2. Molluscs Flashcards
What are the 7 classes of molluscs?
Aplacophora Polyplacophora Monoplacophora Scaphopoda Bivalvia Gastropoda Cephalopoda
Describe the early form of the molluscs shell.
Proteinaceous and chitinous cuticle
Describe how the mollusc shell evolved and what the modern shell is typically like.
Progressively got reinforced with calcium
Modern shell basically has crystals of calcite and aragonite deposited in a framework of layers of protein (conchiolin)
What is the shell secreted by in shelled molluscs?
The mantle
What are radula teeth made of?
Chitin
Describe Aplacophora.
No shell, calcareous spines Worm like, less than 300mm Headless No proper foot Live in marine sediments Most have radula No excretory organs Hermaphrodites
Describe Polyplacophora (chitons).
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)
What is the girdle of the Polyplacophora?
Muscular girdle provides structural support for the plates, tough
Describe Monoplacophora.
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)
Describe Scaphopoda (tusk shells).
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
Describe bivalvia.
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
What are the three gill function types in bivalvia?
Protobranch (most primitive)
Lamellibranch (majority of species)
Septibranch
Describe the gill function type protobranch of bivalvia.
Most primitive
Ctenidia function conventionally, gaseous exchange
2 large ciliated palps, sometimes with tentacles
Move fine food particles to mouth
Describe the gill function type lamellibranch of bivalvia.
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
Describe the gill function type septibranch of bivalvia.
Ctenidia reduced to muscular diaphragms which, in combinations with the inhalant siphon, suck in small animals which are captured by muscular palps
What type of mollusc is the oldest known animal?
Bivalvia
Quahog clam is 405-410 years old
What is the largest and most diverse group of molluscs?
Gastropoda
Describe Gastropoda.
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
What is torsion?
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
Why is torsion useful?
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
How have gastropoda evolved to avoid the excretion interacting with their head?
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
What are the 2 main subclasses of Gastropoda we studied in lectures?
Prosobranchia
Heterobranchia
What are the 2 main superorders of Heterobranchia we studied in lectures?
Opisthobranchia
Pulmonata
Describe the Gastropoda subclass Prosobranchia.
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
Describe the Gastropoda subclass Heterobranchia.
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
What did Heterobranchia evolve from?
Prosobranchia
Describe the Heterobranchia superorder Opisthobranchia.
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
What are Pteropods?
Sea butterflies
Planktonic mucus net feeders
Describe the Heterobranchia superorder Pulmonata.
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
Why are some slugs genetically closer to snails than other slugs?
Slugs have evolved from multiple snail ancestors
Describe Cephalopoda.
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
What are the three main subclasses of Cephalopoda we looked at in lectures and which are extant/ extinct?
Ammonoidea (extinct)
Nautiloidea (extant)
Coleoidea (extant)
What are the four main orders of Coleoidea we studied in lectures?
Sepiida (cuttlefish)
Teuthida, Vampryomorpha (squid)
Octopoda (octopus)
Describe the Cephalopoda subclass Nautiloids.
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
Describe the Cephalopoda subclass Coleoidea.
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
Describe cuttlefish.
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
Describe squids.
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
Describe octopuses.
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
How many arms do decapods have and what are they?
10 arms
Cuttlefish and squid
2 longer arms specialised in catching prey
How many arms do octopods have and what are they?
8 arms
Octopus
Pounces on prey and wraps it with all its arms
Both have 2 arms modified for spermatophore transfer in males
