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Tridacna noae, also known as Noah’s Giant Clam or the Teardrop clam, is a species of giant saltwater clam. Up until recently, T. noae was mistaken as being part of the giant clam species Tridacna maxima. It has a broad distribution, mainly Indo-Pacific.

Populations of T. noae, as all giant clams, are declining due to human exploitation of flesh for consumption, shells, and as aquarium pets. T. noae and all members of Tridacninae are considered endangered, and have been since 1985.

Taxonomy
T. noae are marine bivalve molluscs in the subfamily Tridacninae, 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 Tridacninae, 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 and Japan by Tang (2005) 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-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.