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Biology 320 Invertebrate Zoology Fall 2005 Chapter 12 – Phylum Mollusca Part Four Class Bivalvia • Clams, oysters, mussels, scallops, and shipworms • 8000 described extant spp. • Mostly marine, with 1300 freshwater spp. • Range in size from two mm – one m – Giant clam (Tridacna) lives in South Pacific and weighs close to 300 kg • Common features – Lack radulas – Lateral compression • Facilitates burrowing – Siphons • For ventilation and feeding while being buried • Most are sedentary, but a few can swim • Some participate in symbiotic relationships – Commensals with polychaetes, crustaceans, and echinoderms – Parasites of fish Body Form • Bilaterally symmetrical • Laterally compressed • Essentially lack cephalization • Sensory structures typically located on mantle margin • Ventral blade-like foot • Ventral mass is dorsal to foot • Two lateral mantle cavities are formed by mantle skirts • One gill is housed in each mantle cavity • One anterior and one posterior adductor muscle – Connect valves transversely Shell • Two valves: left and right • Dorsal hinge – Two ligaments • Elastic, so when adductor muscles relax, shell opens – Teeth that interlock to prevent slippage • Oldest part of shell is the umbo • Shell grows outwards from umbo – Mantle lays down concentric rings • Some bivalves (such as cockles and scallops) have ridges on the valve surface – Adds strength to shell Pearls • Formed when a sand grain or parasite gets trapped in extrapallial space • Layer after layer of nacre is added to the particle • Cultured pearls are “seeded” – On average, three years is required to obtain a pearl of marketable size • Freshwater pearl culturing in the Southeastern US is having negative environmental impacts – Siltation – Pollution – Introduction of exotics Gills • Can be used solely for gas exchange, solely for collecting food, or both • Bivalves are classified according to gill structure and function • Three main morphological groups – Protobranchs – primitive – Lamellibranchs – majority; of economic importance – Septibranchs – specialized carnivores Protobranchs • Name means “first gills” • Deposit feeders • Gills used solely for respiration – Small – Located in posterior of mantle cavity – Bipectinate – Cilia create respiratory current and prevent gill fouling • Two palpal tentacles – – – – – Located at posterior end of mantle cavity Elongate Covered with mucus Gather particles from sediment Transfer to labial palp • Labial palp – Covered with ciliated grooves and ridges for sorting – Leads to mouth – Sorting isn’t perfect • Waste (pseudofeces) falls off of palp into mantle cavity – Valves close rapidly to expel wastes Lamellibranchs • Name means “sheet gills” • As organic matter ends up on gills anyway, it was inevitable that some bivalves would adapt to exploit this • Suspension feeders that trap particulate matter on gill surface – Gills are folded and therefore have large surface area – Secrete mucus • Gills have ciliary sorting fields • Labial palps receive strings of mucus from gills • Scallops (Pecten) – Free-living, not attached – Clap valves rapidly to swim away from predators – Large central posterior adductor muscle – Peripheral sensory structures – Ridges on valves • Mussels (Mytilus) – Attached to substrate via a byssus (more later) – Often found in large clusters (beds) – Zebra mussels (Dreissena) are a freshwater invasive in North America • Oysters (Crassostrea, Ostrea) – Cemented to substrate – Produce pearls • Clams – Giant clams (Tridacna) • Live on substrate – Geoducks (Panopea) • Bury • Siphon is so large that it cannot retract into valves Septibranchs • Small group of specialized carnivores • Gills only used for food capture – Form two perforated muscular septa – Contraction of septal muscles elevates septa and water rushes in – Animals / particles are sucked into mantle cavity as water rushes in enlarged inhalant siphon • Gas exchange occurs across mantle surface • Stomach is also adapted for feeding on larger particles / whole organisms – Muscular – Lined w/ cuticle – Functions as a gizzard • Grinds / crushes prey – Enzymes are dumped in Generalized Digestive System • Mouth connected to labial palps • Stomach has: – Sorting fields – Gastric shields – Crystalline style • Huge rod made of solid digestive enzymes • Rubs against gastric shield to release enzymes • In some cases, rotation of rod pulls mucus strings into stomach • Intestine loops through visceral mass several times Burrowing • Pedal hemocoel functions as a hydrostat; controlled by coordinated muscular contractions • Foot elongates and is forced into sediment – Shell acts as penetration anchor • Blood forms terminal anchor in distal foot • Pedal retractor muscles contract – Shell rocks back and forth, and moves forward • Process repeats • Mantle is sealed in several spots – Prevents fouling of mantle cavity Siphons • Extensions of mantle – Ventral inhalant siphon – Dorsal exhalant siphon • Tube length varies – Longest in deep burrowers • Siphons are often grazed upon by fishes and other predators – Regenerated – Can be retracted • Sensory organs are often located on the tips of siphons – Retract when shadows pass overhead • Geoduck clams have huge siphons and can burrow up to one meter • Some live in permanent burrows lined with mucus Attachment • Many attach to rocks, shells, wood, coral, jetties, wharf pilings, etc. • Anterior adductor muscle, siphons, and foot are usually reduced / absent • Attached via: – Byssus • Bundle of strong protein threads secreted by glands in the foot – Cement • Permanently attaches one valve to substrate Boring • Some bore into hard substrates such as rocks, wood, shells, or coral • Animal settles and begins to burrow – Uses valves as a drill – Some use chemicals to soften substrate • Especially true of those that bore into corals • Also secrete chemicals that inhibit nematocyst firing • Permanently enclosed in burrow • Drilling rates (substrate controlled for) vary across species – 2.6 mm / month – 11.4 mm / month • Shipworms – Lamellibranchs that are ecologically important degraders of wood – Elongate, cylindrical, and almost worm-like – Burrows can be as long as two feet – Feed on sawdust, so they can do great damage to piers, pilings, and wooden boats • Symbiotic bacteria to break down cellulose and fix nitrogen (to compensate for low protein diet) Circulation • Open system • Blood travels through mantle for: – Auxiliary respiration – Diffusion for sensory structures • Blood is important to hydrostat – Comprises approximately 50% of soft volume Gas Exchange • Inefficient system when compared with other molluscs – Many extract less than 10% of the available O2 in a given volume of water – Most lack respiratory pigments – Not detrimental as metabolic rates are usually low • Compensated by filtering huge volumes of water – 40 L / hr in oysters • Mantle skirts aid in gas exchange – Especially at low tides when gills are collapsed Nervous System • Not cephalized, but have a bilaterally symmetrical nervous system consisting of ganglia and nerves • Sensory organs (statocysts, ocelli, osphradia, etc.) situated on periphery structures – – – – Foot Siphons Pallial tentacles Mantle margins • Important structures under muscular control: – – – – Pedal retractor, adductor muscles, etc. Pallial tentacles Labial palps Siphons Reproduction • Mostly dioecious • Two gonads surround intestinal loops in visceral mass • External fertilization, sometimes in mantle cavity – Some brood eggs in exhalant chamber • Two gonads surround intestinal loops in visceral mass • Most have a free-swimming larval phase (trochophore, veliger, or both) that later settles • Most freshwater species lack free-swimming larvae – Get swept away with currents – Harder to remain buoyant in FW – Have direct development • Many attach to fish, so that they may be taken upstream to an optimal habitat – Zebra mussels (Dreissena) release parasitic glochidia • Life spans are variable – 20 – 30 yrs is common – Over 150 yrs in some