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(Bi-valv i-a)  (pely-cy-poda)
All of the Taxonomy is
Images are being added and the article is being updated Nov. 24, 2003

Latin;  bi=two   -  two plates (Two halves to the shell)     Pele=hatchet   pod=foot  hatchet foot (shape)

     The Pelecypoda, Bivalva or Lamellibranchia (Latin for leaf-gill) (the only class with three names!!)  is comprised of molluscs known more commonly as just bivalves , because they have two separate halves to their shells.  They all have two-part shells, hinged dorsally.  The head is greatly reduced in size and their foot is laterally compressed. Their mantle cavity is the largest of all known molluscs.  Their gills tend to be very large and not only function for respiration, but aid in food-collecting as well.


  Most bivalves have evolved to become burrowers. They have left the hard substratum of their ancestors and have learned to inhabit the massive mud, silt and sand bottoms of our oceans and freshwaters.  Some bivalves do however live on, or most often in hard substrata such as clay, rocks and wood.  These have become sessile (i.e., once adult, they don't move), or borers (example - the famous shipworms - of various families, including Litihophagidae (litho= wood, phag = eat: wood eater).

Teredo navalis
(Linnaeus, 1758)

NOTE: Shipworms are not a worm at all, but a greatly elongated clam . Its two shells, enclosing only the front end of the body, function as a tool, rather than a protective covering; their ridged and roughened surfaces are used for boring. They are actually a boring clam. Christened by mariners, "termites of the sea," shipworms are parasitic mollusks that thrive in and upon submerged wooden structures, including pilings, bulkheads and the untreated hulls of boats. They are quite destructive and have actually sunk many a wooden ship of old. As they tunne and eat the wood, their tunnel diameter actually increase in diameter due to their growth.

  Most bivalves are marine, and of these the majority live in the littoral, or intertidal zone. However, some species are found in the deepest abyssal zones of the oceans.  Some bivalve species and groups have adapted to living in brackish and freshwater environments. These are found in the freshwater families of the Unionidae (These will be discussed further down the page). Also, some of the "true" mussels (family Mytilidae) such as the infamous Zebra mussels are also found in brackish and fresh water.  Some of the bivalves lead a commensural life style:  living with other marine inhabitants, while still others have evolved to become parasites.

  During periods of low tide or drought, exposed fresh-water bivalves retain precious moisture by keeping totally inactive (which is called "aestivation": their metabolic rate drops to zero, so they can last a long time without water!), retaining fluid within their mantle cavity.

  Bivalves have long played a role in feeding the world's population. Another area where they are important is for man's ornamentation and adornment throughout the ages. Pearls are very economically important as a jewelry item, and many bivalve shells are used in various decorative ways. (See the Man and Mollusc article for details on the many interesting uses man has put molluscs, including bivalves to, over the centuries).

  Bivalves are highly specialized not only in their shape, but in their physiology as well.  Because of this specialization, most remain living in and on "soft bottoms" such as sand, silt and well-oxygenated mud.

Taxonomy of Bivalves

There are a few good taxonomic data bases for one to choose from on the world wide web. Even scientists cannot agree one hundred percent on who is right. With the advent of DNA testing and other scientific equipment that is now available to use to delve into molluscs ever deeper; changes will be continually made as to what order, family, etc. that a mollusc belongs to.

I will show just two of the data bases that I used to help me rewrite my section on bivalves. I will use the OBIS data base to further discuss the deferent subclasses in this article.


This database was compiled by teams at the Academy of Natural Sciences, the Australian Museum, the Muséum National d'Histoire Naturelle, and the California Academy of Sciences, with support from the Alfred P. Sloan Foundation, the National Oceanographic Partnership Program, and the Australian Biological Resources Study. This database is part of the Ocean Biogeographic Information System.

World Biodiversity Database:

ETI, the Expert Center for Taxonomic Identification, is a not for profit foundation, dedicated to improve on a global scale the quantity, quality and accessibility of taxonomic information, based on an initiative of UNESCO.




  • Subclass: Protobranchia
    • Nuculoida
      • Nuculanoidea
      • Nuculoidea
    • Solemyoida
      • Solemyidae Gray, 1889
      • Nucinellidae Vokes, 1956
  • Subclass Protobranchia
    • Order Nuculoida
          • Family Nuculanidae
          • Family Nuculidae
          • Family Phaseolidae
          • Family Yoldiidae
  • Subclass: Pteriomorphia
    • Mytiloida
      • Mytilidae Rafinesque, 1815
    • Arcoida
      • Arcoidea
      • Limopsoidea
    • Pterioida
      • Pterioidea
      • Pinnoidea
    • Limoida
      • Limoidea
    • Ostreoida
      • Pectinina
      • Ostreina



  • Subclass Pteromorphia
    • Order Arcoida
        • Family Arcidae
        • Family Glycymerididae
        • Family Noetidae
        • Family Arcidae
        • Family Glycymerididae
        • Family Noetidae
    • Order Mytiloida
        • Family Mytilidae
        • Family Pinnidae
    • Order Ostreoida
        • Family Ostreidae
    • Order Pterioida
        • Family Anomiidae
        • Family Limidae
        • Family Pectinida
  • Subclass: Palaeoheterodonta
    • Trigonioida
      • Trigonioidea
    • Unionoida
      • Unionoidea
  • Subclass Palaeoheterodonta ¬
    • Order Unionoida ¬
      • Superfamily Unionoidea
          • Family Margaritiferidae
          • Family Unionidae

  • Heterodonta
    • Veneroida
      • Chamoidea
      • Lucinoidea
      • Galeommatoidea
      • Cyamioidea
      • Carditoidea
      • Crassatelloidea
      • Cardioidea
      • Tridacnoidea
      • Mactroidea
      • Solenoidea
      • Tellinoidea
      • Arcticoidea
      • Glossoidea
      • Corbiculoidea
      • Veneroidea
    • Myoida
      • Myoidea
      • Gastrochaenoidea
      • Hiatelloidea
      • Pholadoidea









  • Subclass Heterodonta
    • Order Veneroida
        • Family Arcticidae
        • Family Astartidae
        • Family Cardiidae
        • Family Donacidae
        • Family Kelliidae
        • Family Lasaeidae
        • Family Leptonidae
        • Family Lucinidae
        • Family Mactridae
        • Family Montacutidae
        • Family Petricolidae
        • Family Pharidae
        • Family Psammobiidae
        • Family Scrobiculariidae
        • Family Semelidae
        • Family Solecurtidae
        • Family Solenidae
        • Family Tellinidae
        • Family Thyasiridae
        • Family Turtoniidae
        • Family Ungulinidae
        • Family Veneridae
    • Order Myoida
        • Family Corbulidae
        • Family Hiatellidae
        • Family Myidae
        • Family Pholadidae
        • Family Teredinidae
        • Family Xylophagidae

      • Superfamily Dreissenoidea
        • Family Dreissenidae
      • Superfamily Sphaerioidea
        • Family Corbiculidae
        • Family Pisidiidae
        • Family Sphaeriidae
  • Subclass: Anomalodesmata
    • Pholadomyoida
      • Pholadomyoidea
      • Thracioidea
      • Clavagelloidea
      • Pandoroidea
      • Verticordioidea
      • Poromyoidea
      • Cuspidarioidea
  • Subclass Anomalodesmata
    • Order Pholadomyoida
        • Family Cuspidariidae
        • Family Lyonsiidae
        • Family Pandoridae
        • Family Periplomatidae
        • Family Thraciidae



   In summary, the taxonomy of the Pelecypods (bivalves, lamellibranches) is a twisted, complex affair, to be tackled at your own risk!


