Lucinidae

Lucinidae, common name hatchet shells, is a family of saltwater clams, marine bivalve molluscs.
These bivalves are remarkable for their endosymbiosis with sulphide-oxidizing bacteria.
Characteristics
The members of this family have a worldwide distribution. They are found in muddy sand or gravel at or below low tide mark. But they can also be found at bathyal depths. They have characteristically rounded shells with forward-facing projections. The shell is predominantly white and buff and is often thin-shelled. The shells are equivalve with unequal sides. The umbones (the apical part of each valve) are just anterior to mid-line. The adductor scars are unequal: the anterior are narrower and somewhat longer than the posterior. They are partly or largely separated from the pallial line. The valves are flattened and etched with concentric or radial rings. Each valve bears two cardinal and two plate-like lateral teeth. These molluscs do not have siphons but the extremely long foot makes a channel which is then lined with slime and serves for the intake and expulsion of water. The ligament is external and is often deeply inset. The pallial line lacks a sinus.
Fossil record
An Eocene species Superlucina megameris was the largest lucinid ever recorded, with shell size up to high, over wide and thick.
Symbiosis
Lucinids host their sulfur-oxidizing symbionts in specialized gill cells called bacteriocytes. Lucinids are burrowing bivalves that live in environments with sulfide-rich sediments. The bivalve will pump sulfide-rich water over its gills from the inhalant siphon in order to provide symbionts with sulfur and oxygen. The endosymbionts then use these substrates to fix carbon into organic compounds, which are then transferred to the host as nutrients. During periods of starvation, lucinids may harvest and digest their symbionts as food.
Symbionts are acquired via phagocytosis of bacteria by bacterioctyes. Symbiont transmission occurs horizontally, where juvenile lucinids are aposymbiotic and acquire their symbionts from the environment in each generation. Lucinids maintain their symbiont population by reacquiring sulfur-oxidizing bacteria throughout their lifetime. Although process of symbiont acquisition is not entirely characterized, it likely involves the use of the binding protein, codakine, isolated from the lucinid bivalve, Codakia orbicularis. It is also known that symbionts do not replicate within bacteriocytes because of inhibition by the host. However, this mechanism is not well understood.
Lucinid bivalves originated in the Silurian; however, they did not diversify until the late Cretaceous, along with the evolution of seagrass meadows and mangrove swamps. Lucinids were able to colonize these sulfide rich sediments because they already maintained a population of sulfide-oxidizing symbionts. In modern environments, seagrass, lucinid bivalves, and the sulfur-oxidizing symbionts constitute a three-way symbiosis. Because of the lack of oxygen in coastal marine sediments, dense seagrass meadows produce sulfide-rich sediments by trapping organic matter that is later decomposed by sulfate-reducing bacteria. The lucinid-symbiont holobiont removes toxic sulfide from the sediment, and the seagrass roots provide oxygen to the bivalve-symbiont system.
The symbionts from at least two species of lucinid clams, Codakia orbicularis and Loripes lucinalis, are able to fix nitrogen gas into organic nitrogen.
Genera
The following genera are recognised in the family Lucinidae:
Subfamily Codakiinae Iredale, 1937
Codakia Scopoli, 1777
Ctena Mörch, 1860
Divalucina Iredale, 1936
Epicodakia Iredale, 1930
Epilucina Dall, 1901
Lucinoma Dall, 1901
Subfamily Fimbriinae Nicol, 1950
† Cerkesia Monari, 2003
† Cyclopellatia Cossmann, 1907
Fimbria (traditionally placed in the separate family Fimbriidae)
† Haastina Marwick, 1953
† Mutiella Stoliczka, 1871
† Parvicorbis Cossmann, 1892
† Schafhaeutlia Cossmann, 1897
† Sphaera J. Sowerby, 1822
† Sphaeriola Stoliczka, 1871
Subfamily Leucosphaerinae J. D. Taylor & Glover, 2011
Afrolucina Cosel, 2006
Alucinoma Habe, 1958
Anodontia Link, 1807
Callucina Dall, 1901
Dulcina Cosel & Bouchet, 2008
