IMPLICATIONS OF ALGAL BIOEROSION ON GIANT CLAM SHELL CHEMISTRY

Many organisms commonly bioerode mollusk shells. For example, giant clams, Tridacninae, which live in the shallow, warm waters of coral reefs, are sometimes subjected to a less common shell parasite: Ostreobium boring green algae. Endolithic Ostreobium are adapted to low light environments and find a safe habitat to thrive within giant clam shells. The characteristic complex network of bright green tubes is well documented in coral skeletons and the general green hue has previously been reported in Tridacnids. However, there has been no consideration of the chemical influences Ostreobium may have on the shell material. When sectioned, Tridacna derasa shells, collected alive in early 2016 in Palau, have a visible bright green layer within the shell. To determine the source of organic matter within the shell material, ontogenic pieces of at least 30 g are decalcified in 0.5 M EDTA. The solution is changed every two days and any suspended organic material is separated. Stable carbon isotopes (δ¹³C) of the extracted organic matrix helps to identify the isolate as Ostreobium (-15.9 ± 1.5 ‰VPDB) or Tridacna (Dreier et al., 2014 found -21.4 ± 1.6 ‰VPDB) as it is released from the aragonite. Stable nitrogen isotopes (δ¹⁵N) do not significantly differ. Although the whole shell is being decalcified simultaneously, the bioeroded external layer has more surface area than the internal. Therefore, the interwoven Ostreobium is predominantly detected in the first four to nine of twenty isolations. Understanding this sequence is crucial for quality control in additional isotopic or genetic applications in both modern and fossil shells. Tridacna specimens at the Academy of Natural Sciences of Drexel University, collected as early as 1923, still have the visible bioerosion pattern within the shell suggesting that organic material is present in the shell aragonite. This could pose a problem for the potential use of fossil and sub-fossil shells which may have been subjected to chemical alteration. Caution must be used when analyzing affected specimens.