MOLLUSCA (IV): DIVERSITY- Bivalves etc. (Mollusca study images;
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6) Class BIVALVIA (mussels, oysters, scallops, clams, etc.); 2nd most diverse class (~25,000 marine and freshwater spp)
a) body plan highly modified for: a) burrowing and b) particle-feeding:
b) two shells (valves) joined by flexible ligament (tension or compression); mantle edge fuses to shell margin and links to 2 adductor muscles; may be isomyarian (same size) or anisomyarian (different size)
c) most produce byssal threads to attach as larvae; some still do as adults
d) gill form is critical to understanding the evolution of bivalves. One pair of ctenidia resides inside a large mantle cavity; they may be either:
both have compound latero-frontal cilia for particle capture
e) foot is highly modified for burrowing (except attached or swimming species); burrowing involves "alternating anchors", both shell and foot
f) sexes are separate; gametes spawned freely into water; most have free-swimming trochophore and later bivalved veliger larval stages
g) classification still debated; 4 subclasses based on several traits:
Trait | Paleotaxodonta | Pteriomorpha | Heterodonta | Anomalodesmata |
---|---|---|---|---|
common names | nut shells | mussels, scallops, oysters | 'true' clams | watering pot shells, septibranchs |
hinge teeth | many, small ("taxodont") |
none** | few, large | 1 or none |
adductors | isomyarian | anisomyarian** | isomyarian | isomyarian |
siphons | no | no | typically yes | typically yes |
gill type | protobranch | filibranch | eulamellibranch | eulamellibranch or septibranch (absent) |
7) Class SCAPHOPODA (tusk shells) ~250 burrowing marine spp
Copyright (c) 2017 by A. Richard Palmer. All rights reserved.
(revised Dec. 19, 2016)