Invertebrates Molluscs

• Molluscs


  • Have a soft body enclosed in a calcium carbonate shell
• General characteristics shared by molluscs

  • Mantle- a thin layer of tissue that secretes shell
  • Foot- usually used in locomotion
  • Unique feature called radula
  • Body cavity that is reduced and a simple body plan

  
• Types of molluscs


  • Gastropods
  • Bivalves
  • Cephalopods
  • Other molluscs (Polyplacophora, Scaphopoda)
• Gastropods


  Gastropod means "stomach footed", largest and most common group of molluscs including snails, limpets, abalones and nudibranchs
• Gastropods


  • Have a coiled mass of vital organs enclosed in a dorsal shell, shell rests on a ventral creeping foot and is usually coiled

  
• Nudibranchs


  Gastropods that have lost the shell altogether, one of the most colorful marine organisms
• Nudibranchs


  • Phidiana crassicornis
• Bivalves


  Bivalve means "two valves", enclosed in a shell with two parts
• Bivalves


  • Body is laterally compressed, have an umbo (oldest part of shell), absence of head and radula
• Cephalopods


  Cephalopod means "head foot", like a gastropod with head pushed down toward the foot, foot modified into arms and tentacles with suckers
• Cephalopods


  • Absence of outer shell
• Cephalopods


  • Enteroctopus dofleini
  • Illex illecebrosus
  • Loligo opalescens

  
• Other molluscs


  Polyplacophora, Scaphopoda
• Polyplacophora


  About 800 species
• Scaphopoda


  Have an elongated shell, open at the top and tapered like an elephant tusk, live in sandy and muddy bottoms
• Molluscan feeding and digestion

  1. Molluscan gut has a separate mouth and anus

  2. Digestive gland releases digestive enzymes to break down food

  3. Rasping radula removes minute algae or cuts through large seaweed

  4. Simple digestive system can efficiently process large amounts of hard-to-digest plant material

  5. Extracellular digestion in gut cavity and intracellular digestion in digestive glands
• Gastropod feeding


  Some have a proboscis where mouth and radula is found
• Bivalve feeding


  • Ingest food particles filtered and sorted out by cilia on gills
  • Enzyme secreting rod in stomach, crystalline style, continually rotates food to aid digestion
• Cephalopod feeding


  All are carnivorous that have to digest large prey
• Molluscan nervous system and behavior

  • Gastropods and bivalves have several sets of ganglia or "local brain"
  • Cephalopods have the most complex nervous system compared to all invertebrates
  • Cephalopod eyes are strikingly complex reflecting their nervous system development
• Molluscan reproduction and life history

  1. Most have separate sexes, some are hermaphrodites

  2. In bivalves, chitons, tusk shells, and some gastropods, sperm and eggs released into water for external fertilization

  3. Fertilization is internal in cephalopods and most gastropods, cephalopods use a modified arm to transfer spermatophore to female
• Molluscan larvae


  • Some have a trochophore larva, which usually develops into a veliger larva
  • Many gastropods develop within strings or capsules of eggs
  • Cephalopods lack larvae, young develop from large yolk-filled eggs
  • Female octopuses protect their eggs until they hatch, then usually die
• The trochophore larva is like that of polychaetes
• Mollusks


  • Have a soft body enclosed in a calcium carbonate shell
  • Body is divided into two major parts: Head-Foot region and Visceral mass
  • Mantle - a thin layer of tissue that produces the shell (in some groups is used in locomotion and gas exchange)
  • All mollusks (except bivalves) have a radula
• Types of Mollusks


  • Gastropods (Class Gastropoda)
  • Nudibranchs (sea slugs)
  • Bivalves (Class Bivalvia)
  • Cephalopods (Class Cephalopoda)
  • Chitons (Class Polyplacophora)
  • Scaphopods (Class Scaphopoda)
• Gastropods


  • Largest and most common group of mollusks
  • Have a coiled mass of vital organs enclosed in a dorsal shell
  • Shell rests on a ventral creeping foot and is usually coiled
• Nudibranchs


  • Gastropods that have lost their shell
  • Most are colorful marine organisms, their bright coloration can be an indication of toxicity
  • Many have cerata on their back
  • Some will feed on cnidarians and retain their nematocysts as protection for themselves
• Nudibranchs


  • Phidiana crassicornis
• Bivalves


  • Have two (2) valves (shells) attached dorsally by ligaments
  • Body is laterally compressed
  • The valves are closed by adductor muscles and opened as the muscles relax and the weight of the valves pulls it apart
  • No head or radula
  • Their foot is located ventrally and functions in burrowing and locomotion
  • Umbo- oldest part of a shell
• Cephalopods


  • The foot is modified into a head-like structure and have a ring of tentacles that projects from the anterior end to the head
  • Tentacles are used to capture prey, for defense, reproduction, and locomotion
  • Lack shell or have a small internal shell
  • Most can move by jet propulsion (water is taken into the mantle, the channeled through a small funnel and expelled)
  • Nautiloid Cephalopods have a large coiled shell with multiple chambers filled with gas which can regulate buoyancy and have 60 to 90 tentacles
  • Coleoid Cephalopods includes squids, octopus, and cuttlefish
• Cephalopods


  • Hapalochlaena (Blue-ringed octopus)
  • Enteroctopus dofleini (Pacific giant octopus)
  • Illex illecebrosus and Loligo opalescens (both are squids)
• Chitons


  • Have flattened body covered by eight calcareous plates held together by a girdle
  • Common in intertidal zones where they can attach to rocks and scrape off algae
• Chitons


  • Tonicella lineata
• Scaphopods


  • Have an elongated shell that's open at the top and tapered like an elephant tusk
  • Lives in sandy and muddy bottoms
• Molluscan gut


  Has a separate mouth and anus
• Digestive gland


  Releases digestive enzymes, which break down food into simpler particles
• Radula


  Rasping structure that removes minute algae from surfaces or cuts through large seaweed
• Digestive system


  Can efficiently process large amount of hard-to-digest plant material
• Digestion


  Extracellular digestion in the gut cavity and intracellular digestion in the digestive glands
• Gastropods


  Some have a proboscis where the mouth and radula is found