2. Molluscs

Question 1

What are the 7 classes of molluscs?

Answer 1

Aplacophora
Polyplacophora
Monoplacophora
Scaphopoda
Bivalvia
Gastropoda
Cephalopoda

Question 2

Describe the early form of the molluscs shell.

Answer 2

Proteinaceous and chitinous cuticle

Question 3

Describe how the mollusc shell evolved and what the modern shell is typically like.

Answer 3

Progressively got reinforced with calcium

Modern shell basically has crystals of calcite and aragonite deposited in a framework of layers of protein (conchiolin)

Question 4

What is the shell secreted by in shelled molluscs?

Answer 4

The mantle

Question 5

What are radula teeth made of?

Answer 5

Chitin

Question 6

Describe Aplacophora.

Answer 6

No shell, calcareous spines
Worm like, less than 300mm
Headless
No proper foot
Live in marine sediments
Most have radula 
No excretory organs
Hermaphrodites

Question 7

Describe Polyplacophora (chitons).

Answer 7

8 overlapping plates
Girdle of spiny cuticle 
Can roll into a ball
Less than 400mm
Powerful foot
Marine, especially at shores
Head poorly defined
Kidneys
Radula, algal grazers
6-88 pairs of ctenidia (gills)

Question 8

What is the girdle of the Polyplacophora?

Answer 8

Muscular girdle provides structural support for the plates, tough

Question 9

Describe Monoplacophora.

Answer 9

Simple conical shell 
Less than 30 mm
Weak foot
Marine, deep trenches
Head poorly defined, no eyes or tentacles
Radula, detrital feeders
Mantle cavity around foot
3-6 pairs of ctenidia (gills)

Question 10

Describe Scaphopoda (tusk shells).

Answer 10

Cylindrical shells
Less than 150 mm
Separate sexes
Burrowing foot
Marine sediments
Head has no eyes but has radula
Mantle cavity through body
No ctenidia (gills)
Ciliated contractile tentacles

Question 11

Describe bivalvia.

Answer 11

Pair of shells
Less than 1 m
Separate sexes
Some use foot for burrowing
Others cement to a substrate using byssus 
Head has no eyes, tentacles or radula 
May be tentacles and eyes around mantle 
Ctenidia highly modified

Question 12

What are the three gill function types in bivalvia?

Answer 12

Protobranch (most primitive)
Lamellibranch (majority of species)
Septibranch

Question 13

Describe the gill function type protobranch of bivalvia.

Answer 13

Most primitive
Ctenidia function conventionally, gaseous exchange
2 large ciliated palps, sometimes with tentacles
Move fine food particles to mouth

Question 14

Describe the gill function type lamellibranch of bivalvia.

Answer 14

Used in majority of species

Enlarged, ciliated ctenidia act as filters to catch food, which is passed on by smaller ciliated palps to the mouth

Question 15

Describe the gill function type septibranch of bivalvia.

Answer 15

Ctenidia reduced to muscular diaphragms which, in combinations with the inhalant siphon, suck in small animals which are captured by muscular palps

Question 16

What type of mollusc is the oldest known animal?

Answer 16

Bivalvia

Quahog clam is 405-410 years old

Question 17

What is the largest and most diverse group of molluscs?

Answer 17

Gastropoda

Question 18

Describe Gastropoda.

Answer 18

Torsion
Single coiled shell with operculum generally (some have other shell forms, no shell or no operculum)
Well developed head with radula, eyes and sensory tentacles
Well developed crawling foot
Some are gonochoristic and some are hermaphrodites

Question 19

What is torsion?

Answer 19

Moves organs to be above head
Rotation of visceral hump through 180 degrees
Mantle cavity now faces forward
Anus discharges into mantle cavity over head

Question 20

Why is torsion useful?

Answer 20

Having the mantle cavity at the front allows the well-developed head to be drawn into the shell
Chemoreceptors in the mantle cavity can now monitor the water ahead, rather than behind

Question 21

How have gastropoda evolved to avoid the excretion interacting with their head?

Answer 21

Evolve dorsal aperture in shell for exhaling water and waste
Loss of ctenidium on right, unidirectional water flow
Can use de-torsion 90 degrees to the right, having mantle cavity open to the right, anus to side or rear

Question 22

What are the 2 main subclasses of Gastropoda we studied in lectures?

Answer 22

Prosobranchia

Heterobranchia

Question 23

What are the 2 main superorders of Heterobranchia we studied in lectures?

