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| Chama pacifica Chamidae |
Just last week we did another dissection practical. This time, for bivalves! We got to dissect a Meretrix sp. ("La La", a heterodont), Nuculana sp. (which was really tiny, a protobranch), Anodonata sp. ("Mussel", family Mytillidae, a pteriomorph), Anadara sp. ("Blood Cockle", a heterodont).
Bivalves are molluscs that are laterally compressed and have a hinge, enclosing two hinged parts. The shell is made up of calcium carbonate secreted by the mantle lobes and mantle. The calcium carbonate can be either aragonitic (nacreous inner layer - sheet or lenticular, prismatic, cross lamellar structure - simple or complex) or calcitic (outer foliose, prismatic). The calcium carbonate of a shell is never entirely calcitic though, mostly it is a mix of aragonite and calcite. Certain bivalves though, like oysters have almost all calcitic. On the contrary, some families are entirely aragonitic.
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| Lima vulgaris Limidae with wholly aragonitic shells |
They have a D-larva split into two stages, Prodissoconch I and Prodissoconch II which have the velum to help it with motility with the exception of the Paleoheterodonts which have glochidium/ lasidium larvae.
They are generally split into different clades based on gill type, presence of palps, type of stomach, fusion of mantle & siphon position (higher taxa); hinge teeth type (higher or lower taxa); foot size, position of adductor muscles (lower taxa):
- Protobranch (small group, 3 orders, considered 'ancient', small in size, bipectinate lamellae (simple) gills with palp probocoides (for Nuculidae and Nuculanidae), foot frilled with papillae, do not filter feed but scrape detritus or symbiotic with sulphur oxidising bacteria in gills (only Solemyida))
e.g. Nuculana
- Paleoheterodont (mostly in the family Unionidae but there are 6 other families; mostly are freshwater except one of which is salt-water - Trigoniidae)
- Heterodont (unequal hinge teeth, eulamellibranch gills, mantle edges are fused at the posterior end as an inhalant tube) and Anomalodesmatans (typically lack hinge teeth, aragonitic shells, long siphons, fourth pallial aperture, vestigial gill filaments; Septibranchia is carnivorous with a type II stomach)
e.g. Heart Cockle (Cartiidae), La La Siput (Veneridae), Meretrix (Veneridae)
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| Cordissum cardissa Cartiidae |
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| Dosinia amphismoides Veneridae |
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| Teredo sp. Terenidae |
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| Tellina virgata Tellinidae |
- Pteriomorphs (reduced foot, filibranch gills (Oysters (orders Ostreida, Pectinida) are exceptions - they have eumellibranch gills with the exception of Plicatuloidea (superfamily)), mantle edges generally not fused, taxodont or dysodont hinge teeth)
e.g. Green Mussel Perna sp. (Mytilidae), Hammer Oyster (Isognomonidae), See Hum Anadara sp. (Arcidae), File Shell (Limopsidae), Scallops (Pectinidae), Spiny Oysters (Spondylidae)
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| Lithophaga teres Mytilidae |
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| Isognomon isognomon Isognomidae |
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| Pinna bicolor Pinnidae |
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| Spondylus niobarensis Spondylidae |
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| A related species to Spondylus, the Comptopallium radula |
Notes:
- filibranch is ancestral to eulamellibranch gills
- heterodont/schizodont/isodont/dysodont teeth is more derived than taxodont teeth
Important parts of the bivalves are:
Of the valves
- Hinge teeth: either cardinal or lateral; isodont/schizodont/heterodont/dysodont/taxodont
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| Taxodont hinge of Anadara antiquata Arcidae |
- Umbo: a knobbly protuberance (determines dorsal)
- Lunule (typically anterior but the mouth position determines anterior/posterior)
- Estucheon (typically posterior)
- Periostracum - either secreted by the mantle edge or foot
- Pallial line, where the mantle edge is
- Pallial sinus, a groove where the siphons are retracted into
Anatomy
- Ligament: inner (lamellar), outer (fibrous); can be alivincular, duplivincular, parivincular, multivicular, either amphidetic (either side of the umbo; usually only for alivincular or dulivincular only) or opisthodetic (anterior of umbo; usually for parivincular or multivincular only); e.g. opisthodetic parivincular are typically of the heterodonts, multivincular opisthodetic are typically of the Isognomidae ("Hammer Oysters"), duplivincular amphidetic (Arcidae), alivincular amphidetic (Limopsidae),
- Gills: made up of lamellae with cillia (Protobranchs); when fused, form demibranches connected by tissue between lamella but not always so; might have cillary junctions (filibranch; Pteriomorphs); when filaments are fused with adjacent filaments, form interfilament junctions, exclusively NO cillary junctions (eulamellibranch; Heterodonts); typically one pair of ctenidia on each side of the visceral mass; each ctenidia with a descending and descending lamella for the eulamellibrach condition; Septibranch condition is a poorly-developed septum with vestigial gills but has tentacles on inhalant siphons to suck small prey up
- Adductor muscle - present most of the times in the anterior and posterior position (isomyarian/heteromyarian/monomyarian); typically one pair of adductor muscle(s) on each valve; for example, Green Mussels (Perna viridris) only has posterior adductor mussel, scallops have one large adductor (central) muscle (monomyarian)
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| Saccostrea cucullata Ostreidae with an adductor muscle scar |
- Other retractor muscles
- Mantle (inner, mid, outer folds; outer - secretory, may be sensory; mid - sensory, have tentacles, sometimes eyes; inner - muscular and attached to shells) with type A/B/C fusion
- Visceral mass which contains heart (2 auricles, 1 ventricle; open circulatory but certain families have haemoglobin and is 'bloody'), digestive organs (esophagus/ stomach - crystalline style, typhosole, sorting area/ intestines/ anus), reno-pericardial complex, gonads
- Periocardial complex
- Two ganglia (cerebro-pleural ganglia innervates palps, anterior adductor muscle, otocysts, part of mantle and visceral ganglia innervates gills, heart, pericardium, posterior adductor muscle, most mantle regions, siphons and pallial sense organs)
- Byssus - secreted by foot typically
- Eyes - pallial (marginal or siphonal, innervated by mantle) or cephalic (inner demibranchs of ctenidia, innervated by cerebral ganglia)
- Siphon - for respiratory purposes/ suspension feeding
- Labial palps (2 pairs)/ palp probosoides (not always present)/ mouth; sometimes labial palps can be hypertrophied (such as in Scallops) to helps food items not 'fall out' while swimming
That was a spam of essential terms. Anyway, here're some photographs by Xin Rui during the practical.
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| Meretrix sp. ("La La") which is a heterodont. Photo source: Ong Xin Rui |
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| The internal anatomy of the Meretrix sp. ("La La"), the same as above. For our practical session, we were required to draw the anatomy. Please note that the posterior and anterior ends should be exchanged. Photo source: Ong Xin Rui |
Further reading
[1] About Glochidium/Lasidium larvae: http://mussel-project.uwsp.edu/evol/syst/larvae.html
[2] About hinge types: http://paleo.cortland.edu/tutorial/Bivalves/bivalvemorph.htm
[3] Pictures of bivalves: http://www.nmr-pics.nl/index.htm
I shall update this page again, after I have dug up my bivalve photographs. Everything makes more sense now after this class...
Please correct if you spot any errors. Information taken and summarized from Bivalve lecture notes.
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