''Tridacna noae'', also known as Noah’s giant clam or the Teardrop giant clam, is a species of giant

clam Clam is a common name for several kinds of bivalve molluscs. The word is often applied only to those that are edible and live as infauna, spending most of their lives halfway buried in the sand of the seafloor or riverbeds. Clams have two she ...

. Up until recently, ''T. noae'' was confused with the small giant clam ''

Tridacna maxima The maxima clam (''Tridacna maxima''), also known as the small giant clam, is a species of bivalve mollusc found throughout the Indo-Pacific region.MolluscaBase eds. (2022). MolluscaBase. Tridacna maxima (Röding, 1798). Accessed through: Worl ...

'', but is now known to be its own independent species. It has a broad distribution in the

Indo-Pacific The Indo-Pacific is a vast biogeographic region of Earth. In a narrow sense, sometimes known as the Indo-West Pacific or Indo-Pacific Asia, it comprises the tropical waters of the Indian Ocean, the western and central Pacific Ocean, and the ...

. As with all giant clams, populations of ''T. noae'', are declining due to human exploitation of flesh for consumption, shells, and as aquarium pets. ''T. noae'' and all members of ''

Tridacninae Tridacninae, common name, the giant clams, is a Taxonomy (biology), taxonomic family (biology), subfamily of very large saltwater clams, marine (ocean), marine bivalve molluscs in the family Cardiidae, the cockles. Description This subfamily con ...

'' are considered endangered, and have been since 1985.

Taxonomy

''T. noae'' are marine

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 ...

molluscs 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 taxon, extant species of molluscs are recognized. The number of fossil sp ...

in the subfamily ''

'', also known as giant clams. ''T. noae'' was originally described and named by Röding (1798) based on figures in Chemnitz (1784).

Description

''T. noae'' have a physical appearance typical to that of most bivalves, especially those in the ''

'', or giant clam, subfamily. ''T. noae'' typically have a shell length between 6–20 cm, and shells usually have 5-7 radial ribs. Mantle colors may vary and include brown, yellow, blue, and green. Black hyaline organs, or eyes, are arranged along the border of the mantle, along with a thin, white margin and ocellate spots.

Confusion with ''T. maxima''

''T. noae'' was first recognized as a cryptic species from ''T. maxima'' in

Taiwan Taiwan, officially the Republic of China (ROC), is a country in East Asia, at the junction of the East and South China Seas in the northwestern Pacific Ocean, with the People's Republic of China (PRC) to the northwest, Japan to the nort ...

and

Japan Japan ( ja, 日本, or , and formally , ''Nihonkoku'') is an island country in East Asia. It is situated in the northwest Pacific Ocean, and is bordered on the west by the Sea of Japan, while extending from the Sea of Okhotsk in the north ...

by Tang (200

and Kubo and Iwai (2007) respectively. DNA sequencing characterizes the two as distinct species. ''T. noae'' is even closer related to other members of ''Tridacninae'' than ''T. maxima'' according to rRNA evidence. While ''T. noae'' is very similar in appearance to ''T. maxima'', there are key differences that distinguish between the two, such as shell morphology, mantle pattern, and mantle coloration. The mantles of ''T. noae'' have ocellate spots with a thin, white border around the edge, along with hyaline organs—also known as eyes—that have an arrangement distinctive of the species. Although proven to be an unreliable distinguishing factor, ''T. noae'' usually have a higher number, between 5 and 7, of radial ribs on their shell, compared with between 4 and 6 in ''T. maxima''. Confusion between the two species has led to overestimated population densities of ''T. maxima'' and potentially misleading conclusions in studies of ''T. maxima'' populations.

Distribution

''T. noae'' have a broad distribution, mainly Indo-Pacific, Indo-West Pacific, that overlaps with that of ''T. maxima''. T. noae live in shallow waters, up to 20 meters in depth, and are typically found on the edges and crests of coral reefs, attached by byssal threads. ''T. noae'' populations were first recognized in Taiwan and Japan, but have been observed as far west as

Christmas Island Christmas Island, officially the Territory of Christmas Island, is an Australian external territory comprising the island of the same name. It is located in the Indian Ocean, around south of Java and Sumatra and around north-west of the ...

. Mitochondrial DNA markers break up the distribution of ''T. noae'' into three groups: east Indo-Malay archipelago to Western Australia, Melanesia to Micronesia, and Central Polynesia.

Threats and Conservation

All giant clams, including ''T. noae'', have been listed in Appendix II of the

Convention on International Trade in Endangered Species CITES (shorter name for the Convention on International Trade in Endangered Species of Wild Fauna and Flora, also known as the Washington Convention) is a multilateral treaty to protect endangered plants and animals from the threats of interna ...

since 1985 and on the International Union for Conservation of Nature (IUCN) Red List since 1986. Populations of ''T. noae'' have been on a decline due to human exploitation of the species. Known colloquially as the “teardrop” clam, ''T. noae'' are exploited for their flesh for consumption, their shells for jewelry and collections, and as aquarium pets. Conservation efforts have been made to replenish the wild populations of ''T. noae'' through breeding in captivity, with the additional goal of culture for commercial markets.

References

{{Taxonbar, from=Q20721393 noae Bivalves described in 1798