Lucinidae, common name hatchet shells, is a

family Family (from ) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictabili ...

of saltwater

clam Clam is a common name for several kinds of bivalve mollusc. The word is often applied only to those that are deemed edible and live as infauna, spending most of their lives halfway buried in the sand of the sea floor or riverbeds. Clams h ...

s, marine

bivalve Bivalvia () or bivalves, in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class (biology), class of aquatic animal, aquatic molluscs (marine and freshwater) that have laterally compressed soft bodies enclosed b ...

molluscs Mollusca is a phylum of protostome, protostomic invertebrate animals, whose members are known as molluscs or mollusks (). Around 76,000 extant taxon, extant species of molluscs are recognized, making it the second-largest animal phylum ...

. These bivalves are remarkable for their

endosymbiosis An endosymbiont or endobiont is an organism that lives within the body or cells of another organism. Typically the two organisms are in a mutualism (biology), mutualistic relationship. Examples are nitrogen-fixing bacteria (called rhizobia), whi ...

with sulphide-oxidizing

bacteria Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...

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

Eocene The Eocene ( ) is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (Ma). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene'' comes ...

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 Phagocytosis () is the process by which a cell (biology), cell uses its plasma membrane to engulf a large particle (≥ 0.5 μm), giving rise to an internal compartment called the phagosome. It is one type of endocytosis. A cell that performs ph ...

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 The Silurian ( ) is a geologic period and system spanning 23.5 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the third and shortest period of t ...

; however, they did not diversify until the late

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...

, along with the evolution of

seagrass meadow A seagrass meadow or seagrass bed is an underwater ecosystem formed by seagrasses. Seagrasses are marine (saltwater) plants found in shallow coastal waters and in the brackish waters of estuaries. Seagrasses are flowering plants with stems and ...

s 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, 797/small> * '' 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 ublication 182 319 pp. * Powell A W B, ''New Zealand Mollusca'', William Collins Publishers Ltd, Auckland, New Zealand 1979 {{Authority control Bivalve families Taxa named by John Fleming (naturalist) Extant Silurian first appearances Chemosynthetic symbiosis