The Bivalves consists of five Subclasses, accounting for some 15,000 known species. I will be using the OBIS (see above) taxonaomic classification system to further discuss these subclasses.

NOTE: Where possible in the following section, I will be showing a single representative species in each of the families listed. Occasionally, in cases of a shell being very rare and I am unable to provide an image but there is a web site to refer to , I will them list and link to that site. As with all links, that are not permanent in today's world of change. Should you find a broken link; I sure would appreciate it if you could notify me of this.
Thank you: Avril Bourquin

1. Subclass Protobranchia:  (Pro-to-branch-ia) 
Latin: proto=front    branch=gill: 

Primitive bivalves, their gills are not folded.  Palpal proboscides are frequently present. 



Order Nuculoida:

Shell is aragonic with an interior that is nacerous or porcelaneous The periostracum is smooth. The valves are equal and have a row of sharp teeth along its hinge or border. Large palps used for food collection. Ctenidia are small and used only for gas exchange. Foot is longitudinally grooved and has a plantar sole. (Common Name: Nut Clams)

  Superfamilies, Families & Genus:
  • Nuculanoidea
    • Nuculanidae
        • Ledellinae, Unplaced, Nuculaninae, Yoldiinae
    • Malletiidae
        • Malletia, Pseudomalletia, Bathymalletia
    • Sareptidae
        • Sarepta
    • Neilonellidae
        • Neilonella, Protonucula

Nuculana elenensis

(Sowerby, 1833)
12 mm

Malletia cumingii (Hanley, 1860)
15 mm


Unable to locate a shell or photo


Neilonella dubia Prashad, 1932
6 mm
  • Nuculoidea
    • Nuculidae
      • Nuculinae
        • Nucula, Acila, Pronucula, Nuculoma, Ennucula, Rumptunucula
    • Tindariidae
      • Tindariidae
        • Tindaria, Spinula


Nucula semiornata
(Orbigny, 1846)

5 mm up


Tindaria striata
(King & Broderip, 1831)
12 mm up

Order Solemyoida:
Shell valves are thin, equal in size, elongate and lacking hinge teeth. They have a large ctenidae used for both feeding and gas exchange. Their palps are small.
(Common Name: awning clams)


Superfamilies, Families & Genus:

  • Solemyidae
      • Solemya, Acharax
  • Nucinellidae
      • Nucinella, Huxleyia

Solemya togata
(Poli, 1795)
44 mm


Bulletin #35
Huxleyia cavernicola sp. nov.
0.9 mm


2. Subclass: Pteriomorphia: They possess attenuated, flexed gill filaments.  The filaments are incompletely fused; intercellular junctions are present but the adjacent filaments are joined only by ciliary tufts. They are primitive bivalves.

(WAS: Superorder: Filibranchia:  (Phil-I-branch-ia): (also known as Pteriomorphia):
Latin:  fil=thread    branc=gill



Order Mytiloida:
Shell valves are quite thin, elongated and equal in size.. The valves are uncalcified along the outer edges and hinge teeth are absent. They have one large ctenidia which is used both for feeding and gas exchange. Their palps are small.


Families & Genus:

  • Mytilidae: these are the sea mussels
      • Mytilus, Brachidontes, Mytilaster, Perna, Crenella, Gregariella, Modiolarca, Musculus, Rhomboidella, Lithophaga, Myoforceps, Modiolus, Amygdalum, Dacrydium, Idas, Modiolula, Musculista, Xenostrobus, Bathymodiolus

Mytilus californianus
(Conrad, 1837)
100 mm.

Perna viridus
(Linnaeus, 1758)
50 mm.


Order Arcoida:


Superfamilies, Families & Genus:

  • Arcoidea
    • Noetiidae
      • Noetinae
        • Sheldonella, Noetiella, Estellacar, Verilarca, Spinearca, Didimacar, Striarcinae
      • Striarcinae
        • Striarca , Arcopsis
    • Arcidae (mainly found in sand or mud in shallow and deep water, attached to a rocky or coralline substrate by means of a byssus.)
        • inquirenda, Arca, Trisidos, Scaphula, Barbatia, Anadara, Bathyarca, Bentharca, Porterius, Samacar, Hawaiarca, Mosambicarca
    • Cucullaeidae
        • Cucullaea

Noetia reversa
(Sowerby, 1832)

33 mm.


Arca navicularis

(Bruguière, 1789)
27 mm.


Cucullaea labiata
(Lightfoot, 1786)
75 mm.

  • Limopsoidea
    • Philobryidae
      • Philobrya, Hochstetteria, Neocardia, Adacnarca, Cratis, Notomytilus, Cosa, Micromytilus
    • Limopsidae
      • Limopsis, Lissarca, Empleconia, Nipponolimopsis,Crenulilimopsis
    • Glycymerididae
      • Incertae sedis, Glycymeris, Melaxinaea, Tucetona

Cratis kanekoi
(Hayami & Kase, 1993)
Bulletin #35
1.5 mm


Limopsis sulcata
(Verrill & Bush, 1898)
14 mm


Glycymeris glycymeris
(Linnaeus, 1758)
55 mm


Order Pterioida


Superfamilies, Families & Genus:

  • Pterioidea
    • Pulvinitidae
      • Pulvinites
    • Malleidae
      • Vulsella, Malleus, Neoaviculovulsa
    • Pteriidae
      • Pteria, Pinctada, Electroma
    • Isognomonidae
      • Isognomon, Crenatula


Unable to locate a shell or photo


Vulsella vulsella
(Linnaeus, 1758)

50 mm.

Pinctada margaritifera
(Linnaeus, 1758)

200 - 250 mm.

Isognomon radiatus
(Anton, 1839)

50 mm.
  • Pinnoide
    • Pinnidae
      • Pinna, Atrina, Streptopinna

Pinna nobilis
(Linnaeus, 1758)
60 cm.


Order Limoida


Superfamilies & Family:

  • Limoidea
    • Limidae
      • Lima, Limaria, Limea, Limatula, Ctenoides, Acesta, Limatulella, Divarilima, Fukama

Lima lima
(Linnaeus, 1758)
40 mm.


Order Ostreoida


Superfamilies, Families & Genus:

  • Pectinina
    • Pectinoidea
      • Spondylidae
        • Spondylus
      • Pectinidae
        • Pecten, Pedum, Amusium, Chlamys, Decatopecten, Argopecten, Flexopecten , Lissopecten, Hyalopecten, Nodipecten, Patinopecten, Semipallium, Mimachlamys, Equichlamys, Mesopeplum, Veprichlamys, Notochlamys, Delectopecten, Cryptopecten, Anguipecten, Haumea, Mirapecten, Volachlamys, Juxtamusium, Annachlamys, Gloripallium, Excellichlamys, Bractechlamys, Minnivola, Coralichlamys, Serratovola, Somalipecten, Pseudohinnites, Glorichlamys
      • Entoliidae
        • Pectinella
      • Propeamussiidae
        • Propeamussium, Parvamussium, Cyclopecten, Cyclochlamys, Similipecten

Spondylus americanus
(Hermann, 1781)
Photograph by Jim Miller
10 cm.

Chlamys senatoria nobilis
(Reeve, 1852)
7.5 cm.



