CHAPTER 16

1. CHAPTER 16 Molluscs

3. Characteristics Phylum Mollusca • Over 90,000 living species and 70,000 fossil species • Soft body and belong to the lophotrochozoa protostomes • Include chitons, tusk shells, snails, slugs, nudibranchs, sea butterflies, clams, mussels, oysters, squids, octopuses, and nautiluses

5. Characteristics • May weigh up to 900 kg and grow to nearly 20 m long, but 80% are under 10 cm in size • Herbivorous grazers, predaceous carnivores, filter feeders, and parasites • Most are marine, but some are terrestrial or freshwater aquatic

7. Characteristics Evolution • Fossil evidence • Indicates molluscs evolved in the sea • Most have remained marine • Some bivalves and gastropods • Moved to brackish and freshwater • Snails (gastropods) successfully invaded land • Limited to moist, sheltered habitats with calcium in the soil • Cephalopods • Evolved to become relatively intelligent • Coelom limited to a chamber around the heart • Some believe molluscs arose separately from annelids and coeloms are not homologous

8. Characteristics Economics • Many are used as food • Culturing of pearls and pearl buttons is an important industry • Burrowing shipworms destroy wooden ships and wharfs • Snails and slugs are garden pests • Some snails are intermediate hosts for parasites

9. Form and Function Mollusc Body Plan: Head-Foot and Visceral Mass Portions • Head-foot region contains feeding, cephalic sensory, and locomotor organs • Visceral mass contains digestive, circulatory, respiratory, and reproductive organs • Mantle Cavity • Two folds of skin form protective mantle or pallium • Space between mantle and body wall is the mantle cavity • Mantle cavity houses the gills (ctenidia) or a lung • In most molluscs • Mantle secretes a shell over the visceral mass

11. Form and Function Head-Foot • Most have well-developed head bearing mouth and some sensory organs • Photosensory receptors range from simple to complex eyes • Tentacles may be present • Posterior to mouth is the chief locomotor organ, the foot

12. Form and Function • Radula • Unique to molluscs • Found in all except bivalves and some solenogasters • Protruding, rasping, tongue-like organ • Ribbon-like membrane has rows of tiny teeth (up to 250,000) pointed backward • Radula rasps off particles of food from surfaces • Serves as a conveyor belt to move particles to digestive tract • New rows of teeth replace those that wear away • Pattern and number of teeth are used in classification of molluscs • Some specialized to bore through hard material or harpoon prey

14. Form and Function • Foot • Usually ventral • Functions in attachment to substratum or for locomotion • Modifications include • Attachment disc of limpets • Hatchet foot of clams • Siphon jet of squids • Secreted mucus aids in adhesion or helps molluscs glide on cilia • Snails and bivalves extend the foot hydraulically by engorgement with blood • Burrowers extend the foot into mud or sand, enlarge the tip as an anchor, and draw forward • Free-swimming forms have modified the foot into wing or fin-like swimming agents

15. Form and Function • Mantle and Mantle Cavity • Mantle is a sheath tissue on each side of the body • Secretes the shell when present • Mantle cavity • Houses the gills or lungs that develop from the mantle • Exposed surface of the mantle also functions in gaseous exchange • In aquatic molluscs • Continuous flow of water brings in oxygen and food, and flushes out wastes • Products of digestive, excretory and reproductive systems empty into the mantle cavity

16. Form and Function • Cephalopods • Use head and mantle cavity to create jet propulsion • Mollusc gill has leaf-like filaments • Cilia propel water across the surface • Countercurrent blood movement in gill absorbs oxygen efficiently • In most molluscs, two ctenidia on opposite sides • Form an incurrent and an excurrent chamber

18. Form and Function • Shell • If present, secreted by the mantle and lined by it • Periostracum • Outer horny layer • Composed of conchiolin, a tanned protein • Middle prismatic layer • Closely packed prisms of calcium carbonate • Inner nacreous layer • Next to the mantle; the nacre is laid down in thin layers • Thick periostracum of freshwater molluscs protects against acid from leaf decay in streams

20. Form and Function • Internal Structure and Function • Open circulatory system • Pumping heart, blood vessels, and blood sinuses • Most cephalopods have a closed system with a heart, vessels, and capillaries • Most molluscs have a pair of kidneys or metanephridia • Kidney ducts also discharge sperm and eggs • Nervous system • Pairs of ganglia but generally simpler than in annelids • In air-breathing snails, nervous system produces growth hormones • Sense organs vary and may be highly specialized

21. Form and Function • Reproduction and Life History • Most dioecious, some hermaphroditic • Egg hatches and produces a free-swimming trochophore larva • Larva undergoes direct metamorphosis into a small juvenile in chitons • In many gastropods and bivalves • Intermediate larval stage, the veliger, is a derived state • Trochophore larvae are considered by some to unite molluscs with annelids, marine turbellarians, nemertines, phoronids, etc. in a taxon called Trochozoa within superphylum photrochozoa

24. Classes of Molluscs Class Caudofoveata • Wormlike, marine organisms ranging from two to 140 mm in length • Most burrow • Terminal mantle cavity and gills are near entrance • Feed primarily on microorganisms and detritus • Have no shell but body is covered with calcareous scales • Radula present but may be reduced • Fewer than 120 species

25. Classes of Molluscs Class Solenogasters • Formerly grouped with caudofoveates in Aplacophora • Resemble caudofoveates but have no radula or gills • Foot has a midventral, narrow furrow called the pedal groove • Do not burrow but are bottom dwellers and feed on cnidarians • Approximately 250 species • Class is sometimes called Neomeniomorpha