Epidulcina Cosel & Bouchet, 2008
Gonimyrtea Marwick, 1929
Leucosphaera Taylor & Glover, 2005
Minilucina Cosel & Bouchet, 2008
Myrtina Glover & Taylor, 2007
Neophysema J. D. Taylor & Glover, 2005
Opalocina Glover & J. D. Taylor, 2016
Pseudolucinisca Chavan, 1959
† Rawya Strougo, 1975
Tinalucina Cosel, 2006
Ustalucina J. D. Taylor & Glover, 2021
Subfamily Lucininae J. Fleming, 1828
Austriella Tenison Woods, 1881
Barbierella Chavan, 1938
Bathyaustriella Glover, J. D. Taylor & Rowden, 2004
Bourdotia Dall, 1901
Bretskya Glover & Taylor, 2007
Callucinella Chavan, 1961 †
Cardiolucina
Cavilinga Chavan, 1937
Chavania Glover & J. D. Taylor, 2001
Clathrolucina J. D. Taylor & Glover, 2013
Discolucina Glover & J. D. Taylor, 2007
Divalinga Chavan, 1951
Divaricella von Martens, 1880
Easmithia Glover & J. D. Taylor, 2016
Falsolucinoma Cosel, 2006
Ferrocina Glover & Taylor, 2007
Funafutia Glover & J. D. Taylor, 2001
Gibbolucina Cossmann, 1904
Guyanella J. D. Taylor & Glover, 2016
Here Gabb, 1866
Imparilucina J. D. Taylor & Glover, 2021
Indoaustriella Glover, J. D. Taylor & S. T. Williams, 2008
Jallenia Glover & J. D. Taylor, 2016
Joellina Cosel, 2006
Keletistes P. G. Oliver, 1986
Lamellolucina J. D. Taylor & Glover, 2002
Lamylucina Cosel, 2006
Lepidolucina Glover & Taylor, 2007
Liralucina Glover & Taylor, 2007
Loripes Poli, 1791
Lucina Bruguière, [1797]
Lucinella Monterosato, 1883
Lucinisca Dall, 1901
Megaxinus Brugnone, 1880
Nevenulora Iredale, 1930
Notocina J. D. Taylor & Glover, 2019
Parvidontia Glover & Taylor, 2007
Parvilucina Dall, 1901
† Paslucina Olsson, 1964
Phacoides Agassiz, 1846
Pillucina Pilsbry, 1921
Pleurolucina Dall, 1901
Plicolucina Glover, J. D. Taylor & Slack-Smith, 2003
Pompholigina Dall, 1901
Pusillolucina J. D. Taylor & Glover, 2019
Radiolucina Britton, 1972
Rasta J. D. Taylor & Glover, 2000
Rugalucina J. D. Taylor & Glover, 2019
Scabrilucina J. D. Taylor & Glover, 2013
Semelilucina Cosel & Bouchet, 2008
Stewartia Olsson, A. & Harbison, A. 1953
Troendleina Cosel & Bouchet, 2008
Wallucina Iredale, 1930
Subfamily Milthinae Chavan, 1969
Armimiltha Olsson & Harbison, 1953†
Eomiltha Cossmann, 1912†
Miltha H. Adams & A. Adams, 1857
Milthoidea Marwick, 1931†
Retrolucina J. D. Taylor & Glover, 2018
Subfamily Monitilorinae J. D. Taylor & Glover, 2011
Monitilora Iredale, 1930
Prophetilora Iredale, 1930
Subfamily Myrteinae Chavan, 1969
Elliptiolucina Cosel & Bouchet, 2008
Eulopia Dall, 1901
† Gardnerella Chavan, 1951
Gloverina Cosel & Bouchet, 2008
Graecina Cosel, 2006
Jorgenia J. D. Taylor & Glover, 2009
Myrtea Turton, 1822
Notomyrtea Iredale, 1924
Rostrilucina Cosel & Bouchet, 2008
Solelucina Glover & Taylor, 2007
Taylorina Cosel & Bouchet, 2008
Tellidorella Berry, 1963
Subfamily Pegophyseminae J. D. Taylor & Glover, 2011
Afrophysema J. D. Taylor & Glover, 2005
Bythosphaera J. D. Taylor & Glover, 2005
Cavatidens Iredale, 1930
Cryptophysema J. D. Taylor & Glover, 2005
Euanodontia J. D. Taylor & Glover, 2005
Loripinus Monterosato, 1883
Meganodontia Bouchet & Cosel, 2004
Pegophysema Stewart, 1930
Incertae sedis (Subfamily not yet assigned )
† Amanocina Kiel, 2013
† Cavilucina P. Fischer, 1887
† Claibornites Stewart, 1930
† Cubatea Kiel, 2013
† Dilora Marwick, 1948
† Eophysema Stewart, 1930
† Jagolucina Chavan, 1937
† Jagonoma Chavan, 1946
† Luciniola Skeat & Madsen, 1898
† Mesolinga Chavan, 1951
† Mesomiltha Chavan, 1938
† Microloripes Cossmann, 1912
† Milthona Marwick, 1931
† Myrteopsis Sacco, 1901
† Nymphalucina Speden, 1970
† Pseudomiltha P. Fischer, 1887
† Pterolucina Chavan, 1942
† Pteromyrtea Finlay, 1926
† Saxolucina Stewart, 1930
† Volupia Defrance, 1829
References
Stewart, R. B. (1930). Gabb's California Cretaceous and Tertiary type lamellibranchs. Academy of Natural Sciences of Philadelphia, Special Publications. 3: 1-314, pls 1-17.
Taylor J. & Glover E. (2021). Biology, evolution and generic review of the chemosymbiotic bivalve family Lucinidae. London: The Ray Society [Publication 182]. 319 pp.
Powell A W B, New Zealand Mollusca, William Collins Publishers Ltd, Auckland, New Zealand 1979
Bivalve families
Taxa named by John Fleming (naturalist)
Extant Silurian first appearances
Chemosynthetic symbiosis