Unable to locate a shell or photo




Cyclopecten zacae
(Hertlein, 1935)
(Rare-deep water)
14 mm.
    • Anomioidea
      • Anomiidae
        • Anomiinae
          • Anomia, Enigmonia
        • Placunanomiinae
          • Pododesmus, Patro, Monia
      • Placunidae
        • Placuna

Anomia ephippium
(Linnaeus, 1758)
65 mm.

Placuna sella
(Gmelin, 1791)
19 cm.
  • Ostreina
    • Ostreoidea
      • Ostreidae
        • Ostreinae
          • Ostrea, Pretostrea, Planostrea, Booneostrea, Pustulostrea,
        • Lophinae
          • Lopha , Dendostrea, Alectryonella, Anomiostrea
        • Crassostreinae
          • Crassostrea, Saccostrea, Striostrea
      • Gryphaeidae
          • Pycnodonte, Hyotissa, Neopycnodonte, Parahyotissa

Lopha cristagalli
(Linnaeus, 1758)
9 cm.

Hyotissa imbricata
(Lamarck, 1819)
7.5 cm.
    • Plicatuloidea
      • Plicatulidae
          • Plicatula

Plicatula gibbosa

( Lamarck, 1801)
11 mm.
    • Dimyoidea (freshwater mussels)
      • Dimyidae
          • Dimya

Etheria elliptica
Lamarck, 1807


3. Subclass: Palaeoheterodonta

(WAS: Order: Paleoheterodonta: There are about 1,200 species and it includes the nearly extinct family Trigoniidae (fewer than 6 living species) and the Unionoidea (fresh water bivalves)



Order Trigonioida


Superfamily, & Genus:

  • Trigonioidea
    • Trigoniidae
      • Neotrigonia

Neotrigonia bednalli
(Verco, 1907)
5 cm.


Order Unionoida


Superfamilies, Families:

  • Unionoidea
    • Hyriidae
        • Incertae sedis, Hyridella, Alathyria, Cucumerunio, Lortiella, Velesunio, Westralunio
    • Hyriidae2
        • Hyridella2, Lortiella2, Velesunio2, Alathyria2, Westralunio2,
    • Unionidae
      • Unioninae
          • Oguranodonta

Velesunio ambiguus

(Philippi, 1847)

57 mm.
Hyriidae 2

Hyridella drapeta
(Iredale, 1934)
54 mm.


Oguranodonta ogurae (Kuroda & Habe, 1987
50 mm

  • Muteloidea
    • Muteloidae
        • Iridina, Mutela

Iridina spekei
(Woorward, 1880)
99 mm.


4. Order Heterodonta ('het-er-o-'dän -ta)

(Was Eulamellibranchia  (Eu-la-melli-branch-ia)   
Latin:  eu=well, very    lamella=leaf, layer     branch=gill (also called the Paleoheterodonta and the Heterodonta - two subclasses often just lumped together as the Eulamellibranchs (and no, I don't know why nearly every taxonomic group in the bivalves has at least two names!!)   The reflexed gill filaments are morphologically fused to form true lamellae. )



Order Veneroida: Usually thick-valved, equal valved and isomyarian.


Superfamilies, Families & Genus:

  • Chamoidea
    • Chamidae
        • Incertae, Chama, Pseudochama, Eopseuma, Carditochama

Chama lazarus
(Linnaeus, 1758)
56 mm.
  • Lucinoidea
    • Lucinidae (A large, well-known family of usually white, hard-shelled clams in which the cardinal teeth are small, and the anterior muscle scar is narrow and long. There are no long siphons, so the clams make a tube to the surface with their foot. Many genera and species, worldwide, shallow to deep water. [from A&D, p.320])
        • Codakia, Loripes, Lucina, Anodontia, Myrtea, Ctena, Divaricella, Megaxinus, Austriella, Loripinus, Cavilucina, Phlyctiderma, Callucina, Bourdotia, Cardiolucina, Lucinoma, Pillucina, Notomyrtea, Epicodakia, Nevenulora, Wallucina, Monitilora, Divalucina, Saltocuna, Barbierella, Divalinga, Mesolinga, Rasta, Lamellolucina
    • Ungulinidae
        • Diplodonta, Cycladicama, Ungulina, Microstagon, Fellaniella, Numella
    • Thyasiridae
        • Thyasira, Leptaxinus, Tauraxinus, Parathyasira, Mayorithyas,
    • Mactromyidae
        • Bathycorbis
    • Fimbriidae
        • Fimbria


Loripes lucinalis
(Lamarck, 1818)
15 mm.


Felaniella vilardeboana (Orbigny, 1846)
17 mm.


Thyasira trisinuata Orbigny, 1842
9.5 mm


Unable to locate a shell or photo


Fimbria fimbriata (Linnaeus, 1758)

6.2 mm
  • Galeommatoidea
    • Galeommatidae: (a group with often internalized shells and active crawling behavior)
        • Leiochasmea, Entovalva, Kellia, Lepton, Montacuta, Galeomma, Lasaea, Tellimya, Myllita, Scintilla, Mysella, Pseudopythina, Lionelita, Scintillula, Ephippodonta, Issina, Amphilepida, Pseudokellia, Levanderia , Borniola, Melliteryx, Scintillona, Arthritica, Barrimysia, Fastimysia, Virmysella, Divariscintilla, Ambuscintilla, Marikellia, Achasmea, Hitia , Radobornia, Kaneoha, Cicatellia, Parvikellia,Vermitexta, Fronsella, Pileatona, Paraborniola, Curvemysella, Montacutona


      Myllita deshayesi
      (Orbigny & Récluz, 1850)

      9.0 mm.

  • Cyamioidea
    • Cyamiidae
        • Gaimardia, Cyamiomactra, Legrandina, Eugaimardia
    • Neoleptonidae
        • Neolepton, Puyseguria
    • Sportellidae
        • Sportella, Anisodonta, Basterotia, Isoconcha, Tahunanuia

Gaimardia trapezina
(Lamarck, 1819)
16 mm.

species unknown
2.9 mm.



Unable to locate a shell or photo


  • Carditoidea
    • Carditidae
        • Incertae sedis
          • Cardites tankervillii, Chama australis, Cardita jukesi,
            Cardita (Venericardia) pelseneeri
        • Cardita, Beguina, Venericardia, Cardites, Glans,
          Arcinella, Cardiocardita, Thecalia, Cyclocardia, Carditella,
          Megacardita, Choniocardia, Hamacuna, Carditellopsis

    • Condylocardiidae
      • Condylocardiinae
          • Micromeris, Condylocardia, Benthocardiella, Condylocuna, Cunanax, Austrocardiella, Isodontocardia
      • Cuninae
          • Cuna, Propecuna, Ovacuna, Warrana, Crassacuna,
            Mimicuna, Westaustrocuna

Cardites bicolor
(Lamarck, 1819)
40 mm.

Cuna gambiensis
( Nickles, 1955)
25 mm.
  • Crassatelloidea
    • Crassatellidae
      • Incertae sedis
        • Crassatella striata, Crassatella subradiata, Crassatella erycinea, Crassatella corbuloides, Crassatella jubar, Crassatella lapidea, Crassatella ornata, Crassatella triquetra, Crassatella ziczac, Crassatella compta, Crassatella concinna, Crassatella obscura, Crassatella truncata, Crassatella crebrilirata, Crassatella subquadrata, Crassatella indica
      • Eucrassatella, Talabrica, Salaputium, Bathytormus


Crassatella brasiliensis
(Dall, 1903)
(A rare deep water species)
23 mm.

