sign in sign up
<<
Home , Escargot, Mimic octopus, Sea angel

a. Phylum is the second largest phyla after Arthropoda. There are over 90,000 described living species and some 70,000 fossil species. b. Their name comes from the Latin word ‘molluscus’ meaning soft, after their soft bodies. Although in many species, their soft body is protected by a shell. c. Mollusks are a diverse group that range from fairly simple organisms to some of the most complex and specialized of the invertebrates. They include , , slugs, nudibranchs, sea butterflies, , , , , , , , and . d. Mollusks are found in a great range of habitats, although most require water. Only snails and slugs are terrestrial, yet they too are limited in their range by their need for humidity, shelter, and presence of calcium. e. Mollusks are triploblastic coelomate protostome invertebrates. i. While the molluscan coelom is relatively small, the development of the coelom was a major step in the evolution of larger and more complex forms of . a. There are seven living classes of Mollusks: i. Polyplacophora 1. Contains 600 species of chitons found on rocky marine shorelines. ii. 1. Contains an estimated 40,000 - 150,000 species of nudibranchs, snails and slugs. Also includes unique species such as , sea hares, sea angel, , and Sea Lemon (pictured above). iii. 1. Also called Pelecypoda; contains 8,000 species of clams, oysters, scallops, and mussels. iv.Cephalopoda 1. Contains 786 species of squid, , , and cuttlefish. v. Monoplacophora, Aplacophora and Scaphapoda are rare and/or extremely deep-water creatures, and will not be discussed. a. Mollusks vary in appearance (think of snails, clams, oysters, octopuses, squid, etc.) but all share the following general characteristics: i. Most mollusks have bilateral symmetry, and many have one or more shells called valves. ii. Three-part body plan 1. Most mollusk bodies are composed of three main parts, the head, foot, and visceral mass. a. The head-contains the mouth and a variety of sensory structures i. Most mollusks have well-developed heads, which contain their mouth, photosensory receptors, and a radula. ii. Photosensory receptors range from fairly simple ones to the complex eyes of . 2. The radula is a rasping, tongue-like organ that is found in all mollusks except bivalves. a. It is used for feeding and consists of a ribbon-like membrane that has rows of teeth that point backward. b. This radula scrapes fine particles of food off of hard surfaces. Then it acts as a conveyer belt and takes the food to the mouth of the mollusk. It can also be used as a weapon to puncture prey. c. As the radula wears away anteriorly, new rows of teeth are continuously replaced by secretion at its posterior end. b. The foot is the large, muscular organ used for locomotion and prey capture. i. It varies in shape and function, depending on the type of mollusk. ii. Tentacles are often present as part of the foot.

