Molluscs

Molluscs

Taxonomy, Anatomy, and Diversity

  • Molluscs are a diverse group of animals.
  • Roy Caldwell

Phylogenetic Placement

  • Molluscs are Metazoa, Bilateria, Protostomia, Spiralia, and Trochozoa.
  • The transcript lists various phyla to provide context:
    • Choanoflagellata, Porifera, Placozoa, Ctenophora, Cnidaria, Xenacoelomorpha
    • Vertebrata, Urochordata, Cephalochordata, Pterobranchia, Enteropneusta, Echinodermata, Chaetognatha
    • Platyhelminth, Rhombozoa, Orthonectida, Gastrotricha, Entoprocta, Cycliophora, Nemertea, Annelida, Mollusca
    • Gnathostimulida, Micrognathozoa, Rotifera, Phoronida, Bryozoa, Brachiopoda
    • Priapulida, Loricifera, Kinorhyncha, Nematoda, Nematomorpha
    • Tardigrada, Onychophora, Arthropoda
    • Ecdysozoa, Deuterostomia, Lophophorata, Gnathifera

Mollusc Diversity

  • Gastropoda: ~ 77,000 species
  • Bivalvia: 10,000 species
  • Monoplacopoda: 20 species
  • Scaphopoda (tusk shells): 300–500 species
  • Cephalopoda: 600–700 species
  • Polyplacophora (chitons): 500–600 species
  • Aplacopoda: 250 species
  • Gastropoda Subclasses:
    • Heterobranchia
    • Prosobranchia: ~ 55,000 species, marine snails
    • Pulmonates: ~ 20,000 species, land and freshwater snails and slugs
    • Ophistobranchia: ~ 1,000 species, sea slugs
  • Cephalopoda Subclasses:
    • Ammonoidae (extinct)
    • Nautiloidea: ~ 6 species
    • Coleoidea: ~ 700 species
      • Sepiida – cuttlefish
      • Teuthida – squid
      • Vampyromorpha – squid
      • Octopoda - octopus

Basic Mollusc Body Plan

  • Key features:
    • Shell
    • Mantle
    • Mantle cavity
    • Anus
    • Water
    • Ctenidium (gill)
    • Foot
    • Mouth
    • Radula
    • Digestive gland
    • Gonad
    • Aorta
    • Haemocoel
    • Ventricle
    • Auricle
    • Gut
    • Foot retractor muscle
    • Nerve cord
    • Pericardial coelom
    • Excretory organ
    • Mucus

The Shell

  • Early molluscs had a proteinaceous and chitinous cuticle.
  • Progressively reinforced with calcium.
  • Modern shells consist of crystals of calcite and argonite deposited within a framework of layers of protein (conchiolin).
  • Secreted by the mantle in shelled molluscs.
  • Acts as protection against predators and mechanical damage.
  • Muscles pull the shell down to protect the head and foot when needed.
  • Often conical or helically coiled.

Mouth and Radula

  • Radula is a feeding structure.
  • Radular ribbon contains teeth.
  • Odontophore supports the radula.
  • Muscles protract and retract the odontophore and radular ribbon through the mouth.
  • The radula teeth structure varies between genera/species and can be used to identify mollusc remains from the guts of predators.
  • Mechanism:
    • Contraction of muscles 'a' and 'b' protracts the odontophore and radular ribbon through the mouth.

Heart and Gill (Ctenidium)

  • Components:
    • Heart (Ventricle and Auricle)
    • Gill
    • Mantle
    • Mantle cavity
    • Shell
    • Foot
    • Coelom
    • Excretory organ
    • Rectum
    • Gonad
  • Blood flow:
    • Venous blood goes to the gill.
    • Oxygenated blood goes from the gill to the heart.
    • Arterial blood leaves the heart.

