Mollusk Facts

Mollusk Facts Mollusks may be the most difficult animal group  for the average person to wrap their arms around: this family of  invertebrates  includes creatures as widely divergent in appearance and behavior as snails,  clams, and cuttlefish. Fast Facts: Mollusks Scientific Name: Mollusca (Caudofoveates, Solanogastres, Chitons, Monoplacophorans, Scaphopods, Bivalves, Gastropods, Cephalopods)Common Name: Mollusks or molluscsBasic Animal Group: Invertebrate  Ã‚  Size: Microscopic to 45 feet in lengthWeight: Up to 1,650 poundsLifespan: Hours to centuries- the oldest is known to have lived over 500 yearsDiet:  Mostly herbivore, except for cephalopods who are omnivoresHabitat: Terrestrial and aquatic habitats on every continent and ocean in the worldConservation Status: Several species are threatened or endangered; one is extinct Description Any group that embraces  squids, clams, and slugs present a challenge when it comes to formulating a general description. There are only three characteristics shared by all living mollusks: the presence of a mantle (the rear covering of the body) that secretes calcareous (e.g., calcium-containing) structures; the genitals and anus opening into the mantle cavity; and paired nerve cords. If youre willing to make some exceptions, most mollusks can also be characterized by their broad, muscular feet which correspond to the tentacles of cephalopods, and their shells (if you exclude cephalopods, some gastropods, and the most primitive mollusks). One type of mollusk, the aplacophorans, are cylindrical worms with neither shell nor foot. Getty Images Habitat Most mollusks are marine animals that live in habitats from shallow coastal areas to deep waters. Most stay within the sediments at the bottom of water bodies, although a few- such as cephalopods- are free swimming. Species There are eight different broad categories of mollusks on our planet. Caudofoveates  are small, deep-sea mollusks that burrow into soft bottom sediments. These worm-like animals lack the shells and muscular feet characteristic of other mollusks, and their bodies are covered with scale-like, calcareous spicules.Solanogastres, like caudofoveata, are worm-like mollusks that lack shells. These small, ocean-dwelling animals are mostly blind, and either flattened or cylindrical.Chitons, also known as polyplacophorans, are flat, slug-like mollusks with calcareous plates covering the upper surfaces of their bodies; they live in intertidal waters along rocky coastlines worldwide.Monoplacophorans are deep-sea mollusks equipped with cap-like shells.  They were long believed to be extinct, but in 1952, zoologists discovered a handful of living species.Tusk shells, also known as scaphopods,  have long, cylindrical shells with tentacles extending from one end, which these mollusks use to rope in  prey from the surrounding water.Bivalves are characterized by their hinged shells and live in both marine and freshwater habitats. These mollusks have no heads, and their bodies consist entirely  of a wedge-shaped foot. Gastropods  are  the most diverse family of mollusks,  including over 60,000 species of  snails and slugs that live in marine, freshwater, and terrestrial habitats.  Cephalopods, the most advanced mollusks, include  octopuses, squids, cuttlefish, and nautiluses. Most of the members of this group either lack shells, or have small internal shells. A tusk shell. Getty Images Gastropods or Bivalves Of the roughly 100,000 known mollusk species, about 70,000 are gastropods, and 20,000 are bivalves or 90 percent of the total. It is from these two families that most people derive their general perception of mollusks as small, slimy creatures equipped with calcareous shells. While the snails and slugs of the gastropod family are eaten the world over (including as escargot in a French restaurant), bivalves are more important as a human food source, including clams, mussels, oysters, and other undersea delicacies. The largest bivalve is the giant clam (Tridacna gigas), which reaches a length of four feet and weighs 500 pounds. The oldest mollusk is a bivalve, the ocean quahog (Arctica islandica), native to the northern Atlantic and known to live at least 500 years; it is also the oldest known animal. Bright yellow banana slug. Alice Cahill/Getty Images Octopuses, Squids, and Cuttlefish Gastropods and bivalves may be the most common mollusks, but cephalopods (the family that includes octopuses, squids, and cuttlefish) are by far the most advanced. These marine invertebrates have astonishingly complex nervous systems, which allows them to engage in elaborate camouflage and even display problem-solving behavior- for example, octopuses have been known to escape from their tanks in laboratories, squish along the cold floor, and climb up into another tank containing tasty bivalves. If human beings ever go extinct, it may well be the distant, intelligent descendants of octopuses that wind up ruling the earth- or at least the oceans! The largest mollusk in the world is a cephalopod, the colossal squid (Mesonychoteuthis hamiltoni), known to grow to between 39 and 45 feet and weigh up to 1,650 pounds.   