The Colorful Mantles of Giant Clams

The electric blues of a juvenile giant clam (Tridacna sp.) stand out sharply from the surrounding coral off the island of Pohnpei, Federated States of Micronesia. 
​Source: David Burdick., Public domain, via Wikimedia Commons ​https://commons.wikimedia.org/wiki/File:Reef1427_-_Flickr_-_NOAA_Photo_Library.jpg

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More color variations in the mantle of this Tridacna gigas.
​Photographer Globus 60 Downloade under Standard Rights from Depositphotos.com  

​The polychromic mantle tissues of giant clam give these Bivalves Of Unusual Size (apologies to the writers of the "Princess Bride") a growth advantage in the nutrient-poor, shallow waters of the tropical Indo-Pacific Region.  These colors are the result of a symbiotic relationship between the clams and single celled algae that live within the clams’ tissues.  
 
The mantle is an important organ for all mollusks; in bivalves it covers their internal organs, creates a filter chamber and secretes the shell. Most bivalves minimize the amount of mantle they expose to the external environment, however giant clams have gone in the opposite direction and maximized the amount of mantle that can be reached by the sun.
 
Most bivalved mollusks are filter feeders who pull water into their bodies through a tube-like siphon, run it across their gills to remove phytoplankton and detritus for nourishment and then remove the waste water through an outgoing siphon.  In addition to filter feeding giant clams also obtain nourishment through a symbiotic relationship with photosynthetic dinoflagellates (algae) of the family Symbiodiniaceae that live within their mantles. These organisms are the same algae that form a symbiotic relationship with reef-forming corals and used to be known as zooxanthellae.
 
Giant clams have a number of macro and micro modifications to help dinoflagellates optimize their photosynthetic production of nutrients.  At the macro scale the giant clams’ shells along with their mantles and siphons are oriented toward the surface for maximum exposure to sunlight which is needed by their internal symbiotic algae.  At the microscopic level the mantle contains specialized structures called iridocytes which protect the clam’s tissues from ultraviolet light and allow light to penetrate deeper into the tissue so that more dinoflagellates can use it.  The iridocytes work by both reflecting and absorbing UV light.  Once absorbed, the UV light is converted to visible light waves and remitted, allowing the algae to use it for photosynthesis and that our human eyes see as beautiful iridescence colors.
 
There are a total of seven species of giant clams in two genera.  The Tridacna contains five species while the Hippopus has two species.  Giant clams are the largest living bivalves, growing up to 120 cm (≈ 47 in.) long and weighing more than 200 kg (440 lb.).  They are distributed throughout the tropical Indo-Pacific Region, ranging from the Pitcairn Islands in the eastern Pacific to the East African coast and from the Great Barrier Reef in Australia northward to the island of Okinawa.