Molluscs Flashcards
Mollusca is a v large phylum. Which is the only phylum larger?
Arthropods
What are the features of a generalised mollusc?
> bilaterally symmetrical > poorly cephalised > aquatic > moves & grazes over hard surface > radula = scraping tongue w/ teeth used in feeding
What are the 3 layers of a mollusc shell?
- Periostracum - bright colours
- Ostracum - 2 layers make up bulk of shell
- Hypostracum/Nacre - pearlescent layer where body lies
What are the 6 major mollusc clades?
Aplacophora Polyplacophora Bivalvia Gastropoda Scaphopoda Cephalopoda
Which 2 clades have the greatest diversity?
Bivalves
Gastropods
What are aplacophores?
Worm-like molluscs w/ no shells
Have mix of primitive & specialised features
Which primitive features do aplacophores have?
Which specialised features do aplacophores have?
Poorly developed head
Covered by cuticle w/ calcareous scales/spicules embedded
Reduced foot
Loss of radula & gills
Why might the aplacophores not have ‘lost’ their radula or gills?
If they’re at the base of the mollusc tree
–> might never have had them in the 1st place
What do polyplacophora have?
Many shell plates - always 8 overlapping No cephalic eyes or tentacles Multiple gills Segmented shells Indistinct head
What are polyplacophores adapted for?
When did they evolve?
Adhering to rocks & moving around grazing bacteria & algae
Diverged early from main line of molluscan evolution
Which clade is most like a generalised/ancestral mollusc?
Monoplacophora
How many monoplacophora genera & species are there?
3 genera
11 species
How many pairs of gills do monoplacophores have?
5
Give examples of bivalves.
Clams
Oysters
Mussels
What are the features of bivalves?
>Laterally compressed >Shell of 2 hinged halves >Infaunal or epifaunal >Filter feeders - incurrent and excurrent siphon via 2 tubes >Poorly cephalised
What do bivalves have within their hinge?
Abductin = elastic protein
How can you tell how deep a bivalve burrows?
By the length of their siphons
How do bivalves open & close their shells?
Open = relax muscle Close = contract muscle
Which clade do Blue Mussels belong to?
What specialised structure do they have?
What else do they produce?
Bivalvia
Byssal threads
- pull off predators e.g. whelks
Mussel glue protein - stick themselves into a raft
What are the features of scaphopods?
> Tusk or tooth shells
>Elongated cylindrical tube - open at both ends
What is the scaphopod specialism?
Burrowing marine molluscs
What are the features of gastropods?
> developed head > only terrestrial molluscs > motile - foot for crawling, burrowing, swimming > shell forms deep protective retreat >torsion
What is torsion?
What might it allow?
Twisting of the body through 180 anti-clockwise
- gills, anus & mantle cavity now lies behind head
Withdrawal of head into shell before foot
What is the mantle cavity of a gastropod lined with?
What does this now function as?
Blood vessels
Lung
What are the 3 groups of Gastropoda?
Which group is now known to be polyphyletic?
Prosobranch
Opisthobranch
Pulmonata
Prosobranch
What does ‘prosobranch’ mean?
‘gills in front of the heart’
What does ‘opisthobranch’ mean?
‘gill one behind and to the right of the heart’
What type of torsion do opisthobranchs have?
Not full torsion
= 45 degrees
What are the features of opisthobranchs?
Give examples.
> reduced/absent shell & mantle cavity
many are secondarily bilaterally symmetrical
Sea hares, sea butterflies, sea slugs (e.g. Nudibranchs)
Why are Nudibranchs special?
> Can eat stinging cells from venomous jelly fish
don’t set off stinging cells
–> move them to extremities of their tissues & use to sting predators
What does ‘pulmonata’ mean?
Pallial lung instead of gills
- breathe air
What are the 4 examples of cephalopods?
Cuttlefish
Octopus
Squid
Nautilus
What are the 4 general features of cephalopods?
> head has prehensile tentacles & arms
jet propulsion
carnivorous
majority = 6-70cm
What are the highly derived features of cephalopods?
> ink gland > camouflage > complex NS & behaviour > v developed eyes > vascular system > internal fertilisation via a spermatophore (using male's copulatory appendage)
What is the shell like in squid?
Reduced to ‘pen’ or ‘gladius’
- made of chitin
What is the shell like in cuttlefish?
Internalised as ‘cuttlebone’
- used for buoyancy
What are chromatophores?
What are the reflective cells underneath called?
Pigment-containing light-reflecting cells
Iridocytes
What features do cephalopod eyes have?
Cornea Lens Iris Diaphragm Retina
How do cephalopod eyes differ to mammalian?
Mammals - light focussed by altering shape of lens
Cephalopods - light focussed by moving lens toward or away from retina
What can cephalopods hear?
What is this an adaptation to?
Only v low freqs
Being predated on by whales & dolphins that stun prey w/ sound
- become immune by tuning out sounds
What is the respiratory pigment in cephalopods?
Hemocyanin
- only 25% as efficient in binding O2 as haemoglobin
What are photophores?
Where are they found?
Cells that generate light
On the underside - useful for countershading if can match freq of light coming from above
