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 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, 1937Codakia Scopoli, 1777Ctena Mörch, 1860Divalucina Iredale, 1936Epicodakia Iredale, 1930Epilucina Dall, 1901Lucinoma Dall, 1901
;Subfamily Fimbriinae Nicol, 1950
- † Cerkesia Monari, 2003
- † Cyclopellatia Cossmann, 1907 Fimbria
- † Haastina Marwick, 1953
- † Mutiella Stoliczka, 1871
- † Parvicorbis Cossmann, 1892
- † Schafhaeutlia Cossmann, 1897
- † Sphaera J. Sowerby, 1822
- † Sphaeriola Stoliczka, 1871
- † Rawya Strougo, 1975 Tinalucina Cosel, 2006Ustalucina J. D. Taylor & Glover, 2021
- † Paslucina Olsson, 1964 Phacoides Agassiz, 1846Pillucina Pilsbry, 1921Pleurolucina Dall, 1901Plicolucina Glover, J. D. Taylor & Slack-Smith, 2003Pompholigina Dall, 1901Pusillolucina J. D. Taylor & Glover, 2019
- Radiolucina Britton, 1972Rasta J. D. Taylor & Glover, 2000Rugalucina J. D. Taylor & Glover, 2019Scabrilucina J. D. Taylor & Glover, 2013Semelilucina Cosel & Bouchet, 2008Stewartia Olsson, A. & Harbison, A. 1953Troendleina Cosel & Bouchet, 2008Wallucina Iredale, 1930
;Subfamily Monitilorinae J. D. Taylor & Glover, 2011Monitilora Iredale, 1930Prophetilora Iredale, 1930
;Subfamily Myrteinae Chavan, 1969Elliptiolucina Cosel & Bouchet, 2008Eulopia Dall, 1901
- † Gardnerella Chavan, 1951Gloverina Cosel & Bouchet, 2008Graecina Cosel, 2006Jorgenia J. D. Taylor & Glover, 2009Myrtea Turton, 1822Notomyrtea Iredale, 1924Rostrilucina Cosel & Bouchet, 2008Solelucina Glover & Taylor, 2007Taylorina Cosel & Bouchet, 2008Tellidorella Berry, 1963
;Incertae sedis
- † 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