Biology 320 Invertebrate Zoology Fall 2005

1. Biology 320 Invertebrate ZoologyFall 2005 Chapter 12 – Phylum Mollusca Part Four

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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

16. 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

17. 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

18. 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

20. 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

21. 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

22. 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

23. 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

24. 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)

25. 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

26. 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

27. 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

28. 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

29. 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