Belongs within: Lasaeidae.
The Galeommatidae are a family of small bivalves in which the shell may be completely enclosed by the expanded mantle. The shell itself is often reduced with the valves transverse, irregular and/or gaping. A number of galeommatids live as commensals in the burrows of invertebrates such as echinoderms or crustaceans.
The Leptonidae are a group of small byssiferous bivalves with a generally compressed shell bearing very small beaks.
Bouncing snail-y clams
Published 23 July 2022
For the most part, bivalves are a fairly conservative bunch. They seem to have worked out what they are good at early on in their history and most of them stick to it. There are, however, notable exceptions and perhaps few groups of bivalves are as exceptional as the Galeommatidae.
Galeommatids are small bivalves, less than a centimetre in length, with more or less thin shells. The hinge teeth are generally weak or absent. The valves of the shell are more or less gaping and in life are at least partially covered by the large, reflected mantle. The outer surface of the mantle is warty and bears several slender tentacles, the exact arrangement of tentacles varying by species. The foot is large and extends well outwards from the central body of the animal. In the most extreme cases, you might be forgiven for thinking you were looking at some sort of snail rather than a clam.
Many galeommatids have been found living as symbionts with other invertebrates such as in the burrows of annelids and crustaceans, or crawling on the surface of echinoderms. So far as is known, these relationships are commensal only, the clams using their hosts as a source of shelter and possibly excess food scraps, but species may be very exclusive in their choice of hosts. For instance, Mikkelsen & Bieler (1989) found the species Divariscintilla yoyo and D. troglodytes only in burrows of the mantis shrimp Lysiosquilla scabricauda, never in burrows of other potential hosts in the same area. It seems likely that this commensalism has allowed galeommatids to diversify in soft-bottom habitats, their larger hosts being able to dig into sediments in which the smaller clams would be quickly smothered (Valentich-Scott et al. 2013).
Many galeommatids possess a distinctive ‘hanging-foot’ morphology with the foot divided into two sections, a muscular anterior portion adapted for snail-like crawling, and an elastic posterior section (Bieler & Mikkelsen 1992). The primary byssus gland is located in the anterior section and is connected by a ciliated ventral groove to a terminal adhesive gland in the posterior section. Mikkelsen & Bieler (1989) found that Divariscintilla individuals kept in an aquarium spent most of their time hanging suspended via the posterior part of the foot. Threads produced by the byssus gland were transferred to the terminal adhesor and used to attach to a surface such as the glass of the aquarium (presumably, the clams would normally hang in this manner on the interior wall of the host burrow). When disturbed, hanging clams would rapidly bounce themselves up and down from their attachment point (hence one species being dubbed ‘Divariscintilla yoyo‘). If the clams wished to change their location, they would crawl on the muscular section of the foot, breaking the byssus threads behind them. The elastic part of the foot was not functional in crawling.
The majority of galeommatid clams are hermaphrodites, either protandrous (beginning life as males before maturing into females) or simultaneous. Eggs are not released into the water column but brooded within the ctenidia until larvae are released at a relatively advanced stage of development (whether the parent is able to feed while its gills are so occupied, I don’t know). In a number of species, dwarf males are also present that do not live independently but reside within the mantle cavity of a female (I have seen these males referred to as ‘parasitic’ but I do not know if they are directly so). In this position, they are able to fertilise the female directly. Such behaviour may be seen as a further adaptation to the clam’s commensal lifestyle, contained within the burrow of its host and potentially secluded from more conventional mates. Hidden away in the darkness, they make matryoshkas of themselves.
