the animal is alive the shell is covered by a flap of skin which is part of a brightly-colored mantle. The long narrow aperture of the shell allows the animal to escape inside the shell from predators. The shell is very hard to break.
Cowrie shells have been used as currency by a number of cultures. Up until the 19th century, merchants made a fortune gathering shells in the Indian Ocean and trading them for ivory and gold among people in West Africa who used the cowries for currency.
Describing cowries, David Attenborough wrote: "From the slit along the bottom, it protrudes not only its foot but two sections of its mantle which extend over each flank of the shell and meet at the top. These lay down the marvelously patterned and polished surface characteristics of cowries."
Abalones are greatly valued by humans. Their meat is considered a delicacy, especially in Asia, and the pearly, iridescent inside of their shells in prized in jewelry making. They are also a favorite food of sea otters and the source of nacre, or mother of pearl, a thick iridescent material.
Haliotidae (abalones) Abalones are native to the Pacific. Their shell is ear shaped. The animal crawls around on the bottom of the sea or attaches itself to rocks with the shell facing upwards. It is a grazing animal that feeds on algae and microscopic plants. There are white, red, pink, green and black varieties of abalone. Some species lay 6 million eggs at a time when they spawn. Others are very fussy and breed only in the right conditions.
Abalone have distinctive series of holes through which water enters the gills and waste products are expelled. The animals have powerful muscles designed to prevent predators from prying them off rocks. Their blood doesn't have coagulants like mammals so it is easy for them to bleed to death from a minor cut.
Materials modeled on abalone nacre, or mother or pearl, are as strong as steel at half the weight and perform very well in bullet stopping tests: a tungsten rod forced at 2,000mph made it only halfway through the abalone-like material.
Abalone makes its shell out of calcium carbonate, the same material of which chalk is made, but it is as tough as Kevlar and 3000 times harder than chalk. Its strength is derived from the way the material is organized the into staggered, nanoscale bricks through a subtle play of 15 different proteins. Several teams of scientists are studying the structure and hoping to replicate it but still they admit they don’t even really understand the most basic things about it.
Abalone was once plentiful but now is regarded as an endangered species in many place. Their numbers have been depleted by overfishing. Abalone grow to a foot across. Large abalones, those measuring six inches in length and greater, and found mostly in cool seas in kelp forest. Kelp is their preferred food. They often hang out in crevices and wait for bits of kelp to rain on them.
Image Source: National Oceanic and Atmospheric Administration (NOAA) noaa.gov/ocean ; Wikimedia Commons
Text Sources: Mostly National Geographic articles. Also the New York Times, Washington Post, Los Angeles Times, Smithsonian magazine, Natural History magazine, Discover magazine, Times of London, The New Yorker, Time, Newsweek, Reuters, AP, AFP, Lonely Planet Guides, Compton’s Encyclopedia and various books and other publications.
© 2009 Jeffrey Hays
Last updated March 2011