Aplacophora

  • No shell, calcareous spines
  • Worm-like, <300< 300 mm
  • Headless
  • No proper foot
  • Live in marine sediments
  • Most have radula
  • No excretory organs
  • Hermaphrodites
  • Chaetodermomorpha

Polyplacophora (Chitons)

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

Monoplacophora

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

Scaphopoda (Tusk Shells)

  • Cylindrical shell
  • <150< 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

Bivalvia

  • Pair of shells
  • <1< 1 m
  • Separate sexes
  • Some use foot for burrowing; others are cemented to the substrate (byssus)
  • Head has no eyes, tentacles, or radula
  • May be tentacles and eyes around mantle
  • Ctenidia highly modified

Bivalvia Gill Functions

  • Protobranch (most primitive):
    • Ctenidia (gills) function conventionally for gaseous exchange.
    • Two large ciliated palps (sometimes with tentacles) move fine food particles to the mouth.
  • Lamellibranch (majority of species):
    • Use enlarged, ciliated ctenidia as filters to catch food.
    • Smaller ciliated palps pass food to the mouth.
  • Septibranch:
    • Ctenidia reduced to muscular diaphragms.
    • In combination with the inhalant siphon, suck in small animals captured by muscular palps.

Oldest Animal

  • The oldest animal in the world is a bivalve.
  • Quahog clam, Arctica islandica
  • Was 405−410405-410 years old when found.
  • Was a youngster when Shakespeare was writing his plays.

Gastropoda

  • Largest and most diverse group, ~ 77,000 species
  • Defining feature – torsion (with consequent rearrangements of internal and external morphology)
  • Single coiled shell with operculum (but some have other shell forms or no shell, and some have no operculum)
  • Well-developed head with radula, eyes, sensory tentacles
  • Well-developed crawling foot
  • May be gonochoristic (Prosobranchia Subclass) or hermaphrodites (Heterobranchia Subclass)

Gastropoda - Torsion

  • Rotation of visceral hump through 180°
  • Mantle cavity now faces forward
  • Anus discharges into mantle cavity over the head

Gastropoda - Torsion Advantages

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

Gastropoda - Torsion - Avoiding the Voiding

  • Evolve dorsal aperture in shell for exhalent water and waste
  • Loss of ctenidium on right, unidirectional water flow
  • De-torsion 90° to right, mantle cavity opening to right, anus to side or rear

Gastropoda - Anatomy of a Prosobranch

  • Key features:
    • Posterior aorta
    • Gonad
    • Inhalent current
    • Siphon
    • Gill
    • Exhalent current
    • Anus
    • Seminal receptacle
    • Oviduct
    • Heart
    • Hypobranchial gland
    • Ctenidium
    • Mantle cavity
    • Osphradium
    • Cephalic tentacle
    • Eye
    • Proboscis
    • Mouth
    • Radular apparatus
    • Kidney
    • Stomach
    • Digestive gland
    • Columellar muscle
    • Mantle edge
    • Statocyst
    • Esophagus
    • Foot
    • Operculum

Gastropoda - Superorder Opisthobranchia

  • ~ 1,000 species
  • Sea-slugs, sea-hares
  • Secondary gills or papillate body surface
  • Up to four pairs of tentacles
  • Most are carnivores
  • Includes planctonic mucus net feeders (pteropods) with ciliated wings
  • Examples: Sea butterfly (Pteropod), Sea slug, Sea hare

Gastropoda - Superorder Pulmonata

  • ~ 20,000 species
  • 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

Gastropoda - Pulmonata - Basic Anatomy

  • Key features:
    • mantle edge
    • eye-spot
    • tail
    • head
    • sole
    • foot respiratory reproductive mouth
    • opening
    • fringe pore
    • tentacles
    • mantle cavity with the lung
    • heart
    • kidney
    • salivary gland
    • digestive gland (hepato-pancreas)
    • rectum
    • reproductive system
    • brain
    • foot

Gastropoda - Pulmonata - Slug Morphology

  • Key features:
    • Pneumostome
    • keel
    • tail
    • tubercles
    • mantle
    • respiratory pore
    • head
    • sole
    • foot fringe
    • mantle cavity with the lung
    • heart
    • kidney
    • salivary gland
    • digestive gland
    • rectum
    • reproductive system
    • brain
    • foot

Cephalopoda Body Plan

  • 90° body tilt then elongation
  • Foot elaborated into arms, tentacles and funnel
  • Mantle cavity opens forwards
  • Rapid, directed movement effected by expelling water through maneuverable funnel
  • Prey caught with arms/tentacles, chewed up by beak then radula
  • Gonochoristic and most are short lived (1-2 years)

