Molluscs Flashcards
Molluscs symmetry and cleavage
Bilaterial
Protostomes - Spiral cleavage
Coelomate - triploblastic -(body cavity). Have a reduce coelom
3 body sections
Head
Foot
Visceral mass
What is the visceral mass?
Guts etc.
Contained within shell
covered by mantle
Mantle secretes shell
Excretory system
Metanepridia
what does the mantle cavity (gap) contain?
Gills
Covered in cilia to drive water over gills
Circulatory system
- Open circulatory system
- Haemocoel blood system(combinds blood with coelom)
- Simple system
- Oxygen is taken in via gills → sent to heart which then pumps blood out via ventricles into the coelom
Nerve Ring
Goes round the oesophagus
Nerves going to foot and visceral mass
What is a radula ?
- Feeding apparatus
- Organ covered in teeth which move around (conveyer belt) and scrape food off the substrate
- Some radula modifications e.g. cone snails
Generalised mollusc reproduction
Dioecious (separate sexes)
Some hermaphrodite
Trochophore larvae
3 morphologically diverse groups of molluscs
Gastropods
Bivalves
Cephalopods
How do gastropods differ from the generalised molluscs ?
Development of head (eyes, tentacles etc)
Dorso-ventral elongation of body (grow upward - can lead to coiling)
Shell (Shield - protective retreat)
Torsion
What is torsion?
Rotation of the visceral mass and mantle cavity - 180°
Mantle cavity and visceral mass end up over the head
Nervous system ends up in a knot
Why does torsion happen?
Still unsure as to why gastropods undergo torsion
- Theory 1- protection of veliger larva (where they undergo torsion) - no evidence for protection against predation
- Theory 2- protection of adult - cavity above head, can crawl in for protection
- Theory 3- utilisation of water by gills above head
Disadvantages of torsion
Anus over head
Twisted nervous system
Some gastropods have evolved holes in shells to expel waste e.g. limpet
De-torsion
- Some have undergone de-torsion
- Nudibranchs:
- De-torsion not quite back to original evolutionary torsion
- No shell
- Rhinophore - detect chemical queues
Gastropod shell coiling
- Planispiral (symmetrical)
- Conispiral (asymmetrical)
- Left and right handed coiling
- Most coil right handed (clockwise)
- Left coiling is less common - not due to mutation - can only mate with same coil (left + left etc)
Most specialised group of gastropods?
Pulmonates (land snails and slugs)
why are Pulmonates specialised?
- No gills - terrestrial
- Vascularised mantle cavity
- Functions like a ‘lung’
- Air is drawn in and gaseous exchange takes place over mantle cavity
Pulmonates reproduction
- Hermaphrodites
- complex mating ritual
- ‘Love darts’
- Sequential hermaphroditism
Pulmonate sequential hermaphroditism
Can change sex
E.g. slipper limpet
Pile up - females at bottom and males at top - if bottom ones die top will become female
How have Bivalves developed from classic molluscs?
- Have two shells
- Held together by abductor muscles
- Reduced head
- No radula
- Reduced nervous system
- Foot - either small or enlarged
Bivalves feeding
Most bivalves are filter feeders
Enlarged gills used for feeding
Bivalves eyes
Can have eyes on mantle edge - some simple and some more complex
Bivalve - Byssal threads?
mussels use these to hold onto rocks (may be able to use these to protect against predators)
Bivalve sex
Most are dioecious
Cephalopods orientation of body
On a dorsal ventral axis
Reduced or lost shell (Nautilus only shelled form)
Cephalopod modified foot
Series of tentacles
males uses tentacles for mating
Cephalopod nervous system & eyes
Nervous tissue concentrated into brain.
most well developed nervous system in inverts
Developed eyes - similar to vertebrate eyes (can focus on objects)
Why are cephalopods specialised?
Marine carnivores
Geared up for predation
Cephalopods Radula modification
Modified to form a jaw
Cephalopods colour change
Chromatophores
Muscle cells attached modify how much pigment are in the chromatophores
Used for mating and defence
Nautiloids
Only group of cephalopods that has a fully formed shell
Very ancient group (since cambrian period)
Nautiloids - siphuncle function
Shell has chambers
a central tube runs through these chambers and regulates salt content (SIPHUNCLE)
Water flows in/out by osmosis depending on salt content
Creates buoyancy
Coleoidea - cepholapods
Cuttlefish
Squid
Octopus
Coleoidea features
- Reduction in shell
- Cuttlefish have cuttlebone - used to maintain buoyancy - changes salt content
- Squid - shell reduced to a proteinaceous pen
- Octopuses - shell vestigial / absent
Mimic octopus
Dynamic mimicry
Can alter body shape
Cuttlefish can also mimic