Systematics of Galeommatidae
<==Galeommatidae |--Pseudogaleomma japonica (Adams 1868)TW07 |--PassyaSB93 |--Phlyctaenachlamys lysiosquillinaBRW98 |--Ephippodonta macdougalliBRW98 |--Montacutona compactaBRW98 |--Isoconcha integra [=Benthoquetia integra]BRW98 |--Chlamydoconcha [Chlamydoconchidae, Chlamydoconchoidea]M96 | `--C. orcutti Dall 1884M12 |--Zemyllita Finlay 1927P61 | `--*Z. stowei (Hutton 1873)P61 [=Pythina stoweiP61, Myllita stoweiF27] |--Marikellia Iredale 1936BRW98, P61 | |--*M. solida [=Kellia solida]P61 | `--M. rotunda (Deshayes 1855)P61 |--Melliteryx Iredale 1924P61 | |--*M. acupuncta Hedley 1902P61, K90 [=Erycina acupunctaP61] | `--M. parva (Deshayes 1856)P61 [=Erycina parvaF27] |--Divariscintilla Powell 1932P61 | |--*D. maoria Powell 1932P61 | |--D. troglodytesGW02 | `--D. yoyoGW02 |--Borniola Iredale 1924BRW98, P61 | |--*B. lepida [=Bornia lepida]P61 | |--B. neozelanica Powell 1937P61 | `--B. reniformisHS01 |--Scintilla Deshayes 1855P61 | |--*S. philippinensisP61 | |--S. clarkeana Aldrich 1897SB93 | |--S. stevensoni Powell 1932P61 | `--S. strangei Deshayes 1856BRW98, H09 [=Solecardia strangeiH09] |--GaleommaGW02 | |--G. denticulata Deshayes 1863H09 | |--G. politaBRW98 | |--G. (Paralepida takiiBRW98 | `--G. turtoniGW02 |--Scintillona Finlay 1927P61 | |--*S. zelandicus (Odhner 1924) [=Spaniorinus zelandicus]P61 | |--S. bellerophon Ó Foighil & Gibson 1984V-SOFL13 | |--S. benthicola Dell 1956P61 | |--S. cryptozoica (Hedley 1917)M12 | |--S. daviei Morton 2008M12 | `--S. stigmatica (Pilsbry 1921)V-SOFL13 |--Arthritica Finlay 1927BRW98, P61 | |--*A. bifurca [=Kellia bifurca]P61 | |--A. crassiformis Powell 1933P61 | |--A. helmsi (Hedley 1915) [=Erycina helmsi]F27 | `--A. semenBRW98 |--LeptonBRW98 | |--L. clarkiaeN79 | |--L. meroeum Carpenter 1864C64 | |--L. nitidumN79 | `--L. squamosumJ64 |--Myllitella Finlay 1927P61 | |--*M. vivens Finlay 1927P61 | |--M. finlayi (Marwick 1924) [=Myllita finlayi]F27 | |--M. pinguis Marwick 1928P61 | `--M. tasmanica [=Myllita tasmanica]F27 |--SolecardiaF27 | |--S. alberti (Smith 1884) [=Scintilla alberti]H09 | |--S. aurantiaca (Deshayes 1856) [=Scintilla aurantiaca]H09 | |--S. cuvieri (Deshayes 1856) [=Scintilla cuvieri]H09 | |--S. hyalina (Deshayes 1856) [=Scintilla hyalina]H09 | |--S. purpurascens (Sowerby 1874) [=Scintilla purpurascens]H09 | `--S. turgescens (Deshayes 1856) [=Scintilla turgescens]H09 `--Waldo Nicol 1966V-SOFL13 | i. s.: *W. parasiticus (Dall 1876) [=Lepton parasiticus]V-SOFL13 | W. trapezialis Zelaya & Ituarte 2002V-SOFL13 |--W. digitatus Zelaya & Ituarte 2013V-SOFL13 `--+--W. arthuri Valentich-Scott, Ó Foighil & Li 2013V-SOFL13 `--W. paucitentaculatus Zelaya & Ituarte 2013V-SOFL13
*Type species of generic name indicated
References
[BRW98] Beesley, P. L., G. J. B. Ross & A. Wells (eds) 1998. Fauna of Australia vol. 5. Mollusca: The Southern Synthesis. Part A. Australian Biological Resources Study: Canberra.
