Molluscs Flashcards by Rosie Liddell

Mollusca is a v large phylum. Which is the only phylum larger?

Arthropods

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What are the features of a generalised mollusc?

> bilaterally symmetrical 
> poorly cephalised 
> aquatic
> moves &amp; grazes over hard surface 
> radula = scraping tongue w/ teeth used in feeding

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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

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What are the 6 major mollusc clades?

Aplacophora
Polyplacophora
Bivalvia
Gastropoda
Scaphopoda
Cephalopoda

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Which 2 clades have the greatest diversity?

Bivalves

Gastropods

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What are aplacophores?

Worm-like molluscs w/ no shells

Have mix of primitive & specialised features

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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

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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

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What do polyplacophora have?

Many shell plates - always 8 overlapping
No cephalic eyes or tentacles
Multiple gills 
Segmented shells 
Indistinct head

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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

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Which clade is most like a generalised/ancestral mollusc?

Monoplacophora

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How many monoplacophora genera & species are there?

3 genera

11 species

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How many pairs of gills do monoplacophores have?

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Give examples of bivalves.

Clams
Oysters
Mussels

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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

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What do bivalves have within their hinge?

Abductin = elastic protein

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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