548901005677/Getty Images Diet With the exception of cephalopods, mollusks are by and large gentle vegetarians. Terrestrial gastropods like snails and slugs eat plants, fungi, and algae, while the vast majority of marine mollusks (including bivalves and other ocean-dwelling species) subsist on plant matter dissolved in the water, which they ingest by filter feeding. The most advanced cephalopod mollusks- octopuses, squids, and cuttlefish- feast on everything from fish to crabs to their fellow invertebrates; octopuses, in particular, have gruesome table manners, injecting their soft-bodied prey with venom or drilling holes in the shells of bivalves and sucking out their tasty contents. Behavior The nervous systems of invertebrates in general (and mollusks in particular) are very different from those of vertebrate animals like fish, birds, and mammals. Some mollusks, like tusk shells and bivalves, possess clusters of neurons (called ganglions) rather than true brains, while the brains of more advanced mollusks like cephalopods and gastropods are wrapped around their esophagi rather than isolated in hard skulls. Even more weirdly, most of the neurons of an octopus are located not in its brains, but in its arms, which can function autonomously even when separated from its body. The mouth of a limpet. Getty Images Reproduction and Offspring Mollusks generally reproduce sexually, although some (slugs and snails) are hermaphrodites, they still must mate to fertilize their eggs. Eggs are laid singly or in groups within jelly masses or leathery capsules. The eggs hatch into veliger larva- small, free-swimming larvae- and metamorphose into different stages, depending on the species.   Evolutionary History Because modern mollusks vary so widely in anatomy and behavior, sorting out their exact evolutionary relationships is a major challenge. In order to simplify matters, naturalists have proposed a hypothetical ancestral mollusk that displays most, if not all, of the characteristics of modern mollusks, including a shell, a muscular foot, and tentacles, among other things. We dont have any fossil evidence that this particular animal ever existed; the most any expert will venture is that mollusks descended hundreds of millions of years ago from tiny marine invertebrates known as lophotrochozoans (and even that is a matter of dispute). Extinct Fossil Families Examining the fossil evidence, paleontologists have established the existence of two now-extinct classes of mollusk. Rostroconchians lived in the worlds oceans from about 530 to 250 million years ago, and seem to have been ancestral to modern bivalves; helcionelloidans lived from about 530 to 410 million years ago, and shared many characteristics with modern gastropods. Somewhat surprisingly, cephalopods have existed on earth ever since the Cambrian period; paleontologists have identified over two dozen (much smaller and much less intelligent) genera that plied the worlds oceans over 500 million years ago. Mollusks and Humans Wayne Barrett Anne MacKay / Getty Images Over and above their historical importance as a food source- especially in the far east and the Mediterranean- mollusks have contributed in numerous ways to human civilization. The shells of cowries (a type of small gastropod) were used as money by Native Americans, and the pearls that grow in oysters, as the result of irritation by sand grains, have been treasured since time immemorial. Another type of gastropod, the murex, was cultured by the  ancient Greeks for its  dye, known as imperial purple, and the cloaks of some rulers were woven from long threads secreted by the bivalve species Pinna nobilis. Conservation Status There are over 8,600 species listed in the ICUN, of which 161 are considered Critically Endangered, 140 are Endangered, 86 are Vulnerable, and 57 are Near Threatened. One, the Ohridohauffenia drimica was last seen in 1983 in springs feeding the River Drim in Macedonia, Greece and was listed as extinct in 1996. Additional surveys have failed to find it again. Threats The vast majority of mollusks live in the deep ocean and are relatively safe from the  destruction of their habitat and depredation by humans, but thats not the case for freshwater mollusks (i.e., those that live in lakes and rivers) and