  • Cardioidea
    • Cardiidae
      • Incertae sedis
        • Cardium laevigatum, Cardium rigidum, Cardium (Trachycardium) arenicola, Cardium hiulcum, Cardium fornasinianum, Cardium istmicus, Cardium septuagenarium, Vepricardium monilectum, Cardium (?Fragum) centumliratum, Cardium (Ctenocardium) translatum, Clinocardium californiense bulowi, Cardium (Cerastoderma) iranjanense, Laevicardium (Vepricardium) rudentis
      • Protocardiinae
          • Nemocardium, Lyrocardium, Frigidocardium
      • Cardiinae
          • Incertae sedis, Acanthocardia, Bucardium, Parvicardium,
            Plagiocardium, Papillicardium, Rudicardia, Vepricardium
      • Trachycardiinae
          • Trachycardium, Acrosterigma, Vasticardium
      • Fraginae
          • Fragum, Corculum, Lunulicardia, Ctenocardia, Afrocardium,

      • Laevicardiinae
          • Cerastoderma, Laevicardium, Fulvia, Keenocardium
    • Hemidonacidae
        • Hemidonax


Corculum laevigatum
(Lightfoot, 1786)
36 mm.



Unable to locate
a shell or photo

  • Tridacnoidea
    • Tridacnidae (Giant clams (Cardiidae: Tridacninae) are among the most familiar marine invertebrates. Less well known is the remarkable fact that they are highly derived cardiids (cockles) whose adult morphology has been profoundly restructured by their long evolutionary association with photosymbionts. They have been severly over-harvested throughout much of their collective range and illegal fishing (poaching) remains a serious problem. )
        • Tridacna, Hippopus


      Tridacna (Tridacna) gigas
      ( Linnaeus, 1758 )

      up to 1.5 meters in length 333kg in weight

  • Mactroidea
    • Cardiliidae
        • Cardilia
    • Mactridae: Surf clams like to burrow in rocks, and the shell is consequently very strongly constructed.
      • Incertae sedis
        • Mactra cornea, Mactra opposita, Mactra sulcataria, Mactra symmetrica, Mactra incerta, Mactra, Mactra, Mactra taprobanensis, Mactra thaanumi
      • Mactrinae
        • Mactra, Spisula, Mactrinula, Diaphoromactra
      • Lutrariinae
        • Lutraria, Heterocardia, Meropesta
      • Zenatiinae
        • Zenatina
      • Pteropsellinae
        • Raeta
    • Mesodesmatidae
      • Incertae sedis
        • Caecella transversalis, Paphia macrodon, Mesodesma intermedia
      • Mesodesmatinae
        • Paphies, Coecella
      • Davilinae
        • Davila, Anapella


Unable to locate
a shell or photo


Mactrellona exoleta
(Gray 1837)

Mesodesma donacium
(Lamarck, 1818)
9 cms.
  • Solenoidea
    • Pharidae
      • Siliqua, Cultellus, Phaxas, Ensiculus
    • Solenidae
      • Solen, Neosolen
    • Pharellidae
      • Pharella


Cultellus attenuatus

(Dunker, 1862)
9 cm.


Solen viridis

(Say, 1822)
(See: jaxshells.org)
5 cm.

Pharella javanica
Lamarck, 1818)
  • Tellinoidea
    • Tellinidae (Tellin Clams)
      • Tellininae
        • Tellina, Peronaea, Angulus, Phylloda, Tellinides, Tellinella, Tellidora, Homalina, Eurytellina, Pharaonella, Serratina, Abranda, Macomona, Obtellina, Clathrotellina, Hemimetis, Arcopella, Laciolina, Florimetis
      • Macominae
          • Gastrana, Macoma, Macalia, Psammotreta, Apolymetis, Leporimetis
      • Strigillinae
          • Strigilla, Aeretica
      • Arcopagiinae
          • Arcopagia, Fabulina, Pseudarcopagia, Quadrans, Cyclotellina, Moerella, Merisca, Arcopaginula, Scutarcopagia, Exotica, Semelangulus, Pinguitellina, Quidnipagus, Jactellina, Punipagia, Pistris, Cadella, Loxoglypta
    • Psammobiidae (Sunset Clams)
          • Asaphis, Hiatula, Sanguinolaria, Gari, Psammacola, Soletellina,
            Heteroglypta, Psammotellina, Psammosphaerica

    • Donacidae (Donax Clams)
          • Donax, Hecuba, Heterodonax
    • Solecurtidae (Solecurtus Clams)
          • Solecurtus, Azorinus, Sinonovacula
    • Semelidae
          • Semele, Abra, Ervilia, Cumingia, Theora, Iacra, Leptomya,
            Lonoa, Abrina, Thyellisca, Leptomyaria

Tellina rostrata

(Linnaeus, 1758)

7.5 cm.


Asaphis deflorata (Linnaeus, 1758)
7 cm.


Donax trunculus
(Linnaeus, 1758)
3 cm.


Solecurtus consimilis
(Kuroda & Habe,1961)

6 cm.

Semele laevis
(Sowerby, 1833)
2.1 cm.
  • Arcticoidea
    • Vesicomyidae
        • Vesicomya, Callogonia, Calyptogena
    • Kelliellidae
        • Alveinus, Kelliella
    • Trapeziidae
        • Trapezium, Coralliophaga, Fluviolanatus
Calyptogena magnifica
(Boss & Turner, 1980)
(This is a Deep Sea Vent Clam)
[Collection of Emilio Jorge Power]

Alveinus ojianus
(Yokoyama, 1927)
(shell is only 0.5mm.)

Coralliophaga hyalina sp. nov.
(Bulletin #35)

  • Glossoidea
    • Glossidae
        • Glossus

      Glossus humanus
      (Linnaeus, 1758)
      9 cm.

  • Corbiculoidea (freshwater)
    • Corbiculidae
        • Incertae sedis
          • Cyrena eximia, Batissa triquetra, Cyrena affinis, Cyrena fallax, Cyrena oviformis, Cyrena ventricosa, Batissa australis, Cyrena essingtonensis, Cyrena impressa, Cyrena jukesi, Cyrena placida, Cyrenella moretonensis, Cyrenella sphaericula, Corbicula minor, Corbicula prolongata, Cyrena cyprinaeformis, Corbicula baronialis, Cyrena rugulosa, Cyrena solida, Corbicula deshayesii, Corbicula desolata, Corbicula faba, Corbiculina aramita, Corbiculina esculenta, Corbiculina finkeana, Corbiculina musson, Corbiculina semara
          • Polymesoda, Batissa, Cyrenobatissa


      Corbicula fluminea

      (Müller, 1774)