c. The visceral mass contains the body’s organs. i. Surrounding the visceral mass is a membrane called the mantle. 1. The mantle is a sheath of skin extending from the visceral mass that hangs down on each side of the body, protecting the soft parts 2. The outer surface of the mantle secretes the shell. 1. The space between the mantle and the visceral mass is called the mantle cavity. 2. The mantle cavity plays an important role in the life of a mollusk. It usually houses respiratory organs (gills or lung). 3. Products from the digestive, excretory, and reproductive systems are emptied into the mantle cavity to be flushed out of the body. 4. In cephalopods ( and octopuses) the muscular mantle and its cavity create jet propulsion used in locomotion. Many mollusks can withdraw their head or foot into the mantle cavity, which is surrounded by the shell, for protection. 1.The protective shell of a mollusk is secreted by the mantle. 2.The shell first begins to form during the embryo stage and grows throughout the life of the mollusk, as shown by growth lines or rings on the shell. 3.The shell is composed of 3 layers. a.The outer layer is called the periostracum. i. This layer is especially thick in freshwater species to protect from acidity. ii.This layer is thin or absent in many marine forms. b.The middle layer is called the prismatic layer. i. This is made up of calcium carbonate in a protein matrix. ii.Calcium to make the shell is extracted from water, soil, or food. c. The inner layer is called the nacreous layer, made of nacre. i. This layer is secreted continuously by the mantle surface. This allows for this layer to increase in thickness during the life of the animal. ii.The calcareous nacre is laid down in thin layers. These thin layers produce the iridescent, “mother-of-” rainbow shine found in nautiluses and bivalves. 4. In some such as squid and cuttlefish the shell is internal and greatly reduced; in this case the mantle serves as the animals’ outer covering. a. Most of the internal organs are embedded the in solid tissue of the visceral mass, instead of being free inside a body cavity. i. The digestive tract is complex and highly specialized, according to feeding habits of the various mollusks. ii. The foregut or buccal cavity includes the mouth, radula, salivary glands, and esophagus. It receives and prepares food. iii. The midgut contains the stomach and associated digestive glands used for sorting and crushing food. iv. The hindgut, or intestine, is usually long and coiled for the absorption of nutrients, followed by the formation of feces (undigested residue). i. The nervous system consists of several pairs of ganglia with connecting nerve cords, and it is generally simpler than that of annelids and arthropods. ii.There are various types of highly specialized sense organs. i. Mollusks are the oldest known animals to have evolved excretory structures called nephridia. Nephridia are organs that remove metabolic wastes from the animal’s body. Most mollusks have a pair of nephridia called metanephridia. i. Most mollusks have an open circulatory system with a pumping heart, blood vessels, and blood sinuses. 1.In an open circulatory system blood is not entirely contained within blood vessels; rather it flows through vessels in some parts of the body and enters open sinuses in other parts. 2.An open circulatory system is less efficient at supplying oxygen to all tissues in the body, so it is common in slow-moving animals. ii. The fast-moving cephalopods have a closed circulatory system with several hearts, blood vessels, and capillaries. 1.In a closed circulatory system, blood moves to and from tissues within blood vessels.

iii. Many mollusks have blue blood from a copper- containing pigment called hemocyanin; however some with iron- containing hemoglobin have red blood. i. Gas exchange varies depending on the organism and environment they live in; respiratory organs include ctenidia (modified gills), a simple lung, and the mantle. a. Organisms in Phylum Mollusca do not reproduce asexually. b. Most are dioecious, although some are hermaphroditic. c. In some mollusks, the zygote hatches and undergoes two larval stages, trochophore ↑ trochophore and veliger, before becoming a juvenile. i. The free-swimming trochophore larva that ↓ veliger emerges from the egg in many mollusks is remarkably similar to that seen in annelids. ii. Trochophore larvae are minute, translucent, and have a prominent circlet of cilia used for swimming. d. In many mollusk groups (especially gastropods and bivalves) the trochophore stage is followed by a unique larval stage called a veliger. The free-swimming veliger has the beginnings of a foot, shell, and mantle. a. Polyplacophora means ‘many plate bearers’. i. Their shell contains 8 overlapping plates on the dorsal surface. If detached, they can roll up like a pill bugs or armadillo. b. Chitons have a flattened body with convex upper surface. Most range in size from 2-5 cm; but some species can grow to 30 cm. c. All are marine species; they prefer rocky intertidal zones but have been found at 4000 m depths. d. Chitons are fairly sedentary clinging to rocks or other hard surfaces, moving only short distances to feed. Most feed using radula to scrape algae from rocks. a. ‘Gastro’ means stomach, ‘pod’ means foot. i. Gastropods are known as the stomach-footed mollusks because their large muscular foot is positioned under the rest of its body. b.By far this is the largest and most diverse class in Phylum Mollusca, containing over 70,000 living and more than 15,000 fossil species. i. Some of the more unique species include limpets, , conchs, periwinkles, sea hares, and sea butterflies. c. Gastropods can live on land, in freshwater, or in ocean. They are usually sluggish, sedentary animals because most of them have heavy shells and slow locomotion. Many species secrete mucus from the base of their foot forming a slimy path that they can glide along. Just as lipstick contains fish scales, and bird droppings are in spa facials, cosmetics manufacturers are now including slime as the latest oddity ingredients. i. Most have a single shell (snails, conchs); Some have no shell at all (slugs, nudibranchs aka sea slugs). ii.The shell, when present, is always of one piece (univalve) and may be coiled or uncoiled. iii.The apex is the oldest and smallest whorl. Whorls become successively larger and spiral around a central axis. iv.The shell may be right handed (dextral) or left handed (sinistral), depending on the direction of coiling. 1.The direction of coiling is genetically controlled and dextral shells are far more common. i. Many snails have an operculum, a plate made of tanned protein that covers the shell aperture when the body is withdrawn into the shell. Shells are their chief defense.

