Lecture 14

Question 1

Orientation and anatomy

Answer 1

• Bilateral symmetry
  
  • Dorsal; hinge region
  • Ventral; wide open rim
  • Anterior; short hinge adjacent
  • Posterior; long hinge adjacent
• Bivalves that dig move towards their anterior using their muscular foot
• Siphons, if present, extend from posterior out of the sediment drawing in water

Question 2

Bivalvia - general internal anatomy

Answer 2

• Opening/closing the valves is fundamental to filtration (protection and movement)
• When at least partially open…
  
  • Inhalant siphon draws water into mantle cavity
  • Exhalant siphon expels water from mantle cavity
  • Ctenidia usually act as a filter

Question 3

Ctenidia - forms and functions

Answer 3

• Lamellibranch bivalves have longer and more filamentous ctenidia than other bivalves
  
  • Ctenidia separate entire inhalant/exhalant chamber
  • Indentation at apex forms a ‘food groove’

Question 4

Lemellibranch ctenidia (water currents and ciliary motion)

Answer 4

• Lateral cilia
  
  • Generate water current to drive inhalant water through filaments
• Frontolateral cilia
  
  • Collects food along each filament with the help of mucous directing towards the dorsal or ventral ctenidia margin
• Frontal cilia
  
  • Carry food along ‘food groove’ towards labial palps/mouth

Question 5

Rejection tract

Answer 5

Serves to carry inappropriate particles away from the food groove for ejection as pseudofeces

Question 6

Lemellibranch efficiency

Answer 6

• Modern are highly efficient
  
  • Maximising the surface area of their ctenidia
  • Maximising the degree of separation between inhalant and exhalant space
• Hemolymph flows from afferent to efferent branchial vessels
  
  • Counter-current exchange
  • Maximises respiration/excretion

Question 7

Bivalvia - anatomy and filter feeding

Answer 7

• Food groove carries nutritious prey down the ctenidia towards labial palps and mouth
  
  • Muscular labial palps push food into the mouth
  • Can reject inappropriate sizes
• Deoxygenated and nutrient-poor water exits exhalant siphon

Question 8

Extracellular and intracellular digestion

Answer 8

• Mucous strings with food are dragged into stomach by rotating crystalline style
  
  • Secretes digestive enzymes as it turns
• Digestive gland secretes enzymes and collected small particles for intracellular digestion
  
  • Typhlosole and ciliated sorting area help direct particles by size
• Large particles passed through intestine for excretion and minor absorption

Question 9

Bivalvia/mollusca - generalized circulatory/waste features

Answer 9

Most molluscs, including bivalves and gastropods have relatively comparable circulatory systems
1.) Atrium receives oxygenated hemolymph from efferent branchial vessels
2.) Ventricle pumps hemolymph through arteries to the sinuses
3.) Deoxygenated hemolymph passed through afferent branchial vessels

Question 10

Bivalvia and the art of digging

Answer 10

1.) Foot extends into the substrate, body remains in place. Adductor muscles relax, valves push apart to anchor body. Retractor muscles relax, circular and transverse foot muscles contract. Pedal ganglion contributes to muscle coordination
2.) Foot forms an expanded anchor, water is expelled into sediment. Hemolymph is pumped into base of the foot. Siphons and valves close as shell adductors contract, expelling water
3.) Bivalve recedes into the sediment. Retractor muscles contract. Hemolymph-filled foot remains as an anchor. Adductor muscles slightly relax to push sand/water out of siphons if needed
4.) Valves separate slightly to anchor body in place. Adductor muscles increase relaxation. Siphons are able to function normally. Hemolymph is drained from the receding foot

Question 11

Traditionally grouped into subclasses of:

Answer 11

1.) Prosobranchia
2.) Opisthobranchia
3.) Pulmonata

Question 12

Prosobranchia

Answer 12

Gills in front of the heart

Question 13

Opisthobranchia

Answer 13

Gills behind and to the right of the heart

Question 14

Pulmonata

Answer 14

Air breathing

Question 15

Torsion of the body - Gastropoda

Answer 15

• Rotation of the visceral mass, mantle, and mantel cavity by up to 180 degrees; occurs in all gastropods during early development
  
  • Not all gastropods remain ‘torted’ in their adult lives
• Always occurs in early development
  
  • Usually occurs in the veliger stage

Question 16

Planktonic trochophore larvae

Answer 16

• Apical sensory plate with cilia tuft
• Prototroch-girdle of ciliated cells anterior of the mouth
• Can feed, has a complete digestive system
• Other organs begin to develop
  
  • Trochophore is the only larval stage of some molluscs
    
    • ie. polyplacophora, chitons

Question 17

Metamorphosis to veliger stage

Answer 17

• Unique to some Mollusca
  
  • Includes gastropods and some bivalves
  • Many gastropods skip or rapidly shift from trochophore directly to a veliger
• Develops a foot, shell, operculum and most adult features
• Possesses a characteristic velum for swimming and sometimes feeding
  
  • Two or more large ciliated lobes developed from the prototronch

Question 18

Disadvantages of torsioning

Answer 18

• Growth began to occur mostly in the mid-section of the animal, creating a coiled visceral ‘hump’ we see in extant species
  
  • Hard to gain mass near the head region with the organs clustered in that vicinity
• Risk of fouling increased

Question 19

Why some detort

Answer 19

• Mantle cavity and some organs move partially or fully to the posterior
• Fully deported animals include most nudibranchs
• Partial deported animals include some sea slugs
  May…
  
  • Resolve fouling issues
  • Promote more effective swimming
  • Loss of ability to retract in mantle cavity may have driven chemical defences of many detorted gastropods