Molluscs part 1 - Bivalves Flashcards
What is the age extent of Phylum Mollusca?
Cambrian to recent
How many species are part of phylum Mollusca?
> 120,000 species
What environment are molluscs found in?
Typically marine (although some Bivalve and Gastropods freshwater/ terrestrial)
How many groups can phylum Mollusca be split into?
9
What are the 9 groups of mollusca?
Aplacophora
Polyplacophora
Monoplacophora
Helcionelloida
Gastropoda
Cephalopoda
Lamellibranchia/Pelycopoda/Bivalvia
Rostroconchia
Scaphopoda
What are some examples of how big Molluscs can get?
Architeuthis >10m in length
Mesonychoteuthis hamiltoni >13m in length
What is the structure of molluscs like?
generally unsegmented
serial repetition of internal organs
often with external or secondary internal
skeleton of CaCO3 (Aragonite)
What is the Radula for molluscs?
Composed of chitin and is multiple teeth which act as a rasping feeding organ
What is the basis of the Archimollusc concept?
hypothetical ancestor for all molluscs
consider morphology of all other molluscs
and how they relate to ‘archimollusc’
What is the shell of the Archimollusc like?
cap-like secreted by the mantle
How are the anus and mouth positioned on the Archimollusc?
Mouth and anus at separate ends
The anus opens into the mantle cavity which acts as waste outlet
Where is the visceral mass of the Archimollusc located?
Above the mantle cavity
What does the diversification of molluscs from the Archimollusc link to?
Changes in mode of life and feeding
What does the Archimollusc share characteristics with most?
Tergomya (monoplacophorans) and Polyplacophorans (chitons)
What is the feeding and movement of gastropods, polyplacophorans and
tergomya?
Slowly move on a muscular foot
Radula in mouth (scavengers, herbivores and active predators)
What is the main type of feeding of bivalves?
predominately deposit feeders (rarely
suspension feeders)
What is not present in bivalves?
No head,
No jaws
limited capacity for locomotion
What type of feed gathering is done by cephalopods?
Predatory
How has the radula of cephalopods evolved to be predatory?
Ability to swallow and specialised jaws which can slice and chop
What does the head contain/ allow for in cephalopods?
sense organs and brain
buoyancy, locomotion, tentacules
What is the bivalve the only common mollusc to have?
2 valves shells
How are the valves split with bivalves and how are they connected?
Left and right
With a connecting ligament
How do you determine what side of a bivalves you have?
The side of the pallial sinus is
(left of shell is left)
What is the plane of symmetry for bivalves?
Passes between the valves (leading to relative equivalved nature)
What is the unexplained dominance split between brachiopods and bivalves?
brachiopods dominate Palaeozoic
bivalves in Mesozoic and Cenozoic
What environment are mot Lamellibranchia/Pelycopoda bivalves?
Marine (some freshwater)
What is the mode of life of almost all Lamellibranchia/Pelycopoda bivalves?
Benthic
What is the characteristic mode of life for Lamellibranchia/Pelycopoda bivalves?
infaunal burrowers though many are epifaunal
What is one of the characteristics features of Lamellibranchia/Pelycopoda bivalves?
Enlarged gills which extend from body into the mantle cavity
What are the enlarged gills of Lamellibranchia/Pelycopoda bivalves used for?
filter food and respiration
What is the anatomy of Lamellibranchia/Pelycopoda bivalves?
Gut is dorsal
Mouth at anterior
Anus at posterior
Muscular foot used in burrowing
Cilia cover gills
Siphons
What is the purpose of the cilia covering gills in Lamellibranchia/Pelycopoda?
generate water current
How is water drawn in and expelled from Lamellibranchia/Pelycopoda?
Water in through inhalent siphon
Filtering expelled out exhalent siphon
What is the shell structure of bivalves?
Three layer structure
What are the layers of bivalves?
Outer layer organic periostracum
Two inner layers of CaCO3
How are the valves of bivalves opened and closed?
Elastic ligament open valves
Adductor muscles close valves
What are isomyarian adductor muscle scars?
Equal in size
What are anisomyarian adductor muscle scars?
Unequal in size
What are monomyarian adductor muscle scars?
Single muscle present
What is does the pallial line do?
Connects muscle scars
What does the pallial line mark?
Where the inner most of two calcareous shell layers end
What is the pallial sinus?
Where the pallial line is indented posteriorly
What is the pallial sinus an indicator of?
A pocket which siphons are retracted into
What are the characteristics of infaunal bivalves?
Typically equivalved
Isomyarian/ weakly anisomyarian
Resist shearing forces
Pallial sinus
What happens to characteristics of infaunal bivalves as you get deeper into burrowing?
Gapes
Elongated shells (linked to speed of burrowing)
What will the shell of epifaunal byssally attached?
Shell flattened
What are some examples of epifaunal byssally attached bivalves?
Mytilus
Plagiostoma
What are the characteristics of epifaunal cemented bivalves?
Strongly
Anisomyarian or monomyarian
Irregular
One shell greatly thickened
What is an example of epifaunal cemented bivalves?
Ostrea
What are the characteristics of epifaunal free living bivalves?
monomyarian
or
equilateral symmetry
What are some examples of epifaunal free living bivalves?
Gryphaea
or
Scallop
What do boring bivalves do?
Bore into hard substances (abrasive shell)
What is an example of a boring bivalve?
Petricola or Teredo
