MARB 435 1st Edition Lecture 12Current Lecture: Mollusk part 3Class Bivalvia:About 15,000 extant speciesMostly marine, 10-15% OF SPECIES IN FRESHWATER:CLAMS, SCALLOPS, MUSSELS, OYSTERSMorphology: 2 shell valves with a hinge, a ligament and 1 or 2 adductor muscles Body laterally compressed No cephalization Absence of radulaPearls: Pearls can grow in oysters, scallops and clams and freshwater mussels Mantle secretes shell material around a foreign object to protect itself from the intruder Pearls can grow over many years Pearl oyster farming probably saved the oysters and the oyster trade!Bivalve Classification:Protobranchs: Deposit feeders• Ctenidia mostly for gas exchange• Palp proboscides for food capture lamellibranchs: These notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.Suspension feeders• Ctenidia primarily for feeding• Secrete byssus threads for attachmentAttachment:• Byssus = bundle of strong protein threads extending from the base of the foot• Occurs in many epibenthic species on hard substrates• Byssus is connected to eight byssal retractor muscles• Contraction of byssal retractor muscles pulls the animal to the substratumGiant Clams: Tridacna spp. are the largest species of bivalve Can live up to 100 years or more Harbor photosynthetic symbionts in their mantle tissue Raised in aquaculture for release into the wild and for aquarium tradeBacterial Symbiosis: Some bivalves live in conditions of low nutrients, low oxygen and high level of sulfide May host symbiotic Chemoautotrophic bacteria in their gills that oxidize sulfide to produce energy for carbon fixation Bacteria are transmitted vertically, i. e. through the egg from the mother to the offspringReproduction and Development: Almost all bivalves are gonochoric Fertilization is almost always external Larvae: Trichophore and VeligerDreissena polymorphaZebra Mussel Native to freshwater lakes in southeast Russia Introduced into western Europe and Scandinavia via canals, starting in the 1800s First detected in the Great Lakes in 1986 and spread to various waterways Probably introduced with ballast water An individual female can produce up to 400,000 eggs per year Impacts- Often outcompete native freshwater mussels- Smother other fauna by overgrowing- Damage boats, harbors, etc.- Have invaded water treatment plants- Can block pipesClass Cephalopoda Shell is usually internal or absent (except in Nautilus) Size: few cm to about 20 m (giant squid including arms) Dorso-ventral axis becomes the major body axisNautilus Multiple chambers divided by septa The largest chamber is the body chamber A tube of tissue and calcareous material (= siphuncle) connects the chambers via a central pore Siphuncle pumps water out and gas into the chambers for buoyancy regulationSquids: Strictly carnivorous Prey capture with oral appendages Arms:• short and heavy• covered with suckers Tentacles:• Long and retractile• Suckers only on spatulate ends of tentaclesGas exchange and Circulation: Gills are non-ciliated closed blood vascular system Blood is contained in arteries and veins are connected by capillary beds One systemic heart and two branchial hearts Blood vessels are lined by endothelium Different from other molluscs, similar to vertebratesNervous system and Sensory organs: Nervous system strongly centralized Brain enclosed in a cartilaginous cranium Complex lense eyes Housed in cartilaginous capsule that is fused to the cranium Unusually largeChromatic Organs:chromatophores Expandable pigment cells Surrounded by muscle fibersphotophores Produce bioluminescence Bacterial: produced by symbiotic bacteria Intrinsic: generated by the cephalopod Reproduction and Development: Gonochoric Males produce spermatophores Direct sperm transfer from male to female with heterocotylus Fertilization usually happens in the mantle cavity (external), but is sometimes internal Eggs are large and rich in yolk Cleavage is meroblastic, i.e. does not involve the complete cytoplasm of the oocyte No spiral cleavage Produce