BIOL 1030 – TOPIC 13 LECTURE NOTESTopic 13: More Lophotrochozoans: Mollusks (Ch. 33)I. Phylum Mollusca (clade) – mollusks A. very successful phylum: over 93,000 named, living speciesB. largest number of living species for animals after ArthropodaC. mostly aquatic, but over 35,000 terrestrial species (more than all terrestrial vertebrates)D. includes snails, slugs, clams, oysters, chitons, cuttlefish, octopi, etc.E. from microscopic to 21 m (giant squid – largest invertebrate)F. economically important (food, ornamentation, currency, pests)II. defining characteristics of mollusks A. bilateral symmetry with true coelom; usually have a differentiated headB. unsegmented (some argument as to whether lost during evolution or never present in phylum)C. three main body parts: muscular foot, visceral mass, and mantleD. muscular foot – used for locomotion, food capture, attachment1. may be solid or have tentacles or may be a “wing” or fin in pelagic forms2. covered with soft epithelium3. may secrete mucus to glide onE. visceral mass – contains most of the internal organs, including these systems:1. digestive system – more complicated than in phyla to this point (true coelom)• complete (separate mouth and anus)• mouth, stomach, intestine• radula – rasping “tongue” with rows of microscopic, chitinous “teeth” used to scrape or drill for food2. excretory system – simplest animals with efficient excretory system• nephridia – tubular structures that gather wastes from coelom• walls of nephridia reabsorb useful compounds (sugars, salts, water, etc.)• ability to reabsorb is why the system is considered efficient• wastes discharged into mantle cavity; gills pump wastes out of animal3. circulatory system• some have closed system, most have open system• if open, coelom is generally confined to a small area around the heart• heart may have three chambers• some fast cephalopods have auxiliary hearts to speed blood movement4. reproductive system and life cycle• most have separate sexes; however, many snails are hermaphrodites• trochophore – ciliated free-swimming larva of many marine mollusks1 of 4BIOL 1030 – TOPIC 13 LECTURE NOTES• veliger – second stage in development of most marine snails and bivalves; has beginnings of foot and mantleF. mantle1. folds (often 2) from dorsal body wall2. enclose a cavity between mantle and visceral mass (the mantle cavity)3. mantle cavity - may act as lung or enclose gills• gills are filamentous projections of mantle• gills are rich in blood vessels• gills have a large surface area for gas exchange• continuous pumping keeps flow of water into and out of mantle cavity• may have siphons to move water in (incurrent) and out (excurrent) of mantle cavity bring in oxygenated water and send out deoxygenated water may be used to bring in food (in bivalves) may be used for jet propulsion (in cephalopods)4. shell (if present) – secreted by mantle, usually on outside, but on inside in some snails, squid, and cuttlefish, and lost in slugs, nudibranchs, and octopi• provides protection• one or two valves (shells)• outer layer - horny protein layer; protects inner layers from eroding• middle layer - densely packed crystals of calcium carbonate (calcite or aragonite)• inner layer - pearly - increases in thickness throughout life of mollusk• pearls form between the inner layer and mantle in bivalvesIII. Mollusk taxonomy A. 7 or 8 classes; we will cover 4, all of which appear to be cladesB. Class Polyplacophora (clade) – chitons1. ~1000 living species2. marine; shell is segmented with 8 overlapping, calcareous plates3. body beneath shell is not segmented4. head is greatly reduced5. foot used for locomotion and for holding onto substratesC. Class Gastropods (clade) – snails and slugs1. name means “stomach foot”2. ~70,000 living species3. mostly marine, but abundant in freshwater, and many are terrestrial4. most have a shell2 of 4BIOL 1030 – TOPIC 13 LECTURE NOTES• single shell, if present• operculum found in most marine gastropods – a horny plate that forms a covering “door” when the snail withdraws into its shell• coiling (spiral twisting) of shell due to one side of larva growing faster than the other side• shell has been lost in the course of evolution for slugs and nudibranchs (sea slugs)5. body clearly divisible into head, foot, and visceral mass• head of most has a pair of tentacles with eyes (tentacles lost in more advanced species)• torsion of visceral mass in development leads to asymmetry of visceral mass; anus ends up practically over the head• terrestrial forms may have mantle cavity serve essentially as a lung – area very rich in blood vessels, allowing efficient gas exchange6. feeding – diversity in feeding habits; many are herbivorous (some major agricultural pests); some are active predatorsD. Class Bivalvia (clade) – the bivalves1. includes clams, oysters, scallops, and mussels2. ~20,000 living species3. 2 shells (valves) hinged together4. strong adductor muscles used to pull shells together5. typically with siphons - most are filter feeders (food trapped on mucus on gills)6. complex folded, ciliated gills7. no distinct head or radula8. most have a strong muscular foot (many different adaptations)9. some can move by clapping their shells together (scallops); however, most bivalves are sessile for most of their adult lives, tethered to some substrate by strong threads they secrete or using their foot as an anchor10. scallops have many eyes on soft body parts next to the edges of their shells11. freshwater mussels the most endangered group in North America, and Alabama has the highest diversity of freshwater mussels in the worldE. Class Cephalopoda (clade) – the cephalopods (“head foot”)1. includes octopi, squids, cuttlefish, and nautilus2. ~800 living species3. 0-1 shells, internal or external• octopi – no shell• squid and cuttlefish – internal shell remnant used for support• nautilus – chambered shell, superficially resembling snails4. highly developed nervous system• giant axons –great for research3 of 4BIOL 1030 – TOPIC 13 LECTURE NOTES• elaborate eyes (some up to 40 cm across – largest known eyes)• most intelligent invertebrates; complex behavior• many are skillful hunters5. foot modified into grasping tentacles with suckers6. built for speed – jet propulsion using siphons (they are fast-moving