''Isognomon radiatus'', the radial purse oyster or Lister's tree oyster, is a species of

bivalve Bivalvia (), in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of marine and freshwater molluscs that have laterally compressed bodies enclosed by a shell consisting of two hinged parts. As a group, bival ...

mollusc Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is esti ...

in the

family Family (from la, familia) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its ...

Isognomonidae Isognomonidae is a family (biology), family of medium-sized to large saltwater clams. They are pearl oysters, Marine (ocean), marine bivalve molluscs in the superfamily PterioideaMolluscaBase eds. (2021). MolluscaBase. Isognomonidae Woodring, 192 ...

. It can be found along the Atlantic coast of North America, ranging from southern

Florida Florida is a state located in the Southeastern region of the United States. Florida is bordered to the west by the Gulf of Mexico, to the northwest by Alabama, to the north by Georgia, to the east by the Bahamas and Atlantic Ocean, and to ...

Brazil Brazil ( pt, Brasil; ), officially the Federative Republic of Brazil (Portuguese: ), is the largest country in both South America and Latin America. At and with over 217 million people, Brazil is the world's fifth-largest country by area ...

and

Bermuda ) , anthem = "God Save the King" , song_type = National song , song = " Hail to Bermuda" , image_map = , map_caption = , image_map2 = , mapsize2 = , map_caption2 = , subdivision_type = Sovereign state , subdivision_name = , e ...

Description

Lister's tree oyster is an irregular, fan-shaped shell growing to about long. Each

valve A valve is a device or natural object that regulates, directs or controls the flow of a fluid (gases, liquids, fluidized solids, or slurries) by opening, closing, or partially obstructing various passageways. Valves are technically fittings ...

is a pale brown colour with whitish rays radiating from the umbo, the raised hump that is the first part of the shell to form when the animal is a juvenile. There are numerous sculptured concentric rings on the outside surface. It is attached to a hard substrate by

byssus A byssus () is a bundle of filaments secreted by many species of bivalve mollusc that function to attach the mollusc to a solid surface. Species from several families of clams have a byssus, including pen shells (Pinnidae), true mussels (Mytilid ...

threads and often occurs in clusters.

Distribution and habitat

Lister's tree oyster occurs in southern Florida, the Caribbean Sea and Gulf of Mexico, Bermuda, the West Indies, the Cayman Islands and Brazil. It grows, sometimes in dense patches, on the roots of mangroves and on shallow rocky areas, down to a depth of about . On coral reefs it occurs on rock ledges and exposed rocks in areas of high sedimentation, forming mats of hundreds of individual animals. It also grows on the shells of the Atlantic thorny oyster ('' Spondylus americanus''). In

Little Cayman Little Cayman is one of three Islands that make up the Cayman Islands. It is located in the Caribbean Sea, approximately 60 miles (96 km) northeast of East End, Grand Cayman and five miles (8 km) west of West End, Cayman Brac. Little C ...

it occupies a particular zone on the lower shore and shallow subtidal region of sheltered bays where it is common under large stones, often accompanied by the rock boring urchin (''

Echinometra lucunter ''Echinometra lucunter'', the rock boring urchin, is a species of sea urchin in the family Echinometridae. It is found in very shallow parts of the western Atlantic Ocean and the Caribbean Sea. Description ''Echinometra lucunter'' has an elli ...

''). It also occurs in rock pools. In more exposed locations on the island it is replaced by the closely related flat tree oyster ('' Isognomon alatus''). Lister's tree oyster is a constituent of the characteristic invertebrate fauna of the mangrove swamps fringing the southern part of Florida.

References

{{Taxonbar, from=Q3017904 Pteriida Bivalves described in 1839