ii. Many nudibranchs (sea slugs) that lack a shell have evolved a unique form of defense in which they feed on sea anemones and corals; the undischarged nematocysts of their prey are preserved and transported to the surface of their bodies and used for their own defense. Many of these species also display brilliant colors which they too draw from their prey. i. One unique characteristic of gastropods is that they undergo what is called torsion, in which the visceral mass rotates 180 degrees during development. ii. The result of this is that the gills and the anus will be above their head. In addition, the mantle cavity will also move from a posterior position to an anterior position causing many organs to shift. This will allow for the mantle cavity to be above the mollusk’s head; therefore, the mollusk organism will be able to pull its head into its shell for protection. iii. This happens when the gastropod is at a very young/larval stage. The process of torsion makes the adult gastropod asymmetrical. f. Snails and slugs have a distinct head with a brain, sense organs, and mouth. Most species have a pair of tentacles on their head; this is where the eyes or eyespots are located. g. Gastropods may have gills or a simple lung depending on where they live. Gills are often modified into feathery or comb-like structures called ctenidia. i. Most gastropods are herbivores and use radula to scrap algae off of rocks. ii. Some are filter feeders and use gill cilia to draw in water containing food particles which they filter out. iii. Some are scavengers and feed on dead and decaying organisms. iv. Others are carnivores which feed on a variety of animals including other mollusks, especially bivalves; soft corals, fish, worms, etc. i. Land snails tend to be but many aquatic snails are separate sexes. Species Spotlight Conus geographus Geographic Cone Snail

1. The geographic cone is the most venomous of the 500 known cone snail species. These snails grow to 6 inches long and have intricately patterned brown-and-white shells. They reside in coral reefs of the Indo-Pacific. 2. Cone snails are predatory animals, stinging their victims with a venomous harpoon-like tooth propelled from its proboscis. 3. Geographic Cone Snail venom consists of hundreds of different toxins and is designed to be strong enough to paralyze prey instantly. 3. Symptoms for humans being stung include pain, swelling, numbness, paralysis, and breathing failure. There is no antivenin for a cone snail sting, and treatment is limited to merely keeping victims alive until the toxins wear off. Only about 30 human deaths have been recorded from stings from a cone snail. 4. Ironically, among the compounds found in cone snail venom are proteins which, when isolated, have enormous potential as pain- killing drugs. Research shows they can be up to 10,000 times more potent than morphine without morphine's addictive properties and side-effects. a. ‘Bi’ means two, ‘valve’ means shell. i. Their shells are divided into 2 halves, or valves, connected by a hinge. ii. The animal can tightly hold their shell closed by contracting their adductor muscles which extend between the shells. iii.The bulging part of shell on dorsal side near hinge is called the umbo; this is the oldest part of shell. b. This class was also known as Pelecypoda or “hatchet-footed animals”. c. All species are aquatic; most are marine, many are brackish, and some live in freshwater. Most use their muscular foot to dig down into sand and bury themselves in sediment on the seabed. Others may lie on the sea floor or attach themselves to rocks or other hard surfaces. Video i. Scallops can also swim with a jerky motion by clapping their valves together to create a sort of jet propulsion. The mantle edges can direct the stream of expelled water, so that the animals can swim in virtually any direction. Video d. This group has almost no cephalization; most lack a head and radula. e. Most bivalves are sedentary filter feeders that gather food materials from water brought in via an incurrent siphon. Wastes leave with water through the excurrent siphon. In some bivalves the siphons can be quite long. i. Their ctenidia (specialized gills) serve for respiration and to trap prey (small animals, protists, and organic material). i. Pearl production is a protective measure used by many bivalves when a foreign object (grain of sand, parasite, or other) becomes lodged between the shell and mantle. The mantle secretes many layers of nacre (the same material that lines the inner layer) around the irritating object. Video ii. This process happens naturally but can be cultured by inserting particles of nacre, usually taken from the shells of freshwater clams, between the shell and mantle of a certain species of and by keeping the oysters in enclosures for several years. Meleagrina is an oyster used extensively by the Japanese for pearl farming