  • Veneroidea
    • Petricolidae
        • Petricola, Mysia, Asaphinoides
    • Veneridae (Venus Clams)"The family Veneridae has over 400 living species. It is one of the most colorful of the bivalve groups. Shape varies from circular to triangular, and from side view will appear as either ovate (egg-shaped) or cardioid (heart shaped) shells. Characterististics are: porcelain-like shell that is highly finished; a complex tooth structure in the hinge; a well developed escutchion and lunule; and, a well developed sinus at the pallial line. Spiny forms are comparatively uncommon in this family..."
      • Incertae sedis
        • Venerupis flabagella, Pitar (Pitar) varians, Venus maculata, Venus punctata, Cytherea concentrica var., Cytherea rufa, Venus truncata, Venus variflamma, Venus tessellata, Circe planata, Venus bella, Artemis turgida, Tapes geographica, Chione mitis, Venus fluctifraga, Saxidomus maximus, Venus consobrina, Venerupis chinensis, Venerupis digonea, Chione gibbosula, Chione sphaericula, Chione ustulata, Tapes similis, Dione crocea, Dione rufescens, Meretrix grata, Venus deshayesiana, Circe robillardi, Dosinia conglobata, Dosinia minor, Circe undata, Cytherea ambigua, Gouldia petterdi, Sunetta clessini, Circe gordoni, Callocardia (?) pacifica, Circe amica, Dosinia parva, Callista (Callocardia), Dorisca cookei
      • Unplaced
          • Kyrina, Privigna
      • Venerinae
          • Venus, Antigona, Periglypta, Globivenus
      • Dosiniinae
          • Dosinia, Phacosoma
      • Sunettinae
          • Sunetta, Cyclosunetta
      • Meretricinae
          • Meretrix, Tivela
      • Tapetinae
          • Paphia, Tapes, Irus, Venerupis, Gomphina, Katelysia, Marcia, Ruditapes, Granicorium, Eumarcia
      • Circinae
          • Gafrarium, Circe, Gouldia, Circenita, Microcirce
      • Pitarinae
          • Callista, Lioconcha, Pitar, Callocardia, Amiantis
      • Clementiinae
          • Clementia
      • Chioninae
          • Anomalocardia, Timoclea , Clausinella, Chamelea, Protothaca, Bassina, Placamen, Tawera
      • Samaranginae
          • Samarangia
    • Glauconomidae
          • Glauconome

Petricola denticulata
(Sowerby, 1834
20 mm.

Callista florida
(Lamarck, 1818)

33 mm.

Glauconome chinensis (Gray, 1828)
20 mm.


Order Myoida:
Thin-shelled, burrowing bivalves with well developed syphons. Includes the families:


Superfamilies, Families & Genus:

  • Myoidea
    • Myidae
      • Sphenia, Tugonia, Cryptomya, Tugonella
    • Corbulidae
      • Bithinella, Corbula, Potamocorbula


Sphenia hatcheri
(Pilsbry, 1899)
13 mm.


Corbula caribaea
( Orbigny, 1842
8 mm.
  • Gastrochaenoidea
    • Gastrochaenidae
        • Gastrochaena, Rocellaria, Spengleria, Cucurbitula, Eufistulana

Gastrochaena cuneiformis
(Spengler, 1783)

15 mm.
  • Hiatelloidea
    • Hiatellidae
        • Hiatella, Panopea


    Panopea glycymeris
    (Born, 1778)

    11.8 cms.
    See: Interesting Geoduck Facts
  • Pholadoidea
    • Pholadidae
      • Pholas, Pholadidea, Martesia, Barnea, Jouannetia, Penitella
        Parapholas, Aspidopholas, Nipponopholas
    • Teredinidae: The Teredinidae (shipworms) live in calcareous tubes which erode the wooden structures they live in.
      • Teredininae
        • Teredo, Uperotus, Kuphus, Bankia, Nausitora, Lyrodus, Bactronophorus, Teredora, Teredothyra, Nototeredo, Spathoteredo, Dicyathifer

  • Pholadidae

    Pholadidea melanura
    (Sowerby, 1834)

    40 mm.


    Teredo navalis
    (Linnaeus, 1758)
    12 mm.

    (Note on Shipworms: teredos are specialized molluscs that bore into wood. They are not a worm at all, but a greatly elongated clam . Its two shells enclose only the front end of the body and function as a tool, (rather than a protective covering) and the shell's ridged and roughened surfaces are used for boring into the wood. The burrow is lined with a calcareous coating produced by the clam's mantleand is begun when the animal is in its larval stage and it continues to expanded as it grows. The common shipworm of the Atlantic Ocean, Teredo navalis, may grow up to 2 ft (60 cm) long, although its shells remain only 12 in. (12 mm) long.

    Shipworms then feed on wood particles and minute organisms found in the wood. Unlike most shipworms Teredo navalis is able to subsist on wood alone. They do enormous damage to piers and other wooden structures found in the water. In the days of wooden ships they would eat right through hulls thus sinking the ship! . It was not until the discovery that placing a skin of copper plate on the hull of these wooden ships was the problem controlled. In the 15th and 16th century, the only way to protect a ship was to cover the hull with tar and pitch. This would work for a short time, and the ship had to be beached, periodically so that infested timbers could be replaced and re-pitched.)


5. Subclass: Anomalodesmata

A small, specialized group, in which gills are not present.  The inhalent and suprabranchial (exhalent) cavity are separated by a pumping septum.

(WAS: the Septibranchia  (Sept-a-branch-ia) 
 Latin:  sept=seven, wall    branch=gill (also called Anomalodesmata 



Order Pholadomyoida


Superfamilies, Families & Genus:

  • Pholadomyoidea
    • Pholadomyoidea
      • Pholadomyidae (Piddock Clams)
        • Pholadomya
      • Parilimyidae
        • Parilimya

      Pholadomya candida
      (J. Sowerby, 1823)
      94 mm.


      Unable to locate a shell or photo


    • Thracioidea
      • Thraciidae
        • Thracia, Phragmorisma, Thraciopsis, Parvithracia
      • Periplomatidae
        • Periploma, Takashia
      • Laternulidae
        • Laternula, Exolaternula


    Thracia similis
    (Couthony, 1839)
    20 mm.


    Periploma angasi

    (Crosse & Fisher, 1864
    49 mm.


    Laternula anatina
    (Linnaeus, 1758)
    75 mm.

  • Pandoroidea
    • Cleidothaeridae
      • Cleidothaerus
    • Lyonsiidae
        • Lyonsia, Entodesma
    • Pandoridae
        • Pandora
    • Myochamidae
        • Myochama, Myadora


Cleidothaerus albidus
(Lamarck 1819)

63 mm.
"This image is of a fossilized shell"


Lyonsia patagonica
(Orbigny, 1846)

17 mm.

Pandora inaequivalvis
(Linnaeus, 1758)

25 mm.

Myadora delicata (Cotton, 1931)
9 mm.
  • Verticordioidea
    • Euciroidae
        • Euciroa, Acreuciroa
    • Verticordiidae
        • Verticordia, Haliris, Halicardia, Thracidora, Spinosipella, Vertambitus, Policordia
    • Lyonsiellidae
        • Lyonsiella


    Acreuciroa rostrata


    Vertambitus pygmaea
    ( Kuroda, 1952)


    Unable to locate
    a shell or photo

  • Poromyoidea
    • Poromyidae
      • Poromya, Cetoconcha

        Cetoconcha eximia
        (Pelseneer, 1911)
        12 mm.