Answer 23

Opisthobranchia

Pulmonata

Question 24

Describe the Gastropoda subclass Prosobranchia.

Answer 24

Mainly marine snails
Retain basic Gastropod form (shell, operculum, torsion)
Most have lost right ctenidium
Either gonochoristic or protandrous sequential hermaphrodites
Most are benthic snails eating algae, but can also be deposit feeders and carnivores
Primitive species can have ctenidia on both sides but a separate exhaling hole in the shell
Some species with single ctenidium, draw in water from one side
Advanced spp. may have single ctenidium system but also projected siphon tube or groove

Question 25

Describe the Gastropoda subclass Heterobranchia.

Answer 25

Sea slugs and terrestrial or freshwater snails and slugs Shell reduced, internalised or lost No operculum Detorsion Loss of ctenidium, replaced by secondary gaseous exchange system Simultaneous hermaphrodites

Question 26

What did Heterobranchia evolve from?

Answer 26

Prosobranchia

Question 27

Describe the Heterobranchia superorder Opisthobranchia.

Answer 27

Sea slugs and sea hares Secondary gills or papillate body surface Up to 4 pairs of tentacles Most are carnivores Includes planktonic mucus net feeders (pteropods) with ciliated wings

Question 28

What are Pteropods?

Answer 28

Sea butterflies | Planktonic mucus net feeders

Question 29

Describe the Heterobranchia superorder Pulmonata.

Answer 29

Mainly terrestrial or freshwater Mantle cavity evolved into air-breathing lung with pneumostome Shell thinned or lost Most are grazers of plants or fungi, some carnivores Grazing habits mean often keystone species More developed brain than most molluscs but still quite primitive Often agricultural pests

Question 30

Why are some slugs genetically closer to snails than other slugs?

Answer 30

Slugs have evolved from multiple snail ancestors

Question 31

Describe Cephalopoda.

Answer 31

Largest invertebrates Anatomically diverse Behaviourally intelligent Many believe they have evolved directly form a Monoplacophora type ancestor 90 degree body tilt then elongation Foot elaborated into arms, tentacles and funnel Mantle cavity opens forwards Rapid, directed movement by expelling water through manoeuvrable funnel Prey caught with arms/ tentacles, chewed by beak then radula Gonochoristic Mostly short lived, 1-2 years

Question 32

What are the three main subclasses of Cephalopoda we looked at in lectures and which are extant/ extinct?

Answer 32

Ammonoidea (extinct) Nautiloidea (extant) Coleoidea (extant)

Question 33

What are the four main orders of Coleoidea we studied in lectures?

Answer 33

Sepiida (cuttlefish) Teuthida, Vampryomorpha (squid) Octopoda (octopus)

Question 34

Describe the Cephalopoda subclass Nautiloids.

Answer 34

``` Body enclosed in external shell Only last segment inhabited Siphuncule controls gas/water balance in other chambers to control buoyancy Poor swimmers 80-90 sucker less tentacles 2 pairs of ctenidia and kidneys Nervous system Simple eyes ```

Question 35

Describe the Cephalopoda subclass Coleoidea.

Answer 35

Shell reduced or missing Buoyancy control by changing proportions of water and gas in cells Water forcefully expelled by muscular mantle walls Well developed brain and nervous systems Sophisticated eyes Arms/ tentacles Blood flows through systems of veins and capillaries Skin chromatophores for camouflage and communication, e.g. courtship

Question 36

Describe cuttlefish.

Answer 36

Huge, sophisticated eye Large shell structure Large brain Can change appearance of body: colour, raise bumps on body (texture), flatten tentacles. Blend into background Mixed messages: courtship displays as well as part defence display

Question 37

Describe squids.

Answer 37

Can be colourless Movement in direction of stabilising fins Sperm deposited to female, use of "egg cases" Breeding schools in warmer water (eggs develop faster) "fly" to get away from predators

Question 38

Describe octopuses.

Answer 38

Large optic lobes Sophisticated eyes (lens, iris, muscle, retina, optic nerve) Photoreceptors in eyes point towards the light Some appear to have coloured vision Arms covered in strong suckers No bone structure Very diverse: patterns, change greatly for camouflage Limited known about most deep sea species

Question 39

How many arms do decapods have and what are they?

Answer 39

10 arms Cuttlefish and squid 2 longer arms specialised in catching prey

Question 40

How many arms do octopods have and what are they?

Answer 40

8 arms Octopus Pounces on prey and wraps it with all its arms Both have 2 arms modified for spermatophore transfer in males