Video

Video 2

Video 3 iii. Pearls come in many sizes, shapes, and colors. The larger, more round, and unique colors, the more they are worth. Video i. Most bivalves species have separate sexes and reproduce sexually. They shed sperm and eggs into water where fertilization occurs externally; the fertilized eggs develop into the trochophore and veliger larvae. Species Spotlight

1.Shipworms are bivalves that burrow in wood and feed on particles they excavate, causing great damage to unprotected wooden boats and piers. 2.These strange little clams have a long, wormlike appearance and are nicknamed “termites of the sea.” They use 2 small shells on the end of their body to bore their way through the wood creating sawdust to ingest. 3. Symbiotic bacteria live in a special organ in the bivalve and produce the enzyme cellulase which allows them to digest the wood. 4. Teredo navalis has been transported by ships for so many centuries that its historic native distribution cannot be known with certainty. It is presently found both in Atlantic and Pacific oceans from the tropics and subtropics to cool temperate waters of both the northern and southern hemispheres.

Video Clip Species Spotlight

1.One of the largest mollusks on Earth, Giant clams are capable of reaching 4 feet in length and weighing more than 500 pounds. They live in the warm waters of the South Pacific and Indian Oceans and have lifespans over 100 years. 2.A giant gets only one chance to find a nice home. Once it fastens itself to a spot on a reef, there it sits for the rest of its life. 3. Giant clams gain most of their nutrition as adults from the photosynthetic products of symbiotic dinoflagellates living in their mantle tissue. They also use a siphon to draw in water to filter and consume passing plankton. a. As a result of their unique protist symbionts, no two giant clams have the same coloration.

4. Despite the vast size of the giant clam, giant clams are preyed upon by a number of marine predators, many of which are actually much smaller in size than the giant clam itself. Eels, snails, fish and sea stars are all known to snack on small parts of the giant clam. 5. The excessive harvesting of giant clams by humans has led to rapid declines in the world's giant clam population. Video Video Clip Video a. ‘Cephalo’ means head, ‘pod’ means foot. i. Their modified foot is concentrated in the head region. It takes the form of a funnel for expelling water from the mantle cavity, and the anterior margin is drawn out into a circle or crown of arms or tentacles. b. All of the organisms in this class are found in marine environments and are active predators. i. Cephalopods swim using jet propulsion, in which they forcefully expel water from the mantle cavity through a ventral funnel (or siphon). 1. Squid and cuttlefish are excellent swimmers. Their lateral fins serve as stabilizers, but they are held close to the body for rapid swimming. 2. Nautilus is active at night; its gas-filled chambers keep the shell upright. Although not as fast as squids, it moves surprisingly well. 3. An Octopus has a rather globular body and no fins. An octopus can swim backward by spurting jets of water from its funnel, but it is better adapted to crawling over rocks and coral, using suction discs on its arms to pull itself. Some deep-water octopods have the arms webbed like an umbrella and swim in a medusa-like fashion. ii. Cephalopods bit their prey using a strong beak and radula. Octopus, cuttlefish, and some squid have poison glands to help subdue prey. 1.They catch prey with their arms, and then kill it by biting it with their tough beak, paralyzing the prey with a nerve poison and softening the flesh. They then suck out the flesh. c. Cephalopods are found at various depths. i. Octopuses are often seen in the intertidal zone, lurking among rocks and crevices, but occasionally they are found at great depths. ii. The more active squids are rarely found in very shallow water, and some have been taken at depths of 5000 m. iii.Nautilus is usually found near the bottom in water 50 to 560 m deep, near islands in the southwestern Pacific. d. Cephalopods evolved from shelled ancestors but now the nautilus is the only one to have an external shell. i. The shell of Nautilus, although coiled, is quite different from that of a gastropod. The shell is divided, or chambered, with only the last chamber being inhabited by the living animal. The other chambers are filled with gas, which allows the heavy shells of these organisms to be buoyant. As it grows, it moves forward, secreting behind its body a new septum and chamber. ii. Cuttlefish also have a small, curved shell we call a cuttlebone, but it is entirely enclosed by the mantle. iii.In squids most of the shell has disappeared, leaving only a thin strip called a pen, which is enclosed by the mantle. iv.In Octopus the shell has disappeared entirely. e. Cephalopods use arms or tentacles to capture prey by adhesive secretions or by suckers.

