Introduction to Molluscs: Characteristics, Evolutionary Contributions, and Classes

1. Chapter 16 Molluscs Kingdom: Animalia Phylum: Mollusca Class Gastropoda Class Bivalvia Class Cephalopoda

2. I. Intro to P. Mollusca • + 90,000 living sp. • 70,000 fossil sp. • Chitons, snails, slugs, clams, mussels, oysters, squids, octopuses, nautiluses • Overview • soft body • microscopic → massive • 80% under 10 cm • giant squid - 900 kg, 20 m long • giant clams - 250 kg, 1.5 m dia. • herbivores/predators/filter-feeders/ parasites • marine/freshwater/terrestrial/tropics/polar • burrowers/borers

3. Intro to P. Mollusca A. Evolutionary Contributions • respiratory system • gas exchange though body surface + gills or “lungs” • open circulatory system • ♥ that pumps + vessels + blood sinuses • closed circulatory in cephalopods • highly developed direct eye in cephalopods • photosensitive cells in retina face light • derived from skin

4. Intro to P. Mollusca B. Mollusca Characteristics • bilateral symmetry • bilateral asymmetry • often defined head • ventral body wall forms foot • locomotion • folds of dorsal body wall form mantle w/ mantle cavity • mantle forms gills/lungs and secretes shell • ciliated surface epithelium w/ mucous glands + sensory nerve endings • coelom around ♥, sometimes gonads/kidneys/intestine • complete digestive tract w/ anus • rasping organ (radula)

5. B. Characteristics Continued… • open circulatory system w/ ♥ (3-chambered) + blood vessels + sinuse • 1-2 kidneys (metanephridia) • nervous system w/ paired cerebral/visceral ganglia w/ nerve cords • sensory organs for touch/smell/taste/equilibrium/vision • internal/external ciliary tracts

6. Phylum Mollusca • Coelom limited to space around the heart, gonads, and kidneys

8. P. Mollusca: Form/Func. C. Reproduction and life history • most dioecious, some hermaphroditic • egg hatches and produces a free-swimming larva (trochophore larva) • larva undergoes direct metamorphosis into small juvenile in chitons • many gastropods and bivalves, an intermediate larval stage (veliger) is derived state

9. P. Mollusca: Form/Func. a. Radula • protruding, rasping, tongue-like organ • unique to molluscs • lacking in bivalves • membrane w/ rows of tiny teeth pointed backward • scrapes/tears/cuts food particles from surface • carries particles to digestive tract • some sp. bore through hard material or harpoon prey

10. Radula

11. P. Mollusca: Form/Func. b. Foot • func. attachment or locomotion • usually ventral • modifications • hatchet foot in clams • siphon jet in squids • mucus secretions fucn. adhesion or help glide on cilia • uses • snails/bivalves extend foot hydraulically by engorgement w/ blood • burrowers extend foot into substrate, enlarge tip (anchor), draw forward • free-swimming sp. modified foot into wing/fin-like swimming agents

12. II. 1st Class of Molluscs: Gastropoda A. Class Gastropoda (snails/slugs/conches/sea slugs) • most diverse/abundant • +70,000 living sp., +15,000 fossil sp. • marine → air-breathing terrestrial snails/slugs • sluggish/sedentary animals • terrestrial gastropods restricted by soil mineral content/acidity/ temperature/dryness • intermediate hosts to many parasites • often harmed by larval stages

14. Gastropoda B. Respiration • performed by ctenidia in mantle cavity • have a highly vascular area in mantle that serves as lung • lung opens to outside by small opening (pneumostome)

15. Gastropoda C. Nervous/sensory systems • most have well-developed nervous systems • sense organs include eyes/tactile organs/ chemoreceptors • eyes • simple cups w/ photoreceptors → complex eye w/ lens/cornea

16. Gastropoda D. Reproduction • monoecious/dioecious • some discharge ova/sperm into water • external fertilization • young emerge as veliger larvae or pass this stage inside egg • some sp. ovoviviparous

17. III. 2nd Class of Molluscs: Bivalvia A. Class Bivalvia Characteristics(mussels/clams/scallops/oysters/shipworms) • 1–2 mm → giant South Pacific clams • most sedentary filter feeders • ciliary currents bring in food • lack head/radula/cephalization • marine/freshwater streams/ponds/lakes • native freshwater clams in U.S. most jeopardized animal group • +300 sp. once present • 12 extinct • 42 threatened/endangered • 88 of concern

