MARB 435 1st Edition Lecture 12 Current Lecture Mollusk part 3 Class Bivalvia About 15 000 extant species Mostly marine 10 15 OF SPECIES IN FRESHWATER CLAMS SCALLOPS MUSSELS OYSTERS Morphology 2 shell valves with a hinge a ligament and 1 or 2 adductor muscles Body laterally compressed No cephalization Absence of radula Pearls 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 attachment Attachment 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 substratum Giant 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 trade Bacterial 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 offspring Reproduction and Development Almost all bivalves are gonochoric Fertilization is almost always external Larvae Trichophore and Veliger Dreissena polymorpha Zebra 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 pipes Class 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 axis Nautilus 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 regulation Squids Strictly carnivorous Prey capture with oral appendages Arms short and heavy covered with suckers Tentacles Long and retractile Suckers only on spatulate ends of tentacles Gas 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 vertebrates Nervous 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 large Chromatic Organs chromatophores Expandable pigment cells Surrounded by muscle fibers photophores 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 paralarvae