i. A nautilus has 80 to 90 tentacles that can be extended from the opening of the body compartment of its shell. Its tentacles have no suckers but are made adhesive by secretions. ii. Squid and cuttlefish have eight arms and two tentacles. Both arms and tentacles have suckers, but tentacles bear suckers only at their ends. iii.An octopus has eight arms and no tentacles. f. Cephalopods have the largest brain of any invertebrate. i. Octopuses are intelligent and capable of observational learning and problem solving. Experimenters find it easy to modify octopus behavior patterns by devices of reward and punishment. g. Sense organs are also well developed. Except for Nautilus, cephalopods have highly complex eyes with cornea, lens, chambers, and retina. h. Some cephalopods, especially cuttlefish, have earned the nicknames “masters of disguise”, “Chameleon of the Sea”, and “kings of camouflage”; they can change colors, patterns, and textures to match surroundings or depending on emotions. Octopus Camouflage Video 1, 2 i. Color changes are effected by chromatophores, elastic pigment cells surrounded by small muscles allowing them to expand and contract to expose different colors. ii. Color changes have been observed as camouflage protection, to indicate danger, and during courtship. iii. Many deep sea forms are also bioluminescent. Bioluminescence is light produced by living organisms. It is produced by a chemical reaction when a substance called a luciferin is oxidized. When the light is released, the luciferin becomes inactive until it is replaced by the Googly-eyed glass squid, a close animal. Some animals can relative of the colossal squid. make luciferin themselves, or it may be synthesised by Firefly squid symbiotic bacteria inside their cells. Bioluminescence is common among squid. It is estimated that two-thirds of all squid genera include bioluminescent species. Light organs, or photophores, can be found nearly anywhere on the body of a squid. i. Most cephalopods can also protect themselves by squirting ink. i. An ink sac that empties into the rectum contains an ink gland that secretes sepia, a dark fluid containing the pigment melanin, into the sac. i. Sexes are separate in cephalopods. ii. Spermatozoa are encased in spermatophores (packets of sperm) and stored in a sac that opens into the mantle cavity. One arm of adult males is modified as a hectocotylus, used to pluck a spermatophore from his own mantle cavity and insert it into the mantle cavity of a female near the oviduct opening iii. Eggs are fertilized as they leave the oviduct and are then usually attached to stones or other objects. iv. Juveniles hatch from eggs; no free- swimming larva exists in cephalopods Video Species Spotlight

1. The world’s largest known invertebrate, the giant squid, is very poorly known because no one has ever studied a living specimen. The anatomy has been described from stranded animals, from those captured in nets of fishermen, and from specimens found in the stomach of sperm whales. 2. Most squids grow to about 1 foot in length, but the giant squid can reach a length of 60 feet and weigh more than 3 and a half tons. 3. They have the largest eyes in the animal kingdom: up to 25 cm (10 in) in diameter. 4. They apparently eat fish and other squid, and they are an important food item for sperm whales. 5. They are thought to live on or near the sea bottom at a depth of 1000 m, but some have been observed swimming at the surface.