terrestrial (land-dwelling) species. Perhaps not surprisingly from the perspective of human gardeners, snails and slugs are most vulnerable to extinction today, as they are systematically eradicated by agriculture concerns and picked off by invasive species carelessly introduced into their habitats. Just imagine how easily the average house cat, used to picking off skittering mice, can devastate a near-motionless colony of snails. Lakes and rivers are also prone to the introduction of invasive species, particularly mollusks which travel attached to international seagoing ships. Sources Sturm, Charles F., Timothy A. Pearce, ngel Valdà ©s (eds.). The Mollusks: A Guide to Their Study, Collection, and Preservation. Boca Raton: Universal Publishers for the American Malacological Society, 2006.  Fyodorov, Averkii, and Havrila Yakovlev. Mollusks: Morphology, Behavior, and Ecology. New York: Nova Science Publishers, 2012. Mollusk Facts Mollusks (Mollusca) is a taxonomic phylum that contains a diverse array of organisms, including snails, sea slugs, octopuses, squid, and bivalves such as clams, mussels, and oysters. Between 50,000 and 200,000 species are estimated to belong to this phylum. Imagine the obvious differences between an octopus and a clam, and youll get an idea of the diversity among mollusks. Fast Facts: Mollusks Scientific Name: Mollusca: Gastropoda, Bivalvia, Cephalopoda, Monoplacophora, Scaphopoda, Aplacophora, PolyplacophoraCommon Names: Snails, sea slugs, octopuses, squid, and bivalvesBasic Animal Group: InvertebrateSize: Varies from 0.04 inch (solenogasters and gastropods) to 4 feet (giant clam)  Lifespan: 3 years to over 500Diet:  Carnivore or herbivoreHabitat: Oceans and coastal waterways worldwidePopulation: UnknownConservation Status: Most are classified as Least Concern; mollusks make up nearly one-fourth of all marine animals on Earth.   Description Mollusks have a shell and soft body and usually have a distinguishable head and foot region. Some may have a hard covering, or exoskeleton. Mollusks also have a heart that pumps blood through their blood vessels, digestive system, and a nervous system. In addition to a shell, most mollusks have a muscular foot for creeping or burrowing, and some have a head with sense organs. Their soft body includes lungs or gills for breathing and digestive and reproductive parts. These are surrounded by a skin-like organ called the mantle. Mollusks also have bilateral symmetry- one side is a mirror image of the other- and can have one or two shells. Their organs are in a fluid-filled cavity; indeed, the very word mollusk in Latin means soft. The upper body, or the mantle, is a thin, muscular sheet that covers the internal organs. Most mollusks, particularly those with shells, also have gills in the central part of their body cavity. Despite looking fragile, mollusk shells are quite hard. Scientists are even studying nacre, a material found in mollusk shells, to develop materials that are stronger and lighter than steel. Puneet Vikram Singh, Nature and Concept photographer/Getty Images Habitat and Distribution Mollusks- snails, sea slugs, octopuses, squid, and bivalves- are found in habitats ranging from freshwater lakes and rivers to shallow coastal waters to the deepest parts of the oceans, worldwide. Most live in the bottom sediments, though cephalopods are primarily free-swimming species; some snails and clams are terrestrial. Species Mollusks belong to the kingdom Animalia and there are tens of thousands of invertebrates that fit into the Mollusca phylum. New ones are still being discovered, and researchers continue to modify the number and grouping classifications. One common schema used by researchers includes seven classes: Gastropoda (snails and slugs)  Bivalvia (clams, scallops, oysters, mussels)  Cephalopoda (squid, octopi, nautilus)Monoplacophora (limpets)  Scaphopoda (tusk shells)  Aplacophora (shell-less, worm-like animals)  Polyplacophora (chitons)  Ã‚   Diet and Behavior Many mollusks feed using a radula, essentially a series of teeth on a cartilage base. The radula can be used for complex tasks, from grazing on marine algae or drilling a hole in another animals shell. The radula scrapes tiny plants and animals off rocks or tears food into chunks. The adoption of different feeding habits appears to have had a major influence on molluscan evolution, according to the University of California Museum of Paleontology: The change from grazing to other forms of food acquisition is one of the major features in the radiation of the group. Based on our current understanding of relationships, the earliest mollusks grazed on encrusting animals and detritus. Since mollusks are such a wide-ranging phylum, its