Copyright Pearson Prentice Hall 27-4 Mollusks Photo Credit: © Jeffrey L. Rotman Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall What Is a Mollusk? What Is a Mollusk? Phylum Mollusca: named from the Latin word molluscus, meaning soft. Mollusks are soft-bodied animals that usually have an internal or external shell. Mollusks include snails, slugs, clams, squids, and octopi. Many mollusks share similar developmental stages. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall What Is a Mollusk? Many aquatic mollusks have a free-swimming larval stage called a trochophore. The trochophore larva is also characteristic of annelids, indicating that these two groups may be closely related. Mollusks are an incredibly diverse group, including clams, octopi, and snails, as well as the lesser known cuttlefish, shown here. Many mollusks have a larval stage called a trochophore), shown here, which has at least one band of cilia encircling its body. All mollusks have a soft body, and most mollusks have an internal or external shell. Copyright Pearson Prentice Hall

Form and Function in Mollusks Mollusks have true coeloms surrounded by mesoderm tissue. They have complex, interrelated organ systems that function together to maintain the body as a whole. Copyright Pearson Prentice Hall

Form and Function in Mollusks Body Plan The body plan of most mollusks has four parts: foot, mantle, shell, and visceral mass. The mantle is a thin layer of tissue that covers most of the mollusk's body. The shell is made by glands in the mantle that secrete calcium carbonate. The shell has been reduced or lost in slugs and some other mollusk groups. Just beneath the mantle is the visceral mass, which contains the internal organs. Copyright Pearson Prentice Hall

Form and Function in Mollusks The muscular foot takes many forms flat structures for crawling spade-shaped structures for burrowing tentacles for capturing prey The body plan of most mollusks includes a foot, mantle, shell, and visceral mass. Early mollusks may have looked like the animal shown here. As they evolved, their body parts became adapted for different functions. Squid Snail Clam Copyright Pearson Prentice Hall

Form and Function in Mollusks Feeding  Mollusks can be herbivores, carnivores, filter feeders, detritivores, or parasites. Copyright Pearson Prentice Hall

Form and Function in Mollusks Snails and slugs feed using a flexible, tongue-shaped structure known as a radula. Hundreds of tiny teeth are attached to the radula. The radula is used to scrape algae off rocks or to eat the soft tissues of plants in herbivorous species, and to drill through shells of other animals to tear up and swallow soft tissue inside. Snails use a radula for feeding. The teeth of a radula give it the look and feel of sandpaper. Beneath the radula is a stiff supporting rod of cartilage. When the mollusk feeds, it places the tip of the radula on its food and pulls the sandpapery layer back and forth. Copyright Pearson Prentice Hall

Form and Function in Mollusks Octopi and certain sea slugs use sharp jaws to eat prey. Some octopi also produce poisons to subdue prey. Clams, oysters, and scallops use feathery gills to filter feed. Food is carried by water, which enters the incurrent siphon. A siphon is a tube like structure through which water enters and leaves the body. Copyright Pearson Prentice Hall

Form and Function in Mollusks The water flows over the gills and leaves by the excurrent siphon. As water passes over the gills, plankton become stuck in sticky mucous. Cilia on the gills move the mix of mucous and food into the mouth. Excurrent siphon The anatomy of a clam is typical of bivalves, or two-shelled mollusks. The mantle and part of the foot have been cut away to show internal organs. The adductor muscles are used to open and shut the two exterior shells. The gills exchange oxygen and carbon dioxide between the body and the surrounding water. The arrows show the path of water over the gills. Incurrent siphon Copyright Pearson Prentice Hall

Form and Function in Mollusks Respiration  Aquatic mollusks breathe using feathery gills, called ctenidia, inside their mantle cavity. Gills Copyright Pearson Prentice Hall

Form and Function in Mollusks As water passes through the mantle cavity, oxygen in the water moves into blood flowing through the gills. At the same time, carbon dioxide moves in the opposite direction. Land snails and slugs respire using a mantle cavity that has a large surface area lined with blood vessels. Because this lining must be kept moist so that oxygen and carbon dioxide can diffuse, land snails and slugs tend to live in moist places. Copyright Pearson Prentice Hall

Form and Function in Mollusks Circulation  Some mollusks have open circulatory systems; other mollusks have closed circulatory systems. In an open circulatory system, blood is pumped through vessels by a simple heart. Heart The anatomy of a clam is typical of bivalves, or two-shelled mollusks. Copyright Pearson Prentice Hall

Form and Function in Mollusks Blood leaves the vessels and works its way through different sinuses. A sinus is a large, saclike space. They do not have closed veins or arteries. Blood passes from the sinuses to the gills, where oxygen and carbon dioxide are exchanged. Blood is then pumped back to the heart. Slow-moving mollusks often have open circulatory systems, like snails and clams. Copyright Pearson Prentice Hall

Form and Function in Mollusks Faster-moving mollusks have a closed circulatory system. A closed circulatory system can transport blood through an animal’s body much more quickly than an open circulatory system. Copyright Pearson Prentice Hall