A female sperm whale feeds on a giant squid off the Japanese islands of Ogasawara. Species Spotlight

1. Found in the shallow coral reefs and tide pools of Australia, this one-ounce, golfball-sized octopus is a pale brown to yellow color. Its blue rings only appear as a warning when the animal feels threatened. 2. The Blue Ring Octopus hunts during the day. It eats invertebrates and wounded fish. 3. It is frequently encountered by people wading in tide pools. If provoked or stepped on, it will bite. The bite might be painless, but with a beak that can penetrate a wet-suit, they carry enough poison to kill 26 adults within minutes. Species Spotlight

1. Most octopuses are famous for being able to change their skin color and texture in order to blend in with their background, but this species of octopus is famous for being able to impersonate other sea creatures. 2. The Mimic Octopus is found of the coast of Indonesia. It is striped brown and white and its tentacles are around 25 cm in length and no more than pencil thick. 3. It is often found impersonating venomous sea snakes, lionfish, and sole fish—a strategy used to avoid predators. Scientists believe this creature may also impersonate sand anemones, stingrays, mantis and even jellyfish. 4. Mimic octopuses have been observed shifting between impersonations as it crosses the ocean floor to return to its burrow.

5. This animal is so intelligent that it is able to discern which dangerous sea creature to impersonate that will present the greatest threat to its current possible predator. For example, scientists observed that when the octopus was attacked by territorial damselfishes, it mimicked the banded , a known predator of damselfishes. a. Mollusks are eaten by a large number of animals; they are important in food webs in aquatic ecosystems and even in terrestrial ecosystems. i. Snails are major source of calcium for birds. b. Mollusks have been important to humans for thousands of years. They have been, and still are, used as food, tools, decorations, jewelry, currency, musical instruments, and more. They are even used as roadbed material and in vitamin supplements.

The shells of cowries (especially Monetaria moneta) were used for centuries as a currency in Africa. Video i. Bivalves living in coastal areas, such as clams, oysters, and scallops, are the most commonly eaten mollusks. People also eat octopuses and squid (calamari), whelks, and land snails (escargot). ii. France alone consumes 5 million pounds of escargot (a snail that lives in trees) every year. iii.Today, fisheries in , Japan, and the US alone produce over 1 billion pounds of oyster each year. (a marine gastropod), a great delicacy, can fetch up to three hundred dollars per pound. c. Mollusks are important indicators of the health of ecosystems. i. There are many threatened & endangered species of mollusks. Freshwater bivalves are now the most threatened group of invertebrates in the US. 1. Freshwater clams were once abundant and diverse in streams throughout the eastern United States, but they are now easily the most jeopardized group of animals in the country. Of more than 300 species once present, nearly two dozen are extinct, more than 60 are considered endangered, and as many as 100 more are threatened. 2. Threats include • Hunting Ohio is home to 11 • Harvesting endangered & • threatened • species of damming rivers mussels. • pollution Website Link • sedimentation i. Some mollusks are invasive species.

1.Asian zebra mussels arrived in the US in 1986 via ballast water from a European freighter into Lake Erie but are now found throughout the Great Lakes.

2. These voracious eaters disrupt food chains by competing for same food as native species. They can completely bock water inlet pipes of water treatment plants and power plants and are highly costly to remove. ii. Many gastropod species are necessary intermediate hosts for parasitic flatworms, such as the blood fluke that cause schistosomiasis in humans. iii. Eating mollusks is associated with a risk of food poisoning from toxins that accumulate in mollusks under certain conditions, and many countries have regulations to reduce this risk. At certain times of the year, (usually the warmer months) many species of saltwater mollusks become very poisonous due to an algal bloom known as "red tide." The mollusks filter feed on the tiny creatures (called "dinoflagellates" in the bloom) that produce the toxins. Eating shellfish during a red tide can cause serious illness and even death to humans. iv. Some people are allergic to mollusks and need to be careful about consuming any kind of shelled animals.