26. Classes of Molluscs Class Monoplacophora • Previously considered extinct • Living specimen was discovered in 1952 • About 25 extant species now known • Small molluscs with a rounded shell, resemble limpets • Some organs are repeated: 3–6 pairs of gills, two pairs of auricles, 3–7 pairs of metanephridia, one or two pairs of gonads and 10 pairs of pedal nerves • Radula characteristic of many other molluscs

28. Classes of Molluscs Class Polyplacophora: Chitons • Chitons are somewhat flattened with 7or 8 dorsal plates • About 1000 currently described species • Head and cephalic organs are reduced • Photosensitive structures (esthetes) similar to eyes pierce the plates • Most prefer rocky intertidal surfaces • Chiton radula is reinforced with iron mineral • Scrapes algae from the rocks • Mantle extends around margin

31. Classes of Molluscs • Gills suspended from roof of mantle cavity and grooves • Form a closed chamber • Water flows from anterior to posterior • Pair of osphradia serves as sense organ • Sample water in mantle groove near anus • Blood pumped by a three-chambered heart • Travels through aorta and sinuses to gills • Pair of metanephridia • Carries wastes from pericardial cavity to exterior • Sexes are separate • Trochophore larvae metamorphose into juveniles without a veliger stage

32. Classes of Molluscs Class Scaphopoda • Tusk or tooth shells • Live on the ocean bottom from subtidal zone to 6000 m depth • About 900 living species • Slender body covered with a mantle • Tubular shell is open at both ends • Unique body plan • Mantle is wrapped around the viscera and fused to form a tube • Foot protrudes from larger end to burrow into mud

33. Classes of Molluscs • Foot and ciliary action moves water through mantle cavity • Gills are absent and gaseous exchange occurs via the mantle • Detritus and protozoa are caught on cilia on foot or mucus-covered knobs of tentacles • Radula carries food to a crushing gizzard • Head or captacula lacks eyes, tentacles or osphradia

35. Classes of Molluscs Class Gastropoda • Most diverse class • Over 70,000 living and more than 15,000 fossil species • Snails, limpets, slugs, whelks, conches, periwinkles, sea slugs, sea hares, and sea butterflies • Forms range from marine forms to air-breathing terrestrial snails and slugs • Typically sluggish, sedentary animals • Shells are chief defense • Some produce distasteful or toxic secretions

36. Classes of Molluscs • Gastropod Shells • One-piece univalve, coiled or uncoiled • Apex is smallest and oldest whorl • Whorls become larger and spiral around central axis or columella • Many snails have an operculum covering shell aperture • Giant marine gastropods have shell up to 60 cm long • Some fossil forms are 2 meters long • Terrestrial gastropods are restricted by soil mineral content, temperature, dryness, and acidity

38. Classes of Molluscs • Snails serve as intermediate hosts to many parasites and are often harmed by larval stages • 3 gastropod subclasses • Prosobranchia • Opisthobranchia • Pulmonata

39. Classes of Molluscs • Form and Function • Torsion • Developmental process that changes the relative position of the shell, digestive tract and anus, nerves that lie on both sides of the digestive tract, and the mantle cavity containing the gills • Contraction of a foot retractor muscle pulls shell and viscera 90 counterclockwise • Moves anus to the right side of the body • Recent studies have shown that shell movement is independent of visceral movement

40. Classes of Molluscs • 1st movements of shell rotate it between 90 and 180 degrees into a permanent position • Mantle cavity develops on the right side of the body near the anus, but is initially separate from it • Anus and mantle cavity usually move further to the right and the mantle cavity is remodeled to encompass the anus • Digestive tract moves both laterally and dorsally so that anus lies above head within mantle cavity • After torsion, anus and mantle cavity open above mouth and head

42. Classes of Molluscs • Certain viscera on the left are now on the right side and vice versa • Nerve cords form a figure eight • Varying degrees of detorsion in opistobranchs and pulmonates have been observed • This arrangement resulting from torsion creates fouling • Wastes being washed back over the gills • Developmental sequence is called ontogenetic torsion

43. Classes of Molluscs • Coiling • Coiling or spiral windingof the shell and visceral mass not the same as torsion • Occurs at same larval stage as torsion but had a separate, earlier evolutionary origin • All living gastropods descended from coiled, torted ancestors • Planospiral shell has all whorls in a single plane • Primitive state • Conispiral shape provides more compactness • Each whorl is to the side of the previous one

45. Classes of Molluscs • Shifting the shell upward and back helped balance uneven weight distribution • Gill, auricle and kidney of right side are lost in most species • Loss of the right gill provides one solution to the problem of fouling • Wastes expel to the right

46. Classes of Molluscs • Feeding Habits • Adaptation of the radula provides much variation in gastropod feeding habits • Many are herbivorous and graze, browse or feed on plankton • Some scavenge decaying flesh • Others carnivores that tear prey using radula • Oyster borer alternates rasping with chemical softening of the shell to bore a hole

49. Classes of Molluscs • Species of Conus deliver a lethal sting to secure prey • Venom is a conotoxin • Specific for the neuroreceptors of its preferred prey • Some collect debris as a mucus ball to ingest it • Sea butterflies secrete a mucus net • Digestion usually extracellular in lumen of stomach

50. Classes of Molluscs • Internal Form and Function • Respiration in many performed by ctenidia in mantle cavity • Derived prosobranchs lost one gill and half of remaining gill • Resulting attachment to wall of mantle cavity provided respiratory efficiency • Pulmonates lack gills • Have a highly vascular area in mantle that serves as lung • Lung opens to outside by small opening, the pneumostome • Aquatic pulmonates surface to expel a gas bubble and inhale by curling, thus forming a siphon