18. Bivalvia B. Bivalvia Form/func. • 2 shells or valves held together by hinge ligament • valves drawn together by adductor muscles • umbo = bulge • oldest part of shell • growth occurs outward in rings • pearls produced when irritant lodged btwn shell/mantle • layers of nacre secreted around foreign material

19. B. continued… • freshwater clams sensitive to water quality changes (pollution/sedimentation) • Zebra mussels • serious exotic pest • clog water intake pipes • outcompete native sp. • 20,000 animals /m2 • 30,000-1,000,000 eggs/yr • altering biomass distribution

21. Bivalvia C. Locomotion • slender foot extended out btwn valves • blood pumped into foot • it swells/anchors bivalve in mud • shortening foot pulls clam forward • scallops clap valves to create jet propulsion • mantle edges direct the stream

22. Bivalvia D. Gills • both mantle/gills perform gaseous exchange • gills derived from primitive ctenidia by lengthening filaments to each side • water mvmt through clam • water enters incurrent siphon • enters water tubes via pores • out excurrent siphon

23. Classes of Molluscs: Bivalvia E. Circulatory system • 3-chambered ♥ w/ 2 auricles + 1 ventricle • some blood oxygenated in mantle • returns to ventricle via auricles • rest circulates via sinuses/kidneys/gills before back to auricles

24. Bivalvia F. Feeding • suspended organic matter enters incurrent siphon • gland cells on gills/labial palps secrete mucus to trap particles • food in mucous masses slide via grooves at lower edge of gills • suck in crustaceans by creating sudden inflow of water

25. Bivalvia G. Digestive system • stomach folded into ciliary tracts for sorting particles • rotating style helps free digestive enzymes, roll mucous food mass • dislodged particles directed to digestive gland or engulfed by amoebocytes

26. Bivalvia H. Reproduction and Development • dioecious • gametes discharged out in excurrent flow • fertilization usually external • internal fertilization • freshwater clams • sperm enter incurrent siphon to fertilize eggs in water tubes of gills • embryos develop as trochophore, veliger, spat larval stages • eventually sink to begin independent life on streambed

28. IV. 3rd Class Molluscs: Cephalopoda A. Class Cephalopoda (squid/octopus/nautilus/devilfish/cuttlefish) • 2 cm → giant squid (largest invertebrate known) • mostly marine • octopuses mostly intertidal • squids are deep-sea animals • predators • foot is in head region • modified for expelling water from mantle cavity

29. A. Class Cephalopoda continued… • cuttlefish shell enclosed in mantle • squid shell is thin strip enclosed in mantle (pen) • octopus has completely lost shell

30. Cephalopoda B. Locomotion • swim by forcefully expelling water via ventral funnel/siphon • octopuses crawl on bottom/swim backward by spurting water jets • webbing btwn arms allows swimming w/ medusa-like action • control direction/force of water • determines speed • lateral fins of squids/cuttlefishes stabilize • Active life of cephalopods reflected in respiratory/ circulatory/nervous systems

31. Cephalopoda C. Respiration/circulation • 1 pair of gills • 2 pair in Nautilus • muscular pumping keeps water flowing through mantle cavity when ↑ O2 demand • network of vessels conducting blood through gill filaments • blood goes to systemic circulation before gills • accessory/branchial ♥ at base of each gill ↑ pressure

33. Cephalopoda D. Nervous/sensory systems • largest brain of any invertebrate • giant nerve fibers • well-developed sense organs • complex eyes (cornea/lens/retina) • learn by reward/punishment/ observation of others • lack hearing • tactile/chemoreceptor cells in arms

34. Cephalopoda E. Communication • chemical/visual signals • cells in skin contain pigment granules (chromatophores) • contractions of muscle fibers attached to cell boundary cause cell to expand and change color pattern • deep-water cephalopods have luminescent organs • ink sac empties into rectum • ink gland secretes sepia when animal is alarmed

35. Cephalopoda F. Reproduction • dioecious • ♂ seminal vesicle • 1 arm of ♂ modified as intermittent organ • plucks spermatophore from mantle cavity and inserts it into ♀ • fertilized eggs leave oviduct, attach to substrate • hatch into juveniles w/ no free-swimming larval stage

36. Human eye Anatomy of the human eye PBS evolution of the eye Read Ken Miller Evolutionary Eye Development @ the PBS website