Bivalvia

Bivalvia (historically termed Pelecypoda, "hatchet-foot," or Lamellibranchia, "plate-gills") is a class of exclusively aquatic (marine and freshwater) mollusks comprising 10,000 to 20,000 extant species and more than 40,000 fossil species originating in the early Cambrian (Fordilla, Pojetaia, ~530 Ma). Encompassing clams, cockles, oysters, mussels, scallops, shipworms, and giant clams (Tridacna), bivalves are characterized by a laterally compressed body completely enclosed within two calcareous shell valves (left and right) joined along the dorsal margin by an elastic proteinaceous hinge ligament and interlocking hinge teeth, and pulled shut by one (monomyarian) or two (dimyarian) transverse adductor muscles.

Unlike gastropods and cephalopods, bivalves underwent profound cephalic reduction in adaptation to sedentary suspension feeding and infaunal burrowing: they completely lack a differentiated head, cephalic tentacles, jaws, odontophore, and radula. Instead, sensory organs (including complex mirror-based eyes in scallops and mechanoreceptors) are relocated to the outer mantle margins and siphons, while the ancestral respiratory ctenidia are vastly expanded into folded W-shaped ciliary sheets that filter phytoplankton and organic particulates from the water column.

1. Left vs. Right Valves (Vs. Dorsal/Ventral in Brachiopoda): Whereas brachiopods possess dorsal and ventral valves that are each bilaterally symmetrical across their midline, bivalve shells consist of left and right valves where the plane of symmetry runs between the two valves along the dorsal hinge line (making each valve typically asymmetric from anterior to posterior, except in secondary swimmers such as scallops).
2. Antagonism Between the Hinge Ligament and Adductor Muscles: Bivalves lack muscles that pull their shells open. Instead, shell opening and closing operate as an antagonistic elastic-muscular spring:
   • When the anterior and posterior adductor muscles contract, they pull the two valves tightly closed while compressing the internal resilium (or stretching the external tensilium) of the dorsal hinge ligament, which is composed of cross-linked abductin elastomers reinforced with aragonite nanofibers;
   • When the adductor muscles relax, the stored elastic strain energy in the hinge ligament passively springs the valves open ( explaining why dead bivalve shells gape open).
   • Paramyosin "Catch" Muscle State: Each adductor muscle is divided into a striated quick muscle (for rapid valve snapping) and a smooth catch muscle enriched in dense thick filaments of the coiled-coil protein paramyosin and regulated by the giant kinase twitchin. Once closed, dephosphorylated twitchin cross-links actin and myosin filaments into a tonic catch state that resists tens of newtons of prying force by sea stars (Pisaster) for days while consuming less than 1% of the ATP required by standard tetanic muscle contraction; serotonin release triggers cAMP–PKA phosphorylation of twitchin to immediately unlock the catch.
3. Hydraulic Digging Foot vs. DOPA Byssal Threads:
   • Infaunal Burrowers (Venerida, Cardiida, Solen razor clams): Extend a laterally compressed, blade-like muscular foot into sediment, engorge its hemocoelic sinuses with blood to flare the tip into a terminal anchor, and contract the anterior and posterior pedal retractor muscles to pull the shell downward in a rhythmic two-anchor burrowing cycle, while extending paired posterior inhalant and exhalant siphons to the sediment–water interface.
   • Epifaunal Byssal Attachment (Mytilus Mussels): Intertidal mussels (Mytilus edulis, Mytilus californianus) anchor to wave-battered rocks by secreting acellular collagenous byssal threads from the ventral foot groove. As elucidated by J. Herbert Waite, the terminal adhesive plaque of mussel foot proteins (Mfp-3 and Mfp-5) contains up to 20–30 mol% 3,4-dihydroxyphenyl-L-alanine (L-DOPA)—a post-translationally hydroxylated tyrosine whose catechol side chains displace surface water molecules to form bidentate coordinate bonds with mineral oxides and cross-link via Fe³⁺–catecholate complexes, inspiring synthetic underwater surgical glues and hydrogels.