  • Cuspidarioidea
    • Cuspidariidae
      • Cuspidaria, Cardiomya, Halonympha, Myonera, Pseudoneaera, Protocuspidaria


    Cardiomya alcocki
    (Smith, 1884)
    11 mm.
    VERY RARE ribbed shell, deep water (1000 meters)




Shell & Mantle: (Diagram)

The typical bivalve shell consists of two similar, convex and oval or elongate valves.  These valves are attached and articulate with each other. The shell is made up of three layers:  The periostracum or thin outer layer that is made of horny, organic material called conchiolin, the prismatic or thick middle layer that is made up of calcium carbonate crystals arranged in vertically, and the nacerous or thinner inner layer that is composed of thin horizontally arranged calcium carbonate crystals.  (Diagram)

Dorsally, the shell has a protrusion called the umbo, which rises above the articulation (most commonly called teeth ).  The umbo is the oldest part of the shell. The concentric lines found around the umbo are growth lines, which are usually seasonal, making it a lot easier to tell the age of a bivalve than a gastropod!  The two valves are attached by an elastic band of cartilage-like material called the hinge ligament, which is made up of conchiolin - the same as the periostracum. The hinge ligament is designed to hold both halves of the shell together. The main muscle of a bivalve is called the adductor muscle, and is also used to hold the shell halves together.   When the bivalve s adductor muscles relax, the ligament causes the valves to open. Most species are also equipped with locking teeth or sockets beneath the ligament to prevent lateral slippage. The valves are pulled together through the action of the two strong, adductor muscles.  They are antagonistic to the hinge ligament, as just explained.  On the inside of the shell is a scar that marks where these muscles attach.

In most bivalves, the valves are similar in structure and size; however, in a few families such as the oysters and jingle shells the upper or left valve is always larger than the right valve.  For those bivalves that attach to a substratum, they always do this by their right valve, which is always the smaller of the valves, if there is a difference between them.

In the more ancient bivalves, the adductor muscles are of equal size.  Many families have evolved to the point where the anterior adductor has reduced in size and in some, like the oysters, it has disappeared all together. In these cases, the posterior adductor has shifted to a more central location between the valves. (This large muscle is the part most often eaten by man - for example, the round, white meat we often call a scallop  is in fact on the scallop's adductor muscle.)

Some bivalves can rapidly shut their two valves.  This is to be found in scallops, for example.  Here, the adductor muscle is divided into one section of striated fibers and one section of smooth fibers. The striated fibers cause the rapid closing and the smooth fibers sustain the contraction.

  The bivalve shell exhibits a great variety of shapes, sizes, surface sculpturing and colours. In size they range from a few millimeters (<2mm in the Sphaeriidae) to over four feet (1.3+ meters in the Tridacna gigas).  They come in all colours and colour combinations, and range from smooth as glass to having long sharp spines in their sculpturing (e.g.: the Spondylidae, or "spiny oysters".)

  The mantle greatly overhangs the soft body and forms a large sheet of tissue lying beneath the valves. The edge of the mantle has three folds; an outer, middle and inner one.  The innermost fold is the largest and contains radial and circular muscle tissue.  The middle fold acts as a sensory organ.  The outermost layer is responsible for secreting the shell.  The inner surface of this outer fold lays down the periostracum, and the outer layer lays down the prismatic and nacerous shell layers. The nacerous layer is also secreted by the entire outer surface on the mantle.

The mantle is attached to the shell, in a semicircular line just inside the shell edge, by means of the inner lobes' circular muscle.  This attachment leaves a visible scar on the inside of the shell known as the "pallial line".  The pattern of the pallial lines and adductor muscle scars is extremely useful in identifying very similar species, when you only have the shell.  This attachment prevents foreign particles form getting lodged between the mantle and the shell.  However; sometimes a foreign substance such as a grain of sand or parasite does get in.  To prevent this from irritating the mantle, the mantle lays down concentric layers of nacerous shell around the particle.  This is how a pearl is formed.  Sometimes the pearl becomes totally embedded in the shell itself.  Most bivalves are capable of forming a pearl; however, it is the pearl oyster, Pinctada margaritifera that produces the finest natural pearls man uses for jewels.  Cultured pearls are started when man intentionally inserts a nucleus (a microscopic globule of liquid or solid irritant) in an oyster.  When this pearl is approximately one year old and has a covering of 1 millimeter, this seed pearl is transplanted into another oyster.  Three years after this transplant, the pearl is usually marketable.


The Foot & Locomotion

To facilitate burrowing into the mud or sand where bivalves live, the foot has evolved to become compressed and blade-like. In the more primitive Protobranchia the foot has a flattened sole on its foot.  The edges of this sole fold together to form a sharp edge. It thrusts this sharp-edged foot into the sand or mud then it opens it up again so that the foot now acts as an anchor and the remaining body is pulled down into the soft substratum.  Some of the other bivalves, without this flat sole, can inflate the leading edge of the foot: it then acts as an anchor and is used for digging-in in much the same fashion.

  Bivalve foot movement is accomplished through a combination of changing blood pressures and muscle action.  Attached to the shell, just below the anterior adductor muscle is a pair of protractor muscles that extend from each side of the foot and attach to the opposite valve.  Blood engorges the foot, increasing its blood pressure.   This increased blood pressure in conjunction with the "pedal protraction muscles" (the muscles which manipulate the foot), causes extension.  Withdrawal of the foot is effected by the contraction of a pair of posterior retractors also attached to the foot and shell, and by the contraction of muscle fibers in the foot itself: in other words, the bivalve sort of "inchworms" its way around by contracting and expanding its foot muscle, thereby withdrawing and extending it using a combination of blood-pressure changes and muscles.

  Some bivalves, such as the Cockles (Cardiidae (Cardium)), move along the bottom by means of jumping.  Here the foot is extended then contracted violently, moving backwards in the process.  Thus, the foot acts like a spring always kicking the animal slightly forward.

  The sessile bivalves, such as the oyster and jingle shells (Anomia), have a greatly reduced foot.  Scallops also have a reduced foot and swim in jerky, but often quite effective in the short run, movement through the water by slamming shut their valves.  This sudden closure causes two streams of water to be expelled rapidly from each side of the hinge, causing a form of "jet propulsion".

  The mussels (Mytilidae) live attached to rocks, shells, man-made structures such as piers, or other mussels. They stay attached by means of strong horny threads called byssal threads (Diagram).  A gland in the foot of the mussel produces these threads.  This gland, which is situated just above and behind the small round foot, produces a secretion that flows down the back side of the foot and out to the tip of the foot, which is in contact with the hard substratum. This secretion runs onto the substratum where it hardens as it comes in contact with the water.  A thread is formed. The foot then withdraws and this process is repeated many times on a slightly different area of substratum.  A web of byssal threads thus holds the mussel fast.


Water Circulation  & the Mantle Cavity

The bivalve body has become greatly lengthened dorsal-ventrally (i.e., it has been flattened), and this flattening, in combination with an overhang of the shell, creates an extensive mantle cavity. The mantle cavity extends anteriorly (i.e., to the front), and to each side of the body.

  In some primitive Protobranchs, inhalent water enters the mantle cavity anteriorly, passes over the gills and exits posteriorly. Since these bivalves live buried, sediments get pulled in with the inhalent water. Cilia lining the mantle cavity, foot and gills sweep these sediments to the mantle edge where it accumulates.  Every now and then the valves contract rapidly and flush these sediments out.  Some, such as the Nucula, also have hypobrachial  glands for consolidating the finer sediments that pass through their gills.

  Drawing sediment in with the inhalent water was a major problem for the burrowing bivalves and the solutions made by the primitive species in the Protobranchia group was not very efficient. More advanced bivalves adopted several fundamental strategies to overcome this problem:  In all the bivalves as well as in most of the Protobranchs, the inhalent current returned to the posterior end.  Water enters posteriorly and ventrally, then makes a U-turn through the gills, and passes back out posteriorly and dorsally.  This enables the bivalve to burrow their anterior end into the soft substratum, leaving just the elongated inhalent posterior end protruding through the sediment, clear of excessive sand, mud or silt.