helpful to look at how one of the organisms that belong to this group feeds and how it captures its prey. Consider the deadly blue-ringed octopus. This mollusk hunts small crabs and shrimp during the day, but it will eat bivalves and small fish if it can catch them. The octopus pounces upon its prey, using its tentacles to pull its catch toward its mouth. Then, its beak pierces the crustaceans exoskeleton and delivers the paralyzing venom. The venom is produced by bacteria in octopus saliva, a combination of tetrodotoxin, histamine, taurine, octopamine, acetylcholine, and dopamine. Once the prey is immobilized, this mollusk uses its beak to tear off chunks of the animal to eat. The saliva also contains enzymes that partially digest flesh, so that the octopus can suck it out of the shell. The blue-ringed octopus is immune to its own venom. Reproduction and Offspring Some mollusks have separate genders, with males and females represented in the species. Others are hermaphroditic, which means they have both male and female reproductive organs. The mollusk life cycle varies greatly among different classes of mollusks and among species within classifications. Squids reproduce sexually: The females lay fertilized eggs in the water which hatch into larvae and then develop on their own. Octopuses do the same, except the females carry the eggs with her until they hatch. Clams, mussels, and oysters produce larvae that drop through the sea water and attach to a host to grow to maturity on. The hosts are usually fish, but oysters prefer the shell of an adult oyster. Land snails are hermaphrodites that mate and both partners produce fertilized eggs. The eggs are deposited in the soil; they hatch with a shell but most consume calcium to make it harden.   Mollusks and Humans The primary use that humans have for mollusks and the rest of ocean-living animals is their ability to filter large quantities of water, up to 10 gallons each. Mollusks are also important to humans as a food source- especially in the Far East and the Mediterranean- and have contributed in numerous ways to human civilization. The shells of cowries (a type of small mollusk that belongs to the gastropod family) were used as money by Native Americans, and the pearls that grow in oysters, as the result of irritation by sand grains, have been treasured for centuries. Another type of mollusk, the murex, was cultured by the  ancient Greeks for its  dye, known as imperial purple, and the cloaks of some rulers were woven from long threads secreted by the bivalve species (twin-shelled mollusks)  called Pinna nobilis. Culturing Tahitian pearls. CampPhoto/Getty Images Conservation Status Over 8,600 molluscs are listed in the IUCN Red List, most classified as Least Concern or Data Deficient, although many are threatened or endangered. The phylum represents nearly one-fourth of all the species on the planet. Threats Increasing levels of carbon dioxide are raising the pH level of the worlds oceans, which in turn, increases the acidity of these bodies of water. This greatly weakens mollusks otherwise strong shells and even makes it difficult for them to produce shells in the first place, threatening their survival. If mollusks begin to die out in mass, then fish and other animals that feed off of them may suffer. Northeastern University marine biologist Brian Helmuth gives the example of the common mussel, a member of the family of bivalve mollusks. In addition to the problem of increased acidity in the oceans, which, as noted, makes it harder for these mollusks to produce shells, the increasing temperatures of the oceans and even the sand and air on the surrounding beaches can mean a death sentence for mussels. â€Å"You are sitting there in the blazing sun, you’re not going to be able to move,† Helmuth says. â€Å"You can’t escape the heat, you can’t escape the sun, you can’t go into a crevice like ... a crab.† Mussels can literally start to cook on the rocks if they get too hot. Helmuth adds that global warming is shrinking the habitat in which mussels and other mollusks can live. And since mollusks are such an integral part of the food chain, that could eventually affect many other animals that depend on them for sustenance. Sources Beeler, Carolyn. To Understand Climate Change, Look at it From a Mussels Perspective. PRIs The World, April 25, 2017.Martinez, Andrew J. Marine Life of the North Atlantic. Aqua Quest Publications, Inc.: New York, 2003.The Mollusca. University of California Museum of Paleontology.â€Å"Molluscs.†Ã‚  Biology Education.Mollusks. Factmonster.Phylum: Mollusks. Mollusk Science.Salvini-Plawen, Luitfried. â€Å"Mollusk.†Ã‚  Encyclopà ¦dia Britannica, Encyclopà ¦dia Britannica, Inc., 13 Apr. 2018,