FRAGUM SPP. AS POTENTIAL (PALEO-) ENVIRONMENTAL ARCHIVES

Photosymbiosis in bivalve clams has been documented in Tridacna, Hippopus, Corculum and Fragum genera. It has evolved at least twice and thus there are different anatomical locations within which zooxanthellae live. Photosymbiotic influence on growth can be seen in the microstructure of light and dark increments in the internal layer of Tridacnid shells because these clams secrete their large, dense shells daily. Although Fragum’s relationship with zooxanthellae differs from Tridacna, the goal of this study is to determine if there are similar microstructural characteristics in Fragum unedo and Fragum fragum shells. Shells used in this study were collected alive in 1923 and 1958, respectively. Scanning electron microscopy shows that F. unedo shells are composed entirely of a crossed lamellar microstructure. Also, an underlying, regular pattern of light and dark oscillating increments extends from the margin of growth. F. fragum also has a crossed lamellar microstructure with regular increments apparent predominantly on the margin of growth, similar to F. unedo. F. fragum has an additional prismatic layer on the inside of the shell. Although chemical analysis is needed to support the hypothesis that these increments could be daily, they provide a useful mechanism of aging the shell. Increment counting suggests that F. unedo is likely about 3.37 years old and F. fragum is 2 years old. These Fragum species are thought to have evolved 25 Ma and have a wide range in the Indo-Pacific, therefore a well preserved and well dated collection could provide a robust climate record for the recent Cenozoic. Additionally, the ability to see regular and potentially daily growth increments in another photosymbiotic bivalve may allow for confident chemical analyses (trace element and stable isotope) of these clams at a high resolution of a known time interval.