1 Life History of Aquatic Organisms (AFI-31306) Lecture: Adaptive radiation in molluscs Martin Lankheet, EZO
2 Position in the course THEME Lectures Tutorials Practicals Introduction to Life Life-history trade-offs in History theory zooplankton Niche differentiation and feeding Migration, habitat choice and swimming Introduction to Life History theory Life histories of marine mammals and relevance for population zooplankton ecology Food & food webs Niche adaptation & ecomorphology Adaptive radiation in Lake molluscs Adaptive radiation in fishes Eco-morphological methodology Fish swimming Swimming in 'non-fish' Fish migration Life history trade-offs Density (in) dependent regulation of population numbers Food & food webs assignment Mollusks: adaptation in bivalves and squids Reflection on cyprinid fish adaptations Analysis of larval swimming Migration Feeding ecology of seals Diversity of North Sea fishes Functioning and life history of gastropods, bivalves and cephalopods Cyprinid fish: adaptive radiation 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
3 Phylum Mollusca l Phylum Mollusca includes snails and slugs, oysters and clams, and octopuses, cuttlefish and squids.
4 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.
5 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.
6 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.
7 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
8 Molluscs Phylogenetic Position and Development
9 Phylum Mollusca l Molluscs have a mesoderm lined body cavity a coelom. l They are protostomes l Spiral, determinate cleavage l Schizocoelous coelom development
10 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.
11 Mollusc Body Plan l Most molluscs have separate sexes with gonads located in the visceral mass.
12 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).
13 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.
14 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.
15 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.
16 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.
17 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.
18 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.
19 Mollusc Life Cycle l The trochophore larval stage is followed by a free-swimming veliger larva in most species.
20 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
21 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.
22 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
23 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.
24 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.
25 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.
26 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.
27 Reproduction l Monoecious and dioecious species. l Young may emerge as veliger larvae or pass this stage inside the egg.
28 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.
29 Internal Form and Function l Most have a single nephridium and welldeveloped 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
30 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
31 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.
32 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.
33 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.
34 Class Bivalvia l l l l l Like other molluscs, bivalves have a coelom and an open circulatory system. The mantle cavity of a bivalve contains gills that are used for feeding as well as gas exchange. Pair of kidneys is ventral and posterior to heart. Nervous system has three pairs of widely separated ganglia connected together. Sense organs are poorly developed.
35 Class Bivalvia - Feeding l l l l l Suspended organic matter enters incurrent siphon. Gland cells on gills and labial palps secrete mucus to entangle particles. Food in mucous masses slides to food grooves at lower edge of gills. Cilia and grooves on the labial palps direct the mucous mass into mouth. Some bivalves feed on deposits in sand.
36 Class Bivalvia - Reproduction l Bivalves usually have separate sexes. l Zygotes develop into trochophore, veliger, and spat (tiny bivalve) stages.
37 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.
38 Glochidium Larvae
39 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.
40 Class Cephalopoda - Shells l Shells of Nautilus and early nautiloid and ammonoid cephalopods were made buoyant by a series of gas chambers.
41 Class Cephalopoda - Shells l Cuttlefishes have a small curved shell, completely enclosed by the mantle.
42 Class Cephalopoda - Shells l In squid, the shell has been reduced to a small strip called the pen, which is enclosed in the mantle.
43 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.
44 Class Cephalopoda l Most cephalopods have complex eyes with cornea, lens, chambers, and retina.
45 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.
48 Class Cephalopoda - Reproduction l Sexes are separate in cephalopods. l Juveniles hatch directly from eggs no freeswimming 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.
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