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Phylum Mollusca

Phylum Mollusca. > 100,000 extant species At least 45,000 extinct species Nice fossil history based on shells Fossils from Pre-Cambrian Importance? Shells - collectors, jewelry food. Mollusca characteristics:. 1. Foot 2. Mantle 3. Secretes shell. Shell: 3 layers.

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Phylum Mollusca

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  1. Phylum Mollusca • > 100,000 extant species • At least 45,000 extinct species • Nice fossil history based on shells • Fossils from Pre-Cambrian • Importance? • Shells - collectors, jewelry • food

  2. Mollusca characteristics: • 1. Foot • 2. Mantle • 3. Secretes shell

  3. Shell: 3 layers

  4. Periostracum: horny protein, conchiolin in some

  5. Prismatic layer: calcite crystals w/membranes between

  6. Nacreous layer: CaCo3

  7. Mollusca characteristics: • 1. Foot • 2. Mantle • 3. Secretes shell • 4. External surfaces - ciliated epidermis w/ mucous glands • Food capture, feeding, locomotion, cleaning body surfaces

  8. Cilia move mucous and create water flow • Gas exchange + bring food in • Sorting surfaces separate food particles by size

  9. Cilia over gill surface • Water movement opposite of blood flow

  10. 5. Coelom is reduced • Only pericardial cavity

  11. 6. Open circulatory system • Blood sinuses (no capillaries) • Heart = one or two auricles • collecting chambers • one ventricle • pumping chamber

  12. More circ. system • Hemocyanin pigment in blood (copper) • Blood w/ O2 = blue • Blood w/o O2 = colorless • Pulmonate gastropods have hemoglobin • Cephalopods have closed circulatory system

  13. 7. Digestive system • Sclerotized buccal cavity • Tubular esophagus • Cone-shaped stomach • Long, coiled intestine

  14. Radula • Chitin-toothed • Rasping organ for scraping algae

  15. Stomach • Contains style sac, rotates contents • Pulls strands of mucous from esophagus • Mucous viscosity decreases w/ low pH • Stomach wall is chitinized • Crystalline rod = hyaline mucoprotein • Style has hydrolase digestive enzymes

  16. Stomach, cont. • Sort food particles by size • Intracellular digestion in digestive gland walls • Some extracellular dig. in stomach • Carnivores have no style

  17. Sorting in stomach

  18. Intestine • Fecal compaction • Anus opens into mantle cavity

  19. 8. Nitrogenous waste • Pair of coelomoducts • Open to pericardial cavity • Discharge into mantle cavity via nephridiopores • Probably not homologous to annelid metanephridia (annelid origin = mesoderm; mollusk origin = ectoderm)

  20. Waste product? • Ammonia in aquatic molluscs • Uric acid in terrestrial molluscs

  21. 9. Nervous system - varied • Polyplacophora (chitons) - decentralized, no ganglia • Cephalopods - as developed as in vert’s • Primitive gastropods: • Nerve ring around esophagus, 2 pair of major nerve cords

  22. Reproduction and development • Pair of gonads in coelom • Eggs + sperm into pericardial cavity, outside via coelomoducts • Fert external in sea water • Molluscs mostly dioecious, some gastropods hermaphroditic

  23. Most gastropods, all cephalopods: • Sperm transferred to female’s mantle cavity • Internal fertilization • Hermaphroditic gastropods do reciprocal cross-fertilization

  24. Development • Trochophore larvae = free-swimming eye stomach prototroch ciliated band mouth intestine protonephridium anus

  25. Trochophore larvae • Archaeogastropoda • Polyplacophora • Aplacophora • Most marine bivalves • Develops into veliger larvae • Foot, shell, other structures appear

  26. Phylogenetic significance of trochophore larvae • Hatschik (1878) • Present in molluscs, annelids, other phyla • Promotes ctenophora - trochophore theory of bilateral animals from radial ancestors • body shape, apical sense organs, statocysts, nervous systems

  27. Problem with ctenophora-trochophore connection • Flatworms don’t fit • Degenerate annelids?

  28. 7 mollusca classes • Polyplacophora • Aplacophora • Monoplacophora • Gastropoda • Scaphopoda • Bivalvia • Cephalopoda

  29. Class Polyplacophora • Chitons and oval-flattened beasts - mostly in rocky intertidal zones • All marine, ~ 800 spp. • Mostly 2 - 12 cm • Largest (30 cm)is Cryptochiton stelleri from N. Pacific coast of N. America = Pacific gumshoe chiton

  30. Chiton characteristics: • Most feed on algae and micro-organisms on rock surfaces • Few are predators on small inverts • 1. Rudimentary head • No tentacles or eyes

  31. Characters • 2. Mantle covers dorsal surface • Secretes 8-piece shell • 3. Broad, ventral foot • 4. Many paired gills in mantle cavity • 5. Anterior mouth with radula

  32. Repro: • 6. Dioecious • trochophore larvae, no veliger • external fert. in sea water mouth Gills in mantle cavity mantle foot

  33. Classification of Polyplacophora - 2 orders • Order Lepidopleurida: few genera, Hanleya NE coast • Order Chitonida - most chitons • Chaetopleura (New England - Fl) • Chiton (gulf coast) • Katherina (N. Pacific coast) • Cryptochiton (N. Pacific coast) • Mopalia • Ishnochiton

  34. Class Aplacophora • Solenogasters are worm-like molluscs 0.5 - 30 cm long • Largest is Epimenia verrucova; 30 cm • All marine • Mostly deep waters, 20 - 9000 m • Some crawl and feed on hydroids and corals • Poorly known, seldom seen, ~ 250 spp.

  35. Characteristics: • 1. Worm-like body shape • 2. No shell, mantle, or foot • 3. Cuticle w/layers of imbedded calcareous spicules • 4. Ventral surface has longitudinal pedal groove • 5. Hermaphroditic • 6. Radula well-developed

  36. Pedal groove cloaca

  37. Class Monoplacophora • Originally known only from fossils • Living Neopalina from 3600 m in Pacific Ocean coast of Costa Rica (1952) • Two genera • Neopalina (7 spp.) and Vema

  38. Characteristics: • 1. Dorsal surface covered by flat conical shell. • 2. Ventral surface with mantle, paired gills and foot. • 3. Multiple paired gills, coelomoducts, heart chambers, gonads, and retractor muscles.

  39. Neopalina: Dorsal Ventral

  40. Dissection: bivalve umbo anterior

  41. Remove valve

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