  The second change came about with the sealing off of the mantle edges where openings are not necessary.  This in turn led to the development of the inhalent and exhalent siphons. The mantle edges surrounding these fused edges are often elongated to form actual tubular siphons of varying lengths. This system is very advantageous, as the animal can now remain buried in the sediments with only the tip of its siphon protruding.  The siphon can also be retracted by means of the siphon retractor muscle that was derived from muscle tissue of the innermost mantle fold. (The pallial sinus markings on the shell show where this siphon was to be found.)

  There is a lot of variation to be found amongst different bivalves as to the size, length and shape of this siphon.  Some are short and poorly developed while others are very long and so big that they can no longer be contracted into the shell. Some species have inhalent and exhalent siphons the same length while in others they are quite different. In length

In some, the mantle fusion has been carried to a point where only three apertures are now present.  One aperture for each of the inhalent and exhalent siphonal canals and one for the foot.  A few have a fourth aperture through which the byssal threads pass.



Most bivalves have one pair of long gills that separate the mantle cavity into a ventral inhalent chamber and a dorsal exhalent chamber (also known as the suprabranchial  chamber, for those that like long, fancy words!!).

Cilia provide the power to bring water into the inhalent chamber.  Sediment that enters with the inhalent water gets trapped on the lateral cilia and is swept by the frontal cilia to the midline. It is then moved anteriorly (towards the front) along the gill to the foot (which is, ironically, in the front of the bivalve!!). The sediments are then deposited along the ventral (i.e., bottom) mantle edge. Periodically the valves snap shut, flushing these sediments out.  Fine sediment particles, however, can pass through the gill and they become trapped in the secretions of the hypobrachial glands located over the exhalent chamber.

In the primitive order of Protobranchs (Diagram), the gill is not folded and palpal proboscides are frequently present.  This is the case in the Yolida, Solemya, Nucula, Nuculana, and Malletia.

  Other ancient bivalves appear to have a double set of gills.  The second sets of gills actually arise from a folding of the single gill.  This is the case in the Filibranchia and Eulamellibranchia, where the gills have also taken over the function of obtaining food.  To do this, the gills have many more and greatly lengthened cilia on the gill surface. These long cilia projected somewhat arterially and then become slightly flexed (bent) downward in the middle.  At the angle of the bend, an indentation or notch is formed.  The notches on the adjacent cilia all line up and form a "food groove" that extends along the underside of the gill.  As they developed through time, this flexion increased until the cilia became U-shaped.  Cilia on both sides of the axis, now folded in two, became known as the ascending limb and descending limb. This transformed the single gill into a pair of gills in these bivalves and they became known as the demibranchs.

  In the Filibranchia, bars of tissue, called interlamellar junctions, grew between the two limbs of each U at intervals. However, the adjacent cilia still remained attached only by tufts of cilia.  Each gill was now composed of two lamellae and formed a tight mesh.

  In these bivalves, the frontal cilia carry the food particles, which are trapped on the gill surface, downward to the food groove, and the lateral cilia move the water through the gills.  Between the frontal cilia and the lateral cilia, along the angles of the gill limbs, a new ciliary tract was formed of lateral and frontal cilia.  These cilia prevent large sediments from clogging the gills.

  Inhalent water entering the posterior (back) end of the animal enters the inhalent chamber. It now flows between the filaments and moves up between the two lamellae. From the interlamellar spaces, the water flows into the exhalent chamber and then flows out through the exhalent opening.  In this system the hypobrachial glands became unnecessary as only the very finest of sediments ever pass through the tightly meshed gills.  They eventually disappeared all together.

  The order filibranchia with this gill structure includes the mussels (Mytilus and Modiolus are the mussel genera most commonly eaten), and Ark shells (Arcidae) the oysters- Ostrea, Crassostrea and Spondylus; Anomia; Lima; and the scallops (Pectinidae), and the boring Lithphagia.

  In the Eulamellibranchia (Diagram), the union of filaments even developed further and the ciliary junctions were replaced by actual fusion. The lamellae now consisted of solid sheets of tissue.  The number of interlamellar junctions also increased.  They now extend the length of the lamellae dorso-ventrally (i.e., from top to bottom, vertically) dividing the interlamellar space into vertical water tubes. The tips of the ascending limbs have become fused with the upper surface of the mantle on the outside and the foot on the inside.  This now morphologically separates the inhalent chamber from the exhalent chamber. Instead of blood oxygen diffusion occurring through the lamella, the blood is now carried through the lamellae in vertical vessels that course within the interlamellar junctions.  (And if you can visualize that tangle of concepts, you are pretty smart!!).  Water in the inhalent chamber now circulates between the ridges, and then enters the water tubes through numerous pores (ostia) in the lamella.  Oxygenation takes place as the water flows dorsally through the tubes.  The water then flows into the exhalent or suprabranchial cavity and out the exhalent opening.

  This system was improved upon by many of the Eulamellibranchs.  In these bivalves the surface of the lamella has been increased by folding.  Their gills now have an undulated appearance.

  The order of Eulamellibranchia with their gill filaments morphologically fused includes the Cardiidae (Cockle shells!); the edible Mercenaria (Quahogs); the boring clams- the Petricola (false Angel Wings), Hiatella, Martesia and Teredo (ship worms); the razor clams- Tagelus and Ensis; the little Donax clams; Abra; Pholas (True Angel wings!); Lyonsia; Macoma; the most common freshwater clams- Unionidae; Lampsilidae, Anodonta, and Simpsoniconcha; and the freshwater Sphaeriidae and Magaritidreidae.

In all bivalves, the inner mantle surface plays some role in oxygenation.  In the Septibranchia (Diagram) (which includes the Poromyidae, and the little Spoon Clams - Cuspidariidae, however, the gills have degenerated and modified to become a pair of perforated (full of holes) macular septa that separate the inhalent chamber and the exhalent chamber.   Muscular contractions of this septum move it up and down, which causes water to flow into the inhalent chamber and forcing it out the exhalent chamber.  The mantle has in this order taken over the function of respiration completely.


Circulation (Diagram)

In most bivalves, the heart folds around the rectal portion of the digestive system so that the pericardial sac engulfs the heart as well as a short portion of the digestive tract.  The thin-walled auricles are attached to the muscular ventricle that surrounds the rectum. Ventricular contractions are strong and usually quite slow (approximately about 20 per minute).  Bivalves exhibit a typical molluscan circulatory route through the heart, tissue sinuses, nephridia, and gills.  Minor variances do exist among the different orders and families, but I will not go into these.

  The blood is similar to that of the gastropods; however, some such as the Arcidae (Ark Clams) and Limidae, hemoglobin rather than hemocyanin is present - so these have red, as opposed to the clear or greenish blood most molluscs possess.


Nervous System and Sense Organs:' (Diagram)

Bivalves possess a bilateral and relatively simple nervous system.  They have three pairs of ganglia and two pairs of long nerve cords.

A cerebropleural ganglia is located on both sides of the esophagus and they are connected by a short commisure across the top of the esophagus.  From these ganglia two nerve cords travel to a pair of closely adjacent visceral ganglia located beneath the posterior adductor muscle. Now the second pair of nerve cords pick up and carry the nerve signal to a pair of pedal ganglia located in the foot.

Most bivalve sense organs are located in the margin of the mantle.  Many species possess pallial tentacles, which contain tactile and chemoreception cells.  The entire margin may bear tentacles with, or without eyes (e.g. Pectinidae and Limidae) but usually these are restricted to the inhalent or exhalent aperture or siphons or often they fringe the pedal aperture.

A statocyst is generally found near or embedded in the pedal ganglia.  This statocyst is a small organ of balance and generally consists of a fluid-filled sac containing statoliths (little stones) that help to indicate relative position.

In some bivalves, ocelli (small simple eyes) are present along the edge of the mantle or on the siphons.  In the Spondylus and Pectinidae, the eyes are quite well developed consisting of a cornea, lens and retina.  These eyes most likely cannot form a well - focused image but they can detect changes in light intensity with the photoreceptor cells found in the ocelli.

Bivalves also possess an osphradium, or chemoreception organ which lies directly bellow the posterior adductor muscle in the exhalent chamber. How this sense organ works is not fully understood as yet (another thesis topic for you!!)


Nutrition & Digestive System  (Diagram)

Most bivalves are ciliary feeders (or filter-feeders).  Their gills have taken over the role of trapping food particles as well as respiration.

However; in the ancient order of Protobranchs, the role of food collection is carried out by the elongation of their mouth structure, which is formed into a muscular proboscis and a pair of palps that extend back towards the gills.  This proboscis extends into the surrounding mud or sand and organic detritus is drawn in and carried along its length by means of ciliary action. It is then passed to the palps where it passes through the two lamellae.  Here the detritus is sorted and particles for digestion are sent on to the mouth along a deep oral groove.  Rejected particles are swept to the edge of the lamellae then transferred to the mantle cavity along with the water current.

Food entering the mouth is passed anteriorly to the stomach via ciliary action.  The stomach is surrounded by a large digestive gland and is divided into two regions.  In the first region (dorsal) the esophagus and ducts of the digestive gland enter and it contains a ventral style sac. This dorsal portion of the stomach is lined with chitin except for the large folded and ciliated sorting region, into which the digestive gland opens.  At the apex of the stomach is a tooth-like projection called the gastric shield, which arises from the chitinous girdle.  At the end of this region is the cecum.

Food is passed along the sorting region of this dorsal section.  A few food particles do enter the digestive gland; however most are passed onto the cecum. When the food particles pass out of the cecum, they get enmeshed in great masses of mucus that fills the ventral style sac.  This mass is rotated by the cilia lining the style sac, and along with the muscular action of the sac, this mass is moved dorsally into the upper region of the digestive tract.  The leading edge of this mucus mass is wound around the tooth-like gastric shield and is pressed hard against the chitinous girdle. This winding process causes pieces to be broken off and ground up.  Smaller bits are passed to the digestive gland and coarser bits are passed venrally into a deep groove along the anterior wall of the style sac and they are then passed directly into the intestine. (Diagram)

  The ducts of the digestive gland are ciliated and are divided into an incurrent and excurrent tract.  Food particles enter the tubules of the digestive gland via the incurrent tract.  Here the particles are engulfed by the cells of the gland and are digested intracellulary.  Wastes are dumped into the excurrent tract and are moved by ciliary action back to the stomach where they then get swept into the style sac groove and intestine.  The long intestine loops once or twice around the stomach and then passes through the anterior adductor muscle and becomes the rectum.  The rectum extends through the heart and pericardial cavity and then opens through the anus at the posterior of the suprabrachial cavity.  The intestine only serves in the role of forming feces - no absorption takes place here. Feces leaves as well formed pellets with the exhalent water current.

  In the Filibranchia and Eulamellibranchia, the gills have assumed the function of food acquisition.  The proboscides have disappeared but the lamellae have been retained.  These bivalves have adapted to eating small phytoplankton and very little coarse material ever reaches the stomach.

  Plankton gets trapped in mucous that is on the gill surface and cilia sweep this mass into the food groove, (some bivalves have both a dorsal and a ventral food groove) which runs along the gill.  The food is then sorted in the lamellae and, acceptable food is passed into the mouth and rejected materials are swept to the ventral edge of the mantle and then posteriorly where they accumulate behind the inhalent aperture.  When the valves periodically close, these wastes and water are forced out the inhalent siphon.

  The acceptable food particles are fine enough that they don't require as much grinding and the girdle of chitin has become much reduced.  The style sac and the mucous in this group has consolidated to form a very compact, and often a very long rod called the crystalline style (usually about one inch in length those of the Tridacna or giant  clam may reach a length of 13 inches).  This crystalline style in addition to producing its protein matrix also produces amylase for digestion but basically it acts very much like that to be found in the gastropods.  The projecting tip of the style is rotated by ciliary action and as grinds against the gastric shield the enzymes are shed into the food particles. (The style is constantly replaced at its base and it may spin as rapidly as 11 to 70 times per minute; this rate is affected by temperature, PH, and food pressure as well as the ciliary action.)  This mix passes through the sorting area of the stomach and the finer particles are moved into the digestive glands, of which there are from two to twenty. Here digestion and absorption takes place intracellulary.  Any rejected or waste particles from the digestive gland are passed directly into the intestine.

The Septibranchia, which have lost their gill structure, have become either carnivorous or scavengers.  The pumping action of their septum provides sufficient negative pressure to pull in small animals.  These animals are seized by the much reduced but very muscular lamellae and are passed into the mouth.  The stomach is lined with chitin and it acts as a gizzard, crushing up the animal. The style is also much reduced in this group and may only function in coating harder particles with mucous to protect the intestine from injury.

In the bivalves, one oddity does exist, the "giant clam"Tridacna gigas. This species, besides its regular food procuring and digestive process, literally farms unicellular algae of the family Zooxanthellae, which it encourages to grow within its mantle tissue.  Some of these algae get engulfed and are subsequently digested by phagocytic cells thus providing an additional food source for the Tridacna.



Bivalves posses two nephridia, which are located beneath or just slightly posterior to the pericardial cavity.  The nephridia are folded to form a long U.  One arm is glandular and opens into the pericardial cavity. The other arm forms a bladder and opens through the nephridiopore at the anterior of the suprabrachial cavity.



The majority of bivalves are dioecious (two sexes).  Their two gonads are very closely situated next to each other and they encompass the intestinal loops.  The gonoducts are very simple as there is no copulation amongst bivalves.

In the Protobranchs and Filibranchs, the gonoducts opens directly into the nephridia and provide for the exit of sperm and eggs.

In the Eumellibranchs, the gonoducts opens directly into the mantle cavity very close to the nephridiopore.

A few bivalves such as the Cockles (Cardiidae), Poromyidae, a few of the oysters and scallops (Pectinidae), some of the fresh water clams Sphaeriidae and Unionidae are hermaphroditic (one sex).

In most of the bivalves, sperm and eggs are released into the surrounding water where fertilization occurs.  The eggs and sperm, which were deposited into the suprabrachial chamber, are swept out along with the exhalent current.

In a few of the bivalves, such as the common oyster Ostrea edulis L., fertilization occurs within the suprabranchial chamber itself when sperm is drawn in along with the inhalent current.  The fertilized eggs then develop in the gill filaments.

In some of the freshwater hermaphrodites, self - fertilization may actually occur in the genital ducts before the eggs are deposited into the suprabranchial chamber. The eggs then travel into the water tubes of the gill and there they develop into larvae.


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Musculium lacustre (MŸller, 1774)
Lake orb mussel, capped orb mussels)


Corbicula fluminea
(Müller, 1774)





Margaritifera margaritifera
(Linnaeus, 1758)
Freshwater Pearl Mussel

An excellent presentation on "PACIFIC NORTHWEST FRESHWATER BIVALVES" life history and ecology