Lecture: Adaptive radiation in molluscs Martin Lankheet, EZO

Life History of Aquatic Organisms (AFI-31306)

Lecture: Adaptive radiation in molluscs Martin Lankheet, EZO

THEME Lectures Tutorials Practicals Introduction to Life History theory

• Introduction to Life History theory

• Life histories of marine mammals and relevance for population zooplankton ecology

• Life history trade-offs • Density (in) dependent

regulation of population numbers

• Life-history trade-offs in zooplankton

Niche differentiation and feeding

• Food & food webs • Niche adaptation & eco-

morphology • Adaptive radiation in Lake

molluscs • Adaptive radiation in fishes • Eco-morphological

methodology

• Food & food webs assignment

• Mollusks: adaptation in bivalves and squids

• Reflection on cyprinid fish adaptations

• Feeding ecology of seals • Diversity of North Sea

fishes • Functioning and life history

of gastropods, bivalves and cephalopods

• Cyprinid fish: adaptive radiation

Migration, habitat choice and swimming

• Fish swimming • Swimming in 'non-fish' • Fish migration

• Analysis of larval swimming • Migration

• Larval swimming

Reproductive strategies

• Reproductive and life history strategies in aquatic organisms

• Reproduction in fishes • Physiology of reproduction

• Size of maturity & reproduction

• Physiology of reproduction

Position in the course

Phylum Mollusca

l Phylum Mollusca includes snails and slugs, oysters and clams, and octopuses, cuttlefish and squids.

Phylum Mollusca

l Molluscs evolved in the sea and most molluscs are still marine. l Some gastropods and bivalves inhabit

freshwater. l A few gastropods (slugs & snails) are

terrestrial.

Humans & Molluscs

l Humans use molluscs in a variety of ways: l  As food – mussels, clams, oysters, abalone,

calamari (squid), octopus, escargot (snails), etc.

l  Pearls – formed in oysters and clams. l  Shiny inner layer of some shells used to

make buttons.

Humans & Molluscs

l A few are pests or introduced nuisances: l  Shipworms – burrow through wood,

including docks & ships. l  Terrestrial snails and slugs damage garden

plants. l  Molluscs serve as an intermediate host for

many parasites. l  Zebra mussels – accidentally introduced

into the Great Lakes and reeking havoc with the ecosystem.

Outline

l General body plan l Adaptive radiation l Program:

l  Tomorrow: l  Dissect and study Helix aspersa (garden snail) l  Biologists: Study Bivalves or Cephalopods

l  Wednesday: l  Video l  Presentations/ demonstrations on Bivalves and

Cephalopods

Molluscs Phylogenetic Position and Development

Phylum Mollusca

l  Molluscs have a mesoderm lined body cavity – a coelom.

l  They are protostomes l  Spiral, determinate cleavage l  Schizocoelous coelom development

Mollusc Body Plan

l All molluscs have a similar body plan with three main parts: l  A muscular foot l  A visceral mass – containing digestive,

circulatory, respiratory and reproductive organs.

l  A mantle – houses the gills and in some secretes a protective shell over the visceral mass.

Mollusc Body Plan

l  Most molluscs have separate sexes with gonads located in the visceral mass.

Head-Foot Region

l  Most molluscs have well developed head ends with sensory structures including photosensory receptors that may be simple light detectors or complex eyes (cephalopods).

Head-Foot Region

l  The radula is a rasping, protrusible feeding structure found in most molluscs (not bivalves). l  Ribbon-like

membrane with rows of tiny teeth.

Head-Foot Region

l  The foot of a mollusc may be adapted for locomotion, attachment, or both.

l  Pelagic forms may have a foot modified into wing-like parapodia.

Shells l  When present, the calcareous shell is secreted by the

mantle and is lined by it. It has 3 layers: l  Periostracum – outer organic layer helps to protect inner layers

from boring organisms. l  Prismatic layer – densely packed prisms of calcium carbonate. l  Nacreous layer – iridescent lining secreted continuously by the

mantle – surrounds foreign objects to form pearls in some.

Mantle Cavity

l The space between the mantle and the visceral mass is called the mantle cavity. l  The respiratory organs (gills or lungs) are

generally housed here.

Internal Structure & Function

l Many molluscs have an open circulatory system with a pumping heart, blood vessels and blood sinuses.

l Most cephalopods have a closed circulatory system with a heart, blood vessels and capillaries.

Mollusc Life Cycle

l  Most molluscs are dioecious, some are hermaphroditic.

l  The life cycle of many molluscs includes a free swimming, ciliated larval stage called a trochophore. l  Similar to annelid

larvae.

Mollusc Life Cycle

l  The trochophore larval stage is followed by a free-swimming veliger larva in most species.

Major Mollusc Classes

l  Four major classes of molluscs: l  Class Polyplacophora

– the chitons l  Class Gastropoda –

snails & slugs l  Class Bivalvia – clams,

mussels, oysters l  Class Cephalopoda –

octopus & squid

Class Polyplacophora

l  Class Polyplacophora includes the chitons. l  Eight articulated plates

or valves. l  Can roll up.

l  Live mostly in the rocky intertidal.

l  Use radula to scrape algae off rocks.

l  Gills are suspended from roof of mantle cavity.

Class Gastropoda

l  Gastropoda is the largest of the molluscan classes. l  70,000 named species. l  Include snails, slugs,

sea hares, sea slugs, sea butterflies.

l  Marine, freshwater, terrestrial. l  Benthic or pelagic

Class Gastropoda

l  Gastropods show bilateral symmetry, but due to a twisting process called torsion that occurs during the veliger larval stage, the visceral mass is asymmetrical.

Class Gastropoda, coiling

l  Coiling is not the same as torsion.

l  Early gastropods had a planospiral shell where each whorl lies outside the others. l  Bulky

l  Conispiral shells have each whorl to the side of the preceding one. l  Unbalanced

l  Shell shifts over for better weight distribution.

Gastropod Feeding Habits

l  Most gastropods are herbivores and feed by scraping algae off hard surfaces using the radula.

l  Some are scavengers of dead organisms, again tearing off pieces with radular teeth.

Gastropod Feeding Habits l  Some are carnivores, radula + chemicals to

bore through the shells of other molluscs. l  Snails in the genus Conus feed on fish,

worms, and molluscs. l  Highly modified radula used for prey capture. l  They secrete a toxin that paralyzes their prey.

l  Some are painful, even lethal, to humans.

Reproduction

l Monoecious and dioecious species. l Young may emerge as veliger larvae or

pass this stage inside the egg.

Internal Form and Function l  Pulmonates lack gills.

l  Have a highly vascular area in mantle that serves as lung.

l  Lung opens to outside by small opening, the pneumostome.

l  Aquatic pulmonates surface to expel a gas bubble and inhale by curling, thus forming a siphon.

Internal Form and Function

l  Most have a single nephridium and well-developed circulatory and nervous systems.

l  Sense organs include eyes, statocysts, tactile organs, and chemoreceptors.

l  Eyes vary from simple cups holding photoreceptors to a complex eye with a lens and cornea

Major Groups of Gastropods l  Pulmonata includes land and most freshwater snails

and slugs. l  Ancestral ctenidia have been lost and the

vascularized mantle wall is now a lung. l  Air fills lung by contraction of mantle floor.

l  Anus and nephridiopore open near the pneumostome.

l  Waste is forcibly expelled. l  Monoecious

Class Bivalvia

l  Bivalved molluscs have two shells (valves).

l  Mussels, clams, oysters, scallops, shipworms.

l  Mostly sessile filter feeders.

l  No head or radula.

Class Bivalvia

l  Part of the mantle is modified to form incurrent and excurrent siphons. l  Used to pump water

through the organism for gas exchange and filter feeding.

l  Sometimes used for jet propulsion.

Class Bivalvia - Locomotion

l  Bivalves move around by extending the muscular foot between the shells.

l  Scallops swim by clapping their shells together to create jet propulsion.

http://www.youtube.com/watch?v=u_RfgvIETEY&feature=related http://www.youtube.com/watch?v=vmi_I8QW5eo

Class Bivalvia

l  Like other molluscs, bivalves have a coelom and an open circulatory system.

l  The mantle cavity of a bivalve contains gills that are used for feeding as well as gas exchange.

l  Pair of kidneys is ventral and posterior to heart.

l  Nervous system has three pairs of widely separated ganglia connected together.

l  Sense organs are poorly developed.

Class Bivalvia - Feeding l  Suspended organic matter enters incurrent siphon. l  Gland cells on gills and labial palps secrete mucus to entangle

particles. l  Food in mucous masses slides to food grooves at lower edge of

gills. l  Cilia and grooves on the labial palps direct the mucous mass into

mouth. l  Some bivalves feed on deposits in sand.

Class Bivalvia - Reproduction

l  Bivalves usually have separate sexes.

l  Zygotes develop into trochophore, veliger, and spat (tiny bivalve) stages.

Class Bivalvia - Reproduction

l  In freshwater clams, fertilized eggs develop into glochidium larvae which is a specialized veliger. l  Glochidia live as

parasites on fish and then drop off to complete their development.

Glochidium Larvae

http://www.youtube.com/watch?v=I0YTBj0WHkU&feature=related

Class Cephalopoda

l  Cephalopods include octopuses, squid, nautiluses and cuttlefish.

l  Marine carnivores with beak-like jaws surrounded by tentacles of their modified foot. l  Modified foot is a funnel for expelling water from the

mantle cavity.

Class Cephalopoda - Shells

l  Shells of Nautilus and early nautiloid and ammonoid cephalopods were made buoyant by a series of gas chambers.

Class Cephalopoda - Shells

l  Cuttlefishes have a small curved shell, completely enclosed by the mantle.

Class Cephalopoda - Shells

l  In squid, the shell has been reduced to a small strip called the pen, which is enclosed in the mantle.

Class Cephalopoda

l Cephalopods have a closed circulatory system.

l Nervous and sensory systems are more elaborate in cephalopods than in other molluscs. l  The brain is the largest of any invertebrate.

Class Cephalopoda

l  Most cephalopods have complex eyes with cornea, lens, chambers, and retina.

Class Cephalopoda – Communication l  Visual signals allow cephalopods

to communicate. l  Movement of body and arms l  Color changes effected by

chromatophores (cells in the skin containing pigment granules).

l  Chromatophores can change shape alternately dispersing and concentrating pigment.

Class Cephalopoda - Reproduction

l  Sexes are separate in cephalopods.

l  Juveniles hatch directly from eggs – no free-swimming larvae.

l  One arm of male is modified as an intromittent organ, the hectocotylus. l  Removes a

spermatophore from mantle cavity and inserts it into female.