Mollusks Flashcards
Phylum Mollusca
90,000 living species – most marine, but some are terrestrial or freshwater aquatic
Soft body and belong to the lophotrochozoan protostomes
triploblastic and eucoelomate (true body cavity)
Size range: 1 mm to 18 m, but 80% are < 10 cm in size
Habitat range: 7000 m above sea level to abyssal depths
Locomotion: sessile, sluggish to swift
Diet: herbivores, detritivores, filter feeders & predatory carnivores
Body Form and Function
Simplest body plan consists of:
1. Foot OR head-foot portion
2. Visceral mass portion
3. Shell
Mollusc Body Plan: Head-Foot and Visceral Mass Portions
Head-foot region contains feeding, cephalic sensory, and locomotor organs
Visceral mass contains digestive, circulatory, respiratory, and reproductive organs
Mantle Cavity
Two folds of skin form protective mantle or pallium
Space between mantle and body wall is the mantle cavity
Mantle cavity houses the gills (ctenidia) or a lung
Head-Foot
Well-developed head bearing mouth and some sensory organs
Photosensory receptors range from simple to complex eyes
Tentacles may be present
Posterior to mouth is the chief locomotor organ, the foot
Buccal Mass
Radula - Unique to molluscs
Protruding, rasping, tongue-like organ
Odontophore – cartilage-like supportive structure
Foot
Usually ventral
Functions in attachment to substratum or for locomotion or for a combination of both.
Modifications include Attachment disc of limpets Hatchet foot of bivalves Siphon jet of cephalopods Cephalopod arms
Mantle and Mantle Cavity
Mantle - a sheath tissue on each side of the body
Secretes the shell when present
Mantle cavity
Houses the gills or lungs that develop from the mantle
Exposed surface of the mantle also functions in gaseous exchange
In aquatic molluscs
Continuous flow of water brings in oxygen and food, and
flushes out wastes
Products of digestive, excretory and reproductive systems empty into the mantle cavity
Respiratory organs
Respiratory organs: ctenidia – ciliated molluscan gills housed in mantle cavity Ciliary function: 1. Provide continuous water current into/out of cavity - respiratory - waste elimination - sensory detection - gamete dispersal
- Modified for filter feeding
- bivalves
Shell
If present, secreted by the mantle and lined by it
Periostracum
Outer horny layer
Composed of conchiolin, a tanned protein (=formation of
crosslinking between proteins).
Shell growth occurs at margins.
Middle prismatic layer
Closely packed prisms of calcium carbonate
Inner nacreous layer
Next to the mantle; the nacre is laid down in thin layers
Internal Structure & Function
Circulatory system
Open - pumping heart, blood vessels, and blood sinuses
Closed - most cephalopods have a heart, vessels, and
capillaries
Excretory system
Most molluscs have a pair of kidneys or metanephridia
Kidney ducts also discharge sperm and eggs
Nervous system
Pairs of ganglia but generally simpler than in annelids
Sense organs vary and may be highly specialized
Reproduction
Most dioecious, some hermaphroditic
Gamete exchange:
- internal fertilization
copulation occurs and gametes exchanged in genital ducts
- external fertilization
gametes released to surrounding water results in the
development of free-swimming larval stages
trochophore larva
- may feed, catching small plankton in prototroch, planktotrophic. Can have long planktonic lives
- non-feeding trochophores have large yolk reserves, lecithotrophic. Have short planktonic lives
Life Cycle
Trochophore larval development:
- may exhibit direct metamorphosis into a small juvenile (e.g. in polyplacophorans =chitons)(ancestral)
- In bivalves, most often develop into a second, free-swimming, larval stage. Veliger larva (derived).
- veliger is symmetrical and enclosed within two valves (shell). Velum function: - locomotion - food collection - gas exchange
Veliger larva modification in some freshwater bivalves:
Fertilization is usually internal.
Glochidium larva
microscopic non-swimming shelled larva
- initially “ectoparasitic” (10 – 30 days) on fish
- attaches to fish gills, encyst and feed by phagocytosis
Class Polyplacophora: Chitons
Somewhat flattened with 7- 8 dorsal plates Head and cephalic organs – reduced Most prefer rocky intertidal surfaces Mantle extends around margin Trochophore larvae metamorphose
Class Bivalvia
Mussels, clams, scallops, oysters, and shipworms
Range in size from 1–2 mm in length to the giant South Pacific clams
Most are sedentary filter feeders (gills or ctenidium)
Bivalves lack a head, radula, or other aspects of cephalization
Most are marine
Class Gastropoda
Snails, limpets, slugs, whelks, conches, periwinkles, sea slugs, sea hares, and sea butterflies
Marine forms & air-breathing terrestrial snails and slugs
Typically sluggish, sedentary animals
Shells - chief defense
Some produce distasteful or toxic secretions
Snails - intermediate hosts to many parasites
Pulmonates: mantle cavity functions as lung