1. Guanine-Crystal Mirror Eyes of Scallops (Pecten): Along the middle fold of the mantle margin, scallops (Pecten maximus, Argopecten irradians) bear up to 200 bright blue, 1-mm eyes. As shown by Michael F. Land (1965) and Benjamin Palmer et al. (2017), the scallop eye operates like a reflecting Newtonian telescope: light passes through a weak cornea and lens and through a double-layered retina (consisting of a distal retina with ciliary photoreceptors that hyperpolarize to dimming shadows, and a proximal retina with rhabdomeric photoreceptors that depolarize to light) to strike a hemispherical concave mirror (argentea) tiled with square photonic crystals of guanine, which focuses the image back onto the distal retina to trigger rapid valve closure when a predatory sea star shadow passes overhead.
2. Photosynthetic Tridacna, Chemosynthetic Bathymodiolus, and Xylotrophic Shipworms (Kuphus):
   • Giant Clams (Tridacna gigas, reaching >1.2 m and 200 kg): House extracellular zooxanthellae (Symbiodiniaceae) inside branched tubular diverticula of the stomach (zooxanthellal tube system) extending into the expanded, upwardly turned siphonal mantle; epidermal iridocytes scatter damaging UV/short-wavelength light into optimal photosynthetically active wavelengths down through the mantle tissue.
   • Deep-Sea Bathymodiolus and Giant Shipworm Kuphus polythalamia: Hydrothermal-vent and cold-seep mussels (Bathymodiolus) harbor dual intracellular sulfur-oxidizing (thiotrophic) and methane-oxidizing (methanotrophic) γ-proteobacteria inside specialized gill bacteriocytes. Similarly, wood-boring shipworms (Teredo navalis, Bankia) use rasping denticulate valves at their anterior end to tunnel through submerged timber while harboring nitrogen-fixing, cellulolytic Teredinibacter turnerae symbionts in their gills that secrete glycoside hydrolases directly into the digestive tract.
3. Extreme Longevity and Sclerochronology (Arctica islandica): The North Atlantic ocean quahog Arctica islandica is the longest-lived non-colonial animal ever recorded, with an individual collected off Iceland ("Ming") verified by annual shell growth increments (sclerochronology) and radiocarbon dating at 507 years old (1499–2006 CE), providing centuries-long annual archives of North Atlantic sea surface temperature and Atlantic Meridional Overturning Circulation variability.

How does paralytic shellfish poisoning (PSP) occur in filter-feeding bivalves?

Because single oysters or mussels filter 20 to 100 liters of seawater per day across their ctenidia, they bioaccumulate toxigenic marine dinoflagellates during harmful algal blooms (red tides)—including Alexandrium (producing saxitoxin, causing Paralytic Shellfish Poisoning by blocking human voltage-gated Na_v channels), Karenia brevis (brevetoxin, Neurotoxic Shellfish Poisoning), Dinophysis (okadaic acid, Diarrhetic Shellfish Poisoning), and the diatom Pseudo-nitzschia (domoic acid, Amnesic Shellfish Poisoning). Many bivalve lineages carry fixed amino-acid substitutions in the pore loop of their own voltage-gated Na⁺ channels that confer 1,000-fold resistance to saxitoxin, allowing the clam or mussel to feed unharmed while accumulating lethal neurotoxin doses for vertebrate predators.

What is transmissible cancer (bivalve transmissible neoplasia) in soft-shell clams and mussels?

Unlike almost all vertebrate cancers, which remain confined to the host individual in which they arise, Michael J. Metzger, Stephen P. Goff, and colleagues (2015, 2016) discovered that disseminated leukemia-like hemocyte cancers (hemic neoplasia) in marine soft-shell clams (Mya arenaria), bay mussels (Mytilus trossulus), and cockles (Cerastoderma edule) are clonal transmissible cancers (Bivalve Transmissible Neoplasia, BTN). Malignant polyploid hemocytes shed into seawater survive in the marine environment and are taken up by neighboring filter-feeding bivalves—in some cases even crossing species boundaries from Mytilus trossulus into Mytilus edulis and Mytilus chilensis across both hemispheres.

1. Land, M. F. (1965). Image formation by a concave reflector in the eye of the scallop, Pecten maximus. The Journal of Physiology, 179(1), 138–153. doi:10.1113/jphysiol.1965.sp007653
2. Waite, J. H., & Tanzer, M. L. (1981). Polyphenolic substance of Mytilus edulis: Novel adhesive containing L-dopa and hydroxyproline. Science, 212(4498), 1038–1040. doi:10.1126/science.212.4498.1038
3. Butler, P. G., Wanamaker, A. D., Scourse, J. D., Richardson, C. A., & Reynolds, D. J. (2013). Variability of marine climate on the North Icelandic Shelf in a 1357-year proxy archive based on growth increments in the bivalve Arctica islandica. Palaeogeography, Palaeoclimatology, Palaeoecology, 373, 141–151. doi:10.1016/j.palaeo.2012.01.016
4. Metzger, M. J., Reinisch, C., Sherry, J., & Goff, S. P. (2015). Horizontal transmission of clonal cancer cells causes leukemia in soft-shell clams. Cell, 161(2), 255–263. doi:10.1016/j.cell.2015.02.042
5. Palmer, B. A., Gur, D., Weiner, S., Addadi, L., & Oron, D. (2017). The image-forming mirror in the eye of the scallop. Science, 358(6367), 1172–1175. doi:10.1126/science.aam9506