Bieler, R., & P. M. Mikkelsen. 1992. Preliminary phylogenetic analysis of the bivalve family Galeommatidae. American Malacological Bulletin 9 (2): 157–164.
[C64] Carpenter, P. P. 1864. Supplementary report on the present state of our knowledge with regard to the Mollusca of the west coast of North America. Report of the British Association for the Advancement of Science 33: 517–686.
[F27] Finlay, H. J. 1927. A further commentary on New Zealand molluscan systematics. Transactions and Proceedings of the New Zealand Institute 57: 320–485.
[GW02] Giribet, G., & W. Wheeler. 2002. On bivalve phylogeny: a high-level analysis of the Bivalvia (Mollusca) based on combined morphology and DNA sequence data. Invertebrate Biology 121 (4): 271–324.
[HS01] Hayward, B. W., A. B. Stephenson, M. S. Morley, W. M. Blom, H. R. Grenfell, F. J. Brook, J. L. Riley, F. Thompson & J. J. Hayward. 2001. Marine biota of Parengarenga Harbour, Northland, New Zealand. Records of the Auckland Museum 37: 45–80.
[H09] Hedley, C. 1909. The Marine Fauna of Queensland: Address by the President of Section D. Australasian Association for the Advancement of Science: Brisbane.
[J64] Jeffreys, J. G. 1864. Report of the committee appointed for exploring the coasts of Shetland by means of the dredge. Report of the British Association for the Advancement of Science 33: 70–81.
[K90] Kendrick, G. W. 1990. A Pleistocene molluscan fauna with Anadara trapezia (Deshayes) (Bivalvia: Arcoida) from the Dampier Limestone of Shark Bay, Western Australia. In: Berry, P. F., S. D. Bradshaw & B. R. Wilson (eds) Research in Shark Bay: Report of the France-Australe Bicentenary Expedition Committee pp. 33–48. Western Australian Museum.
Mikkelsen, P. M., & R. Bieler. 1989. Biology and comparative anatomy of Divariscintilla yoyo and D. troglodytes, two new species of Galeommatidae (Bivalvia) from stomatopod burrows in eastern Florida. Malacologia 31 (1): 175–195.
[M96] Morton, B. 1996. The evolutionary history of the Bivalvia. In: Taylor, J. D. (ed.) Origin and Evolutionary Radiation of the Mollusca pp. 337–359. Oxford University Press: Oxford.
[M12] Morton, B. 2012. The biology and functional morphology of Nucula pusilla (Bivalvia: Protobranchia: Nuculidae) from Western Australia, Australia: primitive or miniature simplicity? Records of the Western Australian Museum 27 (2): 85–100.
[N79] Norman, A. M. 1879. The Mollusca of the fiords near Bergen, Norway. Journal of Conchology 2: 8–77.
[P61] Powell, A. W. B. 1961. Shells of New Zealand: An illustrated handbook 4th ed. Whitcombe and Tombs Limited: Christchurch.
[SB93] Skelton, P. W., & M. J. Benton. 1993. Mollusca: Rostroconchia, Scaphopoda and Bivalvia. In: Benton, M. J. (ed.) The Fossil Record 2 pp. 237–263. Chapman & Hall: London.
[TW07] Taylor, J. D., S. T. Williams, E. A. Glover & P. Dyal. 2007. A molecular phylogeny of heterodont bivalves (Mollusca: Bivalvia: Heterodonta): new analyses of 18S and 28S rRNA genes. Zoologica Scripta 36 (6): 587–606.
[V-SOFL13] Valentich-Scott, P., D. Ó Foighil & J. Li. 2013. Where’s Waldo? A new commensal species, Waldo arthuri (Mollusca, Bivalvia, Galeommatidae) from the northeastern Pacific Ocean. ZooKeys 316: 67–80.