Form and Function in Mollusks Excretion  Cells of the body release nitrogen-containing waste into the blood in the form of ammonia. Nephridia remove ammonia from the blood and release it outside the body. Nephridium The anatomy of a clam is typical of bivalves, or two-shelled mollusks. Copyright Pearson Prentice Hall

Form and Function in Mollusks Response  The complexity of the nervous system and the ability to respond to environmental conditions varies among mollusks. Two-shelled mollusks have a simple nervous system consisting of small ganglia near the mouth, a few nerve cords, and simple sense organs, like chemoreceptors and eyespots. Copyright Pearson Prentice Hall

Form and Function in Mollusks Octopi and their relatives have the most highly developed nervous system of all invertebrates. Well-developed brains in these animals allows them to remember things for long periods, and may be more intelligent than some vertebrates. Some octopi are capable of complex behavior and have been trained to perform different tasks for reward or to avoid punishment. Copyright Pearson Prentice Hall

Form and Function in Mollusks Movement  Snails secrete mucus along the base of the foot, and then move over surfaces using a rippling motion of the foot. The octopus draws water into the mantle cavity and then forces the water out through a siphon. Water leaving the body propels the octopus in the opposite direction. Copyright Pearson Prentice Hall

Form and Function in Mollusks Reproduction  Some snails and two-shelled mollusks reproduce sexually by external fertilization: They release enormous amounts of eggs and sperm into open water where they are fertilized and develop into free-swimming larvae. In tentacled mollusks and other snails, fertilization takes place inside the body of the female. Some mollusks are hermaphrodites and usually fertilize eggs from another individual. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Groups of Mollusks Groups of Mollusks The four major classes of mollusks are chitons gastropods bivalves cephalopods Copyright Pearson Prentice Hall

Class Polyplacophora: Chitons Groups of Mollusks Class Polyplacophora: Chitons Chiton's body consists of 8 overlapping shell plates (valves) bound together by a leathery girdle. Lacking eyes or tentacles, chitons can never the less sense the light level through light-sensitive organs in its shell. Scrapes food from rocks with its radula.     Separate sexes. Gametes released into the water column where fertilization takes place. Copyright Pearson Prentice Hall

Class Gastropoda: Gastropods Groups of Mollusks Class Gastropoda: Gastropods Includes pond snails, land slugs, sea butterflies, sea hares, limpets, and nudibranchs. Shell-less or single-shelled mollusks that move by using a muscular foot located on the ventral side.  When threatened, they can pull completely into their coiled shells. Some snails are also protected by a hard disk on the foot that forms a solid “door” at the mouth of their shell when they withdraw. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Groups of Mollusks Land slugs and nudibranchs have no shell, but protect themselves in other ways. Most land slugs spend daylight hours hiding under rocks and logs, hidden from potential predators. Some sea hares can squirt ink into surrounding water producing a smoke screen to confuse predators. Some nudibranchs have body chemicals that taste bad or are poisonous. Other nudibranchs are able to recycle the nematocysts from cnidarians they eat, using them to sting predators. They are usually brightly colored as a warning to predators. Copyright Pearson Prentice Hall

Class Bivalvia: Bivalves Groups of Mollusks Class Bivalvia: Bivalves Bivalves have two shells that are held together by one or two powerful muscles.  Common bivalves include: clams, oysters, mussels, and scallops. Most stay in one place much of the time. Clams burrow into mud or sand and mussels use sticky threads to attach to rocks. Scallops are the least sedentary and can move around rapidly by flapping their shells when threatened. Copyright Pearson Prentice Hall

Class Cephalopoda: Cephalopods Groups of Mollusks Class Cephalopoda: Cephalopods Most active of all mollusks, including octopi, squids, cuttlefishes, and nautiluses. Cephalopods are typically soft-bodied mollusks in which the head is attached to a single foot. The foot is divided into eight or ten tentacles or arms equipped with sucking disks that grab and hold prey. Nautiluses have many more tentacles than other cephalopods (up to 90) and lack suckers. They do have a sticky mucous covering. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Groups of Mollusks Most modern cephalopods have only small internal shells or no shells at all. The only present-day cephalopods with external shells are nautiluses. They can control their depth in water by regulating the amount of gas in their shells. Cuttlefishes have small shells inside their bodies. A squid’s internal shell has evolved into a think supporting rod known as a pen. Octopi have lost their shells completely. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Groups of Mollusks Cephalopods have complex sense organs that help them detect and respond to external stimuli. Cephalopods distinguish shapes by sight and texture by touch. The eyes of many cephalopods are complex. They can be as large as a dinner plate and distinguish objects as small as 0.5 cm from a meter away, allowing them to locate a wide variety of prey. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Ecology of Mollusks Ecology of Mollusks Mollusks feed on plants prey on animals filter algae out of the water eat detritus can be used to monitor water quality rarely develop any form of cancer Some mollusks are hosts to symbiotic algae or to parasites; others are themselves parasites. Mollusks are food for many organisms. Copyright Pearson Prentice Hall