Cephalopoda - Nautiloidae

  • Body enclosed in external shell
  • Only last segment inhabited
  • Siphuncle controls gas/water balance in other chambers to control buoyancy
  • Poor swimmers – water expelled by funnel muscles
  • 80-90 suckerless tentacles
  • Two pairs of ctenidia and kidneys
  • Nervous system and eyes simple (cf. Coleoids)

Cephalopoda - Coleoidea - Sepiida, Cuttlefish

Key Features:

  • gonad
  • ink sac
  • digestive gland
  • shell salivary gland
  • brain
  • stomach
  • buccal mass
  • gill
  • funnel
  • fin

Cephalopoda - Celeoidae - Teuthida, Squid

Key Features:

  • ovary
  • oviducal gland
  • digestive gland
  • fin
  • crop
  • stomach
  • gill
  • ink sac
  • 'pen'
  • funnel
  • Large eye
  • Head
  • Stabilizing fins
  • Siphon
  • Mantle
  • Parrot-like beak and mouth
  • brain
  • buccal mass
  • 8 Arms with 2 rows of suckers on the undersides
  • 2 Long feeding tentacles

Cephalopod - Squid Size

  • Giant squid Architeuthis: Up to 13 m
  • Colossal squid Mesonychoteuthis: Up to 15 m
  • Measurements:
    • Total length
    • Standard length
    • Mantle length
  • Key Features:
    • TENTACLE CLUB
    • TENTACLES
    • FIN
    • MANTLE
    • FUNNEL
    • HEAD
    • ARMS

Cephalopoda - Celeoidae - Octapoda, Octopus

Key Features:

  • oviducal gland
  • posterior salivary gland
  • digestive gland
  • aorta
  • crop
  • ovary
  • kidney
  • brain
  • crop
  • buccal mass
  • ink sac
  • funnel
  • gill
  • branchial heart

Octopus Eye

Key Features:

  • Muscle
  • Lens
  • Iris
  • Retina
  • Space behind lens
  • Optic nerve

Octopus Brain

Key Features:

  • Note large optic lobes
  • Optic stalk
  • Vertical lobe
  • Brachial (arm) lobe
  • Nerve to the arms
  • Optic lobe
  • Optic nerve

Summary

  • Molluscs may be the second largest metazoan phylum.
  • Molluscs evolved from a common ancestor with annelid worms
  • Their basic body form is highly plastic and adaptable, allowing them to inhabit every ecosystem on Earth
  • Bivalve gills in particular demonstrate how organs that originally evolved for one function can be further adapted for use in totally different ways
  • Gastropods are the largest Class within Mollusca and are defined by the process of torsion
  • Torsion (and subsequent detorsion) led to the evolution of a well developed head with sensory organs
  • The Prosobranchia are mainly gonochoristic whereas the Heterobranchia are hermaphrodites
  • The Prosobranchia comprise the largest subclass, and include most of the herbivorous and carnivorous sea snails.
  • The Heterobranchia include the superorders Opisthobranchia (sea slugs) and Pulmonata (land and freshwater snails / slugs)
  • Pulmonates demonstrate a wide range of shell morphologies, from entire shells to a few calcium granules
  • Cephalopods are an ancient lineage descended from primitive ancestors similar to the Monoplacophora
  • Classic features of the Coleoidea are the large brain, the advanced eye structure, powerful jet propulsion, control over their own body patters and colours, multifunctional tentacles and a vascular system with veins and arteries
  • The Coleoidea include the largest and most intelligent of all invertebrates.

Further Reading

  • Brusca, R.C. & Brusca, G.J. Invertebrates. Sinauer.
  • Barnes, R.S.K., Calow, P. Olive, P.J.W., Golding, D.W. & Spicer, J.I. The Invertebrates, A Synthesis. Blacwell Science.
  • Moore, J. An Introduction to the Invertebrates. Cambridge University Press.
  • Anderson, D.T. Invertebrate Zoology. Oxford University Press.
  • Schrope, M. (2005) The undiscovered oceans. New Scientist Vol. 188 No 2525 (12 November), 38-43
  • Young, E. (2003). Monsters of the deep. New Scientist Vol 179 No. 2406 (2nd August), 24-29.
  • Giant squid dissection - www.youtube.com/watch?v=aoWeXjYi9Ns or https://www.youtube.com/watch?v=d83nSFEoz2A