Mollusca Post - October 10, 2024 PDF

2 INTRODUCTION Scaphopoda (1.0%) Mollusca represented by Polyplacophora (1.0%) approx. 93 000 species Cephalopoda (1.0%) Bivalvia (21.5%)...

2 INTRODUCTION Scaphopoda (1.0%) Mollusca represented by Polyplacophora (1.0%) approx. 93 000 species Cephalopoda (1.0%) Bivalvia (21.5%) Monop Aplaco % representation of Polypla extant molluscan classes Cephal Gastropoda *Monoplacophora Bivalvi (0.03%) *Aplacophora (0.40%) (Caudofoveata, Gastro Solenogastres) 3 INTRODUCTION Mollusca are characteristically diverse Size range: 1 mm 18 m (~ 59 ft) Locomotion: sessile, sluggish swift Diet: herbivores, detritovores, filter feeders & predatory carnivores Habitat range: 7000 m above sea level to abyssal depths 4 INTRODUCTION What is a typical mollusc? A “typical” mollusc is a hypothetical mollusc Defining characteristics of the phylum: 1. Mantle – specialized tissue formed by dorsal epithelium Secretes shell(s) 2. Radula – tongue-like organ with teeth used for feeding* 3. “Foot” – ventral body wall muscles developed for locomotion or clinging *radula lost in bivalvia 5 INTRODUCTION Phylum Mollusca Class Polyplacophora Gastropoda Bivalvia Cephalopoda Habitat aquatic/semi- aquatic/terrestrial aquatic aquatic terrestrial /semi-terrestrial Form 1. Shell 7-8 dorsal coiled 2 lateral reduced, plates plates present or absent modified to 2. Foot form “arms”  6 BODY FORM AND FUNCTION Simplest body plan consists of: 1. Foot OR head-foot portion 2. Visceral mass portion 3. Shell Shell Mantle Visceral Mass Mantle cavity Foot or Head Foot 7 BODY FORM AND FUNCTION Respiratory organs: ciliated molluscan gills (ctendia) are housed in the mantle cavity. Ciliary function: 1. Provide continuous water Shell current into/out of cavity Mantle - respiratory - prevent fouling Gills - waste elimination - gamete dispersal - sensory detection Mantle Foot cavity 8 BODY FORM AND FUNCTION Polyplacophora Ventral View Pallial Groove Plates (shell) (Ctenidia) Mantle Foot 9 BODY FORM AND FUNCTION Gastropoda Shell Mantle Foot (Coiled) Cavity Mantle Mantle Cavity Ctenidium Foot 10 BODY FORM AND FUNCTION Bivalvia Siphons (move water into/out of mantle cavity) Shell (valve) Foot Ctenidium Mantle Ctenidia Mantle Cavity 10 BODY FORM AND FUNCTION Cephalopoda Mantle Foot Foot: extensively modified into arms and tentacles (swimming, feeding) Mantle: no shell OR a vestigial shell. Musculature and connective tissue modified for jet propulsion (swimming), chromatophore organs for colour change 11 BODY FORM AND FUNCTION Cephalopoda Tentacle Ctenidium Arms Pen (vestigial shell) Mantle Gut Radula 12 BODY FORM AND FUNCTION 1 Radula Retractor Odontophore 2 Retractor Radula Odontophore Radula teeth 3 Protractor 4 Esophagus Odontophore Protractor 1 2 3 Mouth 4 13 BODY FORM AND FUNCTION Molluscan shell(s) made up of 3 regions: 1. Periostracum – thin, outer layer composed of an organic substance, conchiolin Secretion: mantle edge at shell margin 2. Prismatic layer – middle layer composed of calcium carbonate and protein matrix Secretion: mantle edge at shell margin 3. Nacreous layer – inner calcareous layer composed of nacre (CaCO3 in protein matrix) Secretion: mantle surface 14 BODY FORM AND FUNCTION Pearl production – a by-product of defense Nacre forming materials secreted into extrapallial space extrapallial Mantle attached to shell along space the inner shell edge (pallial line) - foreign object (e.g. sand grain) or parasite between mantle and shell is enclosed in nacre - if object is enfolded by mantle, concentric circles of nacre form a pearl - more commonly, object remains attached to nacreous layer of shell 15 REPRODUCTION AND DEVELOPMENT -the majority of Molluscs are dioecious. (have separate sexes) Exceptions to this strategy; - simultaneous hermaphrodites - eggs/sperm produced by single individual - gonad referred to as ovotestis - reciprocal copulation (some species form mating chains) Aplysia dactylomela 16 REPRODUCTION AND DEVELOPMENT -the majority of Molluscs are dioecious. (have separate sexes) Exceptions to this strategy; - simultaneous hermaphrodites - eggs/sperm produced by single individual - gonad referred to as ovotestis - reciprocal copulation (some species form mating chains) 3 2 1 4 17 REPRODUCTION AND DEVELOPMENT -the majority of Molluscs are dioecious. (have separate sexes) Exceptions to this strategy; - protandrous hermaphrodites (e.g. Crassostrea virginica) - individual changes sex from male to female - occurs as the organism grows older - after changing to female, reversal may occur every few years 18 REPRODUCTION AND DEVELOPMENT 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 mouth trochophore larva stomach - may feed, catching small plankton in prototroch prototroch, planktotrophic. Can have (cilia) long planktonic lives - non-feeding trochophores have large yolk reserves, lecithotrophic. Have short anus planktonic lives 19 REPRODUCTION AND DEVELOPMENT Trochophore larval development - trocophore develops into a second, free-swimming, larval stage. Veliger larva. 20 REPRODUCTION AND DEVELOPMENT Trochophore larval development - may exhibit direct metamorphosis into a small juvenile (e.g. in polyplacophorans, chitons) - In bivalves, most often develop into a second, free-swimming, larval stage. Veliger larva. -veliger is enclosed within two valves (shell). Velum function; - locomotion - food collection - gas exchange Velum (ciliated organ) 21 REPRODUCTION AND DEVELOPMENT Veliger larva modification in freshwater bivalves: Glochidium larva - microscopic non-swimming shelled larva - initially “ectoparasitic” (10 – 30 days) on fish - attaches to fish gills, encyst and feed by phagocytosis adhesive thread hook edge sensory bristles valve (shell) 22 REPRODUCTION AND DEVELOPMENT Glochidia package - ovisac 23 REPRODUCTION AND DEVELOPMENT Glochidia package - ovisac 24 REPRODUCTION AND DEVELOPMENT Glochidia package - ovisac