Chapter 12 Molluscan Success

Evolutionary Perspective Octopuses, squids, and cuttlefish (all cephalopods) are some of the most adept predators of the invertebrate world Class Cephalopoda once numbered approx. 9000 species but now only have about 550 species Numerically, the mollusks are twice are successful as vertebrates There are nearly 100,000 species of mollusc. Most belong to two classes: Gastropoda and Bivalvia

E.P. con’t Molluscs and all animals from this point on are triploblastic. They are the first to possess a coelom Coelom performs multiple functions (refer to previous notes for these functions) Molluscs are protostomate animals Coelom forms by the splitting of the mesoderm. The unique and successful body form of the molluscs resulted in reduced size and importance of the coelom.

Molluscan Characteristics 1. Body of two parts: head-foot and visceral mass 2. Mantle that secretes a calcareous shell and covers the visceral mass 3. Mantle cavity functions in excretion, gas exchange, elimination of digestive wastes, and release of reproductive products 4. Bilateral symmetry 5. Protostome characteristics – trochophose larve, spiral cleavage, and schizocoelous coelom formation 6. Coelom reduced to cavities surrounding heart, nephridia, and gonads 7. Open circulatory system in all but one class (Cephalopods) 8. Radula usually present and used in scraping food

Three Main Regions 1. Head-foot Elongate with an anterior head Contains mouth and nervous and sensory structures Elongate foot – used for attachment and locomotion 2. Visceral Mass Contains the organs of digestion, circulation, reproduction, and excretion Positioned dorsal to the head-foot

Con’t Mantle Usually attaches to the visceral mass, enfolds most of the body, and may secrete a shell that overlies the mantle Shell is secreted in 3 layers: Outer layer – periostracum. Secreted by the mantle’s outer margin cells Middle layer – prismatic. Thickest of the three. Consists of calcium carbonate and organic materials. Secreted by mantle’s outer margin cells Inner layer – nacreous layer. Formed from thin sheets of calcium carbonate alternating with organic matter. Secreted by the entire epithelial border of the mantle. Nacre thickens the shell

Con’t In addition to the three main body regions, there is the mantle cavity and the radula. Mantle cavity – space between mantle and the foot. It is open to the outside. Functions in gas exchange, excretion, elimination of digestive wastes, and release of reproductive products Radula – made up of achitinous and posteriorly curved teeth. Overlays a fleshy tongue like structure called an odontophore. The odontophore can be protruded from the mouth. Food is scraped off and passed posteriorly to the digestive tract

Class Gastropoda Includes snail, slugs, and limpets Over 35,000 living species Largest and most varied molluscan class Found in marine, freshwater, and terrestrial habitats Escargot is considered a delicacy in restaurants

Con’t Cone shells are the only members of the gastropod class that may be seriously harmful to man. The venomous sting of some cone shell species may be deadly even for an adult.

Impacts on Humans Garden pests Serve as an intermediate hosts for trematode parasites of humans Fasciola hepatica, the liver fluke, has a complex life cycle, requiring water snails as intermediate hosts. Humans and other mammals acquire the organism by eating cyst-contaminated water plants. Watercress is a common source of the parasite for humans. The cysts release immature flukes that migrate to the liver and gallbladder. A high load of the parasite may obstruct the biliary tract.

Torsion Torsion positions the gills, anus, and openings from the excretory and reproductive systems the head and nerve cords. Digestive tract becomes U shaped Three advantages: 1. Head enters shell the first. Provides protection from predators. Some snails have a operculum that closes the opening of the shell and prevents dessication 2. Allows clean water from the front of the snail to enter the mantle cavity 3. It makes the snail more sensitive to stimuli coming from the direction in which it moves

Shell Coiling Asymmetrically coiled into compact forms Leaves less room for organs so now organs are single, not paired

Locomotion Flattened foot Often ciliated and covered with gland cells Creeps across the substrate Small gastropods use cilia to propel themselves over mucous trails Larger gastropods use wave-like muscular contractions The foot can be used for clinging or for swimming

Feeding and Digestion Most feed by scraping algae and other organisms from the substrate Others feed on larger plants Some are scavengers, parasites, or predators Some have an extensible proboscis (contains the radula) to get to those hard-to-reach places Digestive tract is ciliated. Digestive gland releases enzymes and acid into the stomach Food trapped on the protostyle is freed and digested Wastes form fecal pellets in the intestines

Maintenance Functions Gastropods have one gill Some have a siphon. Used by burrowing species to extend to the surface of the substrate and bring in water Gills are lost or reduced in land snails – have a rich vascular mantle for gas exchanged between blood and air Open circulatory system. Blood leaves the vessels and directly bathes cells in tissue spaces called sinuses. Heart is a single, muscular mass consisting of ventricle and two auricles. Some have one ventricle, one auricle due to coiling

Con’t Blood acts like a hydraulic skeleton. They contract muscles to force fluid into distant structures to push it forward The nervous system of the modern gastropod is a concentration of nervous tissues into large ganglia, especially in the head Well developed sensory structures: Eyes at/near the base of the tentacles Statocysts are in the foot (used for equilibrium and balance) Osphradia – chemoreceptors in the mantle cavity that detect sediment and chamicals in inhalant water or air. In predatory gastropods, it helps detect prey Nephrida – modifies excretory wastes by selectively reabsorbing certain ions and organis molecules. Aquatic gastropods excrete ammonia, terrestrial gastropods convert ammonia into to uric acid (a semisolid form that helps conserve water)

Reproduction/Development Many marine snails are dioecious. Ducts release gametes into the sea for external fertilization. Other snails are monoecious. Internal, cross-fertilization occurs. Fertilized egg is protected by a capsule. Snails that are monoecious can be protandric. Testes develop first, and after they degenerate, ovaries mature. Eggs are shed singly or in mass for external fertilization

Con’t Eggs are deposited into moist environments (leaf litter) Calcareous shell may encapsulate them