mollusca Flashcards
Mollusca characteristics
- Second biggest group of marine organisms
- Fossilise well
- adapted to every kind of habitat in marine systems
- Most abundant class = gastropods, bivalvia
Mollusca physical characteristics
- ventral
- dorsal
- mantle
- radula
what is the ventral in molluscs
muscular foot
- contains nervous units
- can be used for swimming, digging, holding on, or capturing prey; missing only in Caudofoveata)
what is the dorsal in molluscs
visceral mass containing internal organs
what is the mantle in molluscs
- sheet of tissue covering all or part of the body -> creates a space between it and the visceral mass (mantle cavity) - includes epidermis secreting the calcium carbonate shell
what is the radula in molluscs
- rasping organ containing a ribbon of chitin covered in rows of backwards-facing teeth with a moveable, fleshy base
- as teeth wear out, they are replaced by new rows from behind (lack in all bivalves and some gastropods)
- can be heavily modified
what type of circulatory system do molluscs have
- Open circulation (except cephalopods)
- blood travels freely in spongy sinuses which surround it
class gastropods characteristics
- All snails and slugs
- Head usually well-developed, with sensory appendages
- Large, muscular foot
- All show, to some degree, torsion – body has twisted so that the anus is now above the head
2 different types / subclasses of gastropods
- Prosobranchs
- Opisthobranchs (all have reduced shell)
Prosobranch characteristics
e.g. abalone, littorinids, whelks, heteropods
- Vast majority of marine snails (include heteropods)
- Mostly herbivorous - although some carnivores
- Important grazers e.g. macroalage - live on their food source
- Feed using radula
what are heteropods
- ancient group of pelagic snails
- Usually warm water
- Foot is a laterally flattened, ventral fin
- Shell generally reduced or absent
- Active carnivores
- Telescopic eye – may be better than a fish’s
orders within Opisthobranch
- Order Thecosomata - shelled pteropods
- Order Gymnosomata - naked pteropods
- Order Nudibranchia - sea slugs
gills usually absent - exchange gas across body surface
Order Thecosomata characteristics
- Foot modified into pair of wings to move
- Suspension feeders
- Trap food particles in mucous covered parapodia /mucous nets
- Play important ecological role – produce membrane bound faecal pellets that sink fast / export carbon to deep ocean when they die – help with ocean acidificaton
TGN
theco = thick (shelled)
Order Gymnosomata characteristics
- naked pteropod - Lack shell and mantle cavity
- Streamlined, pair of wings near head
- Hooks, teeth and adhesive tentacles (cephaloconi) or Tentacle bearing suckers (acetabula)
Order Nudibranchia characteristics
- Bilaterally symmetrical (secondary detorsion)
- Grazing carnivores
- Feed on sessile organisms e.g. corals, sponges
- Each family restricted to one type of prey
- Dorsal surface often has projections called cerata (Involved in gas exchange)
what do molluscs with no shell use for defence e.g. Gymnosomes and Nudibranchia
- Primary motor of defence = camoflage
- Chemical defence = toxins (synthesised by the slug / taken from prey – stolen nematocysts = kleptocnidae)
family conidae characteristics e.g dog whelk
- Cone shells
- Ones that eat fish have hollow harpoon connected to venom gland – contain neurotoxins to kill the fish
dog whelk characteristics (family conidae)
- mainly barnacles and mussels, also other bivalves and gastropods
- Important intertidal predator
- Mouth and radula on an extensible proboscis - radula modified to bore holes in shells
- Accessory Boring Organ on the sole of the foot secretes a shell -softening chemical
- Once penetrated, the prey is narcotised before the whelk secretes digestive enzymes into the body of the prey - then ingests the resultant tissue ‘soup’
class bivalve characteristics
clams, oysters, mussels, etc.
- Shell of two halves (valves) - hinged dorsally by calcified teeth + tough ligament which springs the shell open / Shell held closed by adductor muscles
- Dominated by lamellibranchs
- siphons in burrowing species
- Muscular foot often used for digging
- Large gills (ctenidia) - gas exchange + food collection
blue mussel characteristics (class bivalve)
Foot is reduced and used to attach byssal threads to substratum
Class Polyplacophora (chitons) characteristics
- Ovoid / dorso-ventrally flattened
- Shell divided into 8 overlapping plates
- Use foot to move like a snail
- Commonly intertidal
- Move and feed at high tide
- Gills for gas exchange
what are the large gills called in bivalves
ctenidia
what are stollen nematocysts called
kleptocnidae
what are the tentacle characteristics called that Order Gymnosomata have
adhesive tentacles (cephaloconi) or Tentacle bearing suckers (acetabula)
