HEART COCKLE WINDOWS AND TRANSLUCENCY: IMPLICATIONS FOR ENVIRONMENTAL STUDIES

Heart cockles (Fraginae: Corculum) live in shallow coral reefs of the Pacific Ocean, situated in and among sandy substrate, sometimes overlain by a thin layer of sediment. Corculum have a photosymbiotic relationship with dinoflagellate algae, commonly called zooxanthellae, that live underneath the shell. Microstructural windows are thought to allow sunlight to pass through intercostal space on the posterior side of the shell providing an environment for zooxanthellae to flourish. In 2010 (Huber) and 2013 (Huber and Zürich), Corculum cardissa was split into seven different species. On a macroscopic level, species display differences in the translucency of the posterior of the shell; this is used for species diagnosis, in addition to shell morphology. Translucency may also be an indication of the presence of windows, which we infer as a dependence on zooxanthellae. For instance, scanning electron microscopy (SEM) shows that C. cardissa have windows and macroscopically translucent shells, meanwhile, C. impressum do not have microstructural windows and shells are not translucent. Here, we present SEM micrographs and transmitted light images of the intercostal space of five species that will help determine the presence of windows and possible dependence on zooxanthellae in the genus Corculum. We predict a divergence among species: some have windows and harbor zooxanthellae under their shells and others do not have windows and host the zooxanthellae in exposed mantle tissues similar to the gaping mechanism observed in Fragum. We await field studies to confirm these predictions. Understanding how the relationship differs between species is important because clams with a dependence on photosynthetic algae are thought to be important environmental records of solar irradiance. We aim to determine whether Corculum species are recording their local environment congruently and can